Ions keep their cool at crossroads

Apr 15, 2009

Ions keep their cool at crossroads


Down at the crossroadsPhysicists in the US have created a junction through which single ultracold ions can pass without having their temperature raised. The junction is contained within a 2D ion trap and could be useful in building large-scale quantum computers.

Quantum computers, like classical computers, work by processing of bits of information. In classical computers such bits can take on only the values 0 and 1, but in quantum computers they can also take on “superpositions” of both 0 and 1. When many of these quantum bits or “qubits” are combined, a quantum computer can process them simultaneously. In principle, this would enable a quantum computer to work exponentially faster than its classical counterpart for certain operations — however many technical challenges must be overcome before practical quantum computers become a reality.

Scientists are working on many devices to take on the role of qubits, but one of the most promising are trapped ions. Inside a trap, the position and ordering of the ions could be changed by running them through a junction, at which point they can be encouraged to go in one direction or the other. However, it is important that this switching does not result in any heating, because that tends to take the ions out of the required electronic ground state.

Off the heat
Now, Brad Blakestad and colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colorado have created a junction in an ion trap in which there is practically no heating. Constructed from laser-machined alumina, it contains 46 gold-coated electrodes surrounding an X-shaped junction. When the researchers apply a series of voltages to the electrodes, ions are encouraged through the junction a little at a time.

The NIST group managed to get ions through the junction with a 99.99% success rate, and with seven orders of magnitude reduced heating than previous trapped ion systems .

Christopher Monroe, a physicist at the University of Maryland in the US who has worked extensively with trapped ion systems, told physicsworld.com that although it is not surprising that the researchers have shuttled ions without losing coherence of their internal states, it is surprising that the ions have not lost any coherence in their motion.

Exquisite control
“Moreover, to shuttle ions around a multitude of electrodes and around corners requires exquisite control of the applied electrical potentials so that the ions surf smoothly without getting lost (or agitated),” Monroe adds. “The NIST experiment accomplishes all of this, and thus marks an important milestone in one of the only known realistic architectures for a large-scale quantum computer.”


Bernhard Roth, a quantum physicist at the Heinrich Heine University of Düsseldorf, Germany, also thinks the work is important. “In particular the work might be relevant toward efficient large-scale quantum information processing, the main challenge in the field,” he says. “The authors have significantly increased the reliability of the ion transport through an array, have reduced energy gain and preserved coherence. These are all things which are considered essential for the realization large-scale systems.”

The research will be published in Physical Review Letters and a preprint of the paper is available on arXiv.

About the author
Jon Cartwright is a freelance journalist based in Bristol, UK

News and Views

News and Views

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Nature Physics 5, 248 - 249 (2009)
doi:10.1038/nphys1245


Subject Category: Quantum physics

Quantum physics: Schrödinger's cat is still alive
Jörg Wrachtrup1

Jörg Wrachtrup is at the 3rd Institute of Physics, University of Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany.
e-mail: wrachtrup@physik.uni-stuttgart.de



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AbstractStrong coupling between a mechanical oscillator and the spin of an electron could enable cooling of the oscillator to its quantum ground state and measurement of the zero-point fluctuations.

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News and Views
Nature 458, 580-581 (2 April 2009) | doi:10.1038/458580a; Published online 1 April 2009


Solid-state physics: Spin's lifetime extended

Jaroslav Fabian1

Top of pageAbstractElectrons in semiconductors are subject to forces that make their spins flip. According to new evidence, if an ensemble of spins curls into a helix, the collective spin lifetime can be greatly enhanced.

Over the past decade, electron spin — the electron's intrinsic rotation, which is commonly described as 'up' and 'down' and which gives rise to its magnetic moment — has come to the forefront of research in solid-state physics. A whole new field, called spintronics1, 2, 3, 4, has emerged as an umbrella for both applied and fundamental research on spin transport and spin control in metals and semiconductors.

