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arXiv:0812.0999

The macroscopic qubit delusion

Robert Alicki

Published Dec 4, 2008• 1 citation
arXivHTMLPDF

Abstract

There exists a large number of experimental and theoretical results supporting the picture of "macroscopic qubits" implemented, for instance, by Rydberg atoms, Josephson junctions or Bose-Einstein condensates - the systems which should rather emerge in localized semiclassical states. In this note it is shown how, under realistic conditions, the false qubit interpretation can be consistent with the restricted set of experimental data collected for semiclassical systems. The recent experiments displaying semiclassical character of Bose-Einstein condensates and possible quantumness tests for a single system are briefly invoked also.

DOI
10.48550/arxiv.0812.0999

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There exists a large number of experimental and theoretical results supporting the picture of "macroscopic qubits" implemented, for instance, by Rydberg atoms, Josephson junctions or Bose-Einstein condensates - the systems which should rather emerge in localized semiclassical states. In this note it is shown how, under realistic conditions, the false qubit interpretation can be consistent with the restricted set of experimental data collected for semiclassical systems. The recent experiments displaying semiclassical character of Bose-Einstein condensates and possible quantumness tests for a single system are briefly invoked also.
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In this paper I first briefly review the apparently bizarre consequences of attempting to apply quantum mechanics consistently to macroscopic bodies in certain kinds of situations (the ‘Schrödinger's Cat’ paradox). The paradox results from the assumption that the linear superpositions of macroscopically distinct states, which are unambiguously predicted by the formalism, do indeed exist in nature. Until recently, this assumption was experimentally untested. However, recent advances in techniques of microfabrication, cryogenics and noise control have opened up the possibility of indirect and possibly even direct tests, in particular in superconducting systems incorporating Josephson devices. I describe some of these tests, review the current experimental and theoretical situation and discuss the implications of various possible outcomes to the experiments.
There exists a large number of experimental and theoretical results supporting the picture of "macroscopic qubits" implemented by nanoscopic Josephson junctions. On the other hand the standard model of such systems given in terms of a single degree of freedom suggests their semiclassical behavior due to a localization mechanism caused by a strong coupling to an environment. Indeed, such a mechanism is observed in an atomic Bose-Einstein condensate (BEC) placed in a double-well potential - a system mathematically equivalent to a Josephson junction. In this note it is shown, on the example of a Cooper pair box, that replacing the BEC-type model for Cooper pairs by a lattice gas model one can reduce the environmental effects of "dequantization" and can explain the experimental data in particular the existing huge differences between the measured values of relaxation times.
Abstract Quantum effects like the superposition principle contradict our experience of daily life. Decoherence can be viewed as a possible explanation why we do not observe quantum superposition states in the macroscopic world. In this article, we use the qubit ansatz to discuss decoherence in the simplest possible model system and propose a visualization for the microscopic origin of decoherence, and the emergence of a so-called pointer basis. Finally, we discuss the possibility of ‘macroscopic’ quantum effects.