MIT researchers have created a new imaging system that can acquire visual data at a rate of one trillion exposures per second. That’s fast enough to produce a slow-motion video of a burst of light traveling the length of a one-liter bottle, bouncing off the cap and reflecting back to the bottle’s bottom.
Media Lab postdoc Andreas Velten, one of the system’s developers, calls it the “ultimate” in slow motion: “There’s nothing in the universe that looks fast to this camera,” he says.
Picosecond Camera for Time-of-Flight Imaging
Slow art with a trillion frames per second camera
How will the world look with a one trillion frame per second camera? Although such a camera does not exist today, we converted high end research equipment to produce conventional movies at 0.5 trillion (5· 10^11) frames per second, with light moving barely 0.6 mm in each frame. Our camera has the game changing ability to capture objects moving at the speed of light. Inspired by the classic high speed photography art of Harold Edgerton [Kayafas and Edgerton 1987] we use this camera to capture movies of several scenes.
The system relies on a recent technology called a streak camera, deployed in a totally unexpected way. The aperture of the streak camera is a narrow slit. Particles of light — photons — enter the camera through the slit and pass through an electric field that deflects them in a direction perpendicular to the slit. Because the electric field is changing very rapidly, it deflects late-arriving photons more than it does early-arriving ones.
The image produced by the camera is thus two-dimensional, but only one of the dimensions — the one corresponding to the direction of the slit — is spatial. The other dimension, corresponding to the degree of deflection, is time. The image thus represents the time of arrival of photons passing through a one-dimensional slice of space.
The camera was intended for use in experiments where light passes through or is emitted by a chemical sample. Since chemists are chiefly interested in the wavelengths of light that a sample absorbs, or in how the intensity of the emitted light changes over time, the fact that the camera registers only one spatial dimension is irrelevant.
But it’s a serious drawback in a video camera. To produce their super-slow-mo videos, Velten, Media Lab Associate Professor Ramesh Raskar and Moungi Bawendi, the Lester Wolfe Professor of Chemistry, must perform the same experiment — such as passing a light pulse through a bottle — over and over, continually repositioning the streak camera to gradually build up a two-dimensional image. Synchronizing the camera and the laser that generates the pulse, so that the timing of every exposure is the same, requires a battery of sophisticated optical equipment and exquisite mechanical control. It takes only a nanosecond — a billionth of a second — for light to scatter through a bottle, but it takes about an hour to collect all the data necessary for the final video.
Doing the math
After an hour, the researchers accumulate hundreds of thousands of data sets, each of which plots the one-dimensional positions of photons against their times of arrival. Raskar, Velten and other members of Raskar’s Camera Culture group at the Media Lab developed algorithms that can stitch that raw data into a set of sequential two-dimensional images.
The streak camera and the laser that generates the light pulses — both cutting-edge devices with a cumulative price tag of $250,000 — were provided by Bawendi, a pioneer in research on quantum dots: tiny, light-emitting clusters of semiconductor particles that have potential applications in quantum computing, video-display technology, biological imaging, solar cells and a host of other areas.
The trillion-frame-per-second imaging system, which the researchers have presented both at the Optical Society's Computational Optical Sensing and Imaging conference and at Siggraph, is a spinoff of another Camera Culture project, a camera that can see around corners. That camera works by bouncing light off a reflective surface — say, the wall opposite a doorway — and measuring the time it takes different photons to return. But while both systems use ultrashort bursts of laser light and streak cameras, the arrangement of their other optical components and their reconstruction algorithms are tailored to their disparate tasks.
Because the ultrafast-imaging system requires multiple passes to produce its videos, it can’t record events that aren’t exactly repeatable. Any practical applications will probably involve cases where the way in which light scatters — or bounces around as it strikes different surfaces — is itself a source of useful information. Those cases may, however, include analyses of the physical structure of both manufactured materials and biological tissues — “like ultrasound with light,” as Raskar puts it.
As a longtime camera researcher, Raskar also sees a potential application in the development of better camera flashes. “An ultimate dream is, how do you create studio-like lighting from a compact flash? How can I take a portable camera that has a tiny flash and create the illusion that I have all these umbrellas, and sport lights, and so on?” asks Raskar, the NEC Career Development Associate Professor of Media Arts and Sciences. “With our ultrafast imaging, we can actually analyze how the photons are traveling through the world. And then we can recreate a new photo by creating the illusion that the photons started somewhere else.”
“It’s very interesting work. I am very impressed,” says Nils Abramson, a professor of applied holography at Sweden’s Royal Institute of Technology. In the late 1970s, Abramson pioneered a technique called light-in-flight holography, which ultimately proved able to capture images of light waves at a rate of 100 billion frames per second.
But as Abramson points out, his technique requires so-called coherent light, meaning that the troughs and crests of the light waves that produce the image have to line up with each other. “If you happen to destroy the coherence when the light is passing through different objects, then it doesn’t work,” Abramson says. “So I think it’s much better if you can use ordinary light, which Ramesh does.”
Research project website for the trillion frame per second camera
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Angle speeds plastic transistor
Technology Research News
Plastic computer chips have recently received
a lot of attention because they promise to imbue everyday objects with
inexpensive electronic intelligence and enable flexible displays and electronic
Though they are flexible and potentially very inexpensive, organic
electronic devices perform relatively poorly. This is because organic
materials have low charge carrier mobility, which is a measure of how
readily electricity -- or negatively-charged electrons and positively-charged
holes -- moves through the material.
Researchers from Lucent Technologies' Bell Laboratories, Rutgers
University and the University of Illinois have found that the orientation
of crystalline organic semiconductors plays a big role in organic transistor
performance. The researchers have developed a simple lamination manufacturing
process for making transistors from the fragile organic material, and
the resulting transistors have set a record for carrier mobility in organic
The researchers' method could lead to mass production techniques
for organic transistors and light-emitting diodes.
The researchers' field-effect transistor is formed from organic
rubrene crystal and titanium and gold electrodes and has a carrier mobility
of 15.4 square centimeters per volt second, compared to typical organic
semiconductor carrier mobilities of less than one square centimeter per
volt second, according to John Rogers, a professor of materials science
and engineering at the University of Illinois.
The silicon transistors commonly used in today's computer chips
have carrier mobilities of 1,500 square centimeters per volt second, and
other inorganic crystalline semiconductors can have carrier mobilities
an order of magnitude higher than silicon.
The orientation of the molecules within the organic crystal and
the spacing between the molecules contribute to the prototype's relatively
high carrier mobility, said Rogers. Crystal molecules in the prototype
transistor are spaced 1.44 nanometers apart in one direction and 0.72
nanometers in the perpendicular direction. Carrier mobility dropped to
4.4 square centimeters per volt second when the wide spacing of the crystal
was aligned with the electrodes. A nanometer is one millionth of a millimeter.
The rubrene molecule has groups of atoms attached to its sides,
and electrons flow along these side groups and along the backbone of the
molecule. In the high-mobility orientation, the molecules' side groups
are aligned, facilitating electron flow from molecule to molecule. "The
orientation of the molecules relative to the electrodes of the transistors
has a profound impact on the way [the] devices behave," said Rogers.
To test the relationship between orientation and performance in
the organic crystal, the researchers developed a method of making field
effect transistors that allowed them to repeatedly place, remove, rotate
and replace the relatively fragile crystal on the transistor's electrodes.
"We build all components of the transistor -- source/drain electrodes,
gate dielectric, and gate electrode -- out of soft, conformable materials
built on a soft, elastomeric substrate," said Rogers. "We then, at room
temperature and without applied pressure, gently place the organic crystal,
which is grown in a separate process... on the surface of this transistor
To make the transistors, the researchers placed a titanium-gold
gate electrode on a silicone rubber surface, covered it with a thin film
of silicone rubber and placed titanium-gold source and drain electrodes
on top. They then simply placed the organic crystal over the electrodes
and gently pressed one edge of the crystal. This caused the crystal to
adhere to the silicone and metal due to the van der Waals force, which
is the electrostatic attraction between atoms and molecules. "The soft
contact forms very high-performance transistors in a way that avoids all
of the hazards that conventional semiconductor processing poses to the
organics," said Rogers.
The lamination method could be used in practical applications
in three to five years, said Rogers.
Rogers' research colleagues were Bell Laboratories researchers
Vikram Sundar, now at IBM, Jana Zaumseil, now at the University of Cambridge,
Robert Willett, and Takao Someya, now at the University of Tokyo; Vitaly
Podzorov and Michael Gershenson of Rutgers University; and Etienne Menard
of the University of Illinois.
They published the research in the March 12, 2004 issue of Science.
The research was funded by the National Science Foundation (NSF) and the
U.S. Department of Energy.
