Violation of noninvasive macrorealism by a superconducting qubit: Implementation of a Leggett-Garg test that addresses the clumsiness loophole

Abstract

The Leggett-Garg inequality holds for any macrorealistic system that is being measured noninvasively. A violation of the inequality can signal that a system does not conform to our primal intuition about the physical world. Alternatively, a violation can simply indicate that "clumsy" experimental technique led to invasive measurements. Here, we consider a recent Leggett-Garg test designed to try to rule out the mundane second possibility. We tailor this Leggett-Garg test to the IBM 5Q Quantum Experience system and find compelling evidence that qubit Q2 of the system cannot be described by noninvasive macrorealism.

16-qubit IBM universal quantum computer can be fully entangled

“…Hence, after the launch of the IBM Q, many groups tested it and performed quantum computing experiments on the cloud; for instance, see [6][7][8][9][10][11][12].…”

Leggett-Garg macrorealism and the quantum nondisturbance conditions

“…[2] for a review of older experiments and Refs. [13][14][15][16][17][18][19][20] for more recent ones. Furthermore, ways of tightening the loophole have been proposed and implemented, which include special measurement implementations and additional tests involving different evolutions or different preparations [1,11,13,16,18,[21][22][23][24].…”

Measurement of the Entanglement Spectrum of a Symmetry-Protected Topological State Using the IBM Quantum Computer

“…Thus our problem is reduced to solving for the eigenvectors and eigenvalues of µ and picking d non-negative eigenvalues for ρ that minimize the sum in Eq. (18). Let the eigenvalues of µ (ρ) be µ i (ρ i ) and arrange them such that µ i ≥ µ i+1 .…”