Restoring Quantum Coherence Pavia, Italy Decoherence remains the foremost limiting factor on practical implementation of quantum information technologies. In quant-ph 0504195, Buscemi et al. show that for qubit and qutrit systems it is always possible to recover quantum coherence by performing controlled measurements upon the environment, and that the minimal information required to invert qubit decoherence is equivalent to the von Neumann entropy exchange of the system.

Measuring Decoherence in a three-level rf SQUID Qubit U Kansas In cond-mat 0507008, Han et al. perform direct and quantitative measurements of dissipation-induced relaxation in a three-level rf SQUID qubit. "Analysis of the system indicates that the dominant sources of qubit dissipation are the flux bias and magnetometer readout circuits. Since this kind of dissipation-induced qubit decoherence can be greatly suppressed with more sophisticated designs we believe it does not impose a fundamental limit to this type of qubit [...] We are developing advanced designs for qubit bias and readout circuits that are predicted to decrease their contributions to the qubit damping by several orders of magnitude."

Josephson Bifurcation Amplifier for Quantum Measurements Yale In cond-mat 0507248, Devoret et al. construct a new type of amplifier for superconducting qubit readout based on the transition of an rf driven Josephson junction between two distinct oscillation states near a dynamic bifurcation point: "The main advantages of this new amplifier are speed, high-sensitivity, low back-action, and the absence of on-chip dissipation. Using pulsed microwave techniques, we demonstrate bifurcation amplification in nanofabricated Al junctions and verify that the performance predicted by theory is attained."

Flux-Qubit Readout with Frequency Dependent Damping Berkeley, München "Recent experiments on superconducting flux qubits, consisting of a superconducting loop interrupted by Josephson junctions, have demonstrated quantum coherence between two different quantum states. The state of the qubit is measured with a superconducting quantum interference device. Such measurements require the SQUID to have high resolution while exerting minimal backaction on the qubit." In Phys. Rev. B 72, 024513 , Plourde, Wilhelm et al. employ a path-integral approach to analyze the Caldeira-Leggett model, calculating backaction of a shunted symmetric SQUID on a flux qubit. "To test the model, we fabricated a dc SQUID in which each junction is shunted with a thin-film interdigitated capacitor in series with a resistor, and measured the switching distribution as a function of temperature and applied magnetic flux. After accounting for the damping due to the SQUID leads, we found good agreement between the measured escape rates and the predictions of our model. "

arXiv Notables quant-ph Notable submissions to the arXiv this month include a comprehensive overview of solid-state qubits by Esteve and Vion [0505676], two papers by Nori et al. on macroscopic cat states [0506011] and testing Bell inequalities in Josephson qubits [0408089], Wilhelm and Kack present an efficient readout scheme for flux qubits at the degeneracy point [0505537], Greenberger and Svozil derive a quantum information theoretic analysis of time travel [0506027], Wiesniak, Vedral and Brukner on macroscopic entanglement measures [0503037], Brassard et al. on quantum game theory and pseudo-telepathy [0408052], and a novel quantum storage and information transfer method in superconducting qubits by Wang et al. [0506144].

Quantum-Classical Interface Sussex, Liverpool In cond-mat/0505390, Mark Everitt et al. explore nonlinear interaction of a quantum mechanical SQUID ring with its environment. Potential applications include large frequency ratio down-conversion between electromagnetic fields, such as in classical THz communications technologies. "With the now very serious interest being taken in the possibilities of creating quantum technologies such as quantum information processing and quantum computing, much attention is being focused on the application of Josephson effect devices, particularly the SQUID ring. The highly non-perturbative nature of the SQUID ring in the quantum regime means that the ring-environment interaction can be very non-linear, and may lead to unexpected results ..."

Entanglement Extraction from a Solid NEST-INFM, Leeds, Vienna quant-ph/0505107 "It has been a common belief that entanglement cannot exist on a macroscopic scale. This is because decoherence effects from many-particle interaction would destroy all quantum correlations. However, it has been predicted that macroscopic entanglement can exist in solids in the thermodynamical limit – even at high temperature – and it is related to critical phenomena. Here we propose an experimental setup to demonstrate entanglement extraction with present-day technology using optical lattices. This demonstrates that entanglement not only exists in solids, but can even be used for quantum information processing or to violate Bell’s inequalities ..."

An n-qubit controlled phase gate with resonator-coupled SQUIDs U Kansas In quant-ph/0504188, Siyuan Han and Chui-Ping Yang propose a novel method to realize multiqubit controlled phase gates with SQUIDs. The scheme "operates essentially by exchanging a single photon between the controlled SQUIDs and the resonator mode before and after a phase shift performed on the target SQUID."