Timeline: 3-5 years
TRN Categories: Integrated Circuits; Materials Science
Story Type: News
Related Elements: Technical paper, "Elastomeric Transistor
Stamps: Reversible Probing of Charge Transport in Organic Crystals," Science,
March 12, 2004
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In atomic physics
, hyperfine coupling
is the weak magnetic
interaction between electrons
. Hyperfine coupling causes the hyperfine splitting
of atomic or molecular energy levels. The totality of energy levels spawned by hyperfine splitting is called the hyperfine structure
of the atom's or molecule's spectrum
The following terminology has evolved to describe atomic and/or molecular spectra:
- The gross structure is due to the energy difference of electronic orbitals with different principal quantum number n.
- The fine structure occurs only for n>0; it is due to the spin-orbit coupling (the energy difference between the electron spin being parallel or antiparallel to the electron's orbital moment).
- The hyperfine structure is due to an unpaired electron interacting with a nucleus having nuclear spin quantum number I 0. The electron and nucleus (nuclei) are on the same atom or within the same molecule.
- The superhyperfine structure is due to an unpaired electron interacting with a nucleus having I 0. The electron and nucleus (nuclei) are on different atoms or different molecules.
- The spin-spin structure is due to interactions among nuclei having I 0. This phenomenon is especially important in NMR spectra
In first order, hyperfine coupling is a magnetic dipole
-dipole interaction, arising from the interaction of the nuclear magnetic moment
with the magnetic field of the electron.
According to classical thinking, the electron moving around the nucleus has a magnetic dipole moment, because it is charged. The interaction of this magnetic dipole moment with the magnetic moment of the nucleus (due to its spin) leads to hyperfine splitting.
However, due to the electron's spin, there is also hyperfine splitting for s-shell electrons, which have zero orbital angular momentum. In this case, the magnetic dipole interaction is even stronger, as the electron probability density does not vanish inside the nucleus ().
The amount of correction to the Bohr energy levels due to hyperfine splitting of the hydrogen atom is of the order
- is the mass of an electron,
- is the mass of a proton,
- is the fine structure constant , and
- is the speed of light.
For atoms other than hydrogen, the nuclear spin and the total electron angular momentum get coupled, giving rise to the total angular momentum .
The hyperfine splitting is then
- the magnetic dipole moment of the nucleus, and
- is the atomic magnetic field.
This interaction obeys the Lande interval rule: The energy level is split into energy levels, where denotes the total electron angular momentum and denotes the nuclear spin.
Usually, is of order of GHz; the hyperfine splitting is orders of magnitude smaller perturbation than the fine structure.
In a more advanced treatment, one also has to take the nuclear magnetic quadrupole moment into account. This is sometimes (?) referred to as "hyperfine structure anomaly".
The optical hyperfine structure was already observed in 1881 by Albert Abraham Michelson
. It could, however, only be explained in terms of quantum mechanics in the 1920s. Wolfgang Pauli
proposed the existence of a small nuclear magnetic moment in 1924.
In 1935, M. Schiiler and T. Schmidt proposed the existence of a nuclear quadrupole moment in order to explain anomalies in the hyperfine structure.
Hyperfine interactions can be measured, among other ways, in atomic and molecular spectra and in electron paramagnetic resonance spectra of free radicals and transition-metal ions.
As the hyperfine splitting is very small, the transition frequencies usually are not optical, but in the range of radio- or microwave frequencies.
Hyperfine structure gives the 21 cm line observed in HI region in interstellar medium.
Carl Sagan and Frank Drake considered the hyperfine transition of hydrogen to be a sufficiently universal phenomenon so as to be used as a base unit of time and length on the Pioneer plaque and later Voyager Golden Record.
In radio astronomy, heterodyne receivers are widely used in detection of the electromagnetic signals from celestial objects. The separations among various components of a hyperfine structure are usually small enough to fit into the receiver's IF band. Because optical depth varies with frequency, strength ratios among the hyperfine components differ from that of their intrinsic intensities. From this we can derive the object's physical parameters.
process uses the hyperfine splitting of between optical transitions in uranium-235 and uranium-238 to selectively photoionize only the uranium-235 atoms and then separate the ionized particles from the non-ionized ones. Precisely tuned dye lasers
are used as the sources of the necessary exact wavelength radiation.
Use in defining the SI second and meter
The hyperfine structure transition can be used to make a microwave
notch filter with very high stability, repeatability and Q factor
, which can thus be used as a basis for very precise atomic clocks
. Typically, the hyperfine structure transition frequency of a particular isotope of caesium
atoms is used as a basis for these clocks.
Due to the accuracy of hyperfine structure transition-based atomic clocks, they are now used as the basis for the definition of the second. One second is now defined to be exactly 9,192,631,770 cycles of the hyperfine structure transition frequency of caesium-133 atoms.
Since 1983, the meter is defined by declaring the speed of light in a vacuum to be exactly 299,792,458 metres per second. Thus:
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Precision tests of quantum electrodynamics
The hyperfine splitting in hydrogen and in muonium
have been used to measure the value of the fine structure constant α. Comparison with measurements of α in other physical systems provides a stringent test of QED
Qubit in ion-trap quantum computing
The hyperfine states of a trapped ion
are commonly used for storing qubits
in ion-trap quantum computing
. They have the advantage of having very long lifetimes, experimentally exceeding ~10 min (compared to ~1 s for metastable electronic levels).
The frequency associated with the states' energy separation is in the microwave region, making it possible to drive hyperfine transitions using microwave radiation. However, at present no emitter is available that can be focused to address a particular ion from a sequence. Instead, a pair of laser pulses can be used to drive the transition, by having their frequency difference (detuning) equal to the required transition's frequency. This is essentially a stimulated Raman transition.
- G. Herzberg, Atomic Spectra and Atomic Structure. Dover, New York, 1944. See especially chapter 5.
- M. Symons, Chemical and Biochemical Aspects of Electron-Spin Resonance Spectroscopy. Wiley, New York, 1978
- J. A. Weil, J. R. Bolton, and J. E. Wertz, Electron Paramagnetic Resonance: Elementary Theory and Practical Applications. Wiley-Interscience, New York, 2001


(Phys.org) —While quantum states are typically referred to as particles or waves, this is not actually the case. Rather, quantum states have complementary discrete particlelike and continuous wavelike properties that emerge based on the experimental or observational context. In other words, when used to describe quantum states the terms particle and wave are convenient but inaccurate metaphors. This is an important consideration in quantum computing, where photons are used as units of quantum information known as quantum bits, or qubits, which due to quantum superposition (and therefore unlike classical bits) can simultaneously exist in two states. That said, current attempts to devise quantum computers that process photonic qubits universally using particle detectors to count photons and optical circuits to capture quantum wave evolution have been stymied by the fact that ancilla states – fixed qubit states used in reversible quantum computing as input to a gate to give that gate a more specific logic function – consist of many highly-entangled photons, thereby exceeding experimental capabilities. (Entanglement is a uniquely-quantum state in which two or more interacting particles are said to be hypercorrelated – meaning that the state of each individual particle cannot be described independently, and that a change in a property of one particle is instantly reflected in its entangled partner regardless of the distance separating them.)
Recently, however, scientists at The University of Tokyo demonstrated for the first time a two-way conversion between a particlelike single-photon state and a wavelike superposition of coherent states by applying quantum squeezing/unsqueezing as a quantum gate, deriving Gaussian (coherent) operations that are applicable to nonclassical, non-Gaussian quantum states and therefore expanding the hybrid quantum-information processing optical toolbox. (In general, a squeezed coherent state is a quantum state in which the uncertainty principle is saturated. Achieved using a number of methods1, squeezed light is a state in which quantum noise is reduced. Specifically, in a squeezed sate the electric field noise paradoxically falls below that of the vacuum state – a phenomenon that has classical counterpart.) Moreover, the researchers say that their so-called squeezing gate will lead to new applications while forming the basis of a new class of optical quantum processors capable of integrating particlelike and wavelike quantum states.
Prof. Akira Furusawa discussed the paper that he and his co-authors published in Physical Review Letters with Phys.org – including the main challenges in successfully applying a quantum optical squeezing operation upon non-Gaussian quantum states, thereby demonstrating a two-way conversion between a particlelike single-photon state and a wavelike superposition of coherent states. "Previous approaches using direct squeezing operations for nonclassical non-Gaussian states were very difficult because such states are very fragile to losses – and direct squeezing operations inevitably have losses," Furusawa tells Phys.org. "In our approach, the squeezing operation is not direct. Instead, we first prepare a squeezed vacuum by using a conventional optical parametric oscillator and then teleport the squeezing operation to fragile nonclassical non-Gaussian states through linear optics, which have almost no losses."
In quantum teleportation2, qubits (specifying, for example, a photon's precise state) are transmitted between quantum-entangled locations via classical communication systems. "In this case, the essential resource is entanglement between the ancillary squeezed vacuum and nonclassical non-Gaussian states, which are created by a beam splitter with no losses," Furusawa notes. "Our successful teleportation of the squeezing operation to a single-photon state and Schrödinger's-cat" – that is, superposition – "state is the first example of deterministic quantum gate teleportation."
