Quantum Field Theory Resources

Quantum Field Theory Resources

‘Science is the organized skepticism in the reliability of expert opinion.’ - R. P. Feynman (quoted by Smolin, The Trouble with Physics, 2006, p. 307).

‘Science n. The observation, identification, description, experimental investigation, and theoretical explanation of phenomena.’ - http://www.answers.com/

String theory predictions are not analogous to Wolfgang Pauli’s prediction of neutrinos, which was indicated by the solid experimentally-based physical facts of energy conservation and the mean beta particle energy being only about 30% of the total mass-energy lost per typical beta decay event: Pauli made a checkable prediction, Fermi developed the beta decay theory and then invented the nuclear reactor which produced enough decay in the radioactive waste to provide a strong source of neutrinos (actually antineutrinos) which tested the theory because conservation principles had made precise predictions in advance, unlike string theory’s ‘heads I win, tails you lose’ political-type, fiddled, endlessly adjustable, never-falsifiable pseudo-‘predictions’. Contrary to false propaganda from certain incompetent string ‘defenders’, Pauli correctly predicted that neutrinos are experimentally checkable, in a 4 December 1930 letter to experimentalists: ‘... Dear Radioactives, test and judge.’ (See footnote on p12 of this reference.)

note:
neutrino: an elementary particle with zero charge and zero mass;
checkable 的网络定义:Able to be checked;
antineutrino 的网络定义:The anti-particle of a neutrino.
fiddled: Verb. commit fraud and steal from one's employer; n. bowed stringed instrument that is the highest member of the violin family; this instrument has four strings and a hollow body and an unfretted fingerboard and is played with a bow;
endlessly :continuing forever without end;
incompetent :someone who is not competent to take effective action;
propaganda:information that is spread for the purpose of promoting some cause;

Nobel laureate goes to Washington?

Dec 11, 2008
Nobel laureate goes to Washington?
Steven Chu

The Nobel-prize-winning physicist Steven Chu will be Barack Obama’s nomination for Secretary of the Department of Energy (DOE), according to reports from the US President-elect’s transition team.

Chu, 60, is currently the director of the Lawrence Berkeley National Laboratory in California and professor of biophysics at University of California, Berkeley. If his nomination is confirmed by the US senate next year, Chu would be the first working scientist to run the DOE, which has a budget of about $25bn and is one of the largest sources of funding of scientific research in the US. Since 2005, the department has been headed by Samuel Bodman, a former professor of chemical engineering who spent many years as a venture capitalist before joining the DOE.
Unlike some in the outgoing Bush administration, Chu is a firm believer that humans are damaging the Earth’s climate. Indeed, he believes that climate change scenarios laid out in 2007 by the Intergovernmental Panel on Climate Change may be on the conservative side.

Science for a better environment

Chu also believes that science can play an important role in reducing emissions of greenhouse gases. He has been involved in the Berkeley-based Energy Biosciences Institute — a $500m facility, sponsored by oil giant BP, that aims to develop new energy sources from biomass, including biofuels.

Born in St Louis, Missouri, Chu did a PhD in physics at Berkeley and shared the 1997 Nobel Prize in Physics with Claude Cohen-Tannoudji and William Phillips "for development of methods to cool and trap atoms with laser light" — an accomplishment that has led to a renaissance in the study of the quantum mechanics of many-body systems.

In an interview with Physics World magazine earlier this year, Chu expressed his conviction that scientists could work together to save the environment: “Just as in the Second World War, when there were scientists who worked on radar or the bomb because they felt there was an emergency, so there are scientists today who want to work on the energy problem”.

About the author
Hamish Johnston is editor of physicsworld.com

Physics of Information Group at IBM Research

Physics of Information / Quantum Information Group at IBM Research Yorktown

We are in the midst of an information revolution, so much so that even lay people know the basic facts about information—how it can be encoded in bits 0 and 1, stored, retrieved, transmitted, and processed using logic gates like AND and NOT. This revolution is based on our ability to treat information in an abstract way, largely independent of its physical embodiment, which may be as diverse as a hole in a punch card, a voltage in a wire, or the magnetization of a speck of iron oxide. The field of Information Physics treats ways in which it nevertheless fruitful to reintegrate physical laws and principles into the science of information. These include:

Thermodynamics: Processing information consumes energy and generates waste heat, and the amount turns out to depend both on hardware and the nature of the logic operations being performed. The founder of our group, the late Rolf Landauer, during his long career at IBM Research, continually emphasized the connection between information processing and physics, and discovered the connection between logical irreversibility and heat generation now known as Landauer's principle.

