Action at a distance
In physics, action at a distance is the nonlocal interaction of objects that are separated in space.
This term was used most often in the context of early theories of gravity and electromagnetism to describe how an object responds to the influence of distant objects. More generally "action at a distance" describes the failure of early atomistic and mechanistic theories which sought to reduce all physical interaction to collision. The exploration and resolution of this problematic phenomenon led to significant developments in physics, from the concept of a field, to descriptions of quantum entanglement and the mediator particles of the standard model.
Electricity and magnetism
Efforts to account for action at a distance in the theory of electromagnetism led to the development of the concept of a field which mediated interactions between currents and charges across empty space. According to field theory we account for the Coulomb (electrostatic) interaction between charged particles through the fact that charges produce around themselves an electric field, which can be felt by other charges as a force. The concept of the field was elevated to fundamental importance in Maxwell's equations, which used the field to elegantly account for all electromagnetic interactions, as well as light (which, until then, had been a completely unrelated phenomenon). In Maxwell's theory, the field is its own physical entity, carrying momenta and energy across space, and action at a distance is only the apparent effect of local interactions of charges with their surrounding field.
Electrodynamics can be described without fields (in Minkowski space) as the direct interaction of particles with lightlike separation vectors. This results in the Fokker-Tetrode-Schwarzschild action integral. This kind of electrodynamic theory is often called "direct interaction" to distinguish it from field theories where action at a distance is mediated by a localized field (localized in the sense that its dynamics are determined by the nearby field parameters). This description of electrodynamics, in contrast with Maxwell's theory, explains apparent action at a distance not by postulating a mediating entity (the field) but by appealing to the natural geometry of special relativity.
Direct interaction electrodynamics is explicitly symmetrical in time, and avoids the infinite energy predicted in the field immediately surrounding point particles. Feynman and Wheeler have shown that it can account for radiation and radiative damping (which had been considered strong evidence for the independent existence of the field). However various proofs, beginning with that of Dirac have shown that direct interaction theories (under reasonable assumptions) do not admit Lagrangian or Hamiltonian formulations (these are the so-called No Interaction Theorems). Also significant is the measurement and theoretical description of the Lamb shift which strongly suggests that charged particles interact with their own field. Because of these difficulties, and others, it is fields that have been elevated to the fundamental operators in QFT and modern physics has largely abandoned direct interaction theory.
Newton's theory of gravity offered no prospect of identifying any mediator of gravitational interaction. His theory assumed that gravitation acts instantaneously, regardless of distance. Kepler's observations gave strong evidence that in planetary motion angular momentum is conserved. (The mathematical proof is only valid in the case of a Euclidean geometry.) Gravity is also known as a force of attraction between two objects because of their mass.
From a Newtonian perspective, action at a distance can be regarded as: "a phenomenon in which a change in intrinsic properties of one system induces a change in the intrinsic properties of a distant system, independently of the influence of any other systems on the distant system, and without there being a process that carries this influence contiguously in space and time" (Berkovitz 2008).
A related question, raised by Ernst Mach, was how rotating bodies know how much to bulge at the equator. This, it seems, requires an action-at-a-distance from distant matter, informing the rotating object about the state of the universe. Einstein coined the term Mach's principle for this question.
It is inconceivable that inanimate Matter should, without the Mediation of something else, which is not material, operate upon, and affect other matter without mutual Contact…That Gravity should be innate, inherent and essential to Matter, so that one body may act upon another at a distance thro' a Vacuum, without the Mediation of any thing else, by and through which their Action and Force may be conveyed from one to another, is to me so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it. Gravity must be caused by an Agent acting constantly according to certain laws; but whether this Agent be material or immaterial, I have left to the Consideration of my readers.—Isaac Newton, Letters to Bentley, 1692/3
According to Albert Einstein's theory of special relativity, instantaneous action at a distance was seen to violate the relativistic upper limit on speed of propagation of information. If one of the interacting objects were to suddenly be displaced from its position, the other object would feel its influence instantaneously, meaning information had been transmitted faster than the speed of light.
One of the conditions that a relativistic theory of gravitation must meet is to be mediated with a speed that does not exceed c, the speed of light in a vacuum. It could be seen from the previous success of electrodynamics that the relativistic theory of gravitation would have to use the concept of a field or something similar.
This problem has been resolved by Einstein's theory of general relativity in which gravitational interaction is mediated by deformation of space-time geometry. Matter warps the geometry of space-time and these effects are, as with electric and magnetic fields, propagated at the speed of light. Thus, in the presence of matter, space-time becomes non-Euclidean, resolving the apparent conflict between Newton's proof of the conservation of angular momentum and Einstein's theory of special relativity. Mach's question regarding the bulging of rotating bodies is resolved because local space-time geometry is informing a rotating body about the rest of the universe. In Newton's theory of motion, space acts on objects, but is not acted upon. In Einstein's theory of motion, matter acts upon space-time geometry, deforming it, and space-time geometry acts upon matter.
Since the early 20th century, quantum mechanics has posed new challenges for the view that physical processes should obey locality. Whether quantum entanglement counts as action-at-a-distance hinges on the nature of the wave function and decoherence, issues over which there is still considerable debate among scientists and philosophers. One important line of debate originated with Einstein, who challenged the idea that quantum mechanics offers a complete description of reality, along with Boris Podolsky and Nathan Rosen. They proposed a thought experiment involving an entangled pair of observables with non-commuting operators (e.g. position and momentum).
This thought experiment, which came to be known as the EPR paradox, hinges on the principle of locality. A common presentation of the paradox is as follows: two particles interact and fly off in opposite directions. Even when the particles are so far apart that any classical interaction would be impossible (see principle of locality), a measurement of one particle nonetheless determines the corresponding result of a measurement of the other.
After the EPR paper, several scientists such as de Broglie studied local hidden variables theories. In the 1960s John Bell derived an inequality that indicated a testable difference between the predictions of quantum mechanics and local hidden variables theories. To date, all experiments testing Bell-type inequalities in situations analogous to the EPR thought experiment have results consistent with the predictions of quantum mechanics, suggesting that local hidden variables theories can be ruled out. Whether or not this is interpreted as evidence for nonlocality depends on one's interpretation of quantum mechanics.
Non-standard interpretations of quantum mechanics vary in their response to the EPR-type experiments. The Bohm interpretation gives an explanation based on nonlocal hidden variables for the correlations seen in entanglement. Many advocates of the many-worlds interpretation argue that it can explain these correlations in a way that does not require a violation of locality, by allowing measurements to have non-unique outcomes.
- Quantum pseudo-telepathy
- Quantum teleportation
- Wheeler–Feynman absorber theory
- Dynamism (metaphysics)
- Hesse, Mary B. (December 1955). "Action at a Distance in Classical Physics". Retrieved 2012-11-04.
- Barut, A. O. "Electrodynamics and Classical Theory of Fields and Particles"
- Berkovitz, Joseph (2008). "Action at a Distance in Quantum Mechanics". In Edward N. Zalta. The Stanford Encyclopedia of Philosophy (Winter 2008 ed.).
- Einstein, A.; Podolsky, B.; Rosen, N. (1935). "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?". Physical Review 47 (10): 777–780. Bibcode:1935PhRv...47..777E. doi:10.1103/PhysRev.47.777.
- Bell, J.S. (1966). On the problem of hidden variables in quantum mechanics. Reviews of Modern Physics. 38(3). 447-452.
- Rubin (2001). "Locality in the Everett Interpretation of Heisenberg-Picture Quantum Mechanics". Found. Phys. Lett. 14 (4): 301–322. arXiv:quant-ph/0103079. doi:10.1023/A:1012357515678.


The first neotropical rainforest was home of the Titanoboa
Smithsonian researchers working in Colombia's Cerrejón coal mine have unearthed the first megafossil evidence of a neotropical rainforest. Titanoboa, the world's biggest snake, lived in this forest 58 million years ago at temperatures 3-5 C warmer than in rainforests today, indicating that rainforests flourished during warm periods. "Modern neotropical rainforests, with their palms and spectacular flowering-plant diversity, seem to have come into existence in the Paleocene epoch, shortly after the extinction of the dinosaurs 65 million years ago," said Carlos Jaramillo, staff scientist at the Smithsonian Tropical Research Institute. "Pollen evidence tells us that forests before the mass extinction were quite different from our fossil rainforest at Cerrejón. We find new plant families, large, smooth-margined leaves and a three-tiered structure of forest floor, understory shrubs and high canopy."
Historically, good rock exposures and concentrated efforts by paleontologists to understand the evolution of neotropical rainforests—one of the most awe-inspiring assemblages of plant and animal life on the planet—have been lacking. "The Cerrejón mining operation is the first clear window we have to see back in time to the Paleocene, when the neotropical rainforest was first developing," said Scott Wing, a paleontologist from the Smithsonian's National Museum of Natural History.
Some of the more than 2,000 fossil leaves, including the compound leaves and pods of plants in the bean family and leaves of the hibiscus family are among the oldest, reliable evidence of these groups. This was the first time that the plant families Araceae, Arecaceae, Fabaceae, Lauraceae, Malvaceae and Menispermaceae, which are still among the most common neotropical rainforest families, all occurred together.
Many newcomers to modern rainforests remark that the leaves all look the same, a reasonable observation given that most have smooth margins and long "drip-tips" thought to prevent water from accumulating on the leaf surface.