Jaroslav Fabian is at the Institute for Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany.
Email: jaroslav.fabian@physik.uni-regensburg.de

The quantum-optical Josephson interferometer

Nature Physics 5, 281 - 284 (2009)
Published online: 22 March 2009 | doi:10.1038/nphys1223

0811.3762v1
Subject Categories: Quantum physics | Optical physics

The quantum-optical Josephson interferometer
Dario Gerace1,2, Hakan E. Türeci1, Atac Imamoglu1, Vittorio Giovannetti3 & Rosario Fazio3,4


Top of pageThe photon-blockade effect, where nonlinearities at the single-photon level alter the quantum statistics of light emitted from a cavity1, has been observed in cavity quantum electrodynamics experiments with atomic2, 3 and solid-state systems4, 5, 6, 7, 8. Motivated by the success of single-cavity quantum electrodynamics experiments, the focus has recently shifted to the exploration of the rich physics promised by strongly correlated quantum-optical systems in multicavity and extended photonic media9, 10, 11, 12, 13, 14. Even though most cavity quantum electrodynamics structures are inherently dissipative, most of the early work on strongly correlated photonic systems has assumed cavity structures where losses are essentially negligible. Here we investigate a dissipative quantum-optical system that consists of two coherently driven linear optical cavities connected through a central cavity with a single-photon nonlinearity (an optical analogue of the Josephson interferometer). The interplay of tunnelling and interactions is analysed in the steady state of the system, when a dynamical equilibrium between driving and losses is established. Strong photonic correlations can be identified through the suppression of Josephson-like oscillations of the light emitted from the central cavity as the nonlinearity is increased. In the limit of a single nonlinear cavity coupled to two linear waveguides, we show that photon-correlation measurements would provide a unique probe of the crossover to the strongly correlated regime.

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Institute of Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland
CNISM and Dipartimento di Fisica 'A. Volta', Università di Pavia, 27100 Pavia, Italy
NEST CNR-INFM and Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
International School for Advanced Studies (SISSA), via Beirut 2–4, 34014 Trieste, Italy
Correspondence to: Dario Gerace1,2 e-mail: gerace@fisicavolta.unipv.it

Relaxing the requirements for scalable quantum computing

Relaxing the requirements for scalable quantum computing

Fibonacci scheme for fault-tolerant quantum computation

Panos Aliferis and John Preskill

Phys. Rev. A 79, 012332 (Published January 30, 2009)


Quantum Information


To carry out a long calculation on a quantum computer, some form of error correction is necessary. If the error probability of each logical operation is below what is called the “fault-tolerance” threshold, an error correction procedure will actually remove more errors than it introduces, and the overall failure rate can then be made arbitrarily small. The fault-tolerant threshold is typically quoted as 10-4 or 10-5. This is an extremely stringent tolerance, since it says failure must occur in less than 0.01% of the operations.

A few years ago, Emanuel Knill at NIST in Boulder, Colorado, introduced a different approach to error correction that relied primarily on preparing and verifying a (possibly very large) number of auxiliary qubits, called ancillas, in special states that could be used to diagnose the errors in the computer’s qubits, and replace them if necessary. The most attractive feature of these codes was their large error tolerance, which, based on numerical simulations, Knill estimated to be of the order of 1%.

In a paper appearing in Physical Review A, Panos Aliferis, who is at the IBM Watson Research Center, and John Preskill of the California Institute of Technology, rigorously establish a lower bound for the fault-tolerance threshold for one of Knill’s constructions that has relatively small overhead requirements. Their results indicate that fault-tolerant computation should definitely be possible with this scheme, if the error probability per logical operation does not exceed 0.1%. While lower than Knill’s original numerical estimate, this analytical bound is still at least one order of magnitude larger than was thought possible with other codes and it makes the prospect of scalable quantum computing appear that much more feasible. – Julio Gea-Banacloche

Entanglement dies a sudden death

Entanglement dies a sudden death　2007-05-09 18:09

A strange quantum phenomenon that could be a stumbling block to building quantum computers has been observed for the first time by physicists in Brazil. Known as entanglement sudden death (ESD), it involves the rapid decay of the "entangled" pairs of particles that will be central to the operation of quantum computers. Since the particles decay so quickly, the physicists claim that the decay cannot be reversed using the error-correction schemes that have been proposed to increase the lifetimes of entangled particles (Science 316 579).

Decay channel


In the weird world of quantum mechanics, entanglement means that particles can have a much closer relationship than allowed by classical physics. For instance, two photons can be created experimentally 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.

Whereas ordinary computers use bits of information that are either 1 or 0, quantum computers use quantum bits of information, or qubits, that can be in a superposition of both 1 and 0 at the same time. A 1 could represent, say, a horizontally polarized photon, while 0 represent a vertically polarized photon. By combining N such qubits, these could entangled to represent 2N values at the same time, which would, in principle, allow a quantum computer to outperform a classical computer for certain tasks.