Quantum effects: Quantum phenomena like entanglement and interference were neglected in the classical theory of information processing developed by Shannon, Turing, von Neumann and their contemporaries. In retrospect this was a mistake. Including quantum effects, and indeed abstracting them away from any particular physical embodiment, leads to a more coherent and powerful theory of information processing, as well as making possible information-processing feats unachievable with conventional “classical” information, notably quantum cryptography and quantum computational speedups. In place of bits the new quantum information theory has qubits, which are capable of entanglement and superposition, and interact with one another via quantum gates.

Fault-tolerance: Any real physical information processing apparatus, whether man-made or biological, is subject to errors. To make computing systems scalable in the presence of errors, a fault-tolerant architecture is required. This old problem has become acute in the case of quantum computers, where a considerable gap remains to be closed between experimentally available error rates and the thresholds at which fault tolerant architectures would take hold.

Physical Complexity: How can various mathematical notions of complexity, such as time/space complexity, parallel complexity, and algorithmic information, be used to characterize the complexity of physical states, phase transitions, and the behavior of systems at and away from thermal equilibrium. Are there physical systems or dynamics that are uncomputable in the mathematical sense?

Physical Authentication: Can our understanding of the computational complexity be used to authenticate physical objects and evolutions as genuine, rather than forged or simulated?

© Physics of Information Group at IBM Research

Superconducting qubits get entangled

Superconducting qubits get entangled
http://physicsworld.com/cws/article/news/25845

Physicists in the US have taken another step towards the dream of a quantum computer by entangling two superconducting quantum bits (or qubits) for the first time. Circuits made from superconducting elements are promising candidates for a real quantum computer because they are compatible with conventional methods for making integrated circuits (Science 313 1423).

In the weird world of quantum mecahnics, particles can be "entangled" so that they have a much closer relationship than allowed by classical physics. For instance, two photons can be created in an experiment such that if one is polarized in the vertical direction, then the other is always polarized horizontally. By measuring the polarization of one of the pair, we immediately know the state of the other, no matter how far apart they are.

This "spooky action at a distance", which has no classical analogue, could allow multiple bits of information to be processed at the same time in a quantum computer. Such a device could therefore outperform a classical computer by many orders of magnitude. There are currently many rival ways of entangling particles, for example by trapping ions at ultra-low temperatures and manipulating their internal energy states with lasers

However, demonstrating entanglement is hard. In particular, the particles, or qubits, have to be sufficiently isolated from the environment so that the fragile entangled state exists for long enough to allow a calculation to be carried out. Various other conditions also have to be met -- together known as the "DiVincenzo criteria" -- such as being able to measure both qubits at the same time.

Now, however, a team from the University of California, Santa Barbara, has successfully entangled two superconducting qubits for the first time. Electrical circuits made from superconductors are promising candidates for a working quantum computer because they can be made from thin films using conventional microchip fabrication technology. Coupling can be achieved simply by electrical connections between qubits – far easier than the trapped ion approach, where ions need to be shuttled about so they can interact.

Matthias Steffen and colleagues at Santa Barbara were able to entangle two qubits, each made from a Josephson tunnel junction, that meet the DiVincenzo criteria completely with a precision of 87% of theoretical values. The researchers used a delicate method known as "quantum state tomography" to confirm the entanglement, whereby a series of different parameters are measured for the two particles and used to reconstruct the quantum state, much as image “slices” are captured and combined into a three-dimensional picture in tomographic medical imaging.

Although physicists have been able to entangle up to eight ions at the same time -- whereas the present work entangles just two quibits -- Steffen insists superconducting qubits are a viable approach towards quantum computing. "Substituting some of the materials in the fabrication process should translate to a straightforward improvement of our results and in the long run, continued materials research should also help improve qubit performance," he says.

