Measurement-based Quantum Computing with Superconducting Charge Qubits RIKEN Wang, You and Nori report on measurement-based preparation of superconducting cluster states. "The measurement of the current of a few parallel Josephson-junction qubits realizes a novel type of quantum-state selector. Using this selector, one can produce various quantum entangled states and also realize a controlled-NOT gate without requiring an exact control of the interqubit interactions. In particular, cluster states for quantum computation could be produced with only single-qubit measurements."

Measuring the Size of a Schrödinger Cat State München "We propose a measure for the "size" of a Schrödinger cat state, i.e. a quantum superposition of two many-body states with macroscopically distinct properties, by counting how many single-particle operations are needed to map one state onto the other. This definition gives sensible results for simple, analytically tractable cases and is consistent with a previous definition restricted to Greenberger-Horne-Zeilinger-like states. We apply our measure to the experimentally relevant, nontrivial example of a superconducting three-junction flux qubit put into a superposition of left- and right-circulating supercurrent states and find this Schroedinger cat to be surprisingly small."

Symposium for Hans Mooij Kavli|Delft "We see no fundamental reason why superconducting quantum computers cannot be developed into large-scale systems – though, perhaps before then, other applications will also emerge that are much more interesting ..." On Friday, 20^th October 2006 the Kavli Institute of Nanoscience and Delft University of Technology orchestrated a symposium, "The Best of Nanoscience," for Hans Mooij in honor of his attainment of emeritus professor. A program of special guests including Michael Tinkham, David Auston, David DiVincenzo, Seth Lloyd, Yasunobu Nakamura, Hideaki Takeyanagi, John Clarke, Carlo Beenakker, Paul McEuen, Göran Wendin and many other distinguished visitors convened to celebrate Hans's past accomplishments and to highlight future research milestones, followed by a formal reception in honor of the occasion. cf. links to Quantum Transport party committee, TU Delft announcement, featured interview in ∫ Delft Integraal.

Entanglement Demonstrated in Superconducting Qubits UCSB|IBM In Science 313, 5792 and concurrent PhysicsWeb, Scientific American reviews, Martinis et al. report experimental verification of entanglement between two superconducting qubits. "By using simultaneous measurement and state tomography, we demonstrated entanglement between two solid-state qubits. Single qubit operations and capacitive coupling between two super-conducting phase qubits were used to generate a Bell-type state. Full two-qubit tomography yielded a density matrix showing an entangled state with fidelity up to 87%. Our results demonstrate a high degree of unitary control of the system, indicating that larger implementations are within reach."

Symposium on Quantum Technologies Cambridge|MIT "Substantial advances in nanoscale science and engineering have made it possible to engineer a wide range of physical systems whose behaviour is governed by the laws of quantum mechanics. Quantum technologies seek to exploit these quantum effects to develop novel practical applications – from secure communications systems to novel computing devices more powerful than existing computers, new measurement devices more accurate than their classical counterparts, or to facilitate chemical reactions using photonic reagents, which might lead to the discovery new materials – to mention only a few potential applications. This Symposium aims to bring together a range of theoretical and experimental scientists and engineers from academia and industry to discuss the current state of the art of various emerging quantum technologies, and the promises and challenges that lie ahead."

Macroscopic Entanglement in Quantum Computation Tokyo | 東京大学 "We investigate macroscopic entanglement of quantum states in quantum computers, where we say a quantum state is entangled macroscopically if the state has superposition of macroscopically distinct states. When the solutions are such that the problem becomes hard in the sense that classical algorithms take more than polynomial steps to find a solution, macroscopically entangled states are always used in Grover's algorithm and almost always used in Shor's algorithm. Since they are representative algorithms for unstructured and structured problems, respectively, our results support strongly the conjecture that quantum computers utilize macroscopically entangled states when they solve hard problems much faster than any classical algorithms."

Time Reversal and Super-resolving Phase Measurements Queensland "We demonstrate phase super-resolution in absence of entangled states. The key insight is to use the inherent time-reversal symmetry of quantum mechanics: our theory shows that it is possible to measure, as opposed to prepare, entangled states. Our approach is robust, requiring only photons that exhibit classical interference: we experimentally demonstrate high-visibility phase super-resolution with three, four, and six photons using a standard laser and photon counters. Our six-photon experiment demonstrates the best phase super-resolution yet reported with high visibility and resolution."

Selective Qubit Coupling via Stripline Cavity Chalmers "We theoretically investigate selective coupling of superconducting charge qubits mediated by a superconducting stripline cavity with a tunable resonance frequency. The frequency control is provided by a flux biased dc-SQUID attached to the cavity. Selective entanglement of the qubit states is achieved by sweeping the cavity frequency through the qubit-cavity resonances. The circuit is scalable, and allows to keep the qubits at their optimal points with respect to decoherence during the whole operation. We derive an effective quantum Hamiltonian for the basic, two-qubit-cavity system, and analyze appropriate circuit parameters. We present a protocol for performing Bell inequality measurements, and discuss a composite pulse sequence generating a universal control-phase gate."

Entanglement Extraction Leeds Cunha and Vedral review how to obtain spin entangled pairs of fermions from a Fermi gas, outlining the relevant experimental parameters. The experiment can be as "a possible new source of entangled particles; and as a foundational interesting step – testing novel properties of fundamental constituents of matter."

Single-atom Macroscopic Entanglement Resource Texas A&M "We discuss the generation of a macroscopic entangled state in a single atom cavity-QED system. The three-level atom in a cascade configuration interacts dispersively with two classical coherent fields inside a doubly resonant cavity. We show that a macroscopic entangled state between these two cavity modes can be generated under large detuning conditions. The entanglement persists even under the presence of cavity losses."

Nonstatistical Weak Measurements GMU|USC Tollaksen and Aharonov report on nondestructive weak measurement protocols and their application under empirical conditions.