Probing quantumness with joint continuous measurements of noncommuting qubit observables

Luis Pedro García-Pintos and Justin Dressel
Phys. Rev. A 94, 062119 – Published 23 December 2016

Abstract

We analyze the continuous measurement of two noncommuting observables for a qubit, and investigate whether the simultaneously observed noisy signals are consistent with the evolution of an equivalent classical system. Following the approach outlined by Leggett and Garg, we show that the readouts violate macrorealistic inequalities for arbitrarily short temporal correlations. Moreover, the derived inequalities are manifestly violated even in the absence of Hamiltonian evolution, unlike for Leggett-Garg inequalities that use a single continuous measurement. Such a violation should indicate the failure of at least one postulate of macrorealism: either physical quantities do not have well-defined values at all times or the measurement process itself disturbs what is being measured. Nevertheless, for measurements of equal strength we are able to construct a classical stochastic model for a spin that perfectly emulates both the qubit evolution and the observed noisy signals, thus emulating the violations; interestingly, this model also requires an unphysical noise to emulate the readouts, which effectively restricts the ability of an observer to learn information about the spin.

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  • Received 8 July 2016

DOI:https://doi.org/10.1103/PhysRevA.94.062119

©2016 American Physical Society

Physics Subject Headings (PhySH)

Quantum Information, Science & Technology

Authors & Affiliations

Luis Pedro García-Pintos1,* and Justin Dressel1,2

  • 1Institute for Quantum Studies, Chapman University, 1 University Drive, Orange, California 92866, USA
  • 2Schmid College of Science and Technology, Chapman University, 1 University Drive, Orange, California 92866, USA

  • *lpgarciapintos@gmail.com

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Issue

Vol. 94, Iss. 6 — December 2016

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