The work was done by Santa Barbara's Quantum Computation research group, which is led by John Martinis.

Microchip ‘bus’ links up quantum bits

Microchip ‘bus’ links up quantum bits
http://physicsworld.com/cws/article/news/31300

Two independent groups in the US have created “buses” for transferring information between two microchip-based qubits. The buses could allow a number of qubits to be joined together to make powerful quantum computers using standard chip manufacturing processes.

The basic unit of information in a quantum computer is the qubit, which can take the value 0, 1 or — unlike a classical bit — a superposition of 0 and 1 together. When many of these qubits are combined or “entangled” together a quantum computer can process them simultaneously, enabling it to work exponentially faster than its classical counterpart for certain operations.

To achieve this entangling feat, quantum computers need to link remote qubits via a bus that can transmit their states to and fro. Although buses have already been created for trapped ions and atoms — two of the many realizations of qubits — a bus for a superconducting qubit had yet to be realized. Superconducting qubits are particularly promising for practical quantum computers because the whole system can potentially be printed onto a circuit in a similar way to those in present-day computers.

Raymond Simmonds and colleagues from the National Institute of Standards and Technology (NIST) have fabricated two superconducting “phase” qubits — each a super-cooled insulating barrier sandwiched by a pair of tiny metal grains — separated by a bus in the form of a cavity that contains a standing wave. They first prepare one of the qubits in the desired 0, 1 or superposition state with a microwave pulse that changes the quantum oscillations of the phase difference between the barrier’s electrodes. The researchers then use an external field to briefly tune the energy difference across the barrier so that the qubit resonates with the cavity and transfers its state to the standing wave, where it can be stored for up to 10 ns. At the other end of the cavity, the same tuning process transfers the state to the other qubit (Nature 449 438).

Meanwhile, Robert Schoelkopf and colleagues from Yale University used microwaves to prepare superconducting “charge” qubits, which are physically similar to phase qubits but have states defined by the number of paired-up electrons that have tunnelled across the barrier. Their cavity has no initial standing wave, instead relying on the electron-pair tunnelling itself to emit a virtual photon into the cavity and create a combined superposition state between the qubits. This is a similar technique to work performed by the Yale group last week, in which they showed that a superconducting qubit could be used as a single photon source (Nature 449 443).

The three operations — information transfer, storage and combined superposition — by the NIST and Yale groups could eventually underlie the gate operations required to perform calculations in a microchip-based quantum computer. However, this will require many superconducting qubits to be linked together, and in a much more reliable way than the current devices.

Berry’s phase seen in solid-state qubit

Berry’s phase seen in solid-state qubit
http://physicsworld.com/cws/article/news/31942

An international team of physicists is the first to show how information can be stored and manipulated in a solid-state quantum bit using “Berry’s phase” – an esoteric geometrical property of a quantum system.

The team controlled the Berry’s phase of paired electrons in a tiny piece of superconductor by exposing it to pulses of microwave radiation. The breakthrough could help physicists overcome a major barrier to practical quantum computing – the tendency of quantum bits to lose their quantum information content rapidly over time.

If a classical particle such as a stone undergoes a cyclic process – it is heated slightly and then cooled to its original temperature, for example – there is no way of telling from the cooled stone how it was heated, or even if it was heated at all. However, the same does not apply to quantum particles such as electrons, which retain some “memory” of the path taken in a cyclic process. This memory is in the form of a difference in phase between the initial and final quantum states and was first proposed by Michael Berry in 1984.

Berry’s phase is a “geometrical” effect that occurs in the abstract space defined by the orthogonal quantum states of a system. A key property of Berry’s phase is that it is not dependent on the path taken through this space, but only on the area enclosed by the loop. It turns out that this could be very useful to those designing quantum computers in which data are stored and processed in terms of quantum bits (or qubits) of information.
This is because phase plays an important role in quantum information, and manipulating the phase of a qubit corresponds to performing a logical operation. By cycling the system around a closed loop, Berry’s phase, and hence geometry, can be used to perform calculations.

