Post Quantum Cryptography PQCrypto2006 Via the Pontiff – The European Network of Excellence for Cryptology (ECRYPT) and its Asymmetric Techniques Virtual Lab (AZTEC) examine the future of cryptography in the quantum computer era: "Will large quantum computers be built? If so, what will they do to the cryptographic landscape? Anyone who can build a large quantum computer can break today's most popular public-key cryptosystems: e.g., RSA, DSA, and ECDSA. But there are several other cryptosystems that are conjectured to resist quantum computers: e.g., the Diffie-Lamport-Merkle signature system, the NTRU encryption system, the McEliece encryption system, and the HFE signature system. Exactly which of these systems are secure? How efficient are they, in theory and in practice? PQCrypto 2006, the International Workshop on Post-Quantum Cryptography, will look ahead to a possible future of quantum computers, and will begin preparing the cryptographic world for that future."

Quantum Interferometric Sensors LSU Quantum entanglement garners a number of advantages to metrology and remote sensing applications. Dowling, Kapale et al. have issued a recent summary of progress in quantum interferometric sensors, which harness quantum entanglement for improved measurement sensitivity approaching the Heisenberg limit.

Phase-Slip Flux Qubits TU Delft In a recent paper submitted to the New Journal of Physics special issue on solid-state quantum information processing, Mooij and Harmans introduce phase-slip flux qubits, which harness quantum tunnelling to realize a superconducting qubit without the use of Josephson junctions. Phase-slip qubits potentially hold two distinct advantages over traditional flux qubits: lower sensitivity to charge noise-based sources of decoherence, and well-defined separation of energy levels of more than 500 GHz, allowing for extremely rapid excitation of the qubit.



Energy Level Separation Energy levels as a function of applied flux for different fluxoid numbers. A phase-slip event changes the fluxoid number n. The arrow indicates the operating point at f= ½.

Macroscopic EPR Pairs in Superconducting Circuits RIKEN In quant-ph/0508027, Nori et al. propose an efficient approach for deterministic generation of entangled EPR pairs in coupled Josephson nanocicruits. Realization of the experiment would provide an effective means of testing Bell inequality violations, demonstrating nonlocality of quantum entanglement in macroscopic systems.

Delft Scientists Split Electron Pairs in Superconductors TU Delft "Scientists at Stichting FOM and Kavli Institute of Nanoscience Delft have demonstrated that electrons that normally travel through superconductors in pairs can be seperated while retaining their quantum mechanical kinship. The formation of electron pairs - so-called Cooper pairs - is such a fundamental property of current flow in superconductors that the Delft experiment is considered a breakthrough. It lays the foundation for the realization of a superconducting entangler capable of injecting pairs of entangled electrons into nanoelectronic circuits, an important building block of the quantum computer scientists have been dreaming of for years."

Robust Entanglement Innsbruck "It is common belief among physicists that entangled states of quantum systems lose their coherence rather quickly. The reason is that any interaction with the environment which distinguishes between the entangled sub-systems collapses the quantum state. Here we investigate entangled states of two trapped Ca+ ions and observe robust entanglement lasting for more than 20 seconds."

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].