Mar 2010, Volume 5 Issue 1
    

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  • Over the last decades, digital information processing progressed at an enormous speed. It already becomes an indispensable resource both in science and in society. However, for some computational problems, no efficient algorithms are known yet. If quantum mechanical systems are used for information process, an exponential speedup will be obtained for some of these problems instead of today's classical computers. Among the many different physical systems that have been propose [Detail] ...


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  • Research articles
    Spin qubits for quantum simulations
    Xin-hua PENG (彭新华), Dieter SUTER,
    2010, 5(1): 1-25. https://doi.org/10.1007/s11467-009-0067-x
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    The investigation of quantum mechanical systems mostly concentrates on single elementary particles. If we combine such particles into a composite quantum system, the number of degrees of freedom of the combined system grows exponentially with the number of particles. This is a major difficulty when we try to describe the dynamics of such a system, since the computational resources required for this task also grow exponentially. In the context of quantum information processing, this difficulty becomes the main source of power: in some situations, information processors based in quantum mechanics can process information exponentially faster than classical systems. From the perspective of a physicist, one of the most interesting applications of this type of information processing is the simulation of quantum systems. We call a quantum information processor that simulates other quantum systems a quantum simulator.
    This review discusses a specific type of quantum simulator, based on nuclear spin qubits, and using nuclear magnetic resonance for processing. We review the basics of quantum information processing by nuclear magnetic resonance (NMR) as well as the fundamentals of quantum simulation and describe some simple applications that can readily be realized by today’s quantum computers. In particular, we discuss the simulation of quantum phase transitions: the qualitative changes that the ground states of some quantum mechanical systems exhibit when some parameters in their Hamiltonians change through some critical points. As specific examples, we consider quantum phase transitions where the relevant ground states are entangled. Chains of spins coupled by Heisenberg interactions represent an ideal system for studying these effects: depending on the type and strength of interactions, the ground states can be product states or they can be maximally entangled states representing different types of entanglement.
  • Research articles
    Quantum entanglement and disentanglement of multi-atom systems
    Zbigniew FICEK,
    2010, 5(1): 26-81. https://doi.org/10.1007/s11467-009-0078-7
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    We present a review of recent research on quantum entanglement, with special emphasis on entanglement between single atoms, processing of an encoded entanglement and its temporary evolution. Analysis based on the density matrix formalism are described. We give a simple description of the entangling procedure and explore the role of the environment in creation of entanglement and in disentanglement of atomic systems. A particular process we will focus on is spontaneous emission, usually recognized as an irreversible loss of information and entanglement encoded in the internal states of the system. We illustrate some certain circumstances where this irreversible process can in fact induce entanglement between separated systems. We also show how spontaneous emission reveals a competition between the Bell states of a two qubit system that leads to the recently discovered “sudden” features in the temporal evolution of entanglement. Another problem illustrated in details is a deterministic preparation of atoms and atomic ensembles in long-lived stationary squeezed states and entangled cluster states. We then determine how to trigger the evolution of the stable entanglement and also address the issue of a steered evolution of entanglement between desired pairs of qubits that can be achieved simply by varying the parameters of a given system.
  • Research articles
    Brillouin lidar and related basic physics
    Da-he LIU (刘大禾), Jin-wei SHI (石锦卫), Xu-dong CHEN (陈旭东), Min OUYANG (欧阳敏), Wen-ping GONG (弓文平),
    2010, 5(1): 82-106. https://doi.org/10.1007/s11467-009-0071-1
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    The principle of a lidar based on Brillouin scattering is introduced. The basic physics of the Brillouin lidar is discussed. The applications of the Brillouin lidar in remote sensing of the ocean, such as measurement of the sound speed and the bulk viscosity of water and detecting submerged objects are investigated. An actual Brillouin lidar system is developed. Also, several basic problems related to Brillouin lidar are studied in detail. The attenuation coeffcient of a pulsed laser beam with high pulsed energy in water is investigated; it is helpful to reveal the propagation property of a laser beam in water. The investigations on the threshold value of SBS are made theoretically and experimentally. Finally, a novel phenomena is investigated experimentally, in which Stimulated Raman scattering can be enhanced by stimulated Brillouin scattering.
  • Research articles
    A quantitative assessment of stochastic electrodynamics with spin (SEDS): Physical principles and novel applications
    Giancarlo CAVALLERI, Francesco BARBERO, Gianfranco BERTAZZI, Eros CESARONI, Ernesto TONNI, Leonardo BOSI, Gianfranco SPAVIERI, George T. GILLIES,
    2010, 5(1): 107-122. https://doi.org/10.1007/s11467-009-0080-0
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    Stochastic electrodynamics (SED) without spin, denoted as pure SED, has been discussed and seriously considered in the literature for several decades because it accounts for important aspects of quantum mechanics (QM). SED is based on the introduction of the nonrenormalized, electromagnetic stochastic zero-point field (ZPF), but neglects the Lorentz force due to the radiation random magnetic field Br. In addition to that rather basic limitation, other drawbacks remain, as well: i) SED fails when there are nonlinear forces; ii) it is not possible to derive the Schrödinger equation in general; iii) it predicts broad spectra for rarefied gases instead of the observed narrow spectral lines; iv) it does not explain double-slit electron diffraction patterns. We show in this short review that all of those drawbacks, and mainly the first most basic one, can be overcome in principle by introducing spin into stochastic electrodynamics (SEDS). Moreover, this modification of the theory also explains four observed effects that are otherwise so far unexplainable by QED, i.e., 1) the physical origin of the ZPF, and its natural upper cutoff; 2) an anomaly in experimental studies of the neutrino rest mass; 3) the origin and quantitative treatment of 1/f noise; and 4) the high-energy tail (～ 1021 eV) of cosmic rays. We review the theoretical and experimental situation regarding these things and go on to propose a double-slit electron diffraction experiment that is aimed at discriminating between QM and SEDS. We show that, in the context of this experiment, for the case of an electron beam focused on just one of the slits, no interference pattern due to the other slit is predicted by QM, while this is not the case for SEDS. A second experiment that could discriminate between QED and SEDS regards a transversely large electron beam including both slits obtained in an insulating wall, where the ZPF is reduced but not vanished. The interference pattern according to SEDS should be somewhat modified with respect to QED’s.