Another first the researchers achieved was using universal and reversible low-loss broadband squeezing to access for the first time a complete set of deterministic Gaussian operations applicable to nonclassical, non-Gaussian states. "A complete set of deterministic Gaussian operations consists of displacement, rotation, and squeezing in phase space," Furusawa explains. "Displacement can be realized by using an optical modulator and a beam splitter, and rotation by controlling optical path length. Therefore, both operations are very easy to apply – even to nonclassical non-Gaussian states.
The last piece of the complete set is squeezing, where we succeeded – also for the first time."
In short, the scientists' key result – demonstrating the very powerful capability of deterministic quantum gate teleportation – allows non-Gaussian operations that can, in principle, be used to build the elusive universal quantum computer. "We want to hybridize the discrete and continuous quantum protocols to build an efficient and robust quantum computer," Furusawa confirms. "The advantage of using qubit protocols is the robustness coming from the digital processing-like finite dimensionality, while the advantage of continuous-variable protocols is efficiency, because they can allow us to make deterministic operations. (Furusawa points out that it remains an open question if this hybridization has implications for ongoing efforts to integrate quantum mechanics and general relativity, which are described using discrete and continuous mathematics, respectively.)
The paper also describes the notable finding that allows the entire Fock space to be used when processing single photons, thereby possibly helping to construct quantum gates and error correction codes for logical qubits. (The Fock space is a mathematical method for articulating the quantum states of a variable, or a non-specified number of identical particles, from a single particle Hilbert space, which is a generalization of Euclidean space.) "Specifically," says Furusawa, "we're now thinking about constructing quantum gates and error correction codes with the hybrid protocol."
Furusawa adds that the deterministic Gaussian operations accessed made possible by their broadband squeezer will directly lead to applications in this area. "Firstly," he illustrates, "we can construct a quantum non-demolition (QND) gate – in which a measured observable's uncertainty does not increase as the quantum system evolve – that corresponds to a qubit controlled NOT (CNOT) gate." Quantum CNOT gates can be used to simulate any quantum circuit to an arbitrary degree of accuracy, as well as to create and dismantle entangled, or EPR (after the 1935 paper3 by Albert Einstein, Boris Podolsky and Nathan Rosen), states. "Secondly, since the QND gate is a universal entangling gate, it allows more complicated quantum gate teleportation."
In addition, Furusawa tells Phys.org, their next target is a particlelike/wavelike hybrid CNOT gate based on non-Gaussian quantum gate teleportation. "We're also thinking about applying this technology to optical communications – especially a quantum mechanically optimal receiver."
Explore further: Entanglement between particle and wave-like states of light resembles Schrodinger's cat experiment (Update)
More information: Exploring a New Regime for Processing Optical Qubits: Squeezing and Unsqueezing Single Photons, Physical Review Letters 113, 013601 (Published 2 July 2014), doi:10.1103/PhysRevLett.113.013601
1Squeezed light, arXiv:1401.4118v1 [quant-ph]
2Quantum Teleportation and Entanglement by Akira Furusawa and Peter van Loock, Wiley-VCH (2011), ISBN-13:978-3527409303 (Hardcopy), ASIN:B00BP7S3X8 (Kindle), ISBN:9783527635306 (Google EBook)
3Can Quantum-Mechanical Description of Physical Reality Be Considered Complete? Physical Review 47, 777 (15 May 1935), doi:10.1103/PhysRev.47.777


Rydberg atoms are atoms in which one or more of the atom's electrons have been excited into very high energy states. Because the Rydberg electron is so far from the core of the atom, the atom develops exaggerated properties, such as hugh polarizabilities that scale like n7, where n is the principle quantum number. These exaggerated properties lead to strong, tunable interactions among the atoms, which have applications in many different fields of physics.
One of the most important consequences of the strong interactions between Rydberg atoms is the Rydberg excitation blockade, which results from the interactions shifting the energy levels of the atoms. As shown in the figure above, the energy levels deviate from an equidistant ladder. If the shift of the second excited state is great enough such that the excitation laser is out of resonance with the state, then all excitation above the first excitated state is blockaded.
Some of the applications of the Rydberg excitation blockade include quantum computation, quantum cryptography, improved spectroscopic resolution, and atomic clocks. The first proposal to use the blockade for quantum information was in 2000, when Jaksch et. al. suggested a method of generating a fast phase gate (Phys. Rev. Lett. 85, 2208 (2000)) using Rydberg atoms. The motivation discusses this proposal.
As we move toward the goal of quantum computing with Rydberg atoms, we have conducted many interesting studies. We highlight work done converning the Autler-Townes effect with 85Rb. By taking advantage of the long lifetimes of Rydberg atoms (10's of microseconds), and hence small spectroscopic linewidths of Rydberg states, we are able to achieve Autler-Townes spectra with high resolution. These measurements provide a foundation for all later work, as Autler-Townes spectroscopy is a tool for measuring Rabi frequencies with high accuracy.
We have also conducted a spectroscopic measurement of the energy shifts of the second excited state of the Rydberg excitation ladder in different interaction regimes. By applying two sets of excitation pulses with variable frequency (a set because the excitation to Rydberg states is a two-photon excitation), we have measured the lineshape of the 1R - 2R transition. This study is the first spectroscopic proof of the functionality of the Rydberg excitation blockade.
One way of measuring the effectiveness of the Rydberg excitation blockade is to use counting statistics. We have used this method for a range of nD5/2 Rubidium Rydberg states. Counting statistics measurements are particularly useful for measuring blockade effectiveness in small atomic samples and for a variety of different experimental parameters such as excitation Rabi frequencies, detuning, and quantum state.
All atoms have repulsive or attractive forces between them due to temporary dipole moments, when the electrons of an atom leave the positively charged nucleus unshielded. Typically the positively charged nucleus polarizes (induces a dipole) in nearby atoms causing a temporary dipole-dipole interaction. These temporary off-resonant dipole-dipole interactions are typically named "van der Waals" or "London" forces. Two atoms or molecules with permanent dipole moments, e.g. HCl, interact via on-resonant "dipole-dipole" interactions. These permanent dipole-dipole interactions however are always in addition to van der Waals (temporary dipole-dipole) interactions. The similarity between dipole-dipole and van der Waals interactions is often clouded by the naming convention. They are both calculated using the standard interaction potential of two interacting dipoles. Van der Waals are off-resonant, temporary, second-order interactions, and dipole-dipole interactions are on-resonant, permanent, first-order interactions.
Interatomic van der Waals interactions are present in all matter, and play a large role in determining the melting points of all elements. For example, consider the melting point of He, 4K (-269C), as compared to the melting temperature of Radon at 221K (-52C). In general symmetric atoms like He and Radon must first be cooled down significantly in order to condense, because they cannot align themselves into an array of aligned dipoles as effectively as elliptically shaped atoms. Atoms with more electrons like Radon have larger van der Waals interactions, and thus must be heated more to break the van der Waals bonds and become a gas. This is because the electrons have larger orbits away from the nucleus, leaving the nucleus unexposed with a higher probability. Furthermore, the nucleus of heavier atoms will induce larger dipoles in nearby atoms, and hence larger van der Waals interactions.
As briefly mentioned above, there are two interaction regimes for the forces between atoms: the van der Waals regime, and the dipole-dipole regime. We can see how the two regime arise by looking at the Hamiltonian for two particle interactions, shown on the right. Generically, the Hamiltonian contains energies on the diagonal and coupling terms on the off-diagonal. Here, we have a two particle state AA that is coupled to another two particle state BC through an interaction term Vint, and an energy detuning of D. In our case, the interaction term Vint, is the dipole interaction operator. The scaling of this operator is n4/R3.
In the regime of van der Waals interactions, the coupling between the atoms is much less than the energy detuning, D, of the interaction. This leads to energy eigenstates that are shifted in energy by (Vint)2/D. Since D scales like 1/n3, the total scaling of the shift is n11/R6.
Conversely, for dipole-dipole interactions, the energy detuning D is much smaller than the interaction Vint. In this case, the scaling is simply n4/R3.


Diamonds have long been available in pairs—say, mounted in a nice set of earrings. But physicists have now taken that pairing to a new level, linking two diamonds on the quantum level.
A group of researchers report in the December 2 issue of Science that they managed to entangle the quantum states of two diamonds separated by 15 centimeters. Quantum entanglement is a phenomenon by which two or more objects share an unseen link bridging the space between them—a hypothetical pair of entangled dice, for instance, would always land on matching numbers, even if they were rolled in different places simultaneously.
But that link is fragile, and it can be disrupted by any number of outside influences. For that reason entanglement experiments on physical systems usually take place in highly controlled laboratory setups—entangling, say, a pair of isolated atoms cooled to nearly absolute zero.
In the new study, researchers from the University of Oxford, the National Research Council of Canada and the National University of Singapore (NUS) showed that entanglement can also be achieved in macroscopic objects at room temperature. "What we have done is demonstrate that it's possible with more standard, everyday objects—if diamond can be considered an everyday object," says study co-author Ian Walmsley, an experimental physicist at Oxford. "It's possible to put them into these quantum states that you often associate with these engineered objects, if you like—these closely managed objects."