However, the qubits in any practical quantum computer have to interact with their local environments, which will cause the quantum state of the qubit to change, or decay. A photon reflecting from a mirror, for example, could suffer a change to its polarization, and successive interactions could even lead to the entanglement disappearing altogether. Crucially, the gradual nature of the decay means that it should be possible to restore entanglement during the computation process using error-correction schemes.

However, it had been predicted that interactions that appear to have a small effect on a single qubit can have a devastating effect on an entangled system of two qubits. This effect -- entanglement sudden death, or ESD -- is so rapid and complete that error-correction schemes will not be able to restore entanglement. Now, Luiz Davidovich and colleagues at the Federal University of Rio de Janeiro have observed ESD for the first time.


In their experiment, the researchers prepared entangled pairs of photons, which were then sent along two identical paths that were separated such that there could be no mutual interaction between the photons. Each path contained optical equipment that could be used to cause a deliberate and gradual decay of the vertical polarization component of both photons. The researchers then detected both photons with the aid of interference filters to determine their degree of entanglement – or concurrence.

The researchers studied pairs of photons that were entangled in two different ways: one type had a certain combination of horizontal and vertical polarizations, while the other type had a different combination of these polarizations. Both initial states were created with the same degree of entanglement and both were subjected to the same gradual decay of vertical polarization. It turned out that the entangled pairs that were more vertically than horizontally polarized underwent ESD, whereas the pairs that where the opposite was true decayed relatively slowly as expected. Davidovich reckons that the vertically-rich entanglement suffered ESD because in this experiment, vertical polarization is a higher energy state and is therefore more sensitive to decay via interactions with the environment than is the lower-energy state of horizontal polarization.

Davidovich told Physics Web that ESD should also occur in other systems that have been proposed for use in quantum computers including trapped ions and atoms in cavities. However, he does not believe that ESD precludes the development of quantum computers. “It leads to an upper limit for the duration of the quantum computation”, he said. “Calculations must be made faster than the time for which ESD occurs”.

Davidovich explained that ESD precludes the use of error correction: “Error-correction techniques rely on entanglement. ESD implies that the quantum computer becomes classical at a finite instant of time, after which quantum error correction is no longer possible”.

Single photons make the trek from space

Single photons make the trek from space

A team of Italian and Austrian scientists has shown it is possible to send single photons from a satellite to a receiving station on Earth. The work, carried out using the Matera Laser Ranging Observatory in southern Italy, paves the way for global quantum cryptography and more rigorous tests of quantum mechanics.

Quantum cryptography exploits the laws of quantum mechanics to create keys for encoding and decoding messages. These keys are strings of 1s and 0s, which are represented by the quantum states of individual subatomic particles, such as the polarization of photons. In principle quantum keys are uncrackable — this is because a measurement of a quantum system in general alters the state of that system. In other words, an eavesdropper situated between a sender and a receiver cannot intercept and identify a key without corrupting it.

Quantum cryptographic systems are already available commercially and have been used, for example, to make bank transfers. Indeed, physicists have shown how to transmit quantum keys over distances of more than 100 km by sending single photons either along optical fibres or via telescopes. Extending this range significantly is difficult, however — in optical fibres photon scattering causes unacceptably high losses, and telescopes are subject to atmospheric turbulence, which can distort a photon beam.

High expectations
Now, Paolo Villoresi of the University of Padova, along with other scientists in Italy and a group lead by Anton Zeilinger at the University of Vienna in Austria, have shown how to overcome these limitations by extending quantum cryptography into space. The Matera Laser Ranging Observatory is usually used to measure variations in the Earth’s gravity and motion, by measuring the time it takes for laser pulses to return to the observatory having been reflected off a passing satellite. Villoresi and colleagues employ the same basic technique but make the beam deliberately weak so less than one photon from each pulse returns to Earth. Transmitting individual photons is crucial for realizing quantum cryptography to prevent an eavesdropper siphoning off excess photons without altering the key.

By bouncing the beam off the Japanese Ajisai satellite, which orbits at an altitude of about 1500 km, the researchers calculate that they receive an average of just 0.4 photons per pulse (after taking into account losses such as the inefficiency of their photon detector). Crucially, by precisely calculating when each pulse is to return to the observatory (accounting for the changing position of the satellite), they are able to show that these detected photons are those transmitted by the telescope and not stray photons from background sources. “Not only have we shown that it is possible to detect single photons from a satellite, we have also demonstrated that we can do this using existing technology,” says Villoresi. “We are very happy about that.”