S. Joseph Wright, senior scientist at STRI, has noted that all of the areas in the world today with average yearly temperatures greater than 28 C are too dry to support tropical rainforests. If tropical temperatures increase by 3 C by the end of this century as predicted in the 2007 report of the Intergovernmental Panel on Climate Change, "We're going to have a novel climate where it is very hot and very wet. How tropical forest species will respond to this novel climate, we don't know," said Wright.
Based on leaf shape and the size of the cold-blooded Titanoboa, Cerrejón rainforest existed at temperatures up to 30-32 C and rainfall averages exceeded 2500 mm per year.
But Titanoboa's rainforest was not as diverse as modern rainforests. Comparison of the diversity of this fossil flora to modern Amazon forest diversity and to the diversity of pollen from other Paleocene rainforests revealed that there are fewer species at Cerrejón than one would expect. Insect-feeding damage on leaves indicated that they could have been eaten by herbivores with a very general diet rather than insects specific to certain host plants.
"We were very surprised by the low plant diversity of this rainforest. Either we are looking at a new type of plant community that still hadn't had time to diversify, or this forest was still recovering from the events that caused the mass extinction 65 million years ago," said Wing. "Our next steps are to collect and analyze more sites of the same age from elsewhere in Colombia to see if the patterns at Cerrejón hold, and study additional sites that bracket the Cretaceous mass extinction, in order to really understand how the phenomenal interactions that typify modern rainforests came to be."
Articles on the same topic
- Plant fossils give first real picture of earliest Neotropical rainforestsThu, 15 Oct 2009, 16:26:14 EDT
- Plant fossils give first real picture of earliest Neotropical rainforestsfrom Science BlogThu, 15 Oct 2009, 16:49:31 EDT
- Evidence found of neotropical rainforestfrom UPITue, 13 Oct 2009, 9:42:10 EDT
- The first neotropical rainforest was home of the Titanoboafrom Science CentricTue, 13 Oct 2009, 5:56:05 EDT
- First Neotropical Rainforest Was Home Of The Titanoboa -- World's Biggest Snakefrom Science DailyTue, 13 Oct 2009, 0:07:05 EDT
- The first neotropical rainforest was home of the Titanoboafrom PhysorgMon, 12 Oct 2009, 16:07:19 EDT
Latest Science NewsletterGet the latest and most popular science news articles of the week in your Inbox! It's free!
Learn more about
Check out our next project, Biology.Net
From other science news sites
Popular science news articles
- Hearing quality restored with bionic ear technology used for gene therapy
- NASA satellites show drought may take toll on Congo rainforest
- Superconducting qubit array points the way to quantum computers
- Scientists identify source of mysterious sound in the Southern Ocean
- From liability to viability: Genes on the Y chromosome prove essential for male survival
- Criticism of violent video games has decreased as technology has improved, gamers age
- Hummingbirds' 22-million-year-old history of remarkable change is far from complete
- Research clarifies health costs of air pollution from agriculture
- Ancient 'spider' images reveal eye-opening secrets
- New research finds 'geologic clock' that helps determine moon's age


||This article's introduction may be too long for the overall article length. (August 2010)|
Reversible computing is a model of computing where the computational process to some extent is reversible, i.e., time-invertible. In a computational model that uses transitions from one state of the abstract machine to another, a necessary condition for reversibility is that the relation of the mapping from states to their successors must be one-to-one. Reversible computing is generally considered an unconventional form of computing.
There are two major, closely related, types of reversibility that are of particular interest for this purpose: physical reversibility and logical reversibility.
A process is said to be physically reversible if it results in no increase in physical entropy; it is isentropic. These circuits are also referred to as charge recovery logic, adiabatic circuits, or adiabatic computing. Although in practice no nonstationary physical process can be exactly physically reversible or isentropic, there is no known limit to the closeness with which we can approach perfect reversibility, in systems that are sufficiently well-isolated from interactions with unknown external environments, when the laws of physics describing the system's evolution are precisely known.
Probably the largest motivation for the study of technologies aimed at actually implementing reversible computing is that they offer what is predicted to be the only potential way to improve the energy efficiency of computers beyond the fundamental von Neumann-Landauer limit of kT ln(2) energy dissipated per irreversible bit operation.
As was first argued by Rolf Landauer of IBM, in order for a computational process to be physically reversible, it must also be logically reversible. Landauer's principle is the loosely formulated notion that the erasure of n bits of information must always incur a cost of nk ln(2) in thermodynamic entropy. A discrete, deterministic computational process is said to be logically reversible if the transition function that maps old computational states to new ones is a one-to-one function; i.e. the output logical states uniquely defines the input logical states of the computational operation.
For computational processes that are nondeterministic (in the sense of being probabilistic or random), the relation between old and new states is not a single-valued function, and the requirement needed to obtain physical reversibility becomes a slightly weaker condition, namely that the size of a given ensemble of possible initial computational states does not decrease, on average, as the computation proceeds forwards.
The reversibility of physics and reversible computing
Landauer's principle (and indeed, the second law of thermodynamics itself) can also be understood to be a direct logical consequence of the underlying reversibility of physics, as is reflected in the general Hamiltonian formulation of mechanics, and in the unitary time-evolution operator of quantum mechanics more specifically.
In the context of reversible physics, the phenomenon of entropy increase (and the observed arrow of time) can be understood to be consequences of the fact that our evolved predictive capabilities are rather limited, and cannot keep perfect track of the exact reversible evolution of complex physical systems, especially since these systems are never perfectly isolated from an unknown external environment, and even the laws of physics themselves are still not known with complete precision. Thus, we (and physical observers generally) always accumulate some uncertainty about the state of physical systems, even if the system's true underlying dynamics is a perfectly reversible one that is subject to no entropy increase if viewed from a hypothetical omniscient perspective in which the dynamical laws are precisely known.
The implementation of reversible computing thus amounts to learning how to characterize and control the physical dynamics of mechanisms to carry out desired computational operations so precisely that we can accumulate a negligible total amount of uncertainty regarding the complete physical state of the mechanism, per each logic operation that is performed. In other words, we would need to precisely track the state of the active energy that is involved in carrying out computational operations within the machine, and design the machine in such a way that the majority of this energy is recovered in an organized form that can be reused for subsequent operations, rather than being permitted to dissipate into the form of heat.
Although achieving this goal presents a significant challenge for the design, manufacturing, and characterization of ultra-precise new physical mechanisms for computing, there is at present no fundamental reason to think that this goal cannot eventually be accomplished, allowing us to someday build computers that generate much less than 1 bit's worth of physical entropy (and dissipate much less than kT ln 2 energy to heat) for each useful logical operation that they carry out internally.
The motivation behind much of the research that has been done in reversible computing was the first seminal paper on the topic, which was published by Charles H. Bennett of IBM research in 1973. Today, the field has a substantial body of academic literature behind it. A wide variety of reversible device concepts, logic gates, electronic circuits, processor architectures, programming languages, and application algorithms have been designed and analyzed by physicists, electrical engineers, and computer scientists.
This field of research awaits the detailed development of a high-quality, cost-effective, nearly reversible logic device technology, one that includes highly energy-efficient clocking and synchronization mechanisms. This sort of solid engineering progress will be needed before the large body of theoretical research on reversible computing can find practical application in enabling real computer technology to circumvent the various near-term barriers to its energy efficiency, including the von Neumann-Landauer bound. This may only be circumvented by the use of logically reversible computing, due to the Second Law of Thermodynamics.
To implement reversible computation, estimate its cost, and to judge its limits, it is formalized it in terms of gate-level circuits. For example, the inverter (logic gate) (NOT) gate is reversible because it can be undone. The exclusive or (XOR) gate is irreversible because its inputs cannot be unambiguously reconstructed from an output value. However, a reversible version of the XOR gate—the controlled NOT gate (CNOT)—can be defined by preserving one of the inputs. The three-input variant of the CNOT gate is called the Toffoli gate. It preserves two of its inputs a,b and replaces the third c by . With , this gives the AND function, and with this gives the NOT function. Thus, the Toffoli gate is universal and can implement any reversible Boolean function (given enough zero-initialized ancillary bits). More generally, reversible gates have the same number of inputs and outputs. A reversible circuit connects reversible gates without fanouts and loops. Therefore, such circuits contain equal numbers of input and output wires, each going through an entire circuit.
Reversible logic circuits have been first motivated in the 1960s by theoretical considerations of zero-energy computation as well as practical improvement of bit-manipulation transforms in cryptography and computer graphics. Since the 1980s, reversible circuits have attracted interest as components of quantum algorithms, and more recently in photonic and nano-computing technologies where some switching devices offer no signal gain.
- Reverse computation
- Reversible dynamics
- Maximum entropy thermodynamics, on the uncertainty interpretation of the second law of thermodynamics
- Reversible process
- Toffoli gate
- Fredkin gate
- Quantum computing
- Billiard-ball computer
- Three-input universal logic gate
- Reversible cellular automaton
- J. von Neumann, Theory of Self-Reproducing Automata, Univ. of Illinois Press, 1966.
- R. Landauer, "Irreversibility and heat generation in the computing process," IBM Journal of Research and Development, vol. 5, pp. 183-191, 1961.
- C. H. Bennett, "Logical reversibility of computation," IBM Journal of Research and Development, vol. 17, no. 6, pp. 525-532, 1973.
- C. H. Bennett, "The Thermodynamics of Computation -- A Review," International Journal of Theoretical Physics, vol. 21, no. 12, pp. 905-940, 1982.