To entangle relatively large objects, Walmsley and his colleagues harnessed a collective property of diamonds: the vibrational state of their crystal lattices. By targeting a diamond with an optical pulse, the researchers can induce a vibration in the diamond, creating an excitation called a phonon—a quantum of vibrational energy. Researchers can tell when a diamond contains a phonon by checking the light of the pulse as it exits. Because the pulse has deposited a tiny bit of its energy in the crystal, one of the outbound photons is of lower energy, and hence longer wavelength, than the photons of the incoming pulse.
Walmsley and his colleagues set up an experiment that would attempt to entangle two different diamonds using phonons. They used two squares of synthetically produced diamond, each three millimeters across. A laser pulse, bisected by a beam splitter, passes through the diamonds; any photons that scatter off of the diamond to generate a phonon are funneled into a photon detector. One such photon reaching the detector signals the presence of a phonon in the diamonds.
But because of the experimental design, there is no way of knowing which diamond is vibrating. "We know that somewhere in that apparatus, there is one phonon," Walmsley says. "But we cannot tell, even in principle, whether that came from the left-hand diamond or the right-hand diamond." In quantum-mechanical terms, in fact, the phonon is not confined to either diamond. Instead the two diamonds enter an entangled state in which they share one phonon between them.
To verify the presence of entanglement, the researchers carried out a test to check that the diamonds were not acting independently. In the absence of entanglement, after all, half the laser pulses could set the left-hand diamond vibrating and the other half could act on the right-hand diamond, with no quantum correlation between the two objects. If that were the case, then the phonon would be fully confined to one diamond.
If, on the other hand, the phonon were indeed shared by the two entangled diamonds, then any detectable effect of the phonon could bear the imprint of both objects. So the researchers fired a second optical pulse into the diamonds, with the intent of de-exciting the vibration and producing a signal photon that indicates that the phonon has been removed from the system. The phonon's vibrational energy gives the optical pulse a boost, producing a photon with higher energy, or shorter wavelength, than the incoming photons and eliminating the phonon in the process.
Once again, there is no way of knowing which diamond produced the photon, because the paths leading from each diamond to the detectors are merged, so there is no way of knowing where the phonon was. But the researchers found that each of the photon paths leading from the diamonds to the detectors had an interfering effect on the other—adjusting how the two paths were joined affected the photon counts in the detectors. In essence, a single photon reaching the detectors carried information about both paths. So it cannot be said to have traveled down one path from one diamond: the photon, as with the vibrational phonon that produced it, came from both diamonds.
After running the experiment over and over again to gather statistically significant results, the researchers concluded with confidence that entanglement had indeed been achieved. "We can't be 100 percent certain that they're entangled, but our statistical analysis shows that we're 98 percent confident in that, and we think that's a pretty good outcome," Walmsley says.
The catch to using phonons for macroscopic entanglement is that they do not last long—only seven picoseconds, or seven trillionths of a second, in diamond. So the experimenters had to rely on extremely fast optical pulses to carry out their experiment, creating entangled states with phonons and then damping the phonons with the second pulse to test that entanglement just 0.35 picoseconds later.
Because of this brevity, such entanglement schemes may not take over for more established techniques using photons or single atoms, but Walmsley hopes that researchers will consider the possibilities of using fairly ordinary, room-temperature materials in quantum technologies. "I think it gives a new scenario and a new instantiation of something that helps point in that direction," he says.
Indeed, the new study is just the latest to show how quantum mechanics applies in real-world, macroscopic systems. Oxford and NUS physicist Vlatko Vedral, who was not involved in the new research, says it "beautifully illustrates" the point of Austrian physicist Erwin Schrödinger's famous thought experiment in which a hypothetical cat is simultaneously alive and dead. "It can't be that entanglement exists at the micro level (say of photons) but not at the macro level (say of diamonds)," because those worlds interact, Vedral wrote in an email. "Schrödinger used atoms instead of photons and cats instead of diamonds, but the point is the same."


Causality is one of the oldest and most important concepts of Physics. Even recently, at the beginning of the XX century, with the invention of Special Relativity, this concept was in some sense rediscovered. As in a relativistic framework the events can change their temporal order a great effort was made in order to preserve causality in the theory.
There is a general consensus in the scientific community about this concept: For all scientific theories, even for all the theories that will come in the future, causality should be preserved. If causal relations are broken an important number of paradoxes and counter-intuitive results arise. You could even go back in time and kill your grandgrandfather!
In quantum mechanics the discovery of entangled states, that are states with correlations than can act immediately even in they are separated by a distance of millions of light years, challenged this concept. The solution for preserving causality was to accept that quantum systems are intrinsically random and no theory can give a complete description of them.
Very recently, in Reference 1, a paper published in Nature Communications by Ognyan Oreshkov and coworkers, from the University of Vienna, the concept of causality itself is discussed. Just by assuming that quantum mechanics is valid only locally, they show that it is difficult to talk about ‘causal order’. As it has been made before in order to analyze the effects of quantum mechanics the authors decided to illustrate their result with a thought experiment.
The rules of this experiment are:
- There are two parties, Alice and Bob. They are in labs that are far away from each other.
- They both receive one random bit, either 0 or 1.
- They can send information out between their labs.
- They have to guess the bit of each other. This decision should be made at the same time they send their information out.
Obviously, the experiment should be repeated several times, and the goal is to guess the bit of the other party as much times as possible. The ‘figure of merit’ that measures how well we are performing the game is the probability of guessing for both Alice and Bob together, that is a number between 0 and 1.
Let see what can we do in a classical, and causal, framework. It is clear that the success probability will depend in this case on the time order of the events. If Alice sends her information first, she can use it in order to communicate Bob what her bit was. Indeed, Bob will succeed all the time. The problem now is that Alice has no clue about Bob’s bit, so the best she can do is just say something at random. The same problem arises if it is Bob the first in sending the information. So, in the best possible scenario, the probability of success is 1 for one of them, the one that acts second, and ½ for the other one, the one that acts first. That means that the best possible probability in a classical causal framework is ¾.
So, is there any difference in a quantum mechanics framework? Not really, quantum mechanics is also a theory with a definite causal background and has to fulfill the same constrains. But, what happens if we slightly modify quantum mechanics in order to remove the space-time background, making it only valid locally, but not globally? That is the problem analyzed in Ref. 1 by Oreshkov et al. There, the authors performed a similar experiment, where it is assumed that Alice and Bob can make any kind of quantum operation in their labs. In these labs quantum mechanics holds, but there is not any assumption of a preexisting background time, or global causal structure. In this scenario, that differs from normal quantum mechanics, they show that the limit of the probability of success can be enhanced beyond the causal limit.
The rules for the non-causal quantum game are:
- Each laboratory is isolated.
- Quantum mechanics can be applied locally in the labs, but there is no assumption of what happens globally.
- There is also no assumptions about the spatio-temporal location of the experiments. That means that it is not define who makes the measurement before.
- They don’t need to communicate in this case. This is a necessary assumption in this case, because in this case there is not a definite spatio-temporal order, so it is not defined who acts first and can communicate and who is second and can not.
Based on these assumptions the authors create a new framework based on local quantum mechanics for analyzing the possible options of Alice and Bob. The results are surprising, they find a possibility of reaching a success probability of 0,853, that is higher than the ¾ probability of the best causal scenario. Even, without communication between them.
And what does it mean? Is causality broken in this new theory and we can communicate now with our dead grandgrandfather? That could be very interesting for science fiction writers, but it is not like that. The authors claim in their paper that, as quantum mechanics can be applied locally to Alice and Bob’s labs, causality should be preserved. This is due to the noise in the evolution ‘backward in time’ and it is compatible with the Novikov principle.
So, if causality itself is not broken, why is this result interesting? First, the analysis of new possible frameworks is always useful. In general relativity, for instance, when one imposes only local constrains new and interesting features arise, as exotic causal structures. It looks like that something similar happens in the quantum regime. Also, this results imply that if quantum mechanics only works locally new kind of correlations appear, stronger than the ones that are usual in normal quantum mechanics, like entanglement. Even, if these correlations can not break the causal order, as is expectable, the potential implications are huge. We should not forget that entanglement leads to interesting applications as quantum computing, quantum teleportation or cryptography. We can not know which applications these new correlations may have.
Finally, there is a more important question: Are these correlations something real or just a mathematical trick? About this question, the authors mention in the discussion of their paper that maybe these correlations can be found in regimes where the actual theories are untested, such as, for example, those in which quantum mechanics and general relativity become relevant.
So, in my opinion, for the moment this result is purely theoretical, but very interesting in any case. This kind of studies, even if they are just theory, usually open a door to new ways of thinking. Also new theories and potential applications can be realized from it. Only time can show how useful it will be.


