Backward Evolving Quantum States Tel-Aviv In quant-ph 0606208, Vaidman outlines the theoretical limitations on possible manipulations of a backward-evolving quantum state. "The basic concept of the two-state vector formalism, which is the time symmetric approach to quantum mechanics, is the backward evolving quantum state. However, due to the asymmetry of the memory's arrow of time, the possible ways to manipulate a backward evolving quantum state differ from those for a standard, forward evolving quantum state. The similarities and the differences between forward and backward evolving quantum states regarding the no-cloning theorem, nonlocal measurements, and teleportation are discussed. The results are relevant not only in the framework of the two-state vector formalism, but also in the framework of retrodictive quantum theory."

Retrocausation: Experiment and Theory AAAS Causality – the notion that earlier events can affect later events but not vice-versa – undergirds our experience of reality and physical law. Causality is predicated on the forward unidirectionality of time. However, most physical laws are time symmetric; that is, they formally and equally admit both time-forward and time-reverse solutions. Time-reverse solutions are distressing because they would allow the future to influence the past, i.e., reverse causation. Why time-forward solutions are preferentially observed in nature remains an unresolved problem in physics. While the most convincing explanations invoke the second law of thermodynamics, wavefunction collapse or the expansion of the universe, in the end, purely forward causation is an ad-hoc physical assumption. This symposium will explore recent experiments, theory, and philosophical issues connected with reverse causation. In particular, it is hoped that this meeting will help: 1) generate better theoretical models by which established experimental results can be understood; 2) devise new experiments by which the underlying physics may be more clearly exposed; and 3) establish fruitful research collaborations.

Quantum Cosmology From Future to Past CERN, Cambridge In Phys Rev D and concurrent Physics Web overview, Hawking and Hertog apply Feynman's path integral formalism to quantum cosmology. "In this framework, amplitudes for alternative histories for the universe are calculated with final boundary conditions only. This leads to a top-down approach to cosmology, in which the histories of the universe depend on the precise question asked. We study the observational consequences of no boundary initial conditions on the landscape, and outline a scheme to test the theory."

Generation and control of Greenberger-Horne-Zeilinger entanglement in superconducting circuits RIKEN In quant-ph 0510169, Wei, Liu, and Nori propose an efficient approach to generate and control quantum entanglement between three macroscopic coupled superconducting qubits. "By conditionally rotating, one by one, selected Josephson charge qubits, we show that their Greenberger-Horne-Zeilinger (GHZ) entangled states can be deterministically generated. The possibility of using the prepared GHZ correlations to test the macroscopic conflict between the noncommutativity of quantum mechanics and the commutativity of classical physics is also discussed."

Coherent State Evolution in a Superconducting Qubit from Partial-Collapse Measurement UCSB Martinis et al, Science 312, 5779. "Measurement is one of the fundamental building blocks of quantum-information processing systems. Partial measurement, where full wavefunction collapse is not the only outcome, provides a detailed test of the measurement process. We introduce quantum-state tomography in a superconducting qubit that exhibits high-fidelity single-shot measurement. For the two probabilistic outcomes of partial measurement, we find either a full collapse or a coherent yet nonunitary evolution of the state. This latter behavior explicitly confirms modern quantum-measurement theory and may prove important for error-correction algorithms in quantum computation."

Flux qubit decoherence sources RIKEN, VTT, NEC In cond-mat 0606481, Yoshihara, Harrabi, Niskanen, Nakamura and Tsai investigate flux qubit decoherence sources, demonstrating an optimal bias condition at which noise sources are well decoupled and coherence time is primarily limited by energy relaxation of the qubit.

Spheres and connections represent qubits and entanglement bonds in (a) a two-dimensional cluster state. Coloured qubits show a compact CNOT gate, proceeding from the input qubits (yellow), through progression of Y-basis measurements (pink), to output qubits (light blue). (b) Activating additional collision cavities can create higher-dimensional topologies of entanglement such as this helical structure. Blythe and Varcoe, quant-ph 0605190.

Measurement-Based Quantum Computing Sussex In quant-ph 0605190, Blythe and Varcoe present a primer for feedforward quantum computation via crossed atomic beams to create a highly-entangled initial cluster state. Drawing upon previous work by Raussendorf and Briegel, the authors provide specifications for microwave cavity QED based scalable quantum computing architectures. "In a departure from the traditional understanding of a computer as a fixed array of computational elements, we show that cluster state quantum computing is well suited to atomic beam experiments. We show that all of the necessary elements have been individually realised, and that the construction of a truly scalable atomic beam quantum computer may be an experimental reality in the near future."