A shaky hand

The manipulation of any qubit requires contact with the outside world. No matter how carefully this is done, the qubit is subjected to small amounts of noise, which could eventually destroy the quantum nature of the qubit, rendering it useless. However, the effect of noise on manipulating the Berry’s phase of a qubit can be likened to drawing a circle with a shaky hand – as long as the curve joins up and encloses the correct area, the manipulation of the qubit will be sound.
Quantum operations based on Berry’s phase have already been achieved in nuclear magnetic resonance and trapped-ion systems. However, these are large and unwieldy technologies and many physicists believe that it will be difficult to assemble them into practical quantum computers. However, solid-state qubits based on superconductors could, in principle, someday be miniaturized and mass-produced.

Now, Peter Leek and colleagues at ETH Zürich along with researchers in Canada and the US have demonstrated the first solid-state qubit based on Berry’s phase. Their results are reported today in Sciencexpress .

Aluminium qubit

The team’s qubit is a micrometre-sized piece of aluminium, which is a superconductor at very low temperatures. In its lowest energy state the superconductor contains a certain number of paired electrons and its first excited state contains that number plus one pair.
The aluminium was placed at the centre of a millimetre long superconducting wire that functioned as a microwave resonator. The resonant frequency of the resonator was set to be very different from the transition frequencies of the qubit – which served to isolate the qubit from its surrounding environment. However, the resonant frequency changes slightly depending upon the state of the qubit, allowing the researchers to monitor the state of the qubit by injecting a single microwave photon into the resonator.

The team then applied a number of different microwave signals to the resonator. Each signal caused the qubit to follow a path that enclosed a different area. This resulted in a number of different angles of Berry’s phase, which were measured using the single-photon technique.

“This is important research and I am very happy that it has been done”, said Jiannis Pachos of the UK’s University of Leeds, who is a proponent of Berry’s phase for quantum computing. Pachos told physicsworld.com that the next step is for physicists to build a solid-state two-qubit logic gate based on Berry’s phase – something that has already been done for ion-trap qubits.

This, however could prove difficult: “The control of solid-state two-qubit systems is lagging behind other systems”, said Pachos. Peter Leek told physicsworld.com that the team “are thinking of ways to do this at the moment”.

Smith-Purcell effect

Smith-Purcell effect
From Wikipedia, the free encyclopedia

The Smith-Purcell effect was the precursor of the free electron laser (FEL). It was studied by Steve Smith, a graduate student under the guidance of Edward Purcell. In their experiment, they sent an energetic beam of electrons very closely parallel to the surface of a ruled optical diffraction grating, and thereby generated visible light. Smith showed there was negligible effect on the trajectory of the inducing electrons. Essentially, this is a form of Cherenkov radiation where the phase velocity of the light has been altered by the periodic grating.

notes:
*precursor : a substance from which another substance is formed (especially by a metabolic reaction);
*diffraction grating: optical device consisting of a surface with many parallel grooves in it; disperses a beam of light (or other electromagnetic radiation) into its wavelengths to produce its spectrum;
*phase velocity : velocity at which a wave crest propagates ;
*Čerenkov radiation (also spelled Cerenkov or Cherenkov) is electromagnetic radiation emitted when a charged particle (such as an electron) passes through an insulator at a speed greater than the speed of light in that medium. The characteristic "blue glow" of nuclear reactors is due to Čerenkov radiation. It is named after Russian scientist Pavel Alekseyevich Čerenkov, the 1958 Nobel Prize winner who was the first to characterise it rigorously.
*nuclear reactor:(physics) any of several kinds of apparatus that maintain and control a nuclear reaction for the production of energy or artificial elements;

Phys. Rev. Lett. 101, 080502 (2008)

Phys. Rev. Lett. 101, 080502 (2008)
http://www.eng.yale.edu/rslab/index.html

Controlling the Spontaneous Emission of a Superconducting Transmon Qubit

We present a detailed characterization of coherence in seven transmon qubits in a circuit QED architecture. We find that spontaneous emission rates are strongly influenced by far off-resonant modes of the cavity and can be understood within a semiclassical circuit model. A careful analysis of the spontaneous qubit decay into a microwave transmission-line cavity can accurately predict the qubit lifetimes over 2 orders of magnitude in time and more than an octave in frequency. Coherence times T1 and T*2 of more than a µs are reproducibly demonstrated.