- Rolf Drechsler, Robert Wille. From Truth Tables to Programming Languages: Progress in the Design of Reversible Circuits. International Symposium on Multiple-Valued Logic, 2011. http://www.informatik.uni-bremen.de/agra/doc/konf/11_ismvl_reversible_circuit_design_tutorial.pdf
- Mehdi Saeedi, Igor L. Markov, Synthesis and Optimization of Reversible Circuits - A Survey, ACM Computing Surveys, 2012. http://arxiv.org/abs/1110.2574
- Rolf Drechsler and Robert Wille. Reversible Circuits: Recent Accomplishments and Future Challenges for an Emerging Technology. International Symposium on VLSI Design and Test, 2012. http://www.informatik.uni-bremen.de/agra/doc/konf/2012_vdat_reversible_circuits_accompl_chall.pdf
Review of later theoretical work: P.M.B. Vitanyi, Time, space, and energy in reversible computing, Proceedings of the 2nd ACM conference on Computing frontiers, 2005, 435–444.
- Introductory article on reversible computing
- First International Workshop on reversible computing
- Recent publications of Michael P. Frank
- Internet Archive backup of the "Reversible computing community Wiki" that was administered by Dr. Frank
- Recent Workshops on Reversible Computation
- Open-source toolkit for reversible circuit design


Over 400 million transistors are packed on dual-core chips manufactured using Intel's 45nm process. That'll double soon, per Moore's Law. And it'll still be like computing with pebbles compared to quantum computing.
Quantum computing is a pretty complicated subject—uh, hello, quantum mechanics plus computers. I'm gonna keep it kinda basic, but recent breakthroughs like this one prove that you should definitely start paying attention to it. Some day, in the future, quantum computing will be cracking codes, powering web searches, and maybe, just maybe, lighting up our Star Trek-style holodecks.
Before we get to the quantum part, let's start with just "computing." It's about bits. They're the basic building block of computing information. They've got two states—0 or 1, on or off, true or false, you get the idea. But two defined states is key. When you add a bunch of bits together, usually 8 of 'em, you get a byte. As in kilobytes, megabytes, gigabytes and so on. Your digital photos, music, documents, they're all just long strings of 1s and 0s, segmented into 8-digit strands. Because of that binary setup, a classical computer operates by a certain kind of logic that makes it good at some kinds of computing—the general stuff you do everyday—but not so great at others, like finding ginormous prime factors (those things from math class), which are a big part of cracking codes.
Quantum computing operates by a different kind of logic—it actually uses the rules of quantum mechanics to compute. Quantum bits, called qubits, are different from regular bits, because they don't just have two states. They can have multiple states, superpositions—they can be 0 or 1 or 0-1 or 0+1 or 0 and 1, all at the same time. It's a lot deeper than a regular old bit. A qubit's ability to exist in multiple states—the combo of all those being a superposition—opens up a big freakin' door of possibility for computational powah, because it can factor numbers at much more insanely fast speeds than standard computers.
Entanglement—a quantum state that's all about tight correlations between systems—is the key to that. It's a pretty hard thing to describe, so I asked for some help from Boris Blinov, a professor at the University of Washington's Trapped Ion Quantum Computing Group. He turned to a take on Schrödinger's cat to explain it: Basically, if you have a cat in a closed box, and poisonous gas is released. The cat is either dead, 0, or alive, 1. Until I open the box to find out, it exists in both states—a superposition. That superposition is destroyed when I measure it. But suppose I have two cats in two boxes that are correlated, and you go through the same thing. If I open one box and the cat's alive, it means the other cat is too, even if I never open the box. It's a quantum phenomenon that's a stronger correlation than you can get in classical physics, and because of that you can do something like this with quantum algorithms—change one part of the system, and the rest of it will respond accordingly, without changing the rest of the operation. That's part of the reason it's faster at certain kinds of calculations.
The other, explains Blinov, is that you can achieve true parallelism in computing—actually process a lot of information in parallel, "not like Windows" or even other types of classic computers that profess parallelism.
So what's that good for? For example, a password that might take years to crack via brute force using today's computers could take mere seconds with a quantum computer, so there's plenty of crazy stuff that Uncle Sam might want to put it to use for in cryptography. And it might be useful to search engineers at Google, Microsoft and other companies, since you can search and index databases much, much faster. And let's not forget scientific applications—no surprise, classic computers really suck at modeling quantum mechanics. The National Institute of Science and Technology's Jonathan Home suggests that given the way cloud computing is going, if you need an insane calculation performed, you might rent time and farm it out to a quantum mainframe in Google's backyard.
The reason we're not all blasting on quantum computers now is that this quantum mojo is, at the moment, extremely fragile. And it always will be, since quantum states aren't exactly robust. We're talking about working with ions here—rather than electrons—and if you think heat is a problem with processors today, you've got no idea. In the breakthrough by Home's team at NIST—completing a full set of quantum "transport" operations, moving information from one area of the "computer" to another—they worked with a single pair of atoms, using lasers to manipulate the states of beryllium ions, storing the data and performing an operation, before transferring that information to a different location in the processor. What allowed it to work, without busting up the party and losing all the data through heat, were magnesium ions cooling the beryllium ions as they were being manipulated. And those lasers can only do so much. If you want to manipulate more ions, you have to add more lasers.
Hell, quantum computing is so fragile and unwieldy that when we talked to Home, he said much of the effort goes into methods of correcting errors. In five years, he says, we'll likely be working with a mere tens of qubits. The stage it's at right now, says Blinov, is "the equivalent of building a reliable transistor" back in the day. But that's not to say those of tens of qubits won't be useful. While they won't be cracking stuff for the NSA—you'll need about 10,000 qubits for cracking high-level cryptography—that's still enough quantum computing power to calculate properties for new materials that are hard to model with a classic computer. In other words, materials scientists could be developing the case for the iPhone 10G or the building blocks for your next run-of-the-mill Intel processor using quantum computers in the next decade. Just don't expect a quantum computer on your desk in the next 10 years.
Special thanks to National Institute of Standards and Technology's Jonathan Home and the University of Washington Professor Boris Blinov!
Still something you wanna know? Send questions about quantum computing, quantum leaps or undead cats to email@example.com, with "Giz Explains" in the subject line.


University of Utah physicists stored information for 112 seconds in what may become the world's tiniest computer memory: magnetic "spins" in the centers or nuclei of atoms. Then the physicists retrieved and read the data electronically -- a big step toward using the new kind of memory for both faster conventional and superfast "quantum" computers.
"The length of spin memory we observed is more than adequate to create memories for computers," says Christoph Boehme (pronounced Boo-meh), an associate professor of physics and senior author of the new study, published Friday, Dec. 17 in the journal Science. "It's a completely new way of storing and reading information."
However, some big technical hurdles remain: the nuclear spin storage-and-read-out apparatus works only at 3.2 degrees Kelvin, or slightly above absolute zero -- the temperature at which atoms almost freeze to a standstill, and only can jiggle a little bit. And the apparatus must be surrounded by powerful magnetic fields roughly 200,000 times stronger than Earth's.
"Yes, you could immediately build a memory chip this way, but do you want a computer that has to be operated at 454 degrees below zero Fahrenheit and in a big national magnetic laboratory environment?" Boehme says. "First we want to learn how to do it at higher temperatures, which are more practical for a device, and without these strong magnetic fields to align the spins."
As for obtaining an electrical readout of data held within atomic nuclei, "nobody has done this before," he adds.
Two years ago, another group of scientists reported storing so-called quantum data for two seconds within atomic nuclei, but they did not read it electronically, as Boehme and colleagues did in the new study, which used classical data (0 or 1) rather than quantum data (0 and 1 simultaneously). The technique was developed in a 2006 study by Boehme, who showed it was feasible to read data stored in the net magnetic spin of 10,000 electrons in phosphorus atoms embedded in a silicon semiconductor.
The new study puts together nuclear storage of data with an electrical readout of that data, and "that's what's new," Boehme says.
The study was led by Boehme and first author Dane McCamey, a former research assistant professor of physics at the University of Utah and still an adjunct assistant professor. His main affiliation now is with the University of Sydney. Other co-authors were Hans van Tol of the National High Magnetic Field Laboratory in Tallahassee, Fla., and Gavin Morley of University College London.
The study was funded by the National High Magnetic Field Laboratory, the National Science Foundation, the Australian Research Council, Britain's Engineering and Physical Sciences Research Council and the Royal Commission for the Exhibition of 1851, a British funding agency led by Prince Philip.
Of Electronic and Spintronic Memories
Modern computers are electronic, meaning that information is processed and stored by flowing electricity in the form of electrons, which are negatively charged subatomic particles that orbit the nucleus of each atom. Transistors in computers are electrical switches that store data as "bits" in which "off" (no electrical charge) and "on" (charge is present) represent one bit of information: either 0 or 1.
Quantum computers -- a yet-unrealized goal -- would run on the odd principles of quantum mechanics, in which the smallest particles of light and matter can be in different places at the same time. In a quantum computer, one quantum bit or "qubit" could be both 0 and 1 at the same time. That means quantum computers theoretically could be billions of times faster than conventional computers.
McCamey says a memory made of silicon "doped" with phosphorus atoms could be used in both conventional electronic computers and in quantum computers in which data is stored not by "on" or "off" electrical charges, but by "up" or "down" magnetic spins in the nuclei of phosphorus atoms.
