Progress in Quantum Computing IQSA | LT₂₅ | Lorentz Center – I've recently returned from a series of international conferences and workshops on superconductivity, quantum computation, entanglement and quantum coherence. In Sopot, Poland at the International Conference on Quantum Structures, much of the week was spent in long walks on the shores of the Baltic Sea, holding intense discussions on quantum information theory with Lev Levitin, who will be hosting the IQSA meeting at MIT in two years. We also continued ongoing research with Roman Zapatrin (Starlab) to advance the development of adaptive quantum networks for applications in fault-tolerant quantum computation, associative processing and pattern recognition.

Following IQSA, I moved on to the 25^th triennial International Conference on Low-temperature Condensed Matter Physics, where I met with Keith Schwab following presentation of his group's recent experiments with nanomechanical resonators to probe the boundaries of quantum and classical regimes, as well as discussing present and upcoming experiments in superconducting flux qubit systems with Yasu Nakamura, John Clarke, Robert Schoelkopf, and John Martinis.

Upon conclusion of LT₂₅, a satellite conference on Quantum Decoherence in Quantum Information Systems was held at the Lorentz Center, where I met with Vlatko Vedral to discuss long-term research initiatives in multipartite and macroscopic entanglement in condensed matter systems. Jasper van Wezel presented a review of the limits to quantum behavior related to spontaneous symmetry breaking – summarizing recent results on the quantum to classical transition, and future experiments which may elucidate the process of wavefunction collapse. Dirk Bouwmeester was generous enough to offer a tour of the experimental laboratory setup for the MiniGRAIL gravitational wave detector, which has just undergone several modifications, including improvements to the antenna, cryogenic cool-down systems, improved shielding, redesign of the capacitive transducer and fabrication of a new two-stage SQUID module for more stable operation at low temperatures.

Space QUEST: Experiments with quantum entanglement in space Vienna | ESA | ISS In a recent submission to the arXiv,  Zeilinger's group at University of Vienna, Austria has proposed an experiment—Space-QUEST, Quantum Entanglement Science and Technology—for space-to-ground, entangled-photon Bell Inequality violation measurements to verify quantum nonlocality at distances over thousands of kilometers, in a joint operation between the International Space Station and a ground observatory in the European Union.

Entanglement and nonlocality have been pivotal controversies since the birth of quantum mechanics—Einstein's "spooky action at a distance" implies simultaneous, nonlocal correlations between separate entangled particles. J. S. Bell was the first to confirm the phenomenon experimentally in 1964.

Further refinements and increasing precision in succeeding experiments have consistently shown quantum mechanics to be an explicitly nonlocal theory—the outcome Einstein was most averse to accept. However, long-distance relativistic experiments, such as between orbiting satellites, have been technologically cost-prohibitive to date. The paper will be presented at the 2008 IAC Microgravity Sciences and Processes Symposium, under a proposed joint initiative between the European Space Agency and the International Space Station.

"Testing quantum correlations over distances achievable with systems placed in the Earth orbit, or even beyond, would allow to verify both the validity of quantum physics and the preservation of entanglement over distances impossible to achieve on the ground. Using the large relative velocity of two orbiting satellites, one can perform experiments on entanglement where – due to special relativity – both observers can claim that they have performed the measurement on their system prior to the measurement of the other observer. In such an experiment, it is not possible anymore to think of any local realistic mechanisms that potentially influence one measurement outcome according to the other one."

Zeilinger's group has previously conducted proof-of-principle experiments in the Canary Islands with a 144 km free-space link, using an ESA receiver telescope to receive single entangled photons, cf. Nature Physics, 3:481-486 (2007). A more recent experiment in Italy has demonstrated single-photon downlink communications viability from a near-earth orbit satellite, cf. New Journal of Physics, 10:033038 (2008).

The Reality Tests Vienna In Seed (June 2008), the Vienna experimental group discusses physical and philosophical implications of new correlations between entangled photons, which violate an inequality proposed by Leggett for nonlocal realistic theories. This new series of experiments invalidates macrorealism in quantum mechanics by more than 80 orders of magnitude. Preliminary coverage of the experimental results was first presented in Nature 446 (871) and PhysicsWorld, 20 April 2007. According to Časlav Brukner: "Quantum mechanics does not always wash itself out – but to observe its effects for larger and larger objects, we would need more and more accurate measurement devices. We just do not have the sensitivity to observe the quantum effects around us. In essence, we do create the classical world we perceive. There could be other classical worlds completely different from ours."





Quantum networks: Entanglement of distant atoms by projective measurement University of Barcelona | ICFO | Spain Quantum cryptography is rapidly developing into a mature and robust technology for secure data transactions in financial, government and military sector applications. In arXiv 0806.1052, Zippilli et al. quantify the role of photon detector efficiency in quantum repeaters, which will be necessary to scale beyond the point-to-point networks currently employed for secure communications.

Presently, state-of-the-art systems employ atom-photon interaction to generate entanglement between distant nodes across a quantum network through projective measurement. "We assess proposals for entangling two distant atoms by measurement of emitted photons, analyzing how their performance depends on the photon detection efficiency – we believe that these concepts are generally applicable to all systems that may be considered for the creation of distant entanglement, including atomic-ensemble, photonic, and solid state implementations."

The groups's objectives are to quantify the importance of detector efficiency as applied to generating remote entanglement across quantum networks. With minor modifications, these results can be extended to the efficiency of quantum teleportation protocols that are also based on projective quantum measurement. "In all such systems, the detection efficiency will have a similar, important role for the use of the entanglement as a resource in quantum technologies."