Coherence poses the most important challenge for the development of a superconducting quantum computer. As the dephasing time T *2 can never exceed twice the relaxation time T1, it is the relaxation time which ultimately sets the limit on qubit coherence. Although T *2 turned out to be small compared to T1 in the earliest superconducting qubits [1], steady progress over the last decade has significantly reduced this gap [2, 3, 4, 5, 6]. Recently, the transmon, a new type of qubit immune to 1/f charge noise, has been shown to be nearly homogeneously broadened (T *2 \=2T1) [6]. Therefore, under-standing relaxation mechanisms is becoming critical to further improvements in both T1 and T *2 . Progress in this direction will be based on the accurate modeling ofcontributions to T1 and the reliable fabrication of many qubits reaching consistent coherence limits.

One of the main advantages of superconducting qubits is their strong interaction with the wires of an electrical circuit, making their integration with fast control and readout possible and allowing for large, controllable couplings between widely separated qubits [7]. The large coupling also implies a strong interaction between the qubits and their electromagnetic environment, which can lead to a short T1. However, careful control of the coupling to the environment has been shown to allow prevention of circuit dissipation [8, 9]. Relaxation times have been studied in a wide variety of superconducting qubits, created with different fabrication techniques, and measured with a multitude of readout schemes. Typically, values of T1 vary strongly from sample to sample as they can depend on many factors including materials, fabrication, and the design of both readout and control circuitry. In some instances a separation of these compo-nents has been achieved [10, 11, 12, 13], but typically it is difficult to understand the limiting factors, and T1 often varies strongly even among nominally identical qubit samples.

Here, we demonstrate that in a circuit quantum elecodynamics (QED) architecture, where qubits are emedded in a microwave transmission line cavity[3, 14], transmon qubits have reproducible and understandable relaxation times. Due to the simple and well-controlled fabrication of the qubit and the surrounding circuitry, involving only two lithography layers and a single cavity forboth control and readout, we are able to reliably understand and predict qubit lifetimes. This understanding extends to a wide variety of different qubit and cavity parameters. We find excellent agreement between theory and experiment for seven qubits over two orders of magnitude in relaxation time and more than an octave infrequency. The relaxation times are set by either spontaneous emission through the cavity, called the Purcell effect [15], or a shared intrinsic limit consistent with a lossy dielectric. Surprisingly, relaxation times are often limited by electromagnetic modes of the circuit which are far detuned from the qubit frequency. In the circuitQED implementation studied here, the infinite set of cavity harmonics reduces the Purcell protection of the qubitat frequencies above the cavity frequency.

Generally, any discrete-level system coupled to the continuum of modes of the electromagnetic field is subject to radiative decay. By placing an atom in a cavity, the rate of emission can be strongly enhanced [15, 16] or suppressed [17, 18, 19], depending on whether the cavity modes are resonant or off-resonant with the emitter’s transition frequency. This effect is named after E. M. Purcell [15], who considered the effect of a resonant electrical circuit on the lifetime of nuclear spins. Suppression of spontaneous emission provides effective protection from radiative qubit decay in the dispersive regime, where qubit and cavity are detuned [14]. Specifically, the Purcell rate for dispersive decay is given by γ_κ = (g/∆)^2*κ, where g denotes the coupling between qubit and cavity mode, ∆ their mutual detuning, and κ the average photon loss rate.

The suppression and enhancement of decay rates canalternatively be calculated within a circuit model. For concreteness, we consider the case of a qubit capacitively coupled to an arbitrary environment with impedance Z_0(ω), see Fig. 1(a). This circuit may be reduced to a qubit coupled to an effective dissipative element, see Fig. 1(b). Specifically, replacing the coupling capacitor C_g and the environment impedance Z_0 by an effective resistor R = 1/Re[Y (ω)], one finds[8, 9] that the T1 is given by RC, where C is the qubit capacitance. Choosing a purely resistive environment, Z_0 = 50Ω, yields adecay rate γ \= ω^2*Z_0*C^2_g /C. If instead we couple to a parallel LRC resonator, the calculated radiation rate can be reduced to that of the atomic case, γ_κ = (g/∆)^2*κ, thusreproducing the Purcell effect.

