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arXiv:quant-ph/9802015

Quantum Entangled States and Quasiclassical Dynamics in Macroscopic Spin Systems

G. P. Berman, Gary D. Doolen, Gustavo V. López, V. I. Tsifrinovich

Published Feb 5, 1998• 2 citations
arXivHTMLPDF

Abstract

When dealing with macroscopic objects one usually observes quasiclassical phenomena, which can be described in terms of quasiclassical (or classical) equations of motion. Recent development of the theory of quantum computation is based on implementation of the entangled states which do not have a classical analogy. Using a simple example of a paramagnetic spin system we show that the entangled states can be detected in standard macroscopic experiments as a sharp deviation from quasiclassical motion.

DOI
10.48550/arxiv.quant-ph/9802015

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When dealing with macroscopic objects one usually observes quasiclassical phenomena, which can be described in terms of quasiclassical (or classical) equations of motion. Recent development of the theory of quantum computation is based on implementation of the entangled states which do not have a classical analogy. Using a simple example of a paramagnetic spin system we show that the entangled states can be detected in standard macroscopic experiments as a sharp deviation from quasiclassical motion.
In connection with the recent experiments on quantum measurement, such as the “which-way”detection using the Bragg scattering of cold atoms and quantum interference using a small molecular C 60 quantum system, we introduce the quantum measurement problem and its relevant fundamental concepts and ideas, such as quantum coherence, quantum decoherence and quantum entanglement. In order to understand the loss of quantum coherence for a quantum system due to its interaction with another system behaving classically or macroscopically, we discuss some quantum dynamical models to describe the quantum decoherence and quantum dissipation induced by couplings to the external environment or inner motions. In the classical or macroscopic limit, it is shown that the wave function of the total system formed by the quantum system plus another system interacting with it is factorized as an entangled state with correlation between the measured quantum states and the configurations of the external large system or the inner degrees of freedom. In association with a novel viewpoint about quantum measurement, which has been directly verified by recent experiments (e.g.,S. Durr et al., Nature,1998,33:359), it is shown that quantum entanglement is indeed responsible for the quantum decoherence, and thus it can be regarded as a “clean” quantum measurement when the large system behaves as a classical object. These arguments give us a possible solution to the “Schrdinger cat paradox” and to whether the coherent superposition of macroscopic states exists. These discussions are also applied to the quantum decoherence problems in quantum information including quantum computing and quantum communication.
In order to understand quantum decoherence of a quantum system due to its interaction with a large system behaving classically, we introduce the concept of adiabatic quantum entanglement based on the Born-Oppenhemeir approximation. In the adiabatic limit, it is shown that the wave function of the total system formed by the quantum system plus the large system can be factorized as an entangled state with correlation between adiabatic quantum states and quasi-classical motion configurations of the large system. In association with a novel viewpoint about quantum measurement, which has been directly verified by most recent experiments [e.g, S. Durr et.al, Nature 33, 359 (1998)], it is shown that the adiabatic entanglement is indeed responsible for the quantum decoherence and thus it can be regarded as a "clean" quantum measurement when the large system behaves as a classical object. The large system being taken respectively to be a macroscopically distinguishable spatial variable, a high spin system and a harmonic oscillator with a coherent initial state, three illustrations are present with their explicit solutions in this paper.
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Abstract In the standard quantum theory, one can measure precisely only a subset of the incompatible observables. It results in lack of a formal joint probability defining objective realism even if we accept nonlocal or certain faster‐than‐light interactions. Here, a construction of such realism extending the usual single‐copy description to many copies, partially analogous to familiar many worlds, is proposed. Failure of the standard single copy can be easily looked for experimentally. The copies should interact weakly at the macroscopic level, leading to effective collapse to a single identical pointer state. Experimental evidence for this conjecture could be obtained by detecting incomplete collapse in sequential measurements or finding deviations from the single‐copy Born rule when observing simple quantum systems.
The conventional quantum theory gives no real, that is, independent of the observer, natural description. If it is required that it must give an absolute reality of the natural phenomena at least in the macroscopic region, then the quantum theory must be changed. One possibility of that kind is discussed. A model of a quantum theory, which for small systems agrees with the usual theory, but for large systems deviates so much from them that the absolute reality of macroscopic size remains steady, is described. This was achieved by means of an additional term in the motion equation for the density matrix. This additional term plays no role for the small system. For macroscopic ion of the density matrix as if'' all macroscopic size of the system was observed perennially. (tr-auth)