Externally applied electric fields would be used to read and process the data stored as "spins" -- just what McCamey, Boehme and colleagues did in their latest study. By demonstrating an ability to read data stored in nuclear spins, the physicists took a key step in linking spin to conventional electronics -- a field called spintronics.
Spin is an unfamiliar concept to comprehend. A simplified way to describe spin is to imagine that each particle -- like an electron or proton in an atom -- contains a tiny bar magnet, like a compass needle, that points either up or down to represent the particle's spin. Down and up can represent 0 and 1 in a spin-based quantum computer.
Boehme says the spins of atoms' nuclei are better for storing information than the spin of electrons. That's because electron spin orientations have short lifetimes because spins are easily changed by nearby electrons and the temperature within atoms.
In contrast, "the nucleus sits in the middle of an atom and its spin isn't messed with by what's going on in the clouds of electrons around the nucleus," McCamey says. "Nuclei experience nearly perfect solitude. That's why nuclei are a good place to store information magnetically. Nuclear spins where we store information have extremely long storage times before the information decays."
The average 112 second storage time in the new study may not seem long, but Boehme says the dynamic random access memory (DRAM) in a modern PC or laptop stores information for just milliseconds (thousandths of a second). The information must be repeatedly refreshed, which is how computer memory is maintained, he adds.
How to Store and Then Read Data in the Spins of Atomic Nuclei
For the experiments, McCamey, Boehme and colleagues used a thin, phosphorus-doped silicon wafer measuring 1 millimeter square, and placed electrical contacts on it. The device was inside a supercold container, and surrounded by intense magnetic fields. Wires connected the device to a current source and an oscilloscope to record data.
The physicists used powerful magnetic fields of 8.59 Tesla to align the spins of phosphorus electrons. That's 200,000 times stronger than Earth's magnetic field.
Then, pulses of near-terahertz electromagnetic waves were used to "write" up or down spins onto electrons orbiting phosphorus atoms. Next, FM-range radio waves were used to take the spin data stored in the electrons and write it onto the phosphorus nuclei.
Later, other pulses of near-terahertz waves were used to transfer the nuclear spin information back into the orbiting electrons, and trigger the readout process. The readout is produced because the electrons' spins are converted into variations in electrical current.
"We read the spin of the nuclei in the reverse of the way we write information," Boehme says. "We have a mechanism that turns electron spin into a current."
Summarizing the process, Boehme says, "We basically wrote 1 in atoms' nuclei. We have shown we can write and read [spin data in nuclei]," and shown that the information can be repeatedly read from the nuclei for an average of 112 seconds before all the phosphorus nuclei lose their spin information. In a much shorter time, the physicists read and reread the same nuclear spin data 2,000 times, showing the act of reading the spin data doesn't destroy it, making the memory reliable, Boehme says.
Reading out the data stored as spin involved reading the collective spins of a large number of nuclei and electrons, Boehme says. That will work for classical computers, but not for quantum computers, for which readouts must be able to discern the spins of single nuclei, he adds. Boehme hopes that can be achieved within a few years.
- D. R. Mccamey, J. Van Tol, G. W. Morley and C. Boehme. Electronic Spin Storage in an Electrically Readable Nuclear Spin Memory with a Lifetime >100 Seconds. Science, 17 December 2010: Vol. 330 no. 6011 pp. 1652-1656 DOI: 10.1126/science.1197931
Cite This Page:


Physicists at the National Institute of Standards and Technology (NIST) have demonstrated entanglement—a phenomenon peculiar to the atomic-scale quantum world—in a mechanical system similar to those in the macroscopic everyday world. The work extends the boundaries of the arena where quantum behavior can be observed and shows how laboratory technology might be scaled up to build a functional quantum computer.
The research, described in the June 4 issue of Nature, involves a bizarre intertwining between two pairs of vibrating ions (charged atoms) such that the pairs vibrate in unison, even when separated in space. Each pair of ions behaves like two balls connected by a spring (see figure), vibrating back and forth in opposite directions. Familiar objects that vibrate this way include pendulums and violin strings.
The NIST achievement provides insights into where and how "classical" objects may exhibit unusual quantum behavior. The demonstration also showcased techniques that will help scale up trapped-ion technology to potentially build ultra-powerful computers relying on the rules of quantum physics. If they can be built, quantum computers may be able to solve certain problems, such as code breaking, exponentially faster than today's computers. (For further details, see: http://www.nist.gov/public_affairs/quantum/quantum_info_index.html.)
"Where the boundary is between the quantum and classical worlds, no one really knows," says NIST guest researcher John Jost, a graduate student at the University of Colorado at Boulder and first author of the paper. "Maybe we can help answer the question by finding out what types of things can—and cannot be—entangled. We've entangled something that has never been entangled before, and it's the kind of physical, oscillating system you see in the classical world, just much smaller."
Mechanical oscillators like two pendulum-based clocks have previously been synchronized, but their vibrations can still be independent, so that changes in one have no effect on the other. Quantum entanglement—"spooky action at a distance," in Einstein's words—is a far more counterintuitive process: If two objects are entangled, then manipulating one instantaneously affects the other, no matter how far away it is. Entangled objects do not necessarily have identical properties, just properties that are linked in predictable ways.
Jost and colleagues entangled the vibrational motions of two separated mechanical oscillators, each consisting of one beryllium and one magnesium ion. Each pair behaves like two objects connected by a spring 4 micrometers (millionths of a meter) long, with the beryllium and magnesium moving back and forth in opposite directions, first toward each other, then away, then back again. The two pairs perform this motion in unison, even though they are 240 micrometers apart and are located in different zones of an ion trap. The scientists created the desired entangled state at least 57 percent of the time they tried, and have identified ways to improve the success rate.
The NIST experiments suggest that mechanical oscillators can take part in both the quantum and classical worlds, possessing some features of each, depending in part on the energy and other properties of the vibrations. The experiments also achieved the first combined demonstration of arranging different ions into a desired order, separating and re-cooling them while preserving entanglement, and then performing subsequent quantum operations on the ions. These techniques could help scientists build large-scale quantum computers that use hundreds of ions to store data and perform many computational steps. The same NIST group has previously demonstrated the basic building blocks of a quantum computer using ion traps, as well as rudimentary logic operations.
To entangle the motion of the two oscillators, the NIST group first placed four ions together in one trap zone in a particular linear order (Be-Mg-Mg-Be), and entangled the internal energy states of the two beryllium ions. The team then separated the four ions into two pairs, with each pair containing one of the entangled ions. Finally, the scientists transferred the entanglement from the beryllium ions' internal states to the oscillating motions of the separated ion pairs.
The research was funded in part by the Intelligence Advanced Research Projects Activity. The authors include former NIST post-doctoral scholars who are currently at the Weizmann Institute of Science in Israel and Lockheed Martin of Littleton, Colo.
How NIST Entangled Two Mechanical Oscillators
NIST physicists entangled two vibrating mechanical systems each consisting of one beryllium and one magnesium ion, in an experiment that required 14 milliseconds, including verification of results, and involved about 600 laser pulses. The steps below expand on information provided in the figure.
Step 1—Initially, all four ions are placed in the same zone of an ion trap and cooled with lasers to very low temperatures. By tuning the voltages of the trap electrodes scientists arrange the ions in a particular order, with both heavier magnesium ions between the beryllium ions. Using a technique developed for quantum computing several years ago, scientists entangle the two beryllium ions' internal "spin states," which are analogous to tiny bar magnets pointing up or down. Two ultraviolet laser beams, positioned at right angles, cause the ions to oscillate. The lasers are tuned so the difference between their frequencies is very close to the frequency of one of the ions' natural vibrations, the rate at which it likes to oscillate back and forth. Based on differences in their spins, the ions "feel" a differing laser force that causes the ions to oscillate in a particular way. This coupling of the spin states to motion has the global effect of entangling the spins of the beryllium ions in a controlled way.
Step 2—Voltages are then applied to electrode X to separate the ions into two pairs, which are distributed to different trap zones located adjacent to electrodes A and B. The separation and transport boost the energy of motion in the oscillating ions.
Step 3—The magnesium ions are cooled with lasers to remove excess motional energy from the beryllium ions, a process called sympathetic cooling because one type of ion cools the other. This is the first time entangled ions have been re-cooled prior to further operations, a technique expected to be useful in computing.
Step 4—By manipulating laser beam colors and orientations in a sequence of pulses of specific intensity and duration, scientists transfer the entanglement from the beryllium spins to the motion. The two mechanical oscillators are now entangled. Under ideal conditions, the beryllium and magnesium ions are oscillating back and forth in opposite directions, toward each other and then away. The two pairs perform this motion in unison, even though they are 240 micrometers apart and are located in different zones of the trap.
Scientists are not able to measure the entangled motions directly. Instead, to verify the results, they conduct a cleanup procedure partway through the experiment to ensure the entanglement has been transferred successfully from the ions' spin to their mechanical motion. Then, at the end of the experiment, they essentially reverse the entire process to transfer the entanglement from the ion motion back to the spins, to reproduce the initial beryllium spin states, which they can measure through the light scattered by the beryllium ions (spin up scatters laser light, whereas spin down does not).
The above story is based on materials provided by National Institute of Standards and Technology (NIST). Note: Materials may be edited for content and length.