Many important problems in physics—especially low-temperature physics—remain poorly understood because the underlying quantum mechanics is vastly complex. Conventional computers—even supercomputers—are inadequate for simulating quantum systems with as few as 30 particles. Better computational tools are needed to understand and rationally design materials, such as high-temperature superconductors, whose properties are believed to depend on the collective quantum behavior of hundreds of particles.
The NIST quantum simulator permits study of quantum systems that are difficult to study in the laboratory and impossible to model with a supercomputer. The heart of the simulator is a two-dimensional crystal of beryllium ions (blue spheres in the graphic); the outermost electron of each ion is a quantum bit (qubit, red arrows). The ions are confined by a large magnetic field in a device called a Penning trap (not shown). Inside the trap the crystal rotates clockwise.
Nature - Engineered two-dimensional Ising interactions in a trapped-ion quantum simulator with hundreds of spins
The NIST quantum simulator permits study of quantum systems that are difficult to study in the laboratory and impossible to model with a supercomputer. In this photograph of the crystal, the ions are fluorescing, indicating the qubits are all in the same state. Under the right experimental conditions, the ion crystal spontaneously forms this nearly perfect triangular lattice structure.
The NIST simulator consists of a tiny, single-plane crystal of hundreds of beryllium ions, less than 1 millimeter in diameter, hovering inside a device called a Penning trap. The outermost electron of each ion acts as a tiny quantum magnet and is used as a qubit—the quantum equivalent of a “1” or a “0” in a conventional computer. In the benchmarking experiment, physicists used laser beams to cool the ions to near absolute zero. Carefully timed microwave and laser pulses then caused the qubits to interact, mimicking the quantum behavior of materials otherwise very difficult to study in the laboratory. Although the two systems may outwardly appear dissimilar, their behavior is engineered to be mathematically identical. In this way, simulators allow researchers to vary parameters that couldn’t be changed in natural solids, such as atomic lattice spacing and geometry. In the NIST benchmarking experiments, the strength of the interactions was intentionally weak so that the simulation remained simple enough to be confirmed by a classical computer. Ongoing research uses much stronger interactions.
Simulators exploit a property of quantum mechanics called superposition, wherein a quantum particle is made to be in two distinct states at the same time, for example, aligned and anti-aligned with an external magnetic field. So the number of states simultaneously available to 3 qubits, for example, is 8 and this number grows exponential with the number of qubits: 2N states for N qubits.
Crucially, the NIST simulator also can engineer a second quantum property called entanglement between the qubits, so that even physically well separated particles may be made tightly interconnected.
Recent years have seen tremendous interest in quantum simulation; scientists worldwide are striving to build small-scale demonstrations. However, these experiments have yet to fully involve more than 30 quantum particles, the threshold at which calculations become impossible on conventional computers. In contrast, the NIST simulator has extensive control over hundreds of qubits. This order of magnitude increase in qubit-number increases the simulator’s quantum state space exponentially. Just writing down on paper a state of a 350-qubit quantum simulator is impossible—it would require more than a googol of digits: 10 to the power of 100.
Over the past decade, the same NIST research group has conducted record-setting experiments in quantum computing,** atomic clocks and, now, quantum simulation. In contrast with quantum computers, which are universal devices that someday may solve a wide variety of computational problems, simulators are “special purpose” devices designed to provide insight about specific problems.
The presence of long-range quantum spin correlations underlies a variety of physical phenomena in condensed-matter systems, potentially including high-temperature superconductivity. However, many properties of exotic, strongly correlated spin systems, such as spin liquids, have proved difficult to study, in part because calculations involving N-body entanglement become intractable for as few as N ≈ 30 particles. Feynman predicted that a quantum simulator—a special-purpose ‘analogue’ processor built using quantum bits (qubits)—would be inherently suited to solving such problems. In the context of quantum magnetism, a number of experiments have demonstrated the feasibility of this approach but simulations allowing controlled, tunable interactions between spins localized on two- or three-dimensional lattices of more than a few tens of qubits have yet to be demonstrated, in part because of the technical challenge of realizing large-scale qubit arrays. Here we demonstrate a variable-range Ising-type spin–spin interaction, Ji,j, on a naturally occurring, two-dimensional triangular crystal lattice of hundreds of spin-half particles (beryllium ions stored in a Penning trap). This is a computationally relevant scale more than an order of magnitude larger than previous experiments. We show that a spin-dependent optical dipole force can produce an antiferromagnetic interaction , where 0 ≤ a ≤ 3 and di,j is the distance between spin pairs. These power laws correspond physically to infinite-range (a = 0), Coulomb–like (a = 1), monopole–dipole (a = 2) and dipole–dipole (a = 3) couplings. Experimentally, we demonstrate excellent agreement with a theory for 0.05 ≲ a ≲ 1.4. This demonstration, coupled with the high spin count, excellent quantum control and low technical complexity of the Penning trap, brings within reach the simulation of otherwise computationally intractable problems in quantum magnetism.
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Diamonds to dust
One aim of future research is to ultimately confine the light interaction to the atomic scale and to demonstrate selective single atom removal. Credit: Carlo Bradac Photo: Carlo Bradac
Small, it seems, is never quite small enough. In their relentless quest to build ever-more minuscule and compact electronic devices, scientists have attempted to manipulate a variety of materials down to the atomic level.
For many reasons, this has proved tough to achieve. Now, a team of Australian researchers has succeeded in using intense pulses of laser light to move individual atoms in substances as rock-solid as diamonds.
The breakthrough is likely to lead to new types of nano-scale devices measuring just billionths of a metre, including minute sensors, super-small and fast electronic components and data storage systems, quantum computers and perhaps a new generation of high-powered lasers on tiny chips.
The discovery, reported in the British journal Nature Communications, resulted more from serendipity than planning. "To our surprise, we found that ultraviolet lasers could be used to target specific atoms," says team leader Richard Mildren, of Macquarie University in Sydney.
"We knew that UV lasers could eject atoms from the surface of diamonds – even at very low light levels," Associate Professor Mildren explains. "But there was no clue to suggest that this process could be harnessed to remove a single targeted atom."
The telling clue, he says, came from ongoing research using an intense UV laser to slice through small sections of diamond.
Diamonds derive their hardness from the way their carbon atoms are arranged in an extremely rigid grid, known as a crystal lattice. The rigidity results from each atom being bound tightly to four other carbon atoms.
Although diamonds are generally transparent to UV rays, a sliver of the light is absorbed very close to the surface. "We think it may occur in the top one or two rows of atoms," Professor Mildren says. "The surface of diamond is normally covered in oxygen atoms and we suspect the carbon is released in the form of carbon monoxide molecules."
The added energy, he says, is enough to break the chemical bonds that normally bind carbon atoms to the surface.
The scientists found that it takes the energy of two UV light particles, or photons, to dislodge one carbon atom. "Carbon atoms are ejected from the surface one by one," he says. "The rate at which this happens is very predictable."
Exactly how the energy is absorbed, and leads to the bonds being broken, is not yet well understood. "This is something we need to work on."
Not any old light does the trick. The diamonds his team worked on were exposed to a very specific form of light pulses in the UV-C band. These are the sun's harshest rays that are largely filtered out by Earth's ozone layer.
A few seconds after being bombarded with light pulses, the diamonds developed small pits on their surface. "The rate of mass loss in the diamond fell notably for lower light levels," Professor Mildren says. "But the etching process still continued – albeit at a slower and slower pace."
The rate of this etching is so slow that it is not noticeable under normal circumstances. In fact, even under very bright conditions, such as intense sunlight or a UV tanning lamp, it would take roughly the age of the universe – almost 14 billion years – to make an appreciable impact on a diamond.
This is where lasers come in handy. These are basically devices that emit intense beams of light by amplifying the light waves using a process called stimulated emission of electromagnetic radiation. The term laser, in fact, is an acronym for "light amplification by stimulated emission of radiation".
Beams emitted in this way differ from other sources of light because they emit the light coherently. In essence, this means that a "hot" beam can be intensified and concentrated onto a very small area. This allows them to cut or weld through virtually any solid material.
The laser's ability to cut out components on dimensions much smaller than the width of a human hair makes them prized tools in high-tech industries, including electronics and car making.
But at smaller scales – such as the distances between atoms – lasers were, until recently, generally considered to be quite ineffectual.
The problem, Professor Mildren says, is that laser cutting and material processing have relied on the heat produced by a laser's beam, in many cases stripping electrons from their parent atomic nuclei. The smallest cuts that could be made depended on the amount of heat transferred to the surface.
"There are now promising signs that it is possible to use lasers to carve up a material with atomic resolution – that is to pick apart a substance atom by atom by using a light beam to snip the chemical bonds holding the individual atoms together," he explains.
The researchers have experimented with their lasers, for example machining a variety of surfaces.
Examination of the machined surfaces using a high-powered electron microscope showed the formation of a curious pattern of regular nano-structures, Professor Mildren says.
"The key observation came when varying the light beam's polarisation – that is, the direction of the light wave's oscillating movement. The particular shape and orientation of these patterns altered with the way chemical bonds of surface atoms lined up with the polarisation."
