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."