- J. D. Jost, J. P. Home, J. M. Amini, D. Hanneke, R. Ozeri, C. Langer, J. J. Bollinger, D. Leibfried & D. J. Wineland. Entangled mechanical oscillators. Nature, 2009; 459 (7247): 683 DOI: 10.1038/nature08006
Cite This Page:


SSL, or Secure Socket Layer, was first developed by Netscape in the mid-1990's to address the growing need to be able to securely transmit data. It protects data, verifies legitimacy of a website, and is supported by all major browsers. When you log into a banking website, your computer is sent a file called an "SSL certificate" which contains the following data:
Based on the certificate's info, your browser decides whether or not to trust the certificate. This is possible because it uses third-party data, already in your browser, to confirm the certificate wasn't sent by a hacker. Once the certificate is received, the browser checks that the certificate was issued by a trusted third party known as a certificate authority. The browser then uses the public key to encrypt a random, symmetric encryption key and sends it to the server. The web server then decrypts the symmetric encryption key using its private key and uses the symmetric key to decrypt the URL and the HTTP data. Finally, the browser decrypts a response from the server using the symmetric key and displays the information.
Due to the nature of the Internet, the path the content follows between a server and a web browser is not secure. There is always the possibility someone is using a "packet sniffer" to capture data as it passes through a network or, if you're wireless, right out of the air. This is where encryption comes in. Originally, SSL used 40-bit encryption, meaning the value of the key used to decrypt data was selected from 1 out of 1,099,511,627,776 possible values. Today, that level of encryption can be broken almost instantly; so, a 128-bit encryption is commonly used which means 340,282,366,920,938,463,463,374,607,431,768,211,456 possible values; increase it to 256 bits for more security and you have the theoretical number of atoms in the universe. Even with millions of today's top-of-the-line computers working together, brute-force decryption simply takes too long if data is encrypted properly. That said, it's always best to be paranoid because future technologies like quantum computing may render conventional encryption obsolete.
If a brute-force attack won't work, how else can SSL be compromised? No matter how air-tight a security system is, all that work is pointless if users trusted with access have weak passwords or can be tricked into providing their passwords. Although not SSL-specific, it's vital best practices are used to prevent non-technical, "social engineering" attacks.
There is also the possibility that browser and/or server flaws could be exploited. A good way to minimize the risk of a hacker taking advantage of exploits is to subscribe to twitter feeds or blogs related to web security. This way, vulnerabilities can be fixed shortly after they're made public. Another approach would be to establish a list of supported browsers so that you can block or redirect users whose browsers aren't secure.
Flaws in SSL itself could potentially be identified and exploited. SSL supports multiple types of encryption and, in 2008, researchers were able to spoof a certificates by exploiting md5 encryption. This was done with an array of 200 PlayStation 3's and it was made possible because some certificate authorities relied on md5 alone. So, the reliability of an SSL certificate is directly related to the reliability of its certificate authority. If a certificate authority issues an SSL to a hacker's site, users could be fooled into thinking they are on a legitimate site due to successful SSL authentication. Furthermore, some authorities use better encryption methods than others. You can get a certificate from GoDaddy for $70/year or you can spend at least $695 at Symantec. Guess which business takes security more seriously!
First, there's a yearly cost associated with SSL which must be weighed against the security benefit. Is there any data on the site that any hackers might use or is there any motivation for your site to be hacked more than another site? If you're doing financial transactions then you pretty much have to use SSL or users will not feel secure, not to mention it would be an obvious target for hackers. That said, if your site only contains openly shared data and is backed up regularly, the biggest risks might be that an admin's password could be captured or that users might use the same password on other sites that do contain sensitive data.
SSL also uses additional server resources encrypting and decrypting content. Although the difference is minor due to processing power of today's servers, it can be noticeable on high-traffic sites. If you want to mix secure and non-secure content on the same page then users may get a browser warnings, so this limits the ability to host some content elsewhere; for example, a content distribution network. Finally, extra time is needed to purchase the certificate, set up the server, configure the website, and test.
Sometimes SSL is a given, but it can be more of a qualitative question based on the balance between practicality and ideology. Yes, any unencrypted login is vulnerable to attack, but what are the chances? The best thing do is weigh the overall cost of SSL against how sensitive your content is and what might happen, worst case,if it is compromised. If you're not sure whether or not to use SSL but you have the money and don't see any major technical obstacles then go ahead and use it.
A less expensive alternative might be to integrate a service like PayPal that handles authentication outside your website. On the other hand, if SSL's authentication and encryption aren't enough, consider using physical tokens. A physical token is a device that assists with authentication. For example, the device may periodically display a different value used to log in based on the current time. This approach removes the reliance in the certificate authority and allows more control over who has access. It can even be used to establish a VPN connection to the server before the website can be accessed.
When configuring Drupal to use SSL, a good place to start is the Secure Pages modules which lets you define which pages are secure and handles redirects from or to secure pages as needed. If you're using Secure Pages with Drupal 6 then the Secure Pages Prevent Hijack module should be installed to prevent hijacked sessions from access SSL pages. Also, the Auth SSL Redirect module can be used to redirect authenticated users to SSL and it will work in conjunction with Secure Pages. If you're using Ubercart and want to either secure the whole site or just Ubercart pages then another option is Ubercart SSL and it can be extended to secure additional pages. In general, these modules help manage transitions between secure and insecure pages.
[Updated based on comment feedback.]
What do you think, what approaches do you recommend, and what do you recommend against?


Scientists find a way to directly measure quantum states, such as momentum, of photons.
Credit: MPQ, Quantum Dynamics Division.
Quantum computers and communications promise more powerful machines and unbreakable codes. But to make them work, it's necessary to measure the quantum state of particles such as photons or atoms. Quantum states are numbers that describe particle characteristics such as momentum or energy.
But measuring quantum states is difficult and time-consuming, because the very act of doing so changes them, and because the mathematics can be complex. Now, an international team says they found a more efficient way to do it, which could make it simpler to build quantum-mechanical technologies.
In a study detailed in the Jan. 20 issue of the journal Nature Communications, researchers from the University of Rochester and the University of Glasgow took a direct measurement of a photon's 27-dimensional quantum state. These dimensions are mathematical, not dimensions in space, and each one is a number that stores information. To understand a 27-dimensional quantum state, think about a line described in two dimensions. A line would have a direction in the X and Y coordinates — 3 inches left and 4 inches up, for instance. The quantum state has 27 such coordinates. [Quantum Physics: The Coolest Little Particles in Nature]
"We chose 27, kind of to make a point about 26 letters in the alphabet and throwing in one more," said Mehul Malik, now a postdoctoral researcher at theUniversity of Vienna. That means each quantum bit, or "qubit," could store a letter instead of a simple 1 or 0.
Seeing a photon
The group, led by Malik and Robert Boyd, a professor of optics and physics at the University of Rochester, was able to see a photon's states directly. They measured the photon's orbital angular momentum, which is how much the particles of light "twist" as they travel through space.
Ordinarily, finding the quantum state of a photon requires a two-step process. First, scientists have to measure some property of the photon, such as its polarization or momentum. The measurements are performed on many copies of the quantum state of a photon. But that process sometimes introduces errors. To get rid of the errors, the scientists have to look at what results they got that are "disallowed" states — ones that don't follow the laws of physics. But the only way to find them is to search through all the results and discard the ones that are impossible. That eats up a lot of computing time and effort. This process is called quantum tomography. [The 9 Biggest Unsolved Mysteries in Physics]
A light wave is a combination of an electric and magnetic field, each of which oscillates and makes a wave. Each wave moves in time with the other, and they are perpendicular to each other. A beam of light is made up of lots of these waves.
Light can have what is called orbital angular momentum. In a beam with no orbital angular momentum, the peaks of the waves — the electric ones, for example — are lined up. A plane connecting these peaks will be flat. If the beam has orbital angular momentum, a plane connecting these peaks will make a spiral, helical pattern, because the light waves are offset from one another slightly as you go around the beam. To measure the state of the photons, scientists must "unravel" this helical shape of the waves in the beam.
Measuring a photon's quantum state
The team first fired a laser through a piece of transparent polymer that refracted the light, "unraveling" the helix formed by the waves. The light then passed through special lenses and into a grating that makes many copies of the beam. After passing through the grating, the light is spread out to form a wider beam.
After the beam is widened, it hits a device called a spatial light modulator. The modulator carries out the first measurement. The beam then reflects back in the same direction it came from and passes through a beam splitter. At that point, part of thebeam moves toward a slit, which makes a second measurement. [Twisted Physics: 7 Mind-Blowing Experiments]
One of the two measurements is called "weak" and the other "strong." By measuring two properties, the quantum state of the photons can be reconstructed without the lengthy error-correction calculations tomography requires.
In quantum computers, the quantum state of the particle is what stores the qubit. For instance, a qubit can be stored in the photon's polarization or its orbital-angular momentum, or both. Atoms can also store qubits, in their momenta or spins.
Current quantum computers have only a few bits in them. Malik noted that the record is 14 qubits, using ions. Most of the time, ions or photons will only have acouple of bits they can store, as the states will be two-dimensional. Physicists use two-dimensional systems because that is what they can manipulate — it would be very difficult to manipulate more than two dimensions, he said.
Direct measurement, as opposed to tomography, should make it easier to measure the states of particles (photons, in this case). That would mean it is simpler to add more dimensions — three, four or even — as in this experiment, 27 — and store more information.
Mark Hillery, a professor of physics at Hunter College in New York, was skeptical that direct measurement would prove necessarily better than current techniques. "There is a controversy about weak measurements — in particular, whether they really are useful or not," Hillery wrote in an email to LiveScience. "To me, the main issue here is whether the technique they are using is better (more efficient) than quantum-state tomography for reconstructing the quantum state, and in the conclusion, they say they don't really know."