Space-QUEST: Experiments with quantum entanglement in space Vienna | ESA | ISS In a recent submission to the arXiv,  Zeilinger's group at University of Vienna, Austria has proposed an experiment – Space-QUEST, Quantum Entanglement Science and Technology – for space-to-ground, entangled-photon Bell Inequality violation measurements to verify quantum nonlocality at distances over thousands of kilometers, in a joint operation between the International Space Station and a ground observatory in the European Union.
 
Entanglement and nonlocality have been pivotal controversies since the birth of quantum mechanics – Einstein's "spooky action at a distance" implies simultaneous, nonlocal correlations between separate entangled particles. J. S. Bell was the first to confirm the phenomenon experimentally in 1964.

Further refinements and increasing precision in succeeding experiments have consistently shown quantum mechanics to be an explicitly nonlocal theory – the outcome Einstein was most averse to accept. However, long-distance relativistic experiments, such as between orbiting satellites, have been technologically cost-prohibitive to date. The paper will be presented at the 2008 IAC Microgravity Sciences and Processes Symposium, under a proposed joint initiative between the European Space Agency and the International Space Station. 
 
"Testing quantum correlations over distances achievable with systems placed in the Earth orbit, or even beyond, would allow to verify both the validity of quantum physics and the preservation of entanglement over distances impossible to achieve on the ground. Using the large relative velocity of two orbiting satellites, one can perform experiments on entanglement where – due to special relativity – both observers can claim that they have performed the measurement on their system prior to the measurement of the other observer. In such an experiment, it is not possible anymore to think of any local realistic mechanisms that potentially influence one measurement outcome according to the other one."

Zeilinger's group has previously conducted proof-of-principle experiments in the Canary Islands with a 144 km free-space link, using an ESA receiver telescope to receive single entangled photons [Nature Physics, 3:481-486 (2007)]. A more recent experiment in Italy has demonstrated single-photon downlink communications viability from a near-earth orbit satellite [New Journal of Physics, 10:033038 (2008)].

Superconducting Qubits RIKEN | UBC | Sherbrooke – In arXiv 0805.0164, Zagoskin and Blais provide a broad and accessible introduction to quantum information processing with superconducting qubits. "From a physicist's standpoint, the most interesting part of quantum computing research may well be the possibility to probe the boundary between the quantum and the classical worlds. The more macroscopic are the structures involved, the better. So far, the most "macroscopic" qubit prototypes that have been studied in the laboratory are certain kinds of superconducting qubits. To get a feeling for how macroscopic these systems can be, the states of flux qubits which are brought in a quantum superposition corresponds to currents composed of as much as 10⁵ - 10⁶ electrons flowing in opposite directions in a superconducting loop."

Efficient pulsed gates for an oscillator stabilized Josephson qubit IBM Watson In arXiv 0709.1478 and New J. Phys. 10 033027 (2008), Koch, DiVincenzo, Brito and Steffen derive operational specifications for high-fidelity one and two-qubit pulsed gates for a superconducting flux qubit, calculating the Hamiltonian with tunable interaction from initialization to readout.

"The quantitative fact that the values of gate infidelity are at the 1% level – and below – is the major result of this paper."

So, can a "debugged" IBM qubit be used soon for universal quantum computation?

"The short answer is, in our opinion, ultimately yes."

"The answer would certainly be no if the noise threshold for fault-tolerant quantum computation were in the neighborhood of the oft-quoted value of 10⁻⁵. It is not inconceivable for the experiment to get to these values someday, since we find that the infidelities decrease much faster than linearly with the assumed noise levels."

"To get to 10⁻⁵, we would need to get to the very daunting levels of 100nΦ₀ at 1Hz for the 1/f noise amplitudes and 100 f s for timing accuracies; there is optimism that both of these numbers are ultimately attainable. Fortunately, while 10⁻⁵ was the threshold as it was understood ten years ago, much recent work shows that with good designs, much higher thresholds are possible. According to Terhal and Burkard – 1% is, in fact, on the high end of the noise levels for which fault tolerance may be possible."

Photon transmission through sub-wavelength diameter apertures Delft | Optica In Optics Express 16, 10 (abstract, full article) and concurrent TU Delft summary, Photonics review, Adam, Planken et al. report on high time-resolution terahertz mapping of photon transmission through sub-wavelength diameter apertures:

"According to the laws of physics, it is particularly difficult to pass light through a hole smaller than half the wavelength of the light used." The Delft group conducted experiments using extremely high time-resolution measurements in the terahertz (THz) frequency range. The group discovered that even if the hole is up to fifty times smaller than the wavelength used, sufficient light can pass through to allow measurements near the hole – an extremely difficult task using other methods. "Improving the sharpness of THz microscopes, coupled with more sensitive detectors, will improve the viability of creating images of biological cells using this type of measurement."

Prior experiments at Leiden University (Nature 418, 304-306) have also studied photon transmission through sub-wavelength metal films and shown entanglement conservation to be much more robust than expected – surviving the conversion process from surface-plasmon waves, which tunnel through the barrier, before reradiating as photons on the opposite side of the film. "It's a good omen, because it's saying quantum entanglement can survive when you might not expect it to," says Bill Barnes, a photonics expert at the University of Exeter. "If they can survive this, what else can they survive?"

Time Reversal in Bose-Einstein Condensates Toulouse | CNRS In arXiv 0804.3514, Martin, Georgeot, and Shepelyansky of Quantware MIPS Center investigate time reversibility in Bose-Einstein condensates (BEC). "We show that inside the regime of quantum chaos, time-reversal dynamics can be inverted from explosion to collapse. The accuracy of time reversal decreases with the increase of atom interactions inside BEC, until it is completely lost – though, surprisingly, quantum chaos helps to restore time reversibility. Existing experimental setups similar to Ryu, Behinaein, and Wayper can test the fundamental question of BEC time reversal discussed here."