This surprising observation, he says, provided the essential clue that the light was somehow interacting with individual bonds. "It also showed that chemical bonds can be broken before there is any significant dissipation of energy to cause damage to the surrounding area."
Low-cost production of high-quality diamonds from synthetic sources is driving developments in areas such as ultra-fast electronics, quantum computing devices and miniature high-powered diamond lasers.
"Having a new tool to construct and manipulate diamond devices at the ultimate level of resolution is very exciting for developing these future technologies," Professor Mildren says. "We have already shown that it's possible to make diamond structures of less than 20 nanometres – within the size range of large molecules. This is many tens of times smaller than what could previously be achieved, and suitably small to be of immediate use in applications such as super-low friction surfaces and advanced light sources."
The next goal, he says, is to develop ways to treat single or small groups of atoms. "We would like to manipulate surfaces with single-atom precision, or more than 10,000 times smaller than that possible by standard laser machining techniques. This is an area full of interesting challenges for confining a laser beam sufficiently to gain the necessary level of control."
Professor Mildren and colleagues Andrew Lehmann and Carlo Bradac admit the mechanisms behind this process are not yet well understood. "So it is important to study the process in greater detail, asking such questions as: how is the light absorbed? And: how are the chemical bonds broken without significant leakage of energy into surrounding areas?"
That this effect was first detected in diamonds is no coincidence, Professor Mildren says. "Although they have been known for thousands of years, diamonds are only now gaining true importance in science and technology. They have very highly defined bonds that are relatively disconnected from neighbouring atoms. So another key question is this: how many materials other than diamond can we laser-pick apart like this? And what might be the consequences?"
Discover more about UV light and how it relates the electromagnetic spectrum at: http://science.hq.nasa.gov/kids/imagers/ems/uv.html
Please send bright ideas for new topics to firstname.lastname@example.org


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The Bell states are a concept in quantum information science and represent the most simple examples of entanglement. They are named after John S. Bell because they are the subject of his famous Bell inequality. An EPR pair is a pair of qubits which are in a Bell state together, that is, entangled with each other. Unlike classical phenomena such as the nuclear, electromagnetic, and gravitational fields, entanglement is invariant under distance of separation and is not subject to relativistic limitations such as the speed of light[vague].
The Bell states
The degree to which a state is entangled is monotonically measured by the Von Neumann entropy of the reduced density operator of a state. The Von Neumann entropy of a pure state is zero - also for the bell states which are specific pure states. But the Von Neumann entropy of the reduced density operator of the Bell states is maximal
In order to explain this, it is important to first look at the Bell state :
This expression means the following: The qubit held by Alice (subscript "A") can be 0 as well as 1. If Alice measured her qubit in the standard basis the outcome would be perfectly random, either possibility having probability 1/2. But if Bob then measured his qubit, the outcome would be the same as the one Alice got. So, if Bob measured, he would also get a random outcome on first sight, but if Alice and Bob communicated they would find out that, although the outcomes seemed random, they are correlated.
So far, this is nothing special: maybe the two particles "agreed" in advance, when the pair was created (before the qubits were separated), which outcome they would show in case of a measurement.
Hence, followed Einstein, Podolsky, and Rosen in 1935 in their famous "EPR paper", there is something missing in the description of the qubit pair given above—namely this "agreement", called more formally a hidden variable.
But quantum mechanics allows qubits to be in quantum superposition—i.e. in 0 and 1 simultaneously—that is, a linear combination of the two classical states—for example, the states or . If Alice and Bob chose to measure in this basis, i.e. check whether their qubit were or , they would find the same correlations as above. That is because the Bell state can be formally rewritten as follows:
Note that this is still the same state.
John S. Bell showed in his famous paper of 1964 by using simple probability theory arguments that these correlations cannot be perfect in case of "pre-agreement" stored in some hidden variables—but that quantum mechanics predict perfect correlations. In a more formal and refined formulation known as the Bell-CHSH inequality, this would be stated such that a certain correlation measure cannot exceed the value 2 according to reasoning assuming local "hidden variable" theory (sort of common-sense) physics, but quantum mechanics predicts .
There are three specific other states of two qubits which are also regarded as Bell states and which lead to this maximal value of . The four are known as the four maximally entangled two-qubit Bell states:
Bell state measurement
The Bell measurement is an important concept in quantum information science: It is a joint quantum-mechanical measurement of two qubits that determines which of the four Bell states the two qubits are in.
If the qubits were not in a Bell state before, they get projected into a Bell state (according to the projection rule of quantum measurements), and as Bell states are entangled, a Bell measurement is an entangling operation.
Bell-state measurement is the crucial step in quantum teleportation. The result of a Bell-state measurement is used by one's co-conspirator to reconstruct the original state of a teleported particle from half of an entangled pair (the "quantum channel") that was previously shared between the two ends.
Experiments which utilize so-called "linear evolution, local measurement" techniques cannot realize a complete Bell state measurement. Linear evolution means that the detection apparatus acts on each particle independently from the state or evolution of the other, and local measurement means that each particle is localized at a particular detector registering a "click" to indicate that a particle has been detected. Such devices can be constructed, for example, from mirrors, beam splitters, and wave plates, and are attractive from an experimental perspective because they are easy to use and have a high measurement cross-section.
For entanglement in a single qubit variable, only three distinct classes out of four Bell states are distinguishable using such linear optical techniques. This means two Bell states cannot be distinguished from each other, limiting the efficiency of quantum communication protocols such as teleportation. If a Bell state is measured from this ambiguous class, the teleportation event fails.
Entangling particles in multiple qubit variables, such as (for photonic systems) polarization and a two-element subset of orbital angular momentum states, allows the experimenter to trace over one variable and achieve a complete Bell state measurement in the other. Leveraging so-called hyper-entangled systems thus has an advantage for teleportation. It also has advantages for other protocols such as superdense coding, in which hyper-entanglement increases the channel capacity.
In general, for hyper-entanglement in variables, one can distinguish between at most classes out of Bell states using linear optical techniques.
- Nielsen, Michael A.; Chuang, Isaac L. (2000), Quantum computation and quantum information, Cambridge University Press, ISBN 978-0-521-63503-5, pp. 25.
- Kaye, Phillip; Laflamme, Raymond; Mosca, Michele (2007), An introduction to quantum computing, Oxford University Press, ISBN 978-0-19-857049-3, pp. 75.
- On the Einstein Podolsky and Rosen paradox, Bell System Technical Journal, 1964.
- Quantum Entanglement in Electron Optics: Generation, Characterization, and Applications, Naresh Chandra, Rama Ghosh, Springer, 2013, ISBN 3642240704, p. 43, Google Books
- Kwiat, Weinfurter. "Embedded Bell State Analysis"
- Pisenti, Gaebler, Lynn. "Distinguishability of Hyper-Entangled Bell States by Linear Evolution and Local Measurement"


Quantum computing has long been a wacky, borderline fictional, mostly theoretical domain of physics reserved for highly speculative conversation. This is because quantum mechanics, or particle physics as it’s also called, makes some claims completely void of common sense. Particle physicists believe that a subatomic particle called a neutrino can pass through the entire Earth without slowing down, and that particles can be in two different states at the same time, and even that two particles can be entangled in such a way that their properties will match across any distance (imagine if flipping a light switch in Kansas caused a light switch on Saturn to flip as well). Various governments have poured money into the exploration of these theories—a giant sub-atomic roller rink was built in Geneva, Switzerland to test many of them resulting in the discovery of the Higgs Boson. But there hasn’t been much use for these theories in practical application. That is until the concept of quantum computing came about.
If a particle can be in two states at once, then perhaps this could be used to speed up computation by incredible amounts. Traditional bits in computers can either be on (1) or off (0), but quantum bit, or qubits, can be both on and off at the same time (called superposition), allowing them to perform parallel computations at once. Make a device that runs with qubits as the base system and you’ve got a quantum computer. The ability to be both on and off simultaneously allows quantum computers to use a process called annealing and makes a computer extraordinarily faster as it’s able to process all scenarios at once. A quantum computer with just 300 qubits could run more calculations in an instant than there are particles in the whole universe. But all of this is just in theory.
As the theories about quantum computing grew and they became entangled with increased media attention and speculation about their capabilities, the idea of quantum computing morphed into miracle computing. It was believed if quantum computers existed, they could cure disease, power artificially intelligent robots, make time machines function, drive cars, and solve the problems of global warming.
While all the speculation was brewing and more scientists and researchers wrote papers on the matter, one company started to build quantum computers. In 2011, a Canadian company D-Wave (backed by the CIA and individuals like Jeff Bezos of Amazon) sold its first machine to defense contractor Lockheed Martin. In early May, D-Wave sold its second machine, the 439-qubit D-Wave Two, to the Quantum Artificial Intelligence Lab for $15 million. The lab, which is backed by NASA, Google, and the Universities Space Research Association (USRA) will use the device to make advances in machine learning, a field of computer science where computers become more adept at solving problems with the more experience they have.