Jeff Savail, a master's candidate researcher at Canada's Simon Fraser University, worked on a similar direct measurement problem in Boyd's lab, and his work was cited in Malik's study. In an email he said one of the more exciting implications is the "measurement problem." That is, in quantum mechanical systems the question of why some measurements spoil quantum states while others don't is a deeper philosophical question than it is about the quantum technologies themselves. "The direct measurement technique gives us a way to see right into the heart of the quantum state we're dealing with," he said. That doesn't mean it's not useful – far from it. "There may also be applications in imaging, as knowing the wave function of the image, rather than the square, can be quite useful."
Malik agreed that more experiments are needed, but he still thinks the advantages might be in the relative speed direct measurement offers. "Tomography reduces errors, but the post-processing [calculations] can take hours," he said.


Entanglement is one of the defining properties that distinguishes quantum systems from their classical counterparts. It refers to correlations between measurement outcomes on distinct (and potentially distant) degrees of freedom of a system that are stronger than those found in any classical experiment. Quantum entanglement is the key resource that enables the dramatic speedup of calculations in a quantum computer, as well as various other quantum information processing tasks.
In a paper appearing in Physical Review Letters , Haohua Wang and co-workers at the University of California, Santa Barbara, US, Zheijiang University, China, and NEC Corporation, Japan, have experimentally demonstrated entanglement between two spatially separated oscillating electrical circuits. This experiment represents the latest step by these researchers towards the engineering of large scale networks of controlled, entangled systems, which might be useful as quantum computers , or for engineering new states of quantum matter [3, 4].
Harmonic oscillators would seem to be ideal building blocks for constructing highly entangled states. The harmonic oscillator is a particularly well studied exemplar: the classical physics of harmonic oscillators—such as a mass accelerated by the linear restoring force provided by a spring—is understood by high school physics students, whereas the quantum harmonic oscillator is one of the first systems to be dealt with in undergraduate quantum mechanics courses. The quantum dynamics of a harmonic oscillator can be solved exactly, and such solutions are often the starting point in the understanding of quantum field theory. Harmonic oscillators are ubiquitous in physics, and many realizations of such oscillators can be found, ranging from mechanical systems, electrical circuits, and lattice vibrations to elementary excitations of the electromagnetic field (photons).
In the context of the experiment carried out by Wang et al., each harmonic oscillator consists of a coplanar waveguide resonator—equivalent to a circuit comprising a capacitor and an inductor (see Fig. 1). This resonator is superconducting at low temperature and can store excitations for a long time (in other words, it takes a relatively long time for excitations to decay—around ). Excitations of this circuit can be thought of as photons—excitations of the electromagnetic field associated with the circuit elements.
Unfortunately, the simplicity of such harmonic oscillators means that a quantum system consisting solely of linearly coupled oscillators (that is, where the Hamiltonian contains coupling terms that are, at most, bilinear in the coordinate or conjugate momentum of each oscillator) is insufficient for many quantum information processing tasks. Such linear systems are not capable of implementing arbitrary quantum algorithms  (unless augmented with additional resources, such as single photon sources or photon-counting detectors ). Driving a harmonic oscillator with a classical oscillating field only allows the preparation of a restricted class of states, known as coherent states, and with only linear couplings between oscillators, it is not possible to transform such coherent initial states into entangled states of multiple oscillators. One can gain some insight into this restriction by considering the spectrum of a quantum harmonic oscillator: all the energy levels of the oscillator are equally spaced, and so it is not possible to address resonant transitions between a pair of states without also driving transitions between all other states.
Wang et al.’s experiment overcomes these limitations with the inclusion of nonlinear circuit elements: each oscillator is coupled to a superconducting phase qubit, so called because the quantum information is represented by the phase difference between the superconducting condensates on either side of an insulating barrier known as a Josephson junction (see Fig. 1) . The Josephson junction is connected in parallel with a capacitor and an inductance loop. The Josephson junction adds a sinusoidal potential to the Hamiltonian of the system. The circuit is therefore no longer a purely harmonic oscillator, which means that the energy levels of the phase qubit are not equally spaced. This allows one to resonantly address a single pair of states of the phase qubit with an oscillating external field, allowing the preparation of single excitations of the qubit.
By means of a sequence of such driving pulses, together with a bias flux that shifts the qubit energy levels in and out of resonance with the central coupling resonator (denoted by C in Fig. 1), an entangled state can be established between the two phase qubits. Subsequently, this entangled state can be swapped onto the resonators A and B to create a state of the form . More complicated sequences of driving pulses, using higher lying states of the qubit, are used to engineer more general entangled states of the oscillators, such as the -photon NOON state, , which is a superposition of two -photon states, where one state has all photons in resonator A, and the other state has all photons in resonator B . In the final stage of each experiment, the joint state of the two-oscillator system is established with a method called quantum tomography, again with the aid of the phase qubits. Tomography is a process by which the complete state of a quantum system can be determined via repeated preparation and measurement steps. The technique used here is a generalization of a technique pioneered by the University of California, Santa Barbara, group in a recent elegant experiment . With tomographic data, Wang et al. are able to verify the entanglement of the final state, demonstrating entangled NOON states with up to three photons, although the fidelity of these entangled states is reduced somewhat by the short coherence time of the phase qubit. The entanglement in this experiment is truly macroscopic—each resonator is almost a centimeter long, and the resonators are separated by —and therefore the entangled systems are large enough to be easily resolved by the naked eye.
Numerous exciting possibilities are opened up by the techniques developed in this experiment. The ability to deterministically generate NOON states may have applications in Heisenberg limited metrology, that is, quantum assisted measurement, with accuracy beyond that implied by the standard shot noise limit . The superconducting oscillators in Wang et al.’s experiment have a comparatively long coherence time (), which is an order of magnitude larger than the coherence time for the phase qubits. Thus these oscillators may form a useful substrate for a microwave-frequency “linear optical quantum computer” . Such a device may be easier to construct than a conventional, gate model quantum computer, and yet would have equivalent computational power .
Finally, large-scale networks of superconducting oscillators may be used to engineer new, exotic states of quantum matter. Recently, networks have been proposed that allow one to study the physics of a system of photons with broken time reversal symmetry , potentially allowing analogs of the quantum Hall effect for photons. Networks of entangled superconducting elements might also be useful as “protected qubits” [4, 11]; that is, qubits that are inherently protected from environmental noise. These fascinating theoretical proposals might once have seemed far-fetched, but continued experimental progress along the lines reported by Wang et al. gives reason to be optimistic that they may be realized in the not-too-distant future.
- H. Wang, M. Mariantoni, R. C. Bialczak, M. Lenander, E. Lucero, M. Neeley, A. D. O’Connell, D. Sank, M. Weides, J. Wenner, T. Yamamoto, Y. Yin, J. Zhao, J. M. Martinis, and A. N. Cleland, Phys. Rev. Lett. 106, 060401 (2011).
- M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information (Cambridge University Press, Cambridge, 2000)[Amazon][WorldCat].
- J. Koch, A. A. Houck, K. Le Hur, and S. M. Girvin, Phys. Rev. A 82, 043811 (2010); See also the Viewpoint commentary by A. D. Greentree and A. M. Martin, Physics 3, 85 (2010).
- L. B. Ioffe and M. V. Feigel’man, Phys. Rev. B 66, 224503 (2002).
- S. L. Braunstein and P. van Loock, Rev. Mod. Phys. 77, 513 (2005).
- E. Knill, R. Laflamme, and G. J. Milburn, Nature 409, 46 (2001).
- M. H. Devoret, A. Wallraff, and J. M. Martinis, arXiv:cond-mat/0411174v1.
- H. Lee, P. Kok, and J. P. Dowling, J. Mod. Opt. 49, 2325 (2002).
- M. Hofheinz et al., Nature 459, 546 (2009).
- L. Chirolli, G. Burkard, S. Kumar, and D. P. DiVincenzo, Phys. Rev. Lett. 104, 230502 (2010).
- A. Kitaev, arXiv:cond-mat/0609441v2.


In 1965, Gordon Moore predicted that processing power should double every eighteen months.1 Traditionally, this rapid growth has been achieved by shrinking distances between transistors and shortening the distance that information needs to pass through.1 However, the miniaturization of processors and transistors will soon reach a physical barrier.2 With this knowledge, researchers have begun searching for new computing systems that take different approaches to achieving greater efficiency. Many possible computing models have been explored, including optical computing, quantum computing, and perhaps most interestingly, biological computing.
