c o h e r e n c e *

xQIT MIT|RLE The Keck Foundation Center for Extre...

nobody@vox.com(coherence *) — Wed, 14 Mar 2007 14:29:00 -0700

xQIT MIT|RLE The Keck Foundation Center for Extreme Quantum Information Theory has been established at RLE under directive to investigate the fundamental limitations to quantum computing, communications, sensing and control. Seth Lloyd and Jeff Shapiro will lead the initiative.

Read and post comments | Send to a friend

Entangled Quantum Networks ICFO|ICREA|Max-Planck I...

nobody@vox.com(coherence *) — Tue, 27 Feb 2007 14:42:00 -0800

Entangled Quantum Networks ICFO|ICREA|Max-Planck Institute In Nature Physics advance publication 10.1038/nphys549, Acin et al. draw upon the classical percolation methods of statistical mechanics to optimize entanglement distribution through quantum networks. " We argue that there exists an entanglement phase transition in quantum networks which may be exploited to obtain very efficient protocols. This work opens a new set of problems in quantum information theory, which are related to statistical physics, but pose completely new challenges in these fields [...] The work leads to a novel type of critical phenomenon, an entanglement phase transition that we call entanglement percolation. "

Maximizing entanglement in quantum networks. Each node is connected by a state consisting of two copies of the same two-qubit state. The nodes marked in (a) make the optimal measurement for the one-repeater configuration on pairs of qubits belonging to different connections. (b) A triangular lattice is obtained where the maximally entangled state for each connection is the same as for the two-qubit state. Acin et al., Nature Physics, 25 February 2007.

Read and post comments | Send to a friend

Entanglement engineering for quantum metrology I...

nobody@vox.com(coherence *) — Wed, 21 Feb 2007 13:48:00 -0800

Entanglement engineering for quantum metrology Innsbruck Entanglement-assisted metrology has previously been demonstrated to enhance measurement sensitivity and improve fidelity in noisy conditions. In a quant-ph update to Nature 443 (316), Roos et al. obtain precision atomic clock measurements in the presence of magnetic field noise by engineering a decoherence-free subspace to enhance coherence times. " We find that entangled states are not only useful for enhancing the signal-to-noise ratio in frequency measurements – a suitably designed pair of atoms also allows clock measurements in the presence of strong technical noise. The applied technique makes explicit use of nonlocality as an entanglement property, and constitutes a new paradigm for designed quantum metrology."

Signatures for generalized macroscopic superpositions Queensland In quant-ph 0701204 and Phys. Rev. Lett. 97, Cavalcanti and Reid develop signature detection criteria for macroscopic quantum coherence in situations which are not limited to only two macrosopically distinct measurement outcomes. " The criteria provide a means to distinguish a single macroscopic quantum state from one based on a mixture of several microscopic superpositions of pointer-measurement eigenstates." Calculations are provided for the case of Gaussian-squeezed and spin-entangled states.

Read and post comments | Send to a friend

∂-wave throws down the gauntlet Vancouver Pendi...

nobody@vox.com(coherence *) — Fri, 09 Feb 2007 14:37:00 -0800

∂-wave throws down the gauntlet Vancouver Pending third-party referee, peer review or independent verification, D-Wave's press release has been received with expected enthusiasm in the mainstream press and restrained skepticism in the scientific community. "I'll be a bit of a skeptic until I see what they have done. I'm happy these guys are doing it. But the proof of the pudding is in the eating." – Seth Lloyd

Read and post comments | Send to a friend

Nondemolition measurement. a) Schematic represent...

nobody@vox.com(coherence *) — Wed, 24 Jan 2007 14:48:00 -0800

Nondemolition measurement. a) Schematic representation of the qubit excitation pulse (top) and readout pulse (bottom) sequence; b) Probability to detect the oscillator in state h for qubit states; c) Measurement of Rabi oscillations; d) Parameters characterizing measurement errors. Lupascu et al. Nature Physics, 14 January 2007.

Quantum non-demolition measurement of a superconducting two-level system Delft|NTT By minimizing disturbance to the system under investigation, quantum nondemolition measurement (QND) can provide particularly clear signatures of quantum coherence. In Nature Physics and cond-mat 0611505, Lupascu et al. demonstrate nondemolition measurement of superconducting qubits coupled to a nonlinear resonator. "The high correlation between measurement results demonstrates the quantum nondemolition nature of the readout method. The fact that quantum nondemolition measurement is possible for superconducting qubits strengthens the notion that these fabricated mesoscopic systems are to be regarded as fundamental quantum objects. Our results are also relevant for quantum information processing protocols such as state preparation and error correction. " cf. also Kavli Institute announcement in TU Delta (in Dutch).

Read and post comments | Send to a friend

High-speed linear optics quantum computing using a...

nobody@vox.com(coherence *) — Mon, 22 Jan 2007 12:25:00 -0800

High-speed linear optics quantum computing using active feed-forward measurement Vienna In Nature 445, 65-69 and concurrent press summary, Zeilinger's group reports experimental demonstration of feedforward error correction via one-way, highly-entangled cluster states in linear optics. "With present technology, the individual computational step can be operated in less than 150 ns using electro-optical modulators. This is an important result for the future development of one-way quantum computers, whose large-scale implementation will depend on advances in the production and detection of the required highly entangled cluster states."

Retrocausal Signaling UW Via the Pontiff – John Cramer recently conducted a UW seminar outlining recent progress on his plans for an experimental test of retrocausal signaling using entangled photon pairs, notwithstanding Eberhard's theorem. The planned experiment draws upon the thesis of Birgit Dopfer (Zeilinger group, Universität Wien), Wheeler-Feynman theory and Cramer's own transactional interpretation. Concurrent press summary and diagram of the setup are available via SF Gate. cf. also Cramer's online summary of the proposed experiment, Jensen gedanken, double-slit and delayed choice quantum eraser experiments (refs: serafino).

Read and post comments | Send to a friend

High-speed linear optics quantum computing using a...

nobody@vox.com(coherence *) — Mon, 22 Jan 2007 12:25:00 -0800

Read and post comments | Send to a friend

Quantum Transport in Carbon Nanotubes » links – Co...

nobody@vox.com(coherence *) — Sun, 21 Jan 2007 12:30:00 -0800

Quantum Transport in Carbon Nanotubes » links – Condensed matter physics weblog metadatta appended to links. Recent post topics include the Wolf Prize announcement in spintronics and single-quantum dot nanowire light-emitting diodes.

Read and post comments | Send to a friend

Proton Tunneling in Molecular Biophysics Renssela...

nobody@vox.com(coherence *) — Fri, 19 Jan 2007 15:11:00 -0800

Proton Tunneling in Molecular Biophysics Rensselaer RPI researchers have employed the SCOREC supercomputing cluster to conduct advanced modeling of protein folding dynamics which incorporates quantum mechanical effects to study the influence of proton tunneling in enzyme catalysis. The group's initial study of intein's role in C-termini protein folding will be used to develop nanoscale switches for applications ranging from drug delivery to novel sensors.

Read and post comments | Send to a friend

Measurement-based Quantum Computing with Supercond...

nobody@vox.com(coherence *) — Thu, 18 Jan 2007 17:02:00 -0800

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."

Read and post comments | Send to a friend