Research Catherine McGeoch, a professor of computer science at Amherst College, but theory into practice and tested the D-Wave prototype to see if it really was a quantum leap forward. Her findings conclude that the device is fast, but only at specific tasks. “On the largest problem sizes tested, the V5 chip found optimal solutions in less than half a second, while the best software solver, CPLEX, needed 30 minutes to find all optimal solutions,” McGeoch writes in the conclusions section of her academic paper, where CPLEX is a conventional software solver and V5 is the chip in the D-Wave prototype. They received a second V6 chip after most of the study had finished, but they decided to test it anyway, concluding “V6 is three to five times faster than V5″ and “preliminary results suggest that… the hardware can find optimal solutions around 10,000 times faster than CPLEX.”
But these incredible numbers can be a little misleading. This doesn’t say that the D-Wave Two is generally 3,600 to 10,000 times faster than a conventional computer, rather that it solved a specific problem that much faster than the current standard solver CPLEX. As McGeoch told the New Yorker after the many media organizations stated the quantum computer was 3,600 times faster, “the 3,600 number does not give any information about comparative performance of the two types of platforms. It was never intended to.”
Another misleading detail is that the baseline machines that the D-Wave Two was being compared against are simple desktop machines that cost only $1,200. The D-Wave machine wasn’t being compared to state-of-the-art supercomputers, but with something you could more or less pick up at Best Buy. For the cost of one D-Wave Two, you could buy 12,500 of the traditional machines. This doesn’t exactly seem like a fair comparison, and it doesn’t even account for the fact that there need to be incredible conditions to make the D-Wave Two run. Because the machine’s chip requires a near absence of electrical resistivity to function called superconductivity, the machine must be supercooled to nearly absolute zero.
Furthermore, there’s even some doubt as to whether D-Wave’s machines are actually quantum computers. It’s very difficult to tell if a device is actually using a process called quantum tunneling or if a similar effect is being achieved through normal thermal fluctuations. McGeoch even admitted she wasn’t sure how the machine actually operated and simply deferred to previous research that said the D-Wave machine is “at least a little quantum mechanical.”
All things considered, it seems we only have an incredibly expensive machine that looks like a quantum computer. Even at its best, a true quantum computer isn’t the magical solution we might be looking for. Due to the way quantum devices are structured, they may excel at solving specific problems that require multiple calculations simultaneously (like determining how to seat guests at the dinner table so people who dislike each other aren’t placed together), they fall short at doing other computational tasks like running Photoshop or Microsoft Word or browsing Facebook. While this is definitely a simplification, it assists the point that with the current state of the technology, quantum computers will have to be coupled with traditional computers to comprehensively perform tasks. In Google’s announcement surrounding purchasing the D-Wave Two, Director of Engineering Hartmut Neven admitted that in trying to better understand machine learning “we’ve learned some useful principles: e.g., you get the best results not with pure quantum computing, but by mixing quantum and classical computing.”
Even though quantum mechanical devices may be here to the tune of $15 million, there still will be years of research, development, and debate to determine if the technology is the miracle device science fiction hopes it is.
“Launching the Quantum Artificial Intelligence Lab”, Google
Adrian Cho, “Controversial Computer Is at Least a Little Quantum Mechanical”, Science
Gary Marcus, “A Quantum Leap In Computing?”, The New Yorker
Charles Choi, “Google and NASA Launch Quantum Computing AI Lab”, MIT Technology Review
Catherine C. McGeoch, “Experimental Evaluation of an Adiabiatic Quantum System for Combinatorial Optimization”
John Naughton, “Is computing speed set to make a quantum leap?” The Guardian


Squeezed coherent state
In physics, a squeezed coherent state is any state of the quantum mechanical Hilbert space such that the uncertainty principle is saturated. That is, the product of the corresponding two operators takes on its minimum value:
Often, the term squeezed state is used for any such state with in "natural oscillator units". The idea behind this is that the circle denoting a coherent state in a quadrature diagram (see below) has been "squeezed" to an ellipse of the same area.
The most general wave function that satisfies the identity above is the squeezed coherent state (we work in units with )
where are constants (a normalization constant, the center of the wavepacket, its width, and the expectation value of it's momentum). The new feature relative to a coherent state is the free value of the width , which is the reason why the state is called "squeezed".
The squeezed state above is an eigenstate of a linear operator
and the corresponding eigenvalue equals . In this sense, it is a generalization of the ground state as well as the coherent state.
Examples of squeezed coherent states
Depending on at which phase the state's quantum noise is reduced, one can distinguish amplitude-squeezed and phase-squeezed states or general quadrature squeezed states. If no coherent excitation exists the state is called a squeezed vacuum. The figures below give a nice visual demonstration of the close connection between squeezed states and Heisenberg's uncertainty relation: Diminishing the quantum noise at a specific quadrature (phase) of the wave has as a direct consequence an enhancement of the noise of the complementary quadrature, that is, the field at the phase shifted by .
From the top:
- Vacuum state
- Squeezed vacuum state
- Phase-squeezed state
- arbitrary squeezed state
- Amplitude-squeezed state
As can be seen at once, in contrast to the coherent state the quantum noise for a squeezed state is no longer independent of the phase of the light wave. A characteristic broadening and narrowing of the noise during one oscillation period can be observed. The wave packet of a squeezed state is defined by the square of the wave function introduced in the last paragraph. They correspond to the probability distribution of the electric field strength of the light wave. The moving wave packets display an oscillatory motion combined with the widening and narrowing of their distribution: the "breathing" of the wave packet. For an amplitude-squeezed state, the most narrow distribution of the wave packet is reached at the field maximum, resulting in an amplitude that is defined more precisely than the one of a coherent state. For a phase-squeezed state, the most narrow distribution is reached at field zero, resulting in an average phase value that is better defined than the one of a coherent state.
In phase space, quantum mechanical uncertainties can be depicted by Wigner distribution Wigner quasi-probability distribution. The intensity of the light wave, its coherent excitation, is given by the displacement of the Wigner distribution from the origin. A change in the phase of the squeezed quadrature results in a rotation of the distribution.
Photon number distributions and phase distributions of squeezed states
For amplitude squeezed light the photon number distribution is usually narrower than the one of a coherent state of the same amplitude resulting in sub-Poissonian light, whereas its phase distribution is wider. The opposite is true for the phase-squeezed light, which displays a large intensity (photon number) noise but a narrow phase distribution. Nevertheless the statistics of amplitude squeezed light was not observed directly with photon number resolving detector due to experimental difficulty.
For the squeezed vacuum state the photon number distribution displays odd-even-oscillations. This can be explained by the mathematical form of the squeezing operator, that resembles the operator for two-photon generation and annihilation processes. Photons in a squeezed vacuum state are more likely to appear in pairs.
Experimental realizations of squeezed coherent states
There has been a whole variety of successful demonstrations of squeezed states. The most prominent ones were experiments with light fields using lasers and non-linear optics (see optical parametric oscillator). This is achieved by a simple process of four-wave mixing with a crystal; similarly traveling wave phase-sensitive amplifiers generate spatially multimode quadrature-squeezed states of light when the crystal is pumped in absence of any signal. Sub-Poissonian current sources driving semiconductor laser diodes have led to amplitude squeezed light. Squeezed states have also been realized via motional states of an ion in a trap, phonon states in crystal lattices, or atom ensembles. Even macroscopic oscillators were driven into classical motional states that were very similar to squeezed coherent states. Current state of the art in noise suppression, for laser radiation using squeezed light, amounts to 12.7 dB.
Squeezed states of the light field can be used to enhance precision measurements. For example phase-squeezed light can improve the phase read out of interferometric measurements (see for example gravitational waves). Amplitude-squeezed light can improve the readout of very weak spectroscopic signals.
Various squeezed coherent states, generalized to the case of many degrees of freedom, are used in various calculations in quantum field theory, for example Unruh effect and Hawking radiation, and generally, particle production in curved backgrounds and Bogoliubov transformation).
Recently, the use of squeezed states for quantum information processing in the continuous variables (CV) regime has been increasing rapidly. Continuous variable quantum optics uses squeezing of light as an essential resource to realize CV protocols for quantum communication, unconditional quantum teleportation and one-way quantum computing. This is in contrast to quantum information processing with single photons or photon pairs as qubits. CV quantum information processing relies heavily on the fact that squeezing is intimately related to quantum entanglement, as the quadratures of a squeezed state exhibit sub-shot-noise quantum correlations.