Transport of atoms in a quantum conveyor belt NIST Gaithersburg cond-mat/0504606 "An atomic-gas Bose-Einstein condensate (BEC) is a coherent source of matter waves – a collection of atoms, all in the same state, with an extremely narrow momentum spread ... We can easily control the velocity and acceleration of the atomic lattice structure as well as its strength, making it a variable 'quantum conveyor belt.' This allows us to explore situations that are difficult or impossible to achieve in solid state systems. The results are often remarkable and counterintuitive."

Signatures of quantum behavior in single-qubit weak measurements Penn State, UC Riverside quant-ph/0505094 "With the recent surge of interest in quantum computation, it has become very important to develop clear experimental tests for 'quantum behavior' in a system. This issue has been addressed in the past in the form of the inequalities due to Bell and those due to Leggett and Garg. These inequalities concern the results of ideal projective measurements, however, which are experimentally difficult to perform in many proposed qubit designs ... Here, we show that weak continuous measurements, which are often practical to implement experimentally, can yield particularly clear signatures of quantum coherence ..."

Full Protection of Superconducting Qubit Systems from Coupling Errors Munchen, Berkeley quant-ph/0407780 "Solid state qubits realized in superconducting circuits are potentially extremely scalable. However, strong decoherence may be transferred to the qubits by various elements of the circuits that couple individual qubits [...] We propose here an encoding that provides full protection against errors originating from these coupling elements ..."

Asymmetry and Decoherence in a Double-layer Persistent-current Qubit Kavli Institute Delft cond-mat/0405272 "We discuss a superconducting flux qubit design that exploits the symmetries of a circuit to protect the qubit from unwanted coupling to the noisy environment [...] Possibilities for prolonging the relaxation and decoherence times of the studied superconducting qubit are proposed on the basis of the obtained results."

Detection of Macroscopic Entanglement by Correlation of Local Observables University of Tokyo In quant-ph 0504086, Shimizu and Morimae propose a macroscopic entanglement index for unknown and mixed states. "We propose a correlation of local observables on many sites in macroscopic quantum systems. By measuring the correlation one can detect, if any, 'superposition of macroscopically distinct states,' which we call macroscopic entanglement, in arbitrary quantum states that are (effectively) homogeneous. Using this property, we also propose an index of macroscopic entanglement."

Spectroscopy on Two Coupled Superconducting Flux Qubits Kavli Institute Delft In PRL 94, 090501 (2005) Mooij et al. report on spectroscopy measurements of two coupled superconducting flux qubits: "The new results support the notion that superconducting flux qubits can be used to study entanglement in macroscopic quantum systems and for the development of nontrivial two-qubit gates [...] We demonstrate that two macroscopic flux qubits can be coupled to form a quantum mechanical four level system."

2005 Agilent Europhysics Prize Agilent Technologies The 2005 Agilent Technologies Europhysics Prize has been awarded to Awschalom, Dietl, and Ohno for their investigation of solid-state magnetic semiconductors and spin coherence. Spintronics is a promising candidate for scalable quantum computation. "We are proud to recognize these scientists for combining advanced materials engineering, insightful theoretical modeling, ingenious experimental techniques, and international collaboration to attain important breakthroughs in spintronics," said Jim Hollenhorst, director of molecular technology at Agilent Laboratories. Last year the award was shared by Mooij, Nakamura, Devoret and Esteve for their demonstration of superconducting circuits as qubits.

Nonlocal Measurements in Time-Symmetric Quantum Mechanics arXiv Vaidman and Nevo have posted a preprint on nonlocal demolition measurement of backward evolving quantum states which allows for the introduction of novel types of nonlocal variables. The work builds upon upon Aharonov's time-symmetric formalism, which contains the quantum state evolving backward in time from complete measurement performed in the future relative to the time in question. "Demolition measurements of nonlocal backward evolving quantum states require remarkably small resources. This is so because the combined operation of time reversal and teleportation of a local backward evolving quantum state requires only a single quantum channel and no transmission of classical information."

Quantum Interference Effect Transistors PhysicsWeb Cardamone et al. propose a novel approach to single-molecule transistors, the quantum interference effect transistor, or QuIET. Each transistor consists of two electrodes attached to an organic ring molecule in one of two configurations: the presence or absence of quantum interference in the ring determines the state of the transistor. "One potential advantage of the QuIET approach is that it could work in aqueous environments, such as those inside living organisms, because it is made of organic molecules."



Schematic diagrams of two types of QuIET In each, base voltage modulates the coherent suppression of current between emitter (E) and collector (C) leads. In (a), base voltage controls the distance x between the benzene ring and base lead (B), for example an STM tip. This in turn controls the coupling of the ring to the base lead. In (b), a base complex is introduced between the ring and base lead. The electrostatic effect of the base lead's bias on this molecule alters its coupling to the benzene ring.