Biological computing is an altogether very new and very different approach. Rather than attempting to increase the speed of each individual operation, biological computing uses components of living organisms to perform computing tasks faster through massive parallelism, a technique that uses a large number of elements each performing smaller tasks.1 Many recent advances have demonstrated the potential of biological computing, even though research has only begun. For example, Adamatzi and Selim Aki at Queens University demonstrated the ability of slime molds to determine the most efficient paths across networks, and Swiss researchers have successfully programmed human cells to perform binary operations.3 Currently, the preeminent developments in biological computing have occurred in DNA computing. DNA fragments of varying lengths are placed in a solution along with ATP to power the reaction, and the results are analyzed by determining the length and sequence of the output DNA molecule.4 DNA computing allows for the storage of data in a four letter code – “A,” “T,” “C,” and “G” – which is capable of storing far more data more compactly than the binary digit storage of electronic computers.4 In a brilliant example showcasing the potential of DNA computing to revolutionize man-machine interactions, Ehud Shapiro at the Weizmann Institute harnessed DNA computing to diagnose cancerous activity from within the cell and then release an anti-cancer drug based upon the resulting output.4
Advances in biological computing foreshadow a massive revolution in computing technologies by removing physical limitations, improving parallel processing, increasing energy efficiency, and reducing toxicity.1 First, while traditional computational development has relied upon reducing the sizes of and distances between transistors, techniques that will soon face physical limitations, biological computing rapidly increases speed by using more effective parallel processing, which is able to perform 100 times more operations per second than conventional measures.1 Second, biological computing is more energy efficient, relying on energy stored chemically in ATP instead of conventional energy supplies.1 Third, the use of biological components greatly reduces the price and toxicity of computing components, as most biological components are readily available and non-toxic.1 And lastly, biological computing allows for a completely new approach to problem solving: rather than approaching problems sequentially like traditional computers, biological computing is a unique data structure focused upon parallel operations.4 Revolutionizing the computing industry would have groundbreaking impacts in all fields of science, research, technology, and society since computers are crucial for scientific advancement for all scientific and engineering fields.
The decreased toxicity, increased availability, and greater energy efficiency of biological computers may lead to massive benefits for the environment. Traditional computers are major contributors to our carbon footprint; by 2020, the carbon emissions from data centers and Internet services is expected to increase four-fold, surpassing even the carbon footprint of the aviation industry.5 In addition, the production of traditional computers requires enormous amounts of natural resources. A single silicon chip requires 1.6 kilograms of fossil fuels, 72 grams of chemicals, and 32 kilograms of water to manufacture, which is all together over 700 times the weight of the final product.6 The disposal of traditional computers is further complicated by the heavy metals they contain, especially lead, mercury, and cadmium, which can easily leak into and contaminate the environment.6 By replacing the need for silicon and other inorganic materials with readily available organic materials, biological computing can help reduce resource strain. Furthermore, the decreased toxicity allows for safer production, storage, and disposal than silicon-based computers. Finally, the improved energy efficiency of biological computing can allow for a decrease in global energy consumption, reducing the strain on fossil fuels and decreasing the amount of pollutants released into the environment due to energy production. This could help reduce damage to ecosystems, decrease biodiversity loss due to toxicity, and combat climate change by decreasing energy consumption.
In addition to advancing computing, biological computing also allows for unprecedented advances in medicine and biology by allowing closer integration with living material. Biological data is already used to control the chemicals synthesized by various organisms; the development of organic data processing and memory storage greatly compounds this synergy.1 As demonstrated by the earlier research done by Shapiro on cancer diagnoses and treatment, biological computers could provide a means to treat and diagnose genetically based illnesses from within living organisms.1 For instance, Adamatzky Aki, a leading researcher in DNA computing, has suggested the use of a biological implant to detect and treat breast cancer.3 In addition, biological computing could be used to link silicon-based computing and living organisms. Studies on eels have demonstrated that living things can be linked to robots and controlled, providing the ability for humans to study organisms in unprecedented ways and allowing for advances in interactive prosthetics.1 Biological computing could also allow the introduction of computing in harsher natural environments by mimicking the adaptive strategies of resilient life-forms.3 Overall, these advantages could radically change our ability to garner data for a variety of fields, including biology, animal behavior, and studies in extreme environments. In addition, intimate integration with biological tissue could revolutionize the treatment of cancer and other diseases, transform health care, and pave the way for artificially constructed or controlled organisms that create new opportunities in fields ranging from farming to prosthetics.
1. Fulk, Kevin. “Biological computing.” ISRC Future Technology Topic Brief. 2002.
2. Junnarkar, Sandeep. “Tomorrow’s Tech: The Domino Effect.” CNET News. October 24, 2002.
3. Baer Adam. “Why living cells are the future of data processing.” PopSci. November 5, 2012.
4. Tagore, Somnath; Bhattacharya, Saurav; Islam, Ataul; Islam, Lutful. “DNA Computation: Applications and Perspectives.” Journal of Proteomics & Bioinformatics. June 29, 2010: 234-243.
5. Kanter, James. “The Computer Age and its Carbon Footprint.” New York Times. June 13, 2008.
6. Locklear, Fred. “The Environmental Impact of Computing.” Ars Technica. Nov. 12, 2002.


March 17, 2013 | 3
Though the concept of the robot seems to be a modern and a relatively new idea, they have been around for years. The first recording in literature of a possible description of the robot is found in the Iliad in reference to a “a three-legged cauldron that had ears for handles”. Later on, in 1900, we were introduced to Tik-Tok in Frank Baum’s Wizard of Oz. The word robot was first used in 1920 by the Czech writer Karel Čapek in his play R.U.R. (Rossum’s Universal Robots). This would be the first dramatization of a robot under this name. However, robots would come to life and be used for practical purposes in 1962. General Motors was the first company to use a robot for industrial purposes.
Since then, robots have been used in many ways. They have come in all shapes and sizes. They have been used in the medical field, the armed forces, and in the space program.
Now as we face the 21st century, technology evolves more. A new kind of robot is being studied and researched. This robot is called the quantum robot.
The quantum robot is the idea of combining quantum theory with robot technology. In other words, it is a practical use of the combination of quantum computing and robot technology. Quantum computing involves using quantum systems and quantum states to do computations.
A robot is an automated machine that is capable of doing a set of complex tasks. In some applications of robots, the programming used to run the robots may be based on artificial intelligence. Artificial Intelligence is the ability of a computer system to operate in a manner similar to human intelligence. Think of artificial intelligence as if you were training a machine to act like a human. Essentially, quantum robots are complex quantum systems.They are mobile systems with on board quantum computers that interact with their environments. Several programs would be involved in the operation of the robot. These programs would be quantum searching algorithms and quantum reinforcement learning algorithms.
Quantum reinforcement learning is based on superposition of the quantum state and quantum parallelism. A quantum state is a system that is a set of quantum numbers. The four basic quantum numbers represent the energy level, angular momentum, spin, and magnetization. In the superposition of quantum states, the idea is to get one state to look like another.
Let’s say I have two dogs. One dog knows how to fetch a bone (energy level), sit up (angular momentum), give a high five (spin), and shake hands (magnetization). Now, let’s apply the superposition of quantum states. Since one dog has been trained and given the commands, the other dog must learn to mimic or copy what the first dog did. Each time a command is achieved, reinforcement is given. The reinforcement for the dog would be a bone (or no bone if the command is not achieved).
In quantum reinforcement learning, it is slightly different. The idea would be similar to an “If-Then” statement. An example would be if the quantum state has a certain energy level, then the angular momentum is certain value. This idea of “If-Then” statements in the quantum world leads to an idea which can be a topic of its own; Quantum Logic.
Quantum parallelism simply means that computations can happen at the same time. This allows for all of the quantum numbers of the quantum system to be measured at the same time. If there are multiple quantum systems then; by using the concept of parallelism, all systems can be measured at the same time.
Programs used for “quantum searching” are based on quantum random walks. Quantum random walks use probability amplitudes. A probability amplitude allows us to determine that there is more than one possible quantum state. In the classical world, if you type a word “Quantum” in the search engine, you get many results. You may have a tough time finding a needle in a haystack if you use just one word, but if you want to refine your search; let’s say “Quantum Random Walks”, then it narrows the search. The same principle applies in quantum computing to get more refined results. However, you are not necessarily searching for words but you are finding information that may correlate to a quantum state.
What would be the advantages of the Quantum Robot over the Robot?
Quantum robots are more intricate in examining their environments and doing tasks as they apply quantum effects . Because of the complexity in quantum computing, the expectations of the quantum robots would be that they are faster, more accurate, and are able to multitask better than the standard robot.
The quantum robots may be able one day to give us better medical diagnoses and better data interpretation in other research fields such as defense research. In medicine, they may be able to detect pathological changes in the body by being injected through the bloodstream. In the space program, they may be able to examine the delicate environments on other planets. In the military, they may be able to detect changes in the magnetic and electric fields. They may be able to help us detect early warnings of disasters more efficiently.


Quantum computation in diamond
Computers do not necessarily have to perform error-free calculations in order to provide perfect results—they only need to correct their errors in a reliable way. And this will become even more important in the future, when it is hoped that quantum computers will solve some tasks several times faster than conventional PCs with computing processes that are very efficient but also prone to disturbances. An international team headed by physicists from the University of Stuttgart and the Stuttgart Max Planck Institute for Solid State Research has now found a way to control the quantum system of a diamond with a small number of nitrogen impurities particularly well. The researchers can thus specifically address quantum bits, i.e. the smallest computing units of a quantum computer, in the diamond and combine several bits to a computing register. They use the new degree of control for a logic operation, which is essential for a quantum computer, and for error correction.
The physicists already possess quite accurate knowledge about where the strengths of a quantum computer would be: it could carry out searches in large databases, encodings and decodings, or the research tasks in quantum physics much faster than any conceivable conventional computer today. However, there is still no really clear idea of what the blueprint of a quantum computer should look like; neither is there currently a real favourite among the materials from which quantum processors could be made. Possible options here are ions trapped by electric fields, atoms in optical lattices, devices made of superconductors, or diamonds doped with tiny quantities of nitrogen, for example.