Lamb shift spotted in solid qubit

Lamb shift spotted in solid qubit

The Lamb shift was seen in a transmon qubit

A tiny shift in quantum energy levels usually associated with individual atoms has been seen in a solid for the first time by physicists in Switzerland and Canada. The team spotted the Lamb shift in a small piece of superconductor that functions as a quantum bit or “qubit”.
The interactions that cause the Lamb shift are also responsible for making qubits unstable, and therefore the team believes that insights from their experiments could be used to create more robust qubits that could be used in quantum computers.
The Lamb shift is a tiny change in certain atomic energy levels. It occurs because the atom is interacting with the empty space surrounding it by absorbing and emitting “virtual” photons. Discovered in 1947 by the American physicist Willis Lamb, the shift provided important experimental evidence for the then emerging theory of quantum electrodynamics (QED), which describes the interaction of charged particles in terms of the exchange of photons.
While the Lamb shift should also affect electrons in a solid, it has proven difficult to see because electron energy levels in solids are wide bands, rather than discrete atomic levels.

Shifting transmon
Now, Andreas Wallraff and colleagues at ETH Zurich in Switzerland and the University of Sherbrooke in Quebec have spotted the Lamb shift in the energy levels of a qubit called a “transmon”, which is made from two tiny pieces of superconductor connected by two tunnel junctions (Science 322 1357).
The superconductor contains a large number of “Cooper pairs” of electrons that can move through the material without any electrical resistance. The energy levels of the qubit are defined by the precise distribution of Cooper pairs between the two tiny pieces of superconductor.
The team’s transmon is placed in a microwave cavity and its shape was chosen to give it a large electrical dipole moment. This increases the strength at which it interacts with both microwave photons and the virtual photons of the vacuum. In addition, the shape and size of the cavity were designed to enhance the photon’s electric field in the region of the qubit.
Transitions between qubit energy levels occur when electrons in the superconductor collectively absorb or emit photons at certain wavelengths. This process can be enhanced by tuning the frequency of microwave radiation injected into the cavity so that a single photon of the correct wavelength bounces back and forth across the qubit many times.
In their experiment, the team used a cavity to enhance the effect of the virtual photons related to the Lamb shift — which makes it more likely that the qubit absorbs and emits virtual photons. Indeed, Wallraff and colleagues measured a cavity-enhanced shift of 1% in the difference between the two energy levels. This is 10,000 times greater than the Lamb shift seen in hydrogen without a cavity.

Tricky measurement
Despite its relative magnitude Wallraff told physicsworld.com that the tricky part of the experiment was measuring the shift. This is because any measurement on the qubit must be made using photons as a probe — and their presence in the waveguide could cause a shift in the energy levels (the a.c. Stark effect), which would overwhelm the Lamb shift.
To get around this problem, the team used a very small number of probe photons that were off-resonance with the cavity. This means that they remain in the region of the qubit only long enough to measure the transition energy but not cause any a.c. Stark shifts.

The discovery of such a large Lamb shift is a mixed blessing for those trying to design practical qubits. On one hand, the virtual photons induce spontaneous emission in qubits, which limits their usefulness for quantum computing. On the other hand, Wallraff and colleagues have established that the Lamb shift can be minimized in a transmon qubit if it is set far from resonance with the virtual photons. This suggests a way of making qubits more robust, as demonstrated in a recent work by a Robert Schoelkopf and colleagues at at Yale University (Phys Rev Lett 101 080502).
Detecting the Lamb shift in a solid system also suggests the possibility of seeing the effects of other virtual particles such as phonons — which are quantized vibrations in solids. According to Wallraff, such an acoustical Lamb shift due to the mechanical quantum fluctuations of nanometer-scale electromechanical oscillators could similarly affect the energy levels of a qubit.
About the author
Hamish Johnston is editor of physicsworld.com

Qubits are on solid ground

Qubits are on solid ground
http://physicsworld.com/cws/article/news/16975

Physicists in Japan, the Netherlands and the US have taken important steps towards building a quantum computer. Jaw-Shen Tsai, Yasunobu Nakamura and colleagues at the RIKEN and NEC laboratories in Japan and the State University of New York at Stony Brook have “entangled” two quantum bits or “qubits” in a solid-state device for the first time (YA Pashkin et al. 2003 Nature 421 823). Meanwhile, Irenel Chiorescu and co-workers at the Delft University of Technology, working with Nakamura, have demonstrated a new type of superconducting qubit (I Chiorescu et al. 2003 Sciencexpress 1081045).