Quantum Game Theory arXiv Nash equilibria and game theory profoundly affected the outcome of the 20th Century – preventing escalation of Cold War conflict between the US and USSR, for example. Quantum game theoretic approaches similarly hold the potential to influence strategic developments in the coming century. Quantum communications networks are already operating in research laboratories across the globe. With the recent birth of the DARPA/BBN quantum internet, quantum game theory has left the realm of academia and entered the world of practical applications, showing promise to transform politics, economics, conflict and warfare in the decades to come. In a recent PhD thesis, Iqbal reviews the current state of the field. See also "Quantum Pseudo-Telepathy" by Brassard et al, "Classical Rules in Quantum Games" by van Enk, "Quantum Strategies" by Meyer.

Vision 2033

American Association for the Advancement of Science Science, technology and public policy planning for the next thirty years - PDF downloads of the proceedings

Interference in Bose-Einstein Condensates Science Javanainen comments on nondestructive measurement of relative phase difference between two separated BECs, recently conducted by Saba et al. within the Ketterle Group at MIT's Center for Ultra-Cold Atoms: "The experiments open up new ways to manipulate condensates, which are macroscopic objects, as if they were quantum mechanical entities. Measurement devices based on matter-wave interferometry are a potential application."

Quantum Interference in Time arXiv Paulus has posted a preprint of the recent temporal quantum interference experiments, widely reported earlier this month: "The conceptually most important interference experiment is the double-slit scheme, which has played a pivotal role in the development of optics and quantum mechanics. [...] We have realized an intriguing implementation of the double slit in the time domain. The observation of interference and its absence at the same time for the same electron is a beautiful demonstration of the principles of quantum mechanics."

Gaidarzhy Defends Quantized Displacement arXiv In cond-mat 0503502, Gaidarzhy et al. reply to Schwab's critical comments on the Mohanty Group's recently-reported experimental evidence of macroscopic quantum displacement in a nanomechanical oscillator: "In summary, both the objections and the premise of the comment, on the data interpretation, by Schwab et al. are not valid [...] A proper theoretical framework to understand quantized motion of a macroscopic mechanical system of 50 billion atoms, in presence of decoherence and dissipation, is yet to be developed."

Measurement Based Quantum Computation Vienna In Nature 434, Walther et al. report on the first experimental demonstration of Grover's search via one-way quantum computing on entangled photons. Following initialization of a highly-entangled 'cluster state,' irreversible single-qubit measurements are performed in a feedforward process that determines the output of the system. Raussendorf and Briegel's original paper is available via quant-ph 0010033.

AIST Quantum Cryptography Research Tokyo ATIP QUIST reports that AIST, the Japanese National Institute of Advanced Industrial Science and Technology, is establishing a national research center in quantum cryptography and communications this spring under direction of Hideki Imai, University of Tokyo. "Hiroyuki Yoshikawa, Director of AIST, is aiming to establish world-class technology which will contribute to establishing global standards for cryptography technology and procedures for its evaluation."

Macromolecular Quantum Interference Vienna Arndt, Hornberger and Zeilinger provide a summary of recent progress in matter-wave interferometry, outlining near-term experimental objectives as well as theoretical developments towards understanding the mechanisms of decoherence. "In our view, matter-wave interferometry should be feasible for large objects such as proteins, small viruses and nanocrystals with atomic masses of up to 10^6 units. Extrapolating our results to bigger masses and higher temperatures, we believe that neither collisions nor thermal decoherence will be a problem in these cases. No fundamental limit of quantum interference is yet in sight, but much work still has to be done to prepare and manipulate coherent beams of supermassive particles. Carrying out such experiments will be a fascinating challenge."

DARPA Quantum Network Harvard/Boston/BBN Chip Elliot reports on the current status of the DARPA Quantum Network, in continuous operation since June 2004. "The DARPA Quantum Network is the world's first quantum cryptography network, and perhaps also the first QKD system providing continuous operation across a metropolitan area. Four more nodes are now being added to bring the total to 10 QKD nodes." This paper updates quant-ph 0412029 for the upcoming SPIE conference.

Quantum Interference in Time PhysicsWeb, Slashdot Paulus and colleagues have conducted a novel version of the double slit experiment, observing interference fringes with electrons passing through a double slit in time. Separated by femtosecond laser pulses, the 'slit' is composed of two maxima and one minima of the electric field. Slashdot notes several related articles on Zeilinger's interference experiments with fullerenes and biomolecules; see also the "delayed-choice quantum eraser" experiment by Shih and Kim.

Schwab comments on Phys. Rev. Lett. 94, 030402 (2005) arXiv In reply to the recent Gaidarzhy et al. report on "Evidence for Quantized Displacement in Macroscopic Nanomechanical Oscillators," Schwab et al. contend that the Letter contains order-of-magnitude omissions and inaccuracies counter to any interpretation of quantized macroscopic displacement: " ... the evidence, analysis, claims, and conclusions presented are contrary to expectations from fundamentals of quantum mechanics and elasticity theory, and [...] the method used by the authors is unsuitable in principle to observe the quantized energy states of a nanomechanical structure."