Physicists working with Jörg Wrachtrup, professor at the Univ. of Stuttgart and Fellow of the Max Planck Institute for Solid State Research, have been investigating for some time the diamonds which are sporadically interspersed with nitrogen. On the road to the quantum computer, they have now helped the diamonds over several hurdles simultaneously. The Stuttgart-based researchers did this by producing not only a quantum register and thus the counterpart of a conventional processor in a diamond; they were also able to reliably control the register, use it to carry out a logic operation and correct errors in it. “Since we meanwhile understand the quantum mechanics of our system well, we can produce quantum registers using a quite simple approach that doesn’t require complex cryogenic technology or laser systems,” says Jörg Wrachtrup.
A quantum register is in a superposition state of several qubits
A quantum register always contains individual qubits (short for quantum bits), which can be in one of two states just like conventional bits in order to represent a zero or a one. Unlike conventional bits, however, several qubits can be brought into superposition states in which every individual bit virtually floats between “zero” and “one”. This means each superposition state has a different occurence and these are contained in the quantum register as possibilities. These possibilities can be used like the bits of a conventional computer for some parallel computations.
The more quantum bits are combined in a register, the more powerful, but also the more sensitive, is the processor. This is because external disturbances push a qubit only too easily from the floating state between “one” and “zero” towards one of the two options. In the worst case, unwelcome external influences destroy the sensitive superposition and render it useless for parallel computations. The researchers in Stuttgart have now found a remedy for this.
Three nuclear spins are combined to a quantum register via a defect
The nitrogen defect—physicists call it an NV centre (NV: nitrogen vacancy)—can become a trap for one single electron. An electron also has a spin whose orientation also has an effect on the orientation of the nuclear spin. The electron spin can be switched faster than the nuclear spins, but is more prone to the effect of disturbances. The researchers use it for control commands to the nuclear spins that cannot be transmitted with radio frequency pulses. The electron in the defect thus provides the communication between the nuclear spins in the quantum register. Finally, the physicists use it as a tool to help them read the nuclear spins.
A quantum register with fast switch and robust storage device
“In the past, the electron of the NV centre has been used as a storage device in order to expand the quantum register,” says Gerald Waldherr, who played a crucial role in the experiments. “We use the electron solely to control the nuclear spins on which the quantum information is stored.” This allows the researchers to use the advantages of both systems: the quantum register can be switched rapidly using an electron spin. The nuclear spins, in contrast, store the information in a relatively reliable way, as they withstand disturbances well.
Assisted by the electron spin, the physicists now use an ingenious combination of light and radio frequency pulses to manipulate the three nuclear spins into a superposition state initially: they entangle the nuclear spins. Quantum mechanical entanglement creates a kind of virtual bond between quantum particles so that they know of each other’s existence. Only entangled systems are suitable as quantum registers, because only they allow the parallel operation of the quantum computer.
A CNOT gate allows other computing operations
In the next step, the researchers showed that logic operations are possible in this quantum register using a CNOT gate—a logic operation that is particularly important for quantum computers. “All other operations can be realised with the CNOT gate and local operations on individual qubits,” explains Gerald Waldherr. The CNOT gate switches a bit depending on a second bit. If the latter represents a “one”, for example, the first one is set from “zero” to “one” or vice versa; it remains unchanged, however, if the latter is at “zero”. The researchers in Stuttgart carried out exactly this operation on the nuclear spins in their register, by sending a sequence of different radio frequency pulses to the NV centre or the nuclear spins.
The CNOT gate is not only indispensable for the computing power of a quantum computer, it also makes error correction possible. Although nuclear spins are not as sensitive to interferences as electron spins are, they are by no means immune. Gerald Waldherr and his colleagues demonstrated how possible errors in the quantum register can be cancelled for one of the possible superposition states of their quantum register.
To correct the errors, the scientists benefit from the fact that the superposition states are not arbitrary combinations of all possible spin orientations. Rather, in one of these superposition states all qubits are either “one” or “zero”. In another state, two are always “one”. Errors are thus evident immediately. And with the aid of the two intact qubits the original state of the third can be reconstructed. The CNOT operation is the tool of choice for this, because it switches one bit depending on another one. An ingenious sequence of CNOT operations on the three qubits of the quantum register thus not only shows whether one bit deviates from the characteristic pattern of the particular superposition state, it even corrects the error immediately.
The plan is to increase the number of qubits in the quantum register
“Our current work shows that the defect centres in diamonds are significantly more versatile than we originally thought,” says Jörg Wrachtrup. “We have obtained the new findings primarily through a better understanding of the defects and not by investing much into the material.”
The researchers will rely on smart ideas in the future as well, as they try to further improve the prospects of the diamonds in the competition for the most useful quantum register. First they want to increase the number of qubits in their register. To this end, they want to integrate nuclear spins, which find it more difficult to communicate with the electron than the three spins of their current computing register. They could also expand the quantum register, if they succeed in entangling several NV centres and addressing the relevant nuclear spins in the vicinity of the individual centres. They would thus also have networked the nuclear spins, which are controlled by the individual defects. The quantum register would then slowly be approaching a size where it could actually challenge conventional processors for some computing tasks.
Source: Max Planck Institute


Wires made of individual carbon atoms could be used to reduce the size of today’s microchips several-fold. Carbon nanotubes (CNT) were researched in the past few years and used in initial experimental applications. Nano-engineering now has the task of developing production technologies to make CNT applications commonplace even for the mass market.
Sumio Iijima presented the properties of a novel ordered structure of carbon atoms in a paper in Nature in 1991. With their three-dimensional structure and hexagonal arrangement of carbon atoms, the carbon nanotubes (CNT) that he described resemble rolled-up chicken wire. Physicists throughout the whole world were enthusiastic about the material’s promising properties: it was reputed to be stronger than steel and to have a thermal conductivity better than diamond and an electrical conductivity 1000 times higher than copper. Depending on their quantum entanglement, CNT are said to be usable as semiconductors or as conductors.
Via theoretical deductions and individual experiments, fundamental research in recent years has shown that CNT are usable as semiconductors to construct transistors – the basic elements of every computer. CNT could make computer microchips many times smaller. This is attractive particularly against the background that the limits of miniaturisation with conventional chip materials will soon be exhausted and the industry urgently needs alternative technologies for further innovations.
Dimos Poulikakos, Professor at the Laboratory of Thermodynamics in Emerging Technologies (LTNT), says "Although we now understand the properties of CNT relatively well, we are still at the very beginning when it is a question of how to build such systems, which are invisible to the naked eye and extremely vulnerable to external disturbances. That’s why the work we are doing in our laboratory is really basic research in the engineering sciences."
An electrifying, self-organising system
Timo Schwamb, Professor Poulikakos’ doctoral student at the LTNT, published a paper in Nano Letters last November describing possible new ways of positioning CNT in nano-electromechanical systems (NEMS). Dielectrophoresis, a technique well-known from electrical engineering, was used by Schwamb for the first time in his experiments with a high success rate for NEMS with more than two electrodes. This enormously expands the application spectrum of CNTs in nanotechnology. Schwamb applies an inhomogeneous electrical field to a microchip previously treated with a droplet of CNT solution. In this process the electrical circuit has a gap at exactly the place where the CNT is to be positioned. The strongest bipolar forces occur at exactly that point, and ultimately attract and align the CNT.
Schwamb can also use the method described in the paper to incorporate a four-point measuring method into the NEMS. This eliminates results falsifications in the resistance measurement caused by the soldered joints of the CNT that are used. However, because the electrical fields of four electrodes lying in a plane would get in the way, thus disturbing the positioning of the CNT, the doctoral student decided to use a three-dimensional experimental design. Schwamb explains that: "Although we use a little trick to introduce four electrodes for the four-point measuring system, we nevertheless generate an electrical field equivalent to that of only two electrodes and we use this to position the CNT."
This trick works as follows: a gap 0.5 nanometres wide is milled in a three-layer microchip (conductor/insulator/conductor) in such a way that the lowermost layer on both sides projects minimally into the empty space. The two electrodes needed for the four-point measurement can now be “hidden” as it were in the third dimension under the other two electrodes, thus they no longer interfere with the dielectrophoresis.
The LTNT is supported by the EMPA (Swiss Federal Laboratories for Materials Testing and Research) in the difficult work on the chip in the range of a few nanometres. Their experts can use an ion beam to mill gaps and steps in the three-layer silicon chip and can solder tiny contact points between the carrier chip and the CNT in a subsequent step using electron beams. Schwamb summarizes: "For the nanotechnology application we can use the new method to combine two known technologies, dielectrophoresis and the four-point measuring technique, in such a way as to create for the first time the potential to mass-produce nano-devices." This could increase the yield in successfully positioning CNTs across four contact points from approx. 3 percent to 40 percent.
Nano-engineering: an engineering tradition alive in Switzerland
According to Schwamb, the next step will now involve using the new approach to build prototype devices such as transistors and temperature or pressure sensors and to test their properties in a wide variety of ways. However, he says that integrating nano-materials in processes with mass production capability still represents one of the biggest challenges facing nano-technology. However, the most significant benefit of the approach described in the paper is that it can also be used for other nano-particles with interesting properties, and is not limited to CNTs.
Poulikakos also detects a piece of Swiss future in nano-engineering: "Switzerland should not only defend its traditional leading position in the area of engineering achievements in the construction of large machinery but should also position itself as a pioneer in the nano-devices field." However, Poulikakos is still unwilling to make any predictions as to when the first CNT computer with a gigantic computing performance will come onto the market and whether or not this will be in Switzerland.
Cite This Page: