Researches have uncovered "smoking-gun" evidence to confirm the workings of an emerging class of materials that could make possible "spintronic" devices and practical quantum computers far more powerful than today's technologies. The materials are called topological insulators.
For more than 50 years, scientists have debated what turns particular oxide insulators, in which...
Research from North Carolina State Univ. shows that a type of modified titania, or titanium...
Research led by Penn State Univ. and Cornell Univ. physicists is studying "spintorque" in devices that combine a standard magnetic material with a new material known as a topological insulator. The new insulator, which is made of bismuth selenide and operates at room temperature, overcomes one of the key challenges to developing a spintronics technology based on spin-orbit coupling.
Together with teams from Finland and Japan, physicists from the Univ. of Basel in Switzerland were able to place 20 single bromine atoms on a fully insulated surface at room temperature to form the smallest “Swiss cross” ever created. The effort is a breakthrough because the fabrication of artificial structures on an insulator at room temperature is difficult. It is largest number of atomic manipulations ever achieved at room temperature.
Vanadium dioxide is called a "wacky oxide" because it transitions between a conducting metal and an insulating semiconductor and with the addition of heat or electrical current. A device created by Penn State engineers uses a thin film of vanadium oxide on a titanium dioxide substrate to create an oscillating switch that could form the basis of a computational device that uses a fraction of the energy necessary for today’s computers.
Materials that can be used for thermoelectric devices have been known for decades. But, until now, there has been no good explanation for why just a few materials work well for these applications, while most others do not. Now researchers say they have finally found a theoretical explanation for the differences, which could lead to the discovery of new, improved thermoelectric materials.
Manganites show great promise as “go-to” materials for future electronic devices because of their ability to instantly switch from an electrical insulator to a conductor under a wide variety of external stimuli, including magnetic fields, photo-excitations and vibrational excitations. This ultra-fast switching arises from the different ways electrons and electron-spins in a manganite may organize or re-organize in response to such stimuli.
Topological insulators are considered a very promising material class for the development of future electronic devices because they are insulators inside but conductors at the surface. A research team in Germany has discovered how light can be used to alter the physical properties of the electrons in these materials by using it to alter electron spin at the surface.
Topological insulators have been of great interest to physicists in recent years because of unusual properties that may provide insights into quantum physics. But most analysis of such materials has had to rely on highly simplified models. Now, a team of researchers at Massachusetts Institute of Technology has performed a more detailed analysis that hints at the existence of six new kinds of topological insulators.
By applying pressure to a semiconductor, researchers have been able to transform a semiconductor into a “topological insulator” (TI), an intriguing state of matter in which a material’s interior is insulating but its surfaces or edges are conducting with unique electrical properties. This is the first time that researchers have used pressure to gradually “tune” a material into the TI state.
A single layer of tin atoms could be the world’s first material to conduct electricity with 100% efficiency at the temperatures that computer chips operate, according to a team of theoretical physicists led by researchers from SLAC National Accelerator Laboratory and Stanford Univ.
An international team of scientists have discovered a new type of quantum material whose lopsided behavior may lend itself to creating novel electronics. The material is called bismuth tellurochloride, or BiTeCl. It belongs to a class of materials called topological insulators that conduct electrical current with perfect efficiency on their surfaces, but not through their middles.
Researchers at Massachusetts Institute of Technology have succeeded in producing and measuring a coupling of photons and electrons on the surface of an unusual type of material called a topological insulator. This type of coupling had been predicted by theorists, but never observed.
A theoretical study conducted by scientists at Japan’s National Institute of Materials Science reveals the possibility of developing a quantum material to transport zero-resistance edge current above room temperature. This capability, allowed by large spin-orbit coupling, will depend on the construction of a new class of topological materials that the researchers have designed.
An international collaboration at Lawrence Berkeley National Laboratory’s Advanced Light Source has induced high-temperature superconductivity in a toplogical insulator, an important step on the road to fault-tolerant quantum computing.
When scientists found electrical current flowing where it shouldn't be—at the place where two insulating materials meet—it set off a frenzy of research that turned up more weird properties and the hope of creating a new class of electronics. Now scientists have mapped those currents in microscopic detail and found another surprise: Rather than flowing uniformly, the currents are stronger in some places than others.
Researchers not only confirmed several theoretical predictions about topological crystalline insulators (TCIs), but made a significant experimental leap forward that revealed even more details about the crystal structure and electronic behavior of these newly identified materials. The findings reveal the unexpected level of control TCIs can have over electrons by creating mass.
Researchers from the RIKEN Center for Life Science Technologies and Chiba Univ. have developed a high-temperature superconducting wire with an ultrathin polyimide coating only 4 micrometers thick, more than 10 times thinner than the conventional insulation used for high-temperature superconducting wires. The breakthrough should help the development of more compact superconducting coils for medical and scientific devices.
It is well known to scientists that the three common phases of water (ice, liquid and vapor) can exist stably together only at a particular temperature and pressure, called the triple point. Scientists now have made the first-ever accurate determination of a solid-state triple point in a substance called vanadium dioxide, which is known for switching rapidly from an electrical insulator to a conductor.
New research shows that a class of materials being eyed for the next generation of computers behaves asymmetrically at the sub-atomic level. This research is a key step toward understanding the topological insulators that may have the potential to be the building blocks of a super-fast quantum computer that could run on almost no electricity.
A team of theoretical physicists at the U.S. Naval Research Laboratory and Boston College has identified cubic boron arsenide as a material with an extraordinarily high thermal conductivity and the potential to transfer heat more effectively from electronic devices than diamond, the best-known thermal conductor to date.
Researchers have made the first direct images of electrical currents flowing along the edges of a topological insulator. In these strange solid-state materials, currents flow only along the edges of a sample while avoiding the interior. Using an exquisitely sensitive detector they built, the team was able to sense the weak magnetic fields generated by the edge currents and tell exactly where the currents were flowing.
By means of special metamaterials, light and sound can be passed around objects. Researchers have now succeeded in demonstrating that the same materials can also be used to specifically influence the propagation of heat. They have built a structured plate of copper and silicon that conducts heat around a central area without the edge being affected.
Researchers from Dresden have discovered a new material that conducts electric currents without loss of power over its edges and remains an insulator in its interior. The material is made out of bismuth cubes packed in a honeycomb motif that is known from the graphene structure. As opposed to graphene, the new material exhibits its peculiar electrical property at room temperature, giving it promise for applications in nanoelectronics.
Electrons flowing swiftly across the surface of topological insulators are "spin polarized," their spin and momentum locked. This new way to control electron distribution in spintronic devices makes TIs a hot topic in materials science. Now scientists have discovered more surprises: contrary to assumptions, the spin polarization of photoemitted electrons from a topological insulator is wholly determined in three dimensions by the polarization of the incident light beam.
Unlike conventional electrical insulators, which do not conduct electricity, topological insulators have the unique property of conducting electricity on their surface, while acting as an insulator inside. In a step toward understanding and exploiting an exotic form of matter that has been sparking excitement for potential applications in a new genre of supercomputers, scientists are reporting the first identification of a naturally occurring topological insulator that was retrieved from an abandoned gold mine in the Czech Republic.
University of Utah engineers demonstrated it is feasible to build the first organic materials that conduct electricity on their edges, but act as an insulator inside. These materials, called organic topological insulators, could shuttle information at the speed of light in quantum computers and other high-speed electronic devices.
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measures quantum quirk
Technology Research News
Quantum entanglement, which Einstein once
dismissed as impossible, is a physical resource that could transform information
processing. It is key to producing phenomenally powerful quantum computers,
and is the critical component of the most secure form of quantum cryptography.
Until now, however, researchers have had no way to measure entanglement
directly, but have had to rely on indirect measurements or mathematical
Researchers from the Technical University of Gdansk in Poland and the
University of Cambridge in England have come up with a scheme for measuring
entanglement that could give scientists the means to judge the purity
of the primary resource used in quantum information processing.
The scheme could mark the beginning of quantum metrology -- the science
of quantum measurement, said Artur Ekert, a professor of quantum physics
at the University of Cambridge. "Efficient tests for quantum entanglement
will be important in all applications where quantum entanglement is used,"
Entanglement links physical properties, such as polarization or momentum,
of two or more atoms or subatomic particles. It is part of numerous schemes
for secure communication, precise frequency standards, atomic clocks and
When an atom or subatomic particle is isolated from its environment, it
enters into the weird state of superposition, meaning it is in some mixture
of all possible states. For example, a photon can be polarized in one
of two opposite directions. In superposition, however, the photon is polarized
in some mixture of both directions at the same time.
When two or more particles in superposition come into contact with each
other, they can become entangled. A common example is photons that have
their polarizations entangled. When one of the photons is knocked out
of superposition to become, say, vertically polarized, the other photon
leaves superposition at the same instant and also becomes vertically polarized,
regardless of the distance between them.
Existing methods of checking for entanglement involve either indirect
measurements, which are inefficient and leave many entangled states undetected,
or a mathematical estimation, Ekert said.
The researchers' method is similar to the mathematical approach, but works
on the particles directly rather than on a mathematical representation
of them. "We have managed to find a physical operation that mimics the
mathematical one," said Ekert.
Quantum operations alter particles that are used as quantum bits, or qubits,
to represent the 1s and 0s of computing in quantum information systems.
One way to carry out a quantum operation is to use a laser beam to rotate
an atom held in a magnetic trap so that its orientation flips from a position
representing a 1 to a position representing a 0. The basic logic of quantum
computing is made up of many series of these quantum operations.
The researchers' entanglement-detection method could be included in several
proposed architectures for quantum computers, including ion traps, which
hold individual atoms in magnetic fields, and quantum dots, which trap
individual electrons in microscopic specks of semiconductor material,
according to Ekert.
The research is excellent; it is an original idea about how to detect
entanglement in an efficient way, said Vlatko Vedral, a lecturer of physics
at Imperial College and the University of Oxford in England. "One of the
most fundamental issues in quantum information theory is whether two systems
are entangled or not," he said. Scientists have had a good theoretical
understanding of how to detect entanglement, but these methods are not
practical in the physical world because they involve physical impossibilities
like reversing time, he said.
The researchers have come up with a practical method of testing for entanglement,
said Vedral. The basic idea is to mix a bit of noise into the operation
so there will always be a physically possible result, he said. "It turns
out that this mixing can be performed in an efficient way," he added.
Entanglement is crucial for quantum communications, said Vedral. "Some
forms of quantum cryptography depend critically on the presence of entanglement
and cannot be implemented without it," he said.
It's not yet clear how useful being able to measure entanglement will
be for quantum computing because researchers do not know if there is a
direct link between amount of entanglement and the speed of quantum computers,
Vedral said. "Everything indicates that entanglement is an important ingredient,
but how much of it is enough to be clearly better than any classical computer
remains an open question," he said.
The method could be used in practical applications in two to five years,
said Ekert. It is likely to be used first in quantum cryptography and
frequency standards, he said.
Ekert's research colleague was Pawe Horodecki of the Technical University
of Gdansk in Poland. They published the research in the September 16,
2002 issue of Physical Review Letters. The research was funded by the
Polish Committee for Scientific Research, the European Commission, Elsag
SpA, the Engineering and Physical Sciences Research Council and the Royal
Society of London.
Timeline: 2-5 years, 20 years
Funding: Government, Corporate
TRN Categories: Physics; Quantum Computing and Communications
Story Type: News
Related Elements: Technical paper, "Method for Direct Detection
of Quantum Entanglement," Physical Review Letters, September 16, 2002
November 13/20, 2002
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Quantum teleportation, or entanglement-assisted teleportation, is a technique used to transfer quantum information from one quantum system to another. It does not transport the system itself, nor does it allow communication of information at superluminal (faster than light) speed. Neither does it concern rearranging the particles of a macroscopic object to copy the form of another object. Its distinguishing feature is that it can transmit the information present in a quantum superposition, useful for quantum communication and computation.
More precisely, quantum teleportation is a quantum protocol by which a qubit a (the basic unit of quantum information) can be transmitted exactly (in principle) from one location to another. The prerequisites are a conventional communication channel capable of transmitting two classical bits (i.e. one of four states), and an entangled pair (b,c) of qubits, with b at the origin and c at the destination. (So whereas b and c are intimately related, a is entirely independent of them other than being initially colocated with b.) The protocol has three steps: measure a and b jointly to yield two classical bits; transmit the two bits to the other end of the channel (the only potentially time-consuming step, due to speed-of-light considerations); and use the two bits to select one of four ways of recovering c. The upshot of this protocol is to permute the original arrangement ((a,b),c) to ((b′,c′),a), that is, a moves to where c was and the previously separated qubits of the Bell pair turn into a new Bell pair (b′,c′) at the origin.
Suppose Alice has a qubit in some arbitrary quantum state . (A qubit may be represented as a superposition of states, labeled and .) Assume that this quantum state is not known to Alice and she would like to send this state to Bob. Ostensibly, Alice has the following options:
Option 1 is highly undesirable because quantum states are fragile and any perturbation en route would corrupt the state.
Option 2 is forbidden by the no-broadcast theorem.
Option 3 (classical teleportation) has also been formally shown to be impossible. (See the no teleportation theorem.) This is another way to say that quantum information cannot be measured reliably.
Thus, Alice seems to face an impossible problem. A solution was discovered by Bennett, et al. The components of a maximally entangled two-qubit state are distributed to Alice and Bob. The protocol then involves Alice and Bob interacting locally with the qubit(s) in their possession and Alice sending two classical bits to Bob. In the end, the qubit in Bob's possession will be in the desired state.
Assume that Alice and Bob share an entangled qubit AB. That is, Alice has one half, A, and Bob has the other half, B. Let C denote the qubit Alice wishes to transmit to Bob.
Alice applies a unitary operation on the qubits AC and measures the result to obtain two classical bits. In this process, the two qubits are destroyed. Bob's qubit, B, now contains information about C; however, the information is somewhat randomized. More specifically, Bob's qubit B is in one of four states uniformly chosen at random and Bob cannot obtain any information about C from his qubit.
Alice provides her two measured classical bits, which indicate which of the four states Bob possesses. Bob applies a unitary transformation which depends on the classical bits he obtains from Alice, transforming his qubit into an identical re-creation of the qubit C.
Suppose Alice has a qubit that she wants to teleport to Bob. This qubit can be written generally as:
Alice takes one of the particles in the pair, and Bob keeps the other one. The subscripts A and B in the entangled state refer to Alice's or Bob's particle. We will assume that Alice and Bob share the entangled state .
So, Alice has two particles (C, the one she wants to teleport, and A, one of the entangled pair), and Bob has one particle, B. In the total system, the state of these three particles is given by
Alice will then make a partial measurement in the Bell basis on the two qubits in her possession. To make the result of her measurement clear, we will rewrite the two qubits of Alice in the Bell basis via the following general identities (these can be easily verified):
The three particle state shown above thus becomes the following four-term superposition:
Notice all we have done so far is a change of basis on Alice's part of the system. No operation has been performed and the three particles are still in the same state. The actual teleportation starts when Alice measures her two qubits in the Bell basis. Given the above expression, evidently the result of her (local) measurement is that the three-particle state would collapse to one of the following four states (with equal probability of obtaining each):
Alice's two particles are now entangled to each other, in one of the four Bell states. The entanglement originally shared between Alice's and Bob's is now broken. Bob's particle takes on one of the four superposition states shown above. Note how Bob's qubit is now in a state that resembles the state to be teleported. The four possible states for Bob's qubit are unitary images of the state to be teleported.
The crucial step, the local measurement done by Alice on the Bell basis, is done. It is clear how to proceed further. Alice now has complete knowledge of the state of the three particles; the result of her Bell measurement tells her which of the four states the system is in. She simply has to send her results to Bob through a classical channel. Two classical bits can communicate which of the four results she obtained.
After Bob receives the message from Alice, he will know which of the four states his particle is in. Using this information, he performs a unitary operation on his particle to transform it to the desired state :
to recover the state.
to his qubit.
Teleportation is therefore achieved.
Experimentally, the projective measurement done by Alice may be achieved via a series of laser pulses directed at the two particles.
In the literature, one might find alternative, but completely equivalent, descriptions of the teleportation protocol given above. Namely, the unitary transformation that is the change of basis (from the standard product basis into the Bell basis) can also be implemented by quantum gates. Direct calculation shows that this gate is given by
Entanglement can be applied not just to pure states, but also mixed states, or even the undefined state of an entangled particle. The so-called entanglement swapping is a simple and illustrative example.
If Alice has a particle which is entangled with a particle owned by Bob, and Bob teleports it to Carol, then afterwards, Alice's particle is entangled with Carol's.
A more symmetric way to describe the situation is the following: Alice has one particle, Bob two, and Carol one. Alice's particle and Bob's first particle are entangled, and so are Bob's second and Carol's particle:
___ / \ Alice-:-:-:-:-:-Bob1 -:- Bob2-:-:-:-:-:-Carol \___/
Now, if Bob performs a projective measurement on his two particles in the Bell state basis and communicates the results to Carol, as per the teleportation scheme described above, the state of Bob's first particle can be teleported to Carol's. Although Alice and Carol never interacted with each other, their particles are now entangled.
One can imagine how the teleportation scheme given above might be extended to N-state particles, i.e. particles whose states lie in the N dimensional Hilbert space. The combined system of the three particles now has a N3 dimensional state space. To teleport, Alice makes a partial measurement on the two particles in her possession in some entangled basis on the N2 dimensional subsystem. This measurement has N2 equally probable outcomes, which are then communicated to Bob classically. Bob recovers the desired state by sending his particle through an appropriate unitary gate.
A general teleportation scheme can be described as follows. Three quantum systems are involved. System 1 is the (unknown) state ρ to be teleported by Alice. Systems 2 and 3 are in a maximally entangled state ω that are distributed to Alice and Bob, respectively. The total system is then in the state
where Tr12 is the partial trace operation with respect systems 1 and 2, and denotes the composition of maps. This describes the channel in the Schrödinger picture.
Taking adjoint maps in the Heisenberg picture, the success condition becomes
for all observable O on Bob's system. The tensor factor in is while that of is .
The proposed channel Φ can be described more explicitly. To begin teleportation, Alice performs a local measurement on the two subsystems (1 and 2) in her possession. Assume the local measurement have effects
If the measurement registers the i-th outcome, the overall state collapses to
The tensor factor in is while that of is . Bob then applies a corresponding local operation Ψi on system 3. On the combined system, this is described by
where Id is the identity map on the composite system .
Therefore the channel Φ is defined by
Notice Φ satisfies the definition of LOCC. As stated above, the teleportation is said to be successful if, for all observable O on Bob's system, the equality
holds. The left hand side of the equation is:
where Ψi* is the adjoint of Ψi in the Heisenberg picture. Assuming all objects are finite dimensional, this becomes
The success criterion for teleportation has the expression


When atoms of Calcium are brought to a high energy state and are allowed to return to a lower energy state while in a fixed position, each atom emits two photons which fly off in opposite directions. If the spin of the photons is measured in the same axis (vertical, horizontal, front to back or any other axis) then the two photons have opposite direction spins (clockwise and anticlockwise). According to quantum theory, subatomic particles have latent spins in all axes but once the spin is measured along one axis, it is never a fractional one and after measurement along one axis, it is not possible to detect or measure the particle's spin direction along other axis. Many physicists have done this experiment and the spin of the particles measured at various distances. The farthest distance apart between the particles was eleven kilometers. Einstein could not accept that two particles separated in space could influence the behavior or properties of each other. This would have violated his theory of special relativity as one particle would be communicating with the other faster than the speed of light. He called it spooky action at a distance and remarked that god does not play dice. He claimed that the two particles were programmed at birth to have opposite spins.
John Bell got the Nobel prize in physics for thinking up an experiment carried out decades later when more sophisticated technology and instruments became available. These were the ability to measure the direction of spin of particles in the vertical, horizontal and front to back axis (or any other axis). In experiments where the spin direction was measured along the same axis at any angle from the vertical, horizontal or front to back, the experimenter always got opposite results for the two particles. Bell's theorem was simple. The spin direction can be either clockwise or anti-clockwise. If one assumes that the one sided photon is programmed, say clockwise for vertical, anti-clockwise for horizontal and clockwise for front to back, its program would be CAC. In that case the opposite side photon would have to be programmed ACA.
If the experiment was randomized so that two independent experimenters measured the photon spin in any one of the three directions, the results would be CA, CC, CA for vertical axis on one photon and vertical, horizontal and front to back axis on the second. The next set for horizontal axis on the first photon and vertical, horizontal and front to back on the second photon would be AA, AC, AA respectively. For front to back axis on the first photon and three in the same order on the second photon would be CA, CC, CA respectively. Of the nine results, you will notice that, four are the same (AA or CC) and five are different. Thus a series of random tests should give opposite results more than 50% of the time. Yet when repeated experiments are carried out the results always show an agreement of 50% but never greater. This proves that the two photons are not pre-programmed and do not have a fixed spin until measured, but if measured along the same axis they are always opposite. Once the spin on one particle is measured the second entangled particle takes on the opposite spin along the same axis.
This raises strange questions. Was the choice of switching off or on the detector in part one of my article pre-ordained? If so then there is no free will. If the particles are entangled at birth and yet free to choose their spin until measurement, what is reality? By the way despite our Indian habit of entangling all inventions to our scriptures and philosophy, quantum physics has nothing to do with the dance of Shiva, Buddhism or Taoism. If you still believe the opposite, you should nominate me for the Nobel Prize in physics, as I have found two large particles (human beings) entangled at a distance of 9000 miles. They are the president of the US (Bush or Obama) and Manmohan Singh. As soon as a mere thought passes through the president's mind, Manmohan Singh immediately and spontaneously does the same thing not the opposite.
- Iran despite not in violation of the NPT should be referred to the IAEA.
Manmohan Singh'Yes Sahib
- India's Petroleum minister Aiyar should be fired for promoting the IPI gas pipeline.
Manmohan Singh'Jee Huzoor
- India must sign the Civil Nuclear Deal without any guarantees of supply, promise of enrichment technology and agree to intrusive inspections of its nuclear facilities.
Manmohan Singh' Jo Hukoom or I will fall on my sword.
- Pakistan is a victim of terror not its source.
Manmohan Singh in Havana 'India and Pakistan are friends and both are victims of terror.
- Obama thinks India must resume talks with Pakistan to get the US out of the Afghan fire.
Manmohan Singh 'Yes Bwana (at Egypt NAM meeting). What Pakistan does to us in Kashmir is like what we do to it in Baluchistan. India Pakistan Bhai Bhai!
- Fire Kamal Nath from the trade portfolio. He is unwilling to resuscitate the Doha round and agree to our unfair trade practices.
Manmohan Singh 'OK Boss, done.
- We have the right to inspect any arms that you buy from us at exorbitant prices and you need our permission to use them even after you have fully paid for them. We have the right to use your ports, airfields and you have to supply us what we need during war, if necessary. So sign the End User Arms Agreement now.
Manmohan Singh' Bows head and agrees
- You will not get any nuclear enrichment technology as we have changed the terms of the civilian nuclear treaty at the recent G8 meeting.
Manmohan Singh --Your wish is my thought. I will give up my nation's agreements without restriction (with Russia) to be entangled with you as your subordinate. Your foreign policy is mine. I know you are in debt up to your gazoos to China and will not help India if China attacks us, but I will abandon old friends for the entanglement with you.
One psychologist friend said to me My diagnosis is wrong and the above is not quantum entanglement. Maybe Manmohan Singh has no free will and he is a Zombie (Look up psychological definition of Zombie). Another friend who is a comedian said, maybe Manmohan Singh is playing the part of a ventriloquist's dummy. Simultaneously, all three of us said 'you have got to hand it to Sonia. She is like the nuclear strong force which increases with distance, unlike gravity, electromagnetism and the weak force, so she can throw her voice to large distances and alter it to sound like that of a man.


But it's a little more complex than this. We also have quantum mechanics to contend with. The spin of an electron is a vector. But we find that when we measure one of the components of this vector this value is quantised and can only take values +hbar/2 and -hbar/2, where hbar is Planck's constant. We choose units where h-bar is 1 so the z-component of the spin is always measured to be +1/2 or -1/2. If we write these two states as |+> and |-> then because we are dealing with quantum mechanics, the z-component of the spin can be represented by the linear combination a|+>+b|->. This corresponds to a state in which there is a probability |a|² of measuring +1/2 and a probability |b|² of measuring -1/2. This is what might have been written as a.*return (1/2)+b.*return (-1/2) in my earlier Haskell code. But that's just one component of the spin. What about the x- and y-components? Amazingly the state a|+>+b|-> tells us everything we can possibly know about the spin of an electron and we'll call it a spin state.
Suppose we have an electron in the state ψ = a|+>+b|->. What happens if we measure the y-component of its spin? One way to answer that question is to rotate the electron through π/2 so that its x-axis is rotated to align with the z-axis and then measure the z-component of its spin. In order to to that we need to know how to rotate spin states. The rule for rotation through θ about the x-axis is this (in a suitable coordinate frame):
|+> → cos(θ/2)|+>-sin(θ/2)|->
|-> → sin(θ/2)|+>+cos(θ/2)|->
Note how choosing θ=0 gives the identity, as expected. Note also that θ=π maps a|+>+b|-> to b|+>-a|-> so that the probabilities of measuring +1/2 and -1/2 are simply swapped, exactly what you'd expect for turning a state upside down. But there's something else that you should notice - there's an ambiguity. A rotation through 2π should give the same as a rotation through 0 and yet setting θ=2π in that transformation maps a state ψ to -ψ. Now |a|² = |-a|² so the probability of observing spin up or spin down is unaffected. But as I've been showing over previous posts, flipping a sign in a state can make a big difference as soon as you start performing interference experiments. The same goes for any angle: if I rotate through π should I use θ=π or θ = 3π? So can the transformation I've given make sense?
The transformation does make sense if you consider that in any physical process that rotates an electron the transformation will evolve continuously over time. Electrons don't just instantly rotate. In other words, if a rotation is applied to an electron then it will follow a path in SO(3), not just be an instantaneous application of an element of SO(3). And that allows us to resolve the ambiguity: the rotations of electrons are described by the double cover of SO(3) known as SU(2). So a rotation through 360 degrees doesn't return you to the identity although a 720 degree rotation does. The transformation I gave above is completely unambiguous if you continuously rotate an electron around the x-axis tracking a continuous value of θ, after all, the double cover is basically just the set of continuous paths from the identitiy in SO(3) (with homotopic paths considered equivalent).
And that's the bizarre fact: electron rotations aren't described by SO(3), they're described by SU(2). In particular, rotating an electron through 360 degrees does not return it to its original state, but a rotation through 720 degrees does! In a sense, like Dirac's belt, electrons can remember something about the path they took to get where they are, in particular they remember how many twists there were in the path.
What does this mean experimentally? the first thing to note is that this is true not just for electrons but any spin-1/2 fermion. This included protons and neutrons. The stuff I've been talking about manifests itself in a number of ways. In particular, the spin of a particle affects how a magnetic field acts on it. For example, spin-up and spin-down particles can be separated into distinct beams using Stern-Gerlach apparatus. Also, the spin of particles precesses in a magnetic field and this is used on a regular basis in NMR. These two facts allow us to easily manipulate and measure the spin of fermions. In other words, the fact that fermions remember how many twists there are in their rotations isn't just some esoteric nonsense, it's now engineering and the theory is tested repeatedly all over the world.
Every familiar object is invariant under rotations through 360 degrees. So the fact that electrons need to be rotated through 720 degrees to return them to their original state seems like one of the most bizarre facts about the universe I know of. And yet many books that introduce spin just slip in this fact in a routine way as if it were no different to any other.
The fact that the biggest connected cover of SO(3) is the double cover puts a big constraint on the kinds of weird effects like this can happen. We can have a 360 degree rotation multiply by -1, but not by i, because a 720 degree rotation absolutely has to return us to where we started from. But suppose the universe were 2-dimensional. If you remember what I said about SO(2) you may notice that no such constraints apply because SO(2) has an infinite cover. There is a group in which all of the rotations through 360n degrees are distinct for distinct n. This means that a physical system could have its state multiplied by any factor (of modulus 1) when rotated through 360 degrees. Particle that behave this way are called anyons. But we live in a 3D universe so we don't expect any fundamental particles to have this property. However, in quantum mechanics any kind of 'excitation' of a physical system is quantised and can be thought of as a type of particle. These are known as quasiparticles. For example, just as light is made of photons, sound is also quantised as phonons. In the right kind of solid state medium, especially those that arise from some kind of 2D lattice, it seems quite plausible that anyons might arise. This gives rise to the so called fractional quantum hall effect. Anyons might one day play an important role in quantum computing via topological quantum computation.


Are we alone?
1. We have strong evidence that that our solar system is not the only one; we know there are many other Suns with planets orbiting them.
Improved telescopes and detectors have led to the detection of dozens of new planetary systems within the past decade, including several systems containing multiple planets.
One giant leap for bug-kind
2. Some organisms can survive in space without any kind of protective enclosure.
In a European Space Agency experiment conducted in 2005, two species of lichen were carried aboard a Russian Soyuz rocket and exposed to the space environment for nearly 15 days. They were then resealed in a capsule and returned to Earth, where they were found in exactly the same shape as before the flight. The lichen survived exposure to the vacuum of space as well as the glaring ultraviolet radiation of the Sun.
Hot real estate
3. Organisms have been found living happily in scalding water with temperatures as high as 235 degrees F.
More than 50 heat-loving microorganisms, or hyperthermophiles, have been found thriving at very high temperatures in such locations as hot springs in WyomingÕs Yellowstone National Park and on the walls of deep-sea hydrothermal vents. Some of these species multiply best at 221 degrees F, and can reproduce at up to 235 degrees F.
Has E.T. already phoned home?
4. We now have evidence that some form of life exists beyond Earth, at least in primitive form.
While many scientists speculate that extraterrestrial life exists, so far there is no conclusive evidence to prove it. Future missions to Mars, the Jovian moon Europa and future space telescopes such as the Terrestrial Planet Finder will search for definitive answers to this ageless question.
To infinity, and beyond!
5. We currently have the technology necessary to send astronauts to another star system within a reasonable timespan. The only problem is that such a mission would be overwhelmingly expensive.
Even the the unmanned Voyager spacecraft, which left our solar system years ago at a breathtaking 37,000 miles per hour, would take 76,000 years to reach the nearest star. Because the distances involved are so vast, interstellar travel to another star within a practical timescale would require, among other things, the ability the move a vehicle at or near the speed of light. This is beyond the reach of today's spacecraft -- regardless of funding.
Fellowship of the rings
6. All of the gas giant planets in our solar system (Jupiter, Saturn, Uranus and Neptune) have rings.
Saturn's rings are the most pronounced and visible, but they aren't the only ones.
May the force be with you
7. In the "Star Wars" films, the Imperial TIE Fighters are propelled by ion engines (TIE stands for Twin Ion Engine). While these spacecraft are fictional, real ion engines power some of todayÕs spacecraft.
Ion propulsion has long been a staple of science fiction novels, but in recent years it has been successfully tested on a number of unmanned spacecraft, most notably NASAÕs Deep Space 1. Launched in 1998, Deep Space 1 rendezvoused with a distant asteroid and then with a comet, proving that ion propulsion could be used for interplanetary travel.
A question of gravity
8. There is no gravity in deep space.
If this were true, the moon would float away from the Earth, and our entire solar system would drift apart. While itÕs true that gravity gets weaker with distance, it can never be escaped completely, no matter how far you travel in space. Astronauts appear to experience "zero-gravity" because they are in continuous free-fall around the Earth.
9. The basic premise of teleportation -- made famous in TVÕs "Star Trek" -- is theoretically sound. In fact, scientists have already ÒteleportedÓ the quantum state of individual atoms from one location to another.
As early as the late 1990s, scientists proved they could teleport data using photons, but the photons were absorbed by whatever surface they struck. More recently, physicists at the University of Innsbruck in Austria and at the National Institute of Standards and Technology in Boulder, Colorado, for the first time teleported individual atoms using the principle of quantum entanglement.
Experts say this technology eventually could enable the invention of superfast "quantum computers." But the bad news, at least for sci-fi fans, is that experts donÕt foresee being able to teleport people in this manner.
Good day, Suns-shine
10. Tatooine, Luke Skywalker's home planet in the "Star Wars" films, has two Suns -- what astronomers would call a binary star system. Scientists have discovered recently that planets really can form within such systems.
Double-stars, or binary systems, are common in our Milky Way galaxy. Among the more than 100 new planets discovered in recent years, some have been found in binary systems, including16 Cygni B and 55 Cancri A. (But so far, no one has found a habitable planet like Luke Skywalker's Tatooine.)


The latest news from academia, regulators
research labs and other things of interest
Posted: Dec 23, 2013
Graphene can host exotic new quantum electronic states at its edges
(Nanowerk News) Graphene has become an all-purpose wonder material, spurring armies of researchers to explore new possibilities for this two-dimensional lattice of pure carbon. But new research at MIT has found additional potential for the material by uncovering unexpected features that show up under some extreme conditions — features that could render graphene suitable for exotic uses such as quantum computing.
On a piece of graphene (the horizontal surface with a hexagonal pattern of carbon atoms), in a strong magnetic field, electrons can move only along the edges, and are blocked from moving in the interior. In addition, only electrons with one direction of spin can move in only one direction along the edges (indicated by the blue arrows), while electrons with the opposite spin are blocked (as shown by the red arrows).
Under typical conditions, sheets of graphene behave as normal conductors: Apply a voltage, and current flows throughout the two-dimensional flake. If you turn on a magnetic field perpendicular to the graphene flake, however, the behavior changes: Current flows only along the edge, while the bulk remains insulating. Moreover, this current flows only in one direction — clockwise or counterclockwise, depending on the orientation of the magnetic field — in a phenomenon known as the quantum Hall effect.
In the new work, the researchers found that if they applied a second powerful magnetic field — this time in the same plane as the graphene flake — the material’s behavior changes yet again: Electrons can move around the conducting edge in either direction, with electrons that have one kind of spin moving clockwise while those with the opposite spin move counterclockwise.
“We created an unusual kind of conductor along the edge,” says Young, a Pappalardo Postdoctoral Fellow in MIT’s physics department and the paper’s lead author, “virtually a one-dimensional wire.” The segregation of electrons according to spin is “a normal feature of topological insulators,” he says, “but graphene is not normally a topological insulator. We’re getting the same effect in a very different material system.”
What’s more, by varying the magnetic field, “we can turn these edge states on and off,” Young says. That switching capability means that, in principle, “we can make circuits and transistors out of these,” he says, which has not been realized before in conventional topological insulators.
There is another benefit of this spin selectivity, Young says: It prevents a phenomenon called “backscattering,” which could disrupt the motion of the electrons. As a result, imperfections that would ordinarily ruin the electronic properties of the material have little effect. “Even if the edges are ‘dirty,’ electrons are transmitted along this edge nearly perfectly,” he says.
Jarillo-Herrero, the Mitsui Career Development Associate Professor of Physics at MIT, says the behavior seen in these graphene flakes was predicted, but never seen before. This work, he says, is the first time such spin-selective behavior has been demonstrated in a single sheet of graphene, and also the first time anyone has demonstrated the ability “to transition between these two regimes.”
That could ultimately lead to a novel way of making a kind of quantum computer, Jarillo-Herrero says, something that researchers have tried to do, without success, for decades. But because of the extreme conditions required, Young says, “this would be a very specialized machine” used only for high-priority computational tasks, such as in national laboratories.
Ashoori, a professor of physics, points out that the newly discovered edge states have a number of surprising properties. For example, although gold is an exceptionally good electrical conductor, when dabs of gold are added to the edge of the graphene flakes, they cause the electrical resistance to increase. The gold dabs allow the electrons to backscatter into the oppositely traveling state by mixing the electron spins; the more gold is added, the more the resistance goes up.
This research represents “a new direction” in topological insulators, Young says. “We don’t really know what it might lead to, but it opens our thinking about the kind of electrical devices we can make.”
The experiments required the use of a magnetic field with a strength of 35 tesla — “about 10 times more than in an MRI machine,” Jarillo-Herrero says — and a temperature of just 0.3 degrees Celsius above absolute zero. However, the team is already pursuing ways of observing a similar effect at magnetic fields of just one tesla — similar to a strong kitchen magnet — and at higher temperatures.
Philip Kim, a professor of physics at Columbia University who was not involved in this work, says, “The authors here have beautifully demonstrated excellent quantization of the conductance,” as predicted by theory. He adds, “This is very nice work that may connect topological insulator physics to the physics of graphene with interactions. This work is a good example how the two most popular topics in condensed matter physics are connected each other.”
Source: By David L. Chandler, MIT
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IBM researchers have built a prototype optical chip that can transfer a terabit of data per second, using an innovative design requiring 48 tiny holes drilled into a standard CMOS chip, facilitating the movement of light. Much faster and more power-efficient than today's optics, the so-called "Holey Optochip" technology could enhance the power of supercomputers.
Optical chips, which move data with light instead of electrons, are commonly used for interconnects in today's supercomputers and can be found in IBM systems such as Power 775 and Blue Gene. Optical technology is favored over electrical for transmitting high-bandwidth data over longer distances, which is why it's used for telecommunications networks, said IBM Optical Links Group manager Clint Schow.
As speed and efficiency improve, optical technology has become more viable in smaller settings. "I think the number one supercomputer ten years ago had no optics in it whatsoever, and now you're seeing large scale deployments, mostly for rack-to-rack interconnects within supercomputers," Schow told Ars. "It's making its way deeper into the system and getting closer and closer to the actual processor."
With the Holey Optochip, Schow said "our target is the bandwidth that interconnects different processors in the system—not the processor talking to its memory, but a processor talking to another processor in a large parallel system."
The Holey Optochip uses 4.7 watts in delivering nearly one trillion bits per second, enough to download 500 HD movies. At 5.2 mm by 5.8 mm, it's about one-eighth the size of a dime.
IBM built the chip using standard parts so it can make its way to market relatively quickly. "The heart of the chip is a single CMOS, plain-Jane unmodified process chip," Schow said. "That base chip has all the electronic circuit functions to complete the optical link. So it's got drivers that modulate vertical cavity lasers and receiver circuits that convert photocurrent from a detector into a usable electrical signal."
Drilling holes into the chip lets IBM use industry-standard, 850-nanometer vertical cavity surface emitting lasers (VCSEL), and photodiode arrays, both soldered on to the chip. The holes allow optical access through the back of the chip to the transmitter and receiver channels, making it more compact.
"You need the holes because if you have the silicon substrate the chip is made out of, the light can't go through it," Schow said. "You need to make a hole to let the light pass through." An IBM spokesperson further explains that "the optical devices are directly soldered to the front of the CMOS IC (integrated circuit) and the emission/detection of the optical signals is pointed toward the back of the chip. The holes are etched through the chip, one under each laser and detector to allow the optical signals to pass through the chip itself."
A standard optical chip today includes 12 channels (the links between transmitters and receivers), each moving 10 Gigabits per second, he said. The IBM Holey Optochip has 48 channels, each moving 20 gigabits per second, for a total of 960 gigabits, just below a terabit. IBM is unveiling the prototype chip today at the Optical Fiber Communication Conference in Los Angeles, calling it "the first parallel optical transceiver to transfer one trillion bits of information per second."
"That's four times as many channels running twice as fast, and the power efficiency is better by at least a factor of four," Schow said. The whole chip uses more power than current ones, but transmits much more data, resulting in better efficiency as measured by watts per bit.
The speed of each channel itself isn't breaking any records, given that IBM built the prototype chips using standard components. Schow noted that "there's development now that will push channel data rates to 25 gigabits per second in the near future." What's impressive about the Holey Optochip is the design, allowing optimization of density, power, and bandwidth all in one little package.
"You can go really fast if you don't care about power, and you can be really power-efficient if you don't care about speed," Schow said. Getting both facets right can bring an order-of-magnitude improvement to overall performance, he said. This is the second generation of the holey prototype—the first produced speeds of 300 gigabits per second 2010. Back in 2007, Ars reported on a previous, 160Gbps optical networking chip from Big Blue.
Although IBM itself won't be mass-producing the chips, Schow said they could become commercially available within a year or two. Price points could be in the $100 to $200 range, he speculated.
"We're in a group within IBM Research, looking at communications technologies we'll need for future computers, particularly for crunching big data, and analytics applications when you have to have tons of bandwidth in the system," he said. "Our mission is to prototype technologies and show what's possible, to drive the industry to commercial solutions that we can then procure and put into our systems."
IBM researchers also recently made a breakthrough in quantum computing, which could eventually lead to computers exponentially more powerful than today's, as our friends at Wired reported.


USC Scientists Contribute to a Breakthrough in Quantum Computing
Scientists have taken the next major step toward quantum computing, which will use quantum mechanics to revolutionize the way information is processed.
Quantum computers will capitalize on the mind-bending properties of quantum particles to perform complex calculations that are impossible for today’s traditional computers.
Using high-magnetic fields, Susumu Takahashi, assistant professor of chemistry in USC Dornsife, and his colleagues managed to suppress decoherence, one of the key stumbling blocks in quantum computing.
“High-magnetic fields reduce the level of the noises in the surroundings so they can constrain the decoherence very efficiently,” Takahashi said. Decoherence has been described as a “quantum bug” that destroys fundamental properties that quantum computers would rely on.
The research will appear in the online version of Nature magazine today.
Quantum computing uses quantum bits, or qubits, to encode information in the form of ones and zeros. Unlike a traditional computer that uses traditional bits, a quantum computer takes advantage of the seemingly impossible fact that qubits can exist in multiple states at the same time, which is called “superposition.”
While a bit can represent either a one or a zero, a qubit can represent a one and a zero at the same time due to superposition. This allows for simultaneous processing of calculations in a truly parallel system, skyrocketing computing ability.
Though the concepts underpinning quantum computing are not new, problems such as decoherence have hindered the construction of a fully functioning quantum computer.
Think of decoherence as a form of noise or interference, knocking a quantum particle out of superposition — robbing it of that special property that makes it so useful. If a quantum computer relies on a quantum particle’s ability to be both here and there, then decoherence is the frustrating phenomenon that causes a quantum particle to be either here or there.
University of British Columbia researchers calculated all sources of decoherence in their experiment as a function of temperature, magnetic field and by nuclear isotopic concentrations, and suggested the optimum condition to operate qubits, reducing decoherence by approximately 1,000 times.
In Takahashi’s experiments, qubits were predicted to last about 500 microseconds at the optimum condition — ages, relatively speaking.
Decoherence in qubit systems falls into two general categories. One is an intrinsic decoherence caused by constituents in the qubit system, and the other is an extrinsic decoherence caused by imperfections of the system — impurities and defects, for example.
In their study, Takahashi and his colleagues investigated single crystals of molecular magnets. Because of their purity, molecular magnets eliminate the extrinsic decoherence, allowing researchers to calculate intrinsic decoherence precisely.
“For the first time, we’ve been able to predict and control all the environmental decoherence mechanisms in a very complex system — in this case a large magnetic molecule,” said Phil Stamp, University of British Columbia professor of physics and astronomy and director of the Pacific Institute of Theoretical Physics.
Using crystalline molecular magnets allowed researchers to build qubits out of an immense quantity of quantum particles rather than a single quantum object — the way most proto-quantum computers are built at the moment.
“This will obviously increase signals from the qubit drastically so the detection of the qubit in the molecular magnets is much easier,” said Takahashi, who conducted his research as a project scientist in the Institute of Terahertz Science and Technology and the Department of Physics at the University of California, Santa Barbara. Takahashi has been at USC Dornsife since 2010.
Research for the article was performed in collaboration with Phil Stamp and Igor Tupitsyn of the University of British Columbia, Johan van Tol of Florida State University, and Chris Beedle and David Hendrickson of the University of California, San Diego.
The work was supported by the National Science Foundation, the W. M. Keck Foundation, the Pacific Institute of Theoretical Physics at the University of British Columbia, by the Natural Sciences and Engineering Research Council of Canada, the Canadian Institute for Advanced Research and the USC start-up funds.
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Quantum eraser experiment
In quantum mechanics, the quantum eraser experiment is an interferometer experiment that demonstrates several fundamental aspects of quantum mechanics, including quantum entanglement and complementarity.
The double-slit quantum eraser experiment described in this article has three stages:
- First, the experimenter reproduces the interference pattern of Young's double-slit experiment by shining photons at the double-slit interferometer and checking for an interference pattern at the detection screen.
- Next, the experimenter marks through which slit each photon went, without disturbing its wavefunction, and demonstrates that thereafter the interference pattern is destroyed. This stage indicates that it is the existence of the "which-path" information that causes the destruction of the interference pattern.
- Third, the "which-path" information is "erased," whereupon the interference pattern is recovered. (Rather than removing or reversing any changes introduced into the photon or its path, these experiments typically produce another change that obscures the markings earlier produced.)
The quantum eraser experiment described in this article is a variation of Thomas Young's classic double-slit experiment. It establishes that when action is taken to determine which slit a photon has passed through, the photon cannot interfere with itself. When a stream of photons is marked in this way, then the interference fringes characteristic of the Young experiment will not be seen. The experiment described in this article is capable of creating situations in which a photon that has been "marked" to reveal through which slit it has passed can later be "unmarked." A photon that has been "marked" cannot interfere with itself and will not produce fringe patterns, but a photon that has been "marked" and then "unmarked" can thereafter interfere with itself and will cooperate in producing the fringes characteristic of Young's experiment.
This experiment involves an apparatus with two main sections. After two entangled photons are created, each is directed into its own section of the apparatus. It then becomes clear that anything done to learn the path of the entangled partner of the photon being examined in the double-slit part of the apparatus will influence the second photon, and vice-versa. The advantage of manipulating the entangled partners of the photons in the double-slit part of the experimental apparatus is that experimenters can destroy or restore the interference pattern in the latter without changing anything in that part of the apparatus. Experimenters do so by manipulating the entangled photon, and they can do so before or after its partner has passed through the slits and other elements of experimental apparatus between the photon emitter and the detection screen. So, under conditions where the double-slit part of the experiment has been set up to prevent the appearance of interference phenomena (because there is definitive "which path" information present), the quantum eraser can be used to effectively erase that information. In doing so, the experimenter restores interference without altering the double-slit part of the experimental apparatus.
A variation of this experiment, delayed choice quantum eraser, allows the decision whether to measure or destroy the "which path" information to be delayed until after the entangled particle partner (the one going through the slits) has either interfered with itself or not. Doing so appears to have the bizarre effect of causing the outcome of an event after the event has already occurred. In other words, something that happens at time t apparently reaches back to some time t - 1 and acts as a determining causal factor at that earlier time.
First, a photon is shot through a specialized nonlinear optical device: a beta barium borate (BBO) crystal. This crystal converts the single photon into two entangled photons of lower frequency, a process known as spontaneous parametric down-conversion (SPDC). These entangled photons follow separate paths. One photon goes directly to a detector, while the second photon passes through the double-slit mask to a second detector. Both detectors are connected to a coincidence circuit, ensuring that only entangled photon pairs are counted. A stepper motor moves the second detector to scan across the target area, producing an intensity map. This configuration yields the familiar interference pattern.
Next, a circular polarizer is placed in front of each slit in the double-slit mask, producing clockwise circular polarization in light passing through one slit, and counter-clockwise circular polarization in the other slit (see Figure 1). This polarization is measured at the detector, thus "marking" the photons and destroying the interference pattern (see Fresnel–Arago laws).
Finally, a linear polarizer is introduced in the path of the first photon of the entangled pair, giving this photon a diagonal polarization (see Figure 2). Entanglement ensures a complementary diagonal polarization in its partner, which passes through the double-slit mask. This alters the effect of the circular polarizers: each will produce a mix of clockwise and counter-clockwise polarized light. Thus the second detector can no longer determine which path was taken, and the interference fringes are restored.
A double slit with rotating polarizers can also be accounted for by considering the light to be a classical wave. However this experiment uses entangled photons, which are not compatible with classical mechanics.
- Walborn, S. P.; et al. (2002). "Double-Slit Quantum Eraser". Phys. Rev. A 65 (3): 033818. arXiv:quant-ph/0106078. Bibcode:2002PhRvA..65c3818W. doi:10.1103/PhysRevA.65.033818.
- Englert, Berthold-Georg (1999). "REMARKS ON SOME BASIC ISSUES IN QUANTUM MECHANICS". Zeitschrift für Naturforschung 54 (1): 11–32.
- Aharonov, Yakir; Zubairy, M. Suhail (2005). "Time and the Quantum: Erasing the Past and Impacting the Future". Science 307 (5711): pp. 875–879. Bibcode:2005Sci...307..875A. doi:10.1126/science.1107787. PMID 15705840.
- Kim, Yoon-Ho; R. Yu, S.P. Kulik, Y.H. Shih and Marlan Scully (2000). "A Delayed Choice Quantum Eraser". Physical Review Letters 84: 1–5. arXiv:quant-ph/9903047. Bibcode:2000PhRvL..84....1K. doi:10.1103/PhysRevLett.84.1.
- Chiao, R. Y.; P. G. Kwiat; Steinberg, A. M. (1995). "Quantum non-locality in two-photon experiments at Berkeley". Quantum and Semiclassical Optics: Journal of the European Optical Society Part B 7 (3): 6. Retrieved 13 February 2014.


In physics, the Mach–Zehnder interferometer is a device used to determine the relative phase shift variations between two collimated beams derived by splitting light from a single source. The interferometer has been used, among other things, to measure phase shifts between the two beams caused by a sample or a change in length of one of the paths. The apparatus is named after the physicists Ludwig Mach (the son of Ernst Mach) and Ludwig Zehnder: Zehnder's proposal in an 1891 article was refined by Mach in an 1892 article.
The Mach–Zehnder interferometer is a highly configurable instrument. In contrast to the well-known Michelson interferometer, each of the well separated light paths is traversed only once.
If it is decided to produce fringes in white light, then, since white light has a limited coherence length, on the order of micrometers, great care must be taken to simultaneously equalize the optical paths over all wavelengths or no fringes will be visible. As seen in Fig. 1, a compensating cell made of the same type of glass as the test cell (so as to have equal optical dispersion) would be placed in the path of the reference beam to match the test cell. Note also the precise orientation of the beam splitters. The reflecting surfaces of the beam splitters would be oriented so that the test and reference beams pass through an equal amount of glass. In this orientation, the test and reference beams each experience two front-surface reflections, resulting in the same number of phase inversions. The result is that light traveling an equal optical path length in the test and reference beams produces a white light fringe of constructive interference.
Collimated sources result in a nonlocalized fringe pattern. Localized fringes result when an extended source is used. In Fig. 2, we see that the fringes can be adjusted so that they are localized in any desired plane.:18 In most cases, the fringes would be adjusted to lie in the same plane as the test object, so that fringes and test object can be photographed together.
The Mach–Zehnder interferometer's relatively large and freely accessible working space, and its flexibility in locating the fringes has made it the interferometer of choice for visualizing flow in wind tunnels and for flow visualization studies in general. It is frequently used in the fields of aerodynamics, plasma physics and heat transfer to measure pressure, density, and temperature changes in gases.:18,93–95
Mach–Zehnder interferometers are used in electro-optic modulators, electronic devices used in various fibre-optic communications applications. Mach-Zehnder modulators are incorporated in monolithic integrated circuits and offer well-behaved, high-bandwidth electro-optic amplitude and phase responses over a multiple GHz frequency range.
How it works
A collimated beam is split by a half-silvered mirror. The two resulting beams (the "sample beam" and the "reference beam") are each reflected by a mirror. The two beams then pass a second half-silvered mirror and enter two detectors.
The fully silvered and half-silvered surfaces of all mirrors, except the last, face the inbound beam, and the half-silvered surface of the last mirror faces the outbound beam exiting in the same orientation as the original collimated beam. That is, if the original beam is horizontal, the half-silvered surface of the last mirror should face the horizontally outbound beam.
The Fresnel equations for reflection and transmission of a wave at a dielectric imply that there is a phase change for a reflection when a wave reflects off a change from low to high refractive index but not when it reflects off a change from high to low.
In other words:
- A 180 degree phase shift occurs upon reflection from the front of a mirror, since the medium behind the mirror (glass) has a higher refractive index than the medium the light is traveling in (air).
- No phase shift accompanies a rear surface reflection, since the medium behind the mirror (air) has a lower refractive index than the medium the light is traveling in (glass).
We also note that:
- The speed of light is slower in media with an index of refraction greater than that of a vacuum, which is 1. Specifically, its speed is: v = c/n, where c is the speed of light in vacuum and n is the index of refraction. This causes a phase shift increase proportional to (n − 1) × length traveled.
- If k is the constant phase shift incurred by passing through a glass plate on which a mirror resides, a total of 2k phase shift occurs when reflecting off the rear of a mirror. This is because light traveling toward the rear of a mirror will enter the glass plate, incurring k phase shift, and then reflect off the mirror with no additional phase shift since only air is now behind the mirror, and travel again back through the glass plate incurring an additional k phase shift.
Caveat: The rule about phase shifts applies to beamsplitters constructed with a dielectric coating, and must be modified if a metallic coating is used, or when different polarizations are taken into account. Also, in real interferometers, the thicknesses of the beamsplitters may differ, and the path lengths are not necessarily equal. Regardless, in the absence of absorption, conservation of energy guarantees that the two paths must differ by a half wavelength phase shift. Also note that beamsplitters that are not 50/50 are frequently employed to improve the interferometer's performance in certain types of measurement.
Observing the effect of a sample
In Fig. 3, in the absence of a sample, both the sample beam SB and the reference beam RB will arrive in phase at detector 1, yielding constructive interference. Both SB and RB will have undergone a phase shift of (1×wavelength + k) due to two front-surface reflections and one transmission through a glass plate.
At detector 2, in the absence of a sample, the sample beam and reference beam will arrive with a phase difference of half a wavelength, yielding complete destructive interference. The RB arriving at detector 2 will have undergone a phase shift of 0.5×(wavelength) + 2k due to one front-surface reflection and two transmissions. The SB arriving at detector 2 will have undergone a (1×wavelength + 2k) phase shift due to two front-surface reflections and one rear-surface reflection. Therefore, when there is no sample, only detector 1 receives light.
If a sample is placed in the path of the sample beam, the intensities of the beams entering the two detectors will change, allowing the calculation of the phase shift caused by the sample.
Use of the Mach–Zehnder interferometer
The versatility of the Mach–Zehnder configuration has led to its being used in a wide range of fundamental research topics in quantum mechanics, including studies on counterfactual definiteness, quantum entanglement, quantum computation, quantum cryptography, quantum logic, Elitzur-Vaidman bomb tester, the quantum eraser experiment, the quantum Zeno effect, and neutron diffraction. See their respective articles for further information on these topics.
- List of types of interferometers
Related forms of interferometer
Other flow visualisation techniques
- Zehnder, Ludwig (1891). "Ein neuer Interferenzrefraktor". Zeitschrift für Instrumentenkunde 11: 275–285.
- Mach, Ludwig (1892). "Ueber einen Interferenzrefraktor". Zeitschrift für Instrumentenkunde 12: 89–93.
- Zetie, K.P.; Adams, S.F.; Tocknell, R.M. "How does a Mach–Zehnder interferometer work?". Physics Department, Westminster School, London. Retrieved 8 April 2012.
- Ashkenas, Harry I. (1950). The design and construction of a Mach-Zehnder interferometer for use with the GALCIT Transonic Wind Tunnel. Engineer's thesis. California Institute of Technology.
- Hariharan, P. (2007). Basics of Interferometry. Elsevier Inc. ISBN 0-12-373589-0.
- Chevalerias, R.; Latron, Y.; Veret, C. (1957). "Methods of Interferometry Applied to the Visualization of Flows in Wind Tunnels". Journal of the Optical Society of America 47 (8): 703. doi:10.1364/JOSA.47.000703.
- Ristić, Slavica. "Flow visualization techniques in wind tunnels – optical methods (Part II)". Military Technical Institute, Serbia. Retrieved 6 April 2012.
- Paris, M.G.A. (1999). "Entanglement and visibility at the output of a Mach-Zehnder interferometer". Physical Review A 59 (2): 1615–1621. arXiv:quant-ph/9811078. Bibcode:1999PhRvA..59.1615P. doi:10.1103/PhysRevA.59.1615. Retrieved 2 April 2012.
- Haack, G. R.; Förster, H.; Büttiker, M. (2010). "Parity detection and entanglement with a Mach-Zehnder interferometer". Physical Review B 82 (15). arXiv:1005.3976. Bibcode:2010PhRvB..82o5303H. doi:10.1103/PhysRevB.82.155303.


Routing Protocols - List of Routing protocols
Introduction of Routing Protocols
The process of routing governs the path and passage of data traffic in the form of packets and frames. The process of routing is aim to transfer the logical packets from their source to their eventual destination. This process is however monitored by routing protocols. The routing protocols how routers can communicate among themselves. The routing information is circulated that enables the routers to communicate within the computer network.
Border Gateway Protocol (BGP)
The network traffic is forwarded along the desired paths during the process of routing. However this process of routing is governed by crucial routing protocols. Border gateway protocol is the significant routing protocol. Border Gateway Protocol or BGP is capable of maintaining and keeping the track of IP networks which provides network access to autonomous systems (the collection of IPs which illustrates the routing procedure to the internet). Also this protocol substituted the Exterior Gateway Protocol (EGP), the use of this protocol has diminished completely now.
Cisco Discovery Protocol (CDP)
This data link layer network protocol, developed and used by Cisco International. It is most compatible to be used with Cisco network devices; it can be used to share information with the other directly attached Cisco devices. It can serve another purpose of on demand routing. On demand routing enables the CDP to identify the IP addresses and the model and type of the Cisco device connected to the network. This use of CDP in enhanced on demand routing removes the use of other vibrant protocols in the network.
Connectionless Network Service (CNS)
It is network services at the second third layer of the OSI model that is network layer. It is referred to as CNS because it does not require the establishment of circuit and hence the messages are transferred to the destinations independent of each other.
Hot Standby Router Protocol (HSRP)
This redundancy protocol established by Cisco is used as a fault tolerant gateway. The default gateway failover is covered by HSRP by using a simple technique. A multicasting data packet is sent by one HSRP to the other HSRP enabled router. The router with the pre defined IP address and gateway will respond to it quickly. This router is termed as a primary router if it fails to receive the ARP request then the next router receiving the ARP request with the same MAC address is thus successful in accomplishing the default gateway failover.
IGRP/EIGRP (Enhanced Interior Gateway Routing Protocol
EIGPR is a Cisco routing protocol based on its earlier version IGPR. This is termed as a distance vector protocol because it is used in a packet switch networks for communicating. The basic purport of this network protocol is to stabilize the working of the router. Hence it can guide the router in utilizing the bandwidth and power. Moreover the routers associated with using EIGRP can reallocate the route information to IGRP neighbors.
Internet Protocol (IP)
The internet protocol performs the task of delivering eminent data packets from the source to the destination using IP address. It is used to transfer data packets in packet switched network by utilizing internet protocol suits like TCP/IP. Intermediate System-to-Intermediate System (IS-IS)
IS–IS is a network protocol which determines the best and most suitable route for the data packets to be transferred via packet switched network by the network devices such as routers.
Multiprotocol Label Switching (MPLS)
A highly scaleable protocol skeptic mechanism used in high performing telecommunication system which assigns labels to the data packets is known as MPLS. It helps to transfer data between distant nodes by creating virtual links.
Network Address Translation (NAT)
Network address translation is a mechanism which modifies different network addresses into one IP header and it travels across a routing device, this serves the purpose of remapping the discrete addresses from one address legroom to another.
Open Shortest Path First (OSPF)
It is an interior gateway protocol that delivers IP packets to the autonomous systems. It also assembles link state information to form a topology map. This topology map helps the routing tables to make decisions merely based on IP addresses present in IP datagrams.
Quality of Service (QoS)
Quality of service is the term most commonly used in network technologies to refer to the ability to provide recital and performance to data flow .it is a teletraffic engineering terminology. It is specialized in guaranteeing and improving bit rate and multi media streaming capabilities.
Routing Information Protocol (RIP)
This efficient protocol sends the update routing messages to the routers in order to update their routes. Network routing table makes the desirable changes to it when it receives a message to alter its entry level network topologies.
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Single field shapes quantum
Technology Research News
computers, which tap the properties of particles like atoms, photons and
electrons to carry out computations, could potentially use a variety of
schemes: individual photons controlled by optical networks, clouds of atoms
linked by laser beams, and electrons trapped in quantum dots embedded in
Due to the strange nature of quantum particles, quantum computers
are theoretically much faster than ordinary computers at solving certain
large problems, like cracking secret codes.
Chip-based quantum computers would have a distinct advantage: the
potential to leverage the extensive experience and manufacturing infrastructure
of the semiconductor industry. Controlling individual electrons, however,
is extremely challenging.
Researchers have recently realized that it may be possible to control
the electrons in a quantum computer using a single magnetic field rather
than having to produce extremely small, precisely focused magnetic fields
for each electron.
Researchers from the University of Toronto and the University of
Wisconsin at Madison have advanced this idea with a scheme that allows individual
electrons to serve as the quantum bits that store and process computer information.
The scheme is an improvement over existing global magnetic field schemes,
which require each qubit to consist of two or more electrons.
Electrons have two magnetic orientations, spin up and spin down,
which can represent the 1s and 0s of computing. The logic of quantum computing
is based on one-qubit gates and two-qubit gates. One-qubit gates flip individual
spins, changing a 1 to a 0 and vice versa. Two-qubit gates cause two spins
to become linked, or entangled.
The researchers' scheme relies on the interactions of pairs of electrons
to create both types of gates. Tiny electrodes positioned near quantum dots
-- bits of semiconductor material that can trap single electrons -- can
draw neighboring electrons near enough that they exchange energy. If the
electrons interact long enough, they swap spin orientations. The challenge
is finding a way to use the interaction to flip the spin of one electron
without flipping the spin of the other.
The scheme does so by taking a pair of electrons through eleven
incremental steps using the electron interaction and the global magnetic
field. "We first turn on the exchange interactions... through small electrodes
to generate a swap gate, then turn on the global magnetic field," said Lian-Ao
Wu, a research associate at the University of Toronto.
The eleven steps -- four electron interactions and seven pulses
of the magnetic field -- alter the spins. Because the magnetic field diminishes
in strength over distance each electron is exposed to a different strength.
By tuning the field, the researchers can make the process cancel out the
changes to one spin while flipping the other, according to Wu.
The researchers' scheme could be implemented using a pair of square,
100-nanometer-diameter aluminum nanowires separated by a thin insulating
layer. A row of quantum dots in a zigzag pattern would be positioned parallel
to the wires, with half of the dots 200 nanometers from the wires and the
other half 300 nanometers away. A nanometer is one millionth of a millimeter,
or the span of 10 hydrogen atoms.
The ability to build such a quantum computer depends on developments
in nanotechnology, said Wu. "It is still hard to design a complete control
scheme of the exchange interactions," he said. "Once such obstacles are
overcome, our scheme should offer significant simplifications and flexibility."
The on-chip conducting wires called for in the researchers' scheme
have been used in physics experiments involving controlling beams of atoms
and Bose-Einstein condensates, which are small clusters of atoms induced
to behave as one quantum entity, according to Wu.
The researchers are working on reducing the number of steps required
for their quantum logic circuit, combining their scheme with quantum error
correction techniques, and reducing the engineering challenge of implementing
the design, said Wu. The scheme would require making the aluminum wires
with a precision of a single layer of atoms, but optimizing the scheme should
make it possible to loosen the requirements to several atomic layers, which
is technologically feasible, according to Wu.
"The main challenge is [achieving a] high degree of control of the
exchange interactions," he said.
The technique could be used practically in 10 to 20 years, said
Wu's research colleague was Daniel A. Lidar at the University of
Toronto and Mark Friesen at the University of Wisconsin at Madison. The
work appeared in the July 15, 2004 issue of Physical Review Letters.
The research was funded by the Defense Advanced Research Projects Agency
(DARPA), the National Science Foundation (NSF), and the Army Research Office/Advanced
Research and Development Activity (ARO/ARDA).
Timeline: 10-20 years
TRN Categories: Quantum Computing and Communications
Story Type: News
Related Elements: Technical paper, "One-Spin Quantum Logic
Gates from Exchange Interactions and a Global Magnetic Field," Physical
Review Letters, July 15, 2004
November 3/10, 2004
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According to experts, the chips used to power data center servers will continue to get smaller and faster, and Moore's Law of doubled performance every two years will continue unabated for at least the next 10 years. Also, new features like security and wireless communications will be bundled into microprocessors, ultimately easing the jobs of people in the data center who implement and maintain the servers.
First discovered in 1824 as a means of conducting electricity, silicon has been used as an essential semiconductor building block virtually since the first integrated circuit was designed at Texas Instruments in 1958.
After 2010, though, things could get very interesting.
"Everyone in the field is pretty comfortable that Moore's Law and silicon will dominate through the end of the decade," says Nathan Brookwood, a semiconductor analyst at Insight 64, an independent consultancy in Saratoga, Calif. "After that, though, people are a little worried about whether silicon will be able to go on indefinitely."
Some believe that silicon eventually will start to run out of steam; after all, one can cram only so many things into a tiny space before reaching a point of diminishing returns. Experts have been debating for years when that limit will be reached. There's been plenty of research going on in various areas of chip design and fabrication to help overcome the obstacles. Nanotechnology, quantum computing and other technologies have come to the fore as possible silicon replacements, Brookwood says.
In the meantime, semiconductor makers are trying to do all they can to keep silicon alive. Although research in new areas is ongoing, to actually mass-produce anything other than silicon will cost billions of dollars in new semiconductor manufacturing and design equipment. As the economy continues to spiral downward, these are not costs that chip makers are eager to bear.
One method of prolonging silicon's life is to put two or more processors on a single piece of silicon. Called chip multiprocessing, this is expensive because all the components in each chip must be replicated. A less expensive approach -- one already being used by Intel and about to be adopted by Sun and others -- is called simultaneous multithreading. With this technique, some parts of the chip are replicated but the device can switch among multiple threads while sharing a lot of the underlying chip resources. Thus one chip can do the work of two or more, cutting down the number of chips needed to do the same amount of work.
Other approaches are being tried, too. In April, IBM and Sony announced a deal to jointly develop chips based on silicon-on-insulator and other types of materials. Silicon-on-insulator places a level of insulation between the transistor and its silicon substrate, reducing distortion and improving switching by 20 to 35 percent. Another up-and-coming area is called a "system on a chip," in which the central processing unit, communications features and memory are integrated onto one chip.
In the meantime, though, data center staffers will continue to see familiar, if welcome, improvements in chip and server technologies.
Mike Splain, chief technology officer for Sun's processor and network products group, expects several things to happen during the next few years with high-end servers. One is a migration from software-based recovery to hardware-based, so error recovery is more automatic and totally guaranteed.
Another trend will be the use of doubled-up processor cores -- using multiple threads in one CPU. "You won't get exactly a doubling every two years, but it will be some factor of that," Splain says. "So you might see a true doubling every three to four years" in the highest-end machines. He also expects the machines to become much smaller because of this doubling-up, so customers "will get more space back in their data centers."
For its part, Intel has committed to expanding its NetBurst architecture, now used in Intel's Pentium and Xeon chips as well as the Itanium line, to be able to handle 10 GHz, up from 2.8 GHz today, according to Mike Graf, product line manager for the Itanium processor family. Intel is positioning Itanium as the highest-end chip in its lineup, making it the basis for machines with 32-plus processors. Distributed databases, scientific technical computing and other high-performance niches are its target markets.
Intel is currently shipping its second-generation Itanium. At the Intel Developer's Forum in September, the company laid out plans for what's ahead. By summer 2003, the "Madison" generation of the chip will debut, with twice the cache (6M bytes versus the current 3M bytes), and those chips will be 30 to 50 percent faster than the current generation. Also, Itanium will feature multithreading and hyperthreading, which allows the computer to execute multiple threads in parallel. This, in turn, improves transaction rates by up to 30 percent over systems that don't have hyperthreading, Intel says. Multithreading and hyperthreading are already features in the Xeon family and will now move to Itanium.
Within about two years, the company will debut a chip with 1 billion transistors on a single processor, Graf says. Today's top-of-the-line processor has about 221 million transistors, and the Madison generation will sport around a half-billion.
Perhaps just as important, future generations of Itanium -- and there are five in development -- will be compatible at both the hardware and software levels with the existing chip set. This should translate into fewer installation problems with device drivers and other issues down the road.
The goal is to provide IT shops, especially in these troubled economic times, the means for "doing more with less," Graf says.
All told, Intel will "take technology traditionally in the high end of the market and bring it into the mainstream," says Tony Massimini, chief of technology at Semico Research Corp. in Phoenix, an independent consultancy specializing in semiconductor research. "They will keep pumping out chips in high volume and low price, and this will look very attractive" to IT shops, he says.
Although Massimini doesn't expect anything radically different for server chips in the next few years, he did say that Intel will be "pushing wireless" features a great deal. "With greater wireless connectivity for notebooks and desktops, that will put a load on the data center guys" to support those features from the server side, he adds.
According to Massimini, at its recent developer forum Intel "alluded" to the idea improving security by embedding some features into its chips, through something code-named La Grande, although the company didn't provide a timeframe for doing this. Intel's Graf wouldn't disclose any information about this project, but said that security is an issue the company is aware of and working on.
For more information:
- Search390.com's Best Web Links on processors/servers
- Learn more about Moore's Law and Intel processors in this tip: Intel brings economies of scale to high-end space with Itanium 64-bit processor