Simultaneous State Measurement of Coupled Josephson Phase Qubits NIST, UCSB In Science [25 Feb 2005], McDermott et al. report on simultaneous precision measurement of antiphase oscillation between flux qubits, consistent with entanglement of the two qubit states. "One of the many challenges of building a scalable quantum computer is single-shot measurement of all the quantum bits. Here, we exploit the simultaneous single-shot measurement of two coupled Josephson phase qubits to directly probe interaction of the qubits in the time domain [...] thereby opening the possibility for the full characterization of multi-qubit gates and elementary quantum algorithms."

Quantum Leap canada.com A brief overview of the history and development of quantum cryptography, from its origins through a chance encounter between Charles Bennett and Giles Brassard while swimming in the warm tropical waters of the Caribbean, to the next-generation satellite-based quantum encryption links of tomorrow. "Together, Brassard and Bennett would go on to found a field of science - quantum information processing - whose effects on society some say could even rival the impact that the steam engine had in its time."

Schrodinger's Kittens Mohanty Group In Physical Review Letters [ 28 Jan 2005 ], the Mohanty Group at Boston University reports evidence of [center-of-mass displacement] quantum nanomechanical oscillations in silicon antennae of ~ 50 billion atoms, making this the largest observation of quantum mechanical behavior to date. "It's a truly macroscopic quantum system," says Alexei Gaidarzhy, the paper's lead author and a graduate student in the BU College of Engineering's Department of Aerospace and Mechanical Engineering. "When it's a new phenomenon, it's best not to be guided by expectations based on conventional wisdom. The philosophy here is to let the data speak for itself."

Mainstreaming Quantum Crypto? MagiQ/Cavium MagiQ Technologies has signed a deal with Cavium Networks to boost performance and lower costs through standardized engineering protocols, incorporating Cavium's network security chips into MagiQ's servers and creating designs for networking boards. Will quantum encryption go mainstream?

Coupling Flux Qubits You, Nakamura, and Nori "A central problem for implementing efficient quantum computing is how to realize fast operations, both one- and two-bit ones. However, this is difficult to achieve for a collection of qubits, especially for those separated far away, because the interbit coupling is usually much weaker than the intrabit coupling. Here we present an experimentally feasible method to effectively couple two flux qubits via a common inductance and treat both single and coupled flux qubits with more realistic models which include the loop inductance. The main advantage of our proposal is that a strong interbit coupling can be achieved using a small inductance, so that two-bit operations as fast as one-bit ones can be easily realized. We also show the flux dependence of the transitions between states for the coupled flux qubits." [ arXiv ]

Interpretation of Quantum Mechanics: Current Status and Future Directions Perimeter Institute Lecture Series A technical but pedagogical introduction to the problems associated with developing a consistent understanding of the nature of objective reality in light of quantum theory.

New Life for Schrodinger's Cat The observation of quantum superpositions of distinct macroscopic states by groups at Stony Brook and Delft represents a milestone in experimental quantum physics. Both teams have reported spectroscopic evidence for currents of microamps flowing through a superconducting ring in opposite directions at the same time. Physics World article by Tony Leggett.

Superconducting Quantum Bits Quantum Transport Group Superconducting devices can be used to explore the boundaries between the quantum and classical worlds, and hold potential applications in quantum information. Physics World article by Hans Mooij.

UNESCO Physics for Tomorrow The launch conference for the International Year of Physics was held at UNESCO Headquarters in Paris, 13-15 Jan 2005. I've recently posted a summary and photos from the event.

RSA INFOSEC Awards My briefing to the UNISCA First Committee, "Converging Technologies: The Future of the Global Information Society," was selected to receive the 2004 RSA Information Security Award for Outstanding Achievement in Government Policy. RSA Awards summaries are available online via RSA Conference website, press release and news links.

Adaptive Quantum Networks quantum topology » neural networks [IJTP] Our paper on adaptive quantum networks appears in this month's International Journal of Theoretical Physics. We introduce a novel model of superposed adaptive quantum networks, with considerations for high-dimensional dissipative quantum systems in both quantum computation and molecular biology. A preprint of the article is available via quant-ph 0311016.

Quantum networks, quantum registers and developments in quantum computing Future Salon News summaries of recent developments in quantum information science and technology.

Room-temperature Bose-Einstein condensation?

Hideyo OKUSHI, AIST Tsukuba Diamond Research Center, Japan The AIST Tsukuba Diamond Research Center has observed extremely sharp 235-nm exciton emission in fabricated single-crystal diamond film semiconductors at 300K. If the exciton lifetimes are long enough it is possible that Bose-Einstein condensation can occur in these diamond films, even at room temperature.