Photons, atoms or trapped ions can be used as qubits but it should be easier to build working devices using solid-state qubits. Quantum computing works on two basic quantum mechanical principles. The first is the superposition of states, which is a one-particle phenomenon. The second is entanglement, which involves two or more particles.
The spin of a particle can point in two opposite directions, “up” and “down”, but the particle can also exist in a superposition of these states. This superposition also holds true for two-particle states including entangled states. When two particles are entangled they behave as one, regardless of how far apart they are.
For quantum computing to work, however, these entangled states must be made to interact in a controlled manner.
Tsai and co-workers used micron-sized “boxes” of superconducting material that were connected to a Josephson junction - a type of superconducting “reservoir” - via a capacitor. A Cooper pair of electrons can tunnel from the junction onto the box. The box is the qubit, which can exist in two states: one state has an excess of Cooper pairs while the other has no excess Cooper pairs. The qubits are made to interact using the capacitor, which leads to a mixing of two-particle states and thus entanglement of the qubit pair.
Although the team has not yet measured a specific entangled state, they have shown that the qubit pair is strongly entangled. “This result shows that it is indeed possible to construct a quantum logic gate using such a solid-state device,” Tsai told PhysicsWeb. “A quantum computer could be made using such gates as the basic units.”
The superconducting flux qubit developed by the Delft-NEC team, on the other hand, comprises three Josephson junctions in a loop. The two quantum states in this system are macroscopic currents consisting of billions of Cooper pairs travelling around the loop in opposite directions. The qubit can undergo hundreds of oscillations between these two states, and can be read with a superconducting quantum interference device.
About the author
Belle Dumé is Science Writer at PhysicsWeb

Quantum entanglement gets a laser-like lift

Quantum entanglement gets a laser-like lift
http://physicsworld.com/cws/article/news/2612

Lasers have been used to amplify light for many years, but physicists have now achieved a similar feat with pairs of 'entangled' photons for the first time. The phenomenon could lead to a reliable method for creating such pairs, which could be the basis of future quantum computers and encryption techniques. Antia Lamas-Linares and co-workers at the University of Oxford, UK, exploited quantum effects to boost the number of entangled photons created when an ultraviolet laser passes through a crystal (A Lamas-Linares et al 2001 Nature 412 887).

Under certain circumstances, an ultraviolet photon can spontaneously split into two lower-energy infrared photons - this is known as down-conversion. The polarizations of these two photons are intimately related: a measurement of the polarization of one photon would reveal the polarization of the other, even if they were widely separated. This is an example of 'entanglement' - a correlation that can exist between quantum particles that is much stronger than those allowed in classical physics.
But entangled photon pairs of this kind arise rarely in ultraviolet beams. In order to create more pairs, Lamas-Linares and colleagues shone a pulsed ultraviolet laser through a crystal of barium borate. As expected, one of the millions of photons split into two infrared photons via the down-conversion process. These photons left the crystal at an angle to the direction of travel of the laser pulse, and mirrors then reflected them back into the crystal. Meanwhile, the laser pulse that passed through the crystal was also reflected back towards it. The mirrors were arranged so that the reflected laser pulse reached the crystal at exactly the same time as the entangled photons.
The quantum interaction of the entangled photons and the reflected pulse sparked the production of another pair of entangled photons. Classically, this would lead to two photon pairs, but because this is a quantum interference process, it can produce a maximum of four pairs of photons or a minimum of zero. This is analogous to the reinforcement or cancellation of light waves in a diffraction pattern, and can result in a four-fold increase in the number of entangled photons produced. The phenomenon can also multiply the number of entangled photons by sixteen if it is applied to an even rarer system composed of four entangled photons.
"Currently available sources of entangled photons are extremely weak, but the laser action for entangled photons can produce very bright sources of entangled photon pairs", Lamas-Linares told PhysicsWeb. "Laser action will also create far more complicated entangled states that involve many photons and are likely to play an important role in the realization of several recent theoretical developments in quantum information."
The amplification demonstrated by Lamas-Linares and co-workers is analogous to the light that bounces between the mirrors at the ends of a laser cavity. In practice, however, the light in a conventional laser cavity undergoes many reflections, whereas the initial entangled pair in the Oxford experiment is reflected only once.
"We are now working on a system in which the laser passes through the crystal many times", says Lamas-Linares. The refined set-up could lead to fluxes containing up to 100 entangled photons.
About the author
Katie Pennicott is Editor of PhysicsWeb