Quantum Coherence, Noise and Decoherence in Nanostructures Max-Planck Institute "The modern field of nanoelectronics has brought about novel physical phenomena and created new challenges for their interpretation within quantum theory. Among the most fundamental concepts are quantum coherence and interference effects. An improved understanding of these phenomena is needed both from a fundamental point of view as well as for a variety of potential applications – ranging from highly sensitive detectors to quantum information devices and single-electron logic circuits operating at room temperature. The goal of this meeting is to bring together leading scientists from different subfields of condensed matter physics in order to advance the understanding of decoherence in nanostructures."

Constructive Role of Noise in Complex Systems Max-Planck Institute "Noise is inevitably present in any dissipative systems, and all living organisms operate in the noisy environment. Understanding the role of noise is crucial both in fundamental research in nonlinear physics, and in many applications in engineering, biology and medicine. Recent developments in statistical physics and nonlinear dynamics have shed light on a new, sometimes counterintuitive role which noise plays in nonlinear systems: in a wide range of systems, random forces may bring a system to a more ordered state. This meeting will focus on recent developments in the field of noise and fluctuations in complex nonlinear systems, as well as on applications of new noise-mediated phenomena and theoretical methodologies in experimental physics, biological physics, neuroscience and medicine."

Macroscopic Quantum Coherence and Computing MQC² Workshop "The aim of the workshop is to report on the recent theoretical and experimental results on the macroscopic quantum coherence of mesoscopic systems, as well as on solid state realization of qubits and quantum gates. Particular attention will be given to coherence effects in Josephson devices. Other physical systems, including quantum dots, optical, atomic, and molecular devices, exhibiting macroscopic quantum coherence, will also be discussed."

Quantum Communications in Telecom Networks IEEE "Quantum Cryptography, which carries a promise of fundamentally secure communications, has reached a point of relative maturity and first commercial offerings. Its broad deployment, however, is impeded by many technical challenges. This conference will bring together researchers from universities, industry and government labs, commercial QC system manufacturers, service providers, and funding agencies to discuss the novel physics of single-photon sources, interactions between photonic and material qubits, distant entanglement, single photon detection, fundamental physical constraints on the performance of QC links and networks, and resulting trade-offs among key rate, distance and cryptographic security."

Entanglement Distribution Revealed by Macroscopic Observations Vienna "Observation of quantum entanglement between increasingly larger macroscopic objects is one of the most promising avenues of experimental quantum physics. Eventually, all these developments will lead to a complete understanding of the simultaneous coexistence of a macroscopic classical world and an underlying quantum realm." In quant-ph 0603208, Kofler and Brukner compute multipartite entanglement measures to reveal quantum correlations in the collective properties of two separated objects – "The present work demonstrates that macroscopic properties can reveal entanglement between two or more macroscopic samples. On the fundamental side, our method demonstrates that there is no principal reason why purely quantum correlations could not have an effect on the global properties of objects."

Macroscopic Einstein-Podolsky-Rosen Pairs in Superconducting Circuits RIKEN In quant-ph 0508027, Wei et al. introduce an efficient method of creating EPR pairs in capacitively-coupled Josephson nanocircuits: "A possible application of the deterministically generated EPR pairs is to test Bell's Inequality at the macroscopic level. The approach proposed can be easily modified to engineer quantum entanglement in other fixed-interaction solid-state systems."

Quantum Computing with Superconducting Qubits NATO ASI Geller, Wilhelm et al. provide a concise overview of research efforts currently underway to develop scalable superconducting quantum circuits in Superconducting Qubits I: Architectures and Superconducting Qubits II: Decoherence – "Josephson junctions have demonstrated enormous potential as qubits for scalable quantum computing architectures. Here we discuss the current approaches for making multi-qubit circuits and for performing quantum information processing with them."

Defense and Security Applications of Quantum Information SPIE Applied technologies that compute, store, and distribute information based upon quantum mechanical entanglement, superposition, and interference phenomena are currently being pursued and realized in multiple parallel architectures, with high-impact assessment in the fields of cryptography, communications, computation and metrology. The SPIE Defense and Security Symposium, the largest unclassified international meeting of its kind, was held from 17-21 April, 2006.

Entanglement as a function of effective coupling between light and mirror (k) and effective duration of coupling (x-axis). Maximal entanglement is shown in red.

High-Temperature Macroscopic Entanglement PRL Via Raitio – Aires Ferreira, Ariel Guerreiro, and Vlatko Vedral have published novel results on high-temperature macroscopic entanglement in Phys. Rev. Lett. 96, 060407 [arXiv, physicsweb]. "Can entanglement and the quantum behavior in physical systems survive at arbitrary high temperatures? In this Letter we show that this is the case for a electromagnetic field mode in an optical cavity with a movable mirror in a thermal state [...] Entanglement between a macroscopic mirror and a cavity mode field can arise due to radiation pressure at arbitrarily high temperatures as the system evolves in time. This is very surprising because it is commonly believed that high temperature completely destroys entanglement."