- Loudon, Rodney, The Quantum Theory of Light (Oxford University Press, 2000), [ISBN 0-19-850177-3]
- D. F. Walls and G.J. Milburn, Quantum Optics, Springer Berlin 1994
- C W Gardiner and Peter Zoller, "Quantum Noise", 3rd ed, Springer Berlin 2004
- D. Walls, Squeezed states of light, Nature 306, 141 (1983)
- R. E. Slusher et al., Observation of squeezed states generated by four wave mixing in an optical cavity, Phys. Rev. Lett. 55 (22), 2409 (1985)
- G. Breitenbach, S. Schiller, and J. Mlynek, "Measurement of the quantum states of squeezed light", Nature, 387, 471 (1997)
- Entanglement evaluation with Fisher information - http://arxiv.org/pdf/quant-ph/0612099
- S. Machida et al.,Observation of amplitude squeezing in a constant-current–driven semiconductor laser, Phys. Rev. Lett. 58, 1000–1003 (1987) - http://link.aps.org/doi/10.1103/PhysRevLett.58.1000
- T. Eberle et al., Quantum Enhancement of the Zero-Area Sagnac Interferometer Topology for Gravitational Wave Detection, Phys. Rev. Lett., 22 June 2010 - http://arxiv.org/abs/1007.0574
- S. L. Braunstein and P. van Loock, “Quantum information with continuous variables,” Rev. Mod. Phys., vol. 77, no. 2, pp. 513–577, Jun. 2005. http://link.aps.org/doi/10.1103/RevModPhys.77.513
- A. Furusawa, J. L. Sørensen, S. L. Braunstein, C. A. Fuchs, H. J. Kimble, and E. S. Polzik, “Unconditional Quantum Teleportation,” Science, vol. 282, no. 5389, pp. 706–709, 1998. http://www.sciencemag.org/content/282/5389/706.abstract
- N. C. Menicucci, S. T. Flammia, and O. Pfister, “One-Way Quantum Computing in the Optical Frequency Comb,” Phys. Rev. Lett., vol. 101, no. 13, p. 130501, Sep. 2008. http://link.aps.org/doi/10.1103/PhysRevLett.101.130501


Computer network Communication Devices
Introduction to computer network devices
Learning about network types and configuration remains incomplete unless we get to know the devices which help in communication between computers in any given network. Without the communication devices networks cannot be formed so knowing their names and what are their uses are equally important. To develop LAN network following network communication devices are required which are listed below:
NIC is Network Interface Card; this is the most important device in building network.These adapters are the most common part of computers which are used in our homes and offices.Nic is also referred to LAN, i.e. is Local area network card. Communication mediums (cables) are attached to this card to build network. This device has unique Mac address. To build network unique IP address is assign to this LAN card to begun communication.In case of developing WLAN, instead of LAN card we use Wireless card. Its functionality is same as simple LAN card; it is just wireless communication device which connects to router for communication.
Router is intelligent device which routes data to destination computers. It helps in connecting two different logical and physical networks together. In small network server is connected to router along with clients for communication. With routers network communication is not possible; it is soul of network without which distribution if internet and other network data to entire network is impossible. It works very same when it comes to use wireless network using wireless network router. It performs all functions similarly without using any medium like cables etc.Router uses software known as routing table. Routing table is used to store source and destination address. Major companies which know for manufacturing routers and wireless routers are Tp Link, Cisco systems, Nortel, D link etc.
If we talk about networks on larger scale hub(s) are required to build network. All computers are connected directly to the hub as hub performs as centralized device the network. When data is sent to the hub it broadcasts the data to all the ports of the hub and then it is sent to destination computer on the network. If hubs fails to perform its routine functions it will halt the working of the entire network until it is put back in normal condition.
Switch is another important device when we talk about computer network on broader spectrum.It is used at the same place as hub is but the only difference between the two is that switch possess switching table with in it. Switching tables store the MAC addresses of every computer it is connected to and send the data to only requested address unlike hub which broadcasts the data too all the ports. Switches can be considered advance form of hubs.
As name suggests it some kind of passing through to some thing. Interestingly gateways can be software or it can also be device. Gateway device connects LAN with internet. Its basic functionality is to provide security to the network. By using gateways incoming/out going traffic can be monitored for any malicious activity within the network which can be harmful to network integrity.
Modems can be of two types. One modem is very common in every computer which we use to connect to internet using our telephone line by dialing to our ISP and the other one is used to connect to DSL. Functions however are same for both types of modems; they are used for modulation and demodulation, they are used to convert analog signals into digital and digital signals into analog so that signals can be travelled on telephone lines.
Cables are obviously used to connect communication devices with each other to form network. There different types of cables, commonly used cables are 10baseT/CAT5 , coaxial cable, Ethernet and fiber optical cable. Fiber optical is the most expensive as it enables the data transfer at speed of light. It is costly solution which is mostly get adopted by corporate sector. However in recent developments optical fiber cable is now being used in home networking and also used as medium to connect to internet.
- Network Configuration
- What is Network address translator (Nat) ?
- Types of network cables
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The recent list of Universe Today’s Top 10 Stories of 2010 included the story Faster than Light Pulsars Discovered – which on further reading made it clear that the phenomenon being studied wasn’t exactly moving faster than light.
Anyhow, this prompted me to look up different ways in which apparent superluminal motion might be generated, partly to reassure myself that the bottom hadn’t fallen out of relativity physics and partly to see if these things could be adequately explained in plain English. Here goes…
1) Cause and effect illusions
The faster than light pulsar story is essentially about hypothetical light booms – which are a bit like a sonic booms, where it’s not the sonic boom, but the sound source, that exceeds the speed of sound – so that individual sound pulses merge to form a single shock wave moving at the speed of sound.
Now, whether anything like this really happens with light from pulsars remains a point of debate, but one of the model’s proponents has demonstrated the effect in a laboratory – see this Scientific American blog post.
What you do is to arrange a line of light bulbs which are independently triggered. It’s easy enough to make them fire off in sequence – first 1, then 2, then 3 etc – and you can keep reducing the time delay between each one firing until you have a situation where bulb 2 fires off after bulb 1 in less time than light would need to travel the distance between bulbs 1 and 2. It’s just a trick really – there is no causal connection between the bulbs firing – but it looks as though a sequence of actions (first 1, then 2, then 3 etc) moved faster than light across the row of bulbs. This illusion is an example of apparent superluminal motion.
There are a range of possible scenarios as to why a superluminal Mexican wave of synchrotron radiation might emanate from different point sources around a rapidly rotating neutron star within an intense magnetic field. As long as the emanations from these point sources are not causally connected, this outcome does not violate relativity physics.
2) Making light faster than light
You can produce an apparent superluminal motion of light itself by manipulating its wavelength. If we consider a photon as a wave packet, that wave packet can be stretched linearly so that the leading edge of the wave arrives at its destination faster, since it is pushed ahead of the remainder of the wave – meaning that it travels faster than light.
However, the physical nature of ‘the leading edge of a wave packet’ is not clear. The whole wave packet is equivalent to one photon – and the leading edge of the stretched out wave packet cannot carry any significant information. Indeed, by being stretched out and attenuated, it may become indistinguishable from background noise.
Also this trick requires the light to be moving through a refractive medium, not a vacuum. If you are keen on the technical details, you can make phase velocity or group velocity faster than c (the speed of light in a vacuum) – but not signal velocity. In any case, since information (or the photon as a complete unit) is not moving faster than light, relativity physics is not violated.
3) Getting a kick out of gain media
You can mimic more dramatic superluminal motion through a gain medium where the leading edge of a light pulse stimulates the emission of a new pulse at the far end of the gain medium – as though a light pulse hits one end of a Newton’s Cradle and new pulse is projected out from the other end. If you want to see a laboratory set-up, try here. Although light appears to jump the gap superluminally, in fact it’s a new light pulse emerging at the other end – and still just moving at standard light speed.
4) The relativistic jet illusion
If an active galaxy, like M87, is pushing out a jet of superheated plasma moving at close to the speed of light – and the jet is roughly aligned with your line of sight from Earth – you can be fooled into thinking its contents are moving faster than light.
If that jet is 5,000 light years long, it should take at least 5,000 years for anything in it to cross that distance of 5,000 light years. A photon emitted by a particle of jet material at point A near the start of the jet really will take 5,000 years to reach you. But meanwhile, the particle of jet material continues moving towards you nearly as fast as that photon. So when the particle emits another photon at point B, a point near the tip of the jet – that second photon will reach your eye in much less than 5,000 years after the first photon, from point A. This will give you the impression that the particle crossed 5,000 light years from points A to B in much less than 5,000 years. But it is just an optical illusion – relativity physics remains unsullied.
5) Unknowable superluminal motion
It is entirely possible that objects beyond the horizon of the observable universe are moving away from our position faster than the speed of light – as a consequence of the universe’s cumulative expansion, which makes distant galaxies appear to move away faster than close galaxies. But since light from hypothetical objects beyond the observable horizon will never reach Earth, their existence is unknowable by direct observation from Earth – and does not represent a violation of relativity physics.
And lastly, not so much unknowable as theoretical is the notion of early cosmic inflation, which also involves an expansion of space-time rather than movement within space-time – so no violation there either.
I’m not sure that the above is an exhaustive list and I have deliberately left out other theoretical proposals such as quantum entanglement and the Alcubierre warp drive. Either of these, if real, would arguably violate relativity physics – so perhaps need to be considered with a higher level of skepticism.