Entanglement reaches new levels

Entanglement reaches new levels
http://physicsworld.com/cws/article/news/23734


Two rival teams of physicists in the US and Austria have succeeded in entangling the largest number of particles ever. Dietrich Leibfried and colleagues at the National Institute of Standards and Technology (NIST) in Colorado have entangled six beryllium ions while Hartmut Haffner and co-workers at Innsbruck University have independently entangled eight calcium ions. The results are the latest step on the long road to large-scale quantum computers and may also be important for quantum cryptography and ultra-sensitive measurement techniques.
Entangled ions
Entanglement allows particles to have a much closer relationship than is possible in classical physics: if two particles are entangled, we can know the state of one particle by measuring the state of the other. For example, two particles can be entangled such that the spin of one particle is always "up" when the spin of the other is "down", and vice versa. An additional feature of quantum mechanics is that the particle can exist in a superposition of both these states at the same time. By taking advantage of such quantum phenomena, a quantum computer could, in principle, outperform a classical computer for certain tasks.
Using lasers and ultra-cold electromagnetic traps, the NIST scientists entangled six beryllium ions so that all their nuclei were collectively spinning in both clockwise and anticlockwise directions at the same time (Nature 438 639). These states are also known as "cat" states after Erwin Schrödinger's famous thought experiment in which a cat was somehow both alive and dead at the same time. Using similar techniques, the Austrian scientists entangled eight calcium ions that were more robust because they were stable even if some particles were removed (Nature 438 643).
The new results break the previous record of five entangled photons achieved last year. Moreover, the entangled states can be produced "on demand" and made available for further tasks without being destroyed -- something that has never been done before. The number of particles entangled could be increased even further, leading the way to large-scale quantum computers.
Cat states could be used to correct errors in quantum computation and so make fault-tolerant quantum computers. These entangled states are also more sensitive to decoherence -- the transition from quantum to classical behaviour that occurs when the particles interact with their environment -- than other types of superpositions. They could therefore be useful in applications such as precision spectroscopy and quantum cryptography, which allows data to be transmitted with complete security.
About the author
Belle Dumé is science writer at PhysicsWeb

Anton Zeilinger scoops first Isaac Newton medal

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Anton Zeilinger scoops first Isaac Newton medal

The Austrian physicist and quantum-computing pioneer Anton Zeilinger has been awarded the inaugural Isaac Newton medal by the Institute of Physics. Zeilinger was honoured for "his pioneering conceptual and experimental contributions to the foundations of quantum physics, which have become the cornerstone for the rapidly-evolving field of quantum information".
Anton Zeilinger
Zeilinger has pioneered the study of entanglement — a property of quantum theory that allows two or more particles to display much stronger correlations than are possible in classical physics. Entanglement is what, in principle, could allow quantum computers to outperform conventional computers at some tasks.
His achievements include the first demonstration of quantum communication based on the entanglement of photons in 1995, the first "quantum teleportation" in 1997 and the first quantum cryptography performed with entangled photons in 2000. Earlier this year he led a team that managed to transmit an entangled photon 144 km in free-space, opening the door to satellite-based quantum communication.
Zeilinger, 62, was born in Austria and is currently professor of experimental physics at the University of Vienna as well as scientific director of the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences.
The Isaac Newton Medal is one of ten new honours launched for 2008 by the Institute. Awarded for "outstanding contributions to physics", the medal differs from the Institute's other 23 awards in not being restricted to physicists working in the UK or Ireland, or to those with strong connections to the two countries.