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

TL;DR

This study tests macroscopic realism using a superconducting flux qubit, providing strong evidence against certain objective collapse theories and supporting the existence of macroscopic superpositions.

Contribution

The paper introduces a noise-tolerant experimental protocol to test macroscopic realism and applies it to a superconducting flux qubit, strengthening the evidence for macroscopic superpositions.

Findings

Rules out theories denying superpositions of macroscopic currents

Demonstrates superpositions over 170 nA currents for ~10 ns

Addresses the 'clumsiness loophole' with control experiments

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.