Weak measurement has provided new insight into the nature of quantum measurement, by demonstrating the ability to extract average state information without fully projecting the system. For single-qubit measurements, this partial projection has been demonstrated with violations of the Leggett–Garg inequality. Here we investigate the effects of weak measurement on a maximally entangled Bell state through application of the Hybrid Bell–Leggett–Garg inequality (BLGI) on a linear chain of four transmon qubits. By correlating the results of weak ancilla measurements with subsequent projective readout, we achieve a violation of the BLGI with 27 s.d.s. of certainty.
White, T. C. et al. Preserving entanglement during weak measurement demonstrated with a violation of the Bell–Leggett–Garg inequality. NPJ Quantum Information 2, 15022 (2016). doi: 10.1038/npjqi.2015.22
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