Abstract

Quantum coherence marks a deviation from classical physics, and has been studied as a resource for metrology and quantum computation. Finding reliable and effective methods for assessing its presence is then highly desirable. Coherence witnesses rely on measuring observables whose outcomes can guarantee that a state is not diagonal in a known reference basis. Here we experimentally measure a novel type of coherence witness that uses pairwise state comparisons to identify superpositions in a basis-independent way. Our experiment uses a single interferometric set-up to simultaneously measure the three pairwise overlaps among three single-photon states via Hong-Ou-Mandel tests. Besides coherence witnesses, we show the measurements also serve as a Hilbert-space dimension witness. Our results attest to the effectiveness of pooling many two-state comparison tests to ascertain various relational properties of a set of quantum states.

Highlights

  • Quantum coherence is a resource for quantum metrological advantage and quantum computational speedup, and is central to phenomena such as superfluidity, superconductivity, and Bose-Einstein condensation [1,2]

  • Aside from coherence witnesses, we show the measurements serve as a Hilbert-space dimension witness

  • In a HOM test, two photons impinge on different input ports of a 50/50 beam splitter, and the probability that photons come out bunched in a single output port pb is given by pb = (1 + rAB)/2 [41,43,47]

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Summary

Introduction

Quantum coherence is a resource for quantum metrological advantage and quantum computational speedup, and is central to phenomena such as superfluidity, superconductivity, and Bose-Einstein condensation [1,2]. It is possible to create superposition states of the various degrees of freedom of photons [6]: polarization, path [7], transverse mode structure [8,9,10,11,12], time of arrival [13,14,15,16], frequency [17], etc This flexibility, associated with their fast propagation and resistance to decoherence, has resulted in the use of photons for tests of the foundations of quantum theory [16,18,19], demonstrations of cryptographic key distribution [11,15,20,21,22], and other quantum communication protocols [23]. Photons are the perfect information carriers to connect different nodes of a distributed quantum computer, and may even yield scalable, fully photonic quantum computers [26]

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