A strict experimental test of macroscopic realism in a superconducting flux qubit

TLDR

Macroscopic realism modifies quantum theory to allow macroscopic objects to be described independently of measurement while preserving microscopic quantum behavior, with objective collapse theories as a prominent example. The study aims to test whether macroscopic realism theories accurately describe nature by implementing a noise‑tolerant experimental protocol. The protocol was executed on a superconducting flux qubit, incorporating a noise‑tolerant design and control experiments to close the clumsiness loophole. The experiment ruled out macroscopic realism theories denying coherent superpositions of 170 nA currents over ~10 ns with ~84 σ significance, providing strong evidence for macroscopic superposition.

Abstract

Abstract Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurement-independent manner, while largely preserving a fully quantum mechanical description of the microscopic world. Objective collapse theories are examples which aim to solve the quantum measurement problem through modified dynamical laws. Whether such theories describe nature, however, is not known. Here we describe and implement an experimental protocol capable of constraining theories of this class, that is more noise tolerant and conceptually transparent than the original Leggett–Garg test. We implement the protocol in a superconducting flux qubit, and rule out (by ∼84 s.d.) those theories which would deny coherent superpositions of 170 nA currents over a ∼10 ns timescale. Further, we address the ‘clumsiness loophole’ by determining classical disturbance with control experiments. Our results constitute strong evidence for the superposition of states of nontrivial macroscopic distinctness.