Testing contextuality on quantum ensembles with one clean qubit

TL;DR

Contextuality tests have been contentious for finite-precision experiments and may require special quantum states. This paper measures correlations directly on quantum ensembles using a probe qubit, then reports a contextuality-inequality violation under fair sampling.

• Problem

  Contextuality tests have been contentious for finite-precision experiments, and prior approaches have involved creating special quantum states.

  Source: p. 1

• Method

  The protocol uses an ancillary probe qubit to measure correlations on ensembles, choosing a maximally mixed system so only the probe qubit is clean.

  Source: p. 2

• Results

  β = 5.2 ± 0.1, violating inequality (2); the authors interpret the ensemble experiment as revealing correlations unexplained by NCHV theory under fair sampling.

  Source: pp. 3–4

• Takeaways & Limitations

  The protocol measures ensemble correlations without repeated projective measurements on individual systems and extends to similar inequalities on higher-dimensional systems.

  Source: p. 4

  The inequality's preparation independence requires all terms to be estimated using the same preparation.

  Source: p. 1