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arXiv:2105.11881 · PRX Quantum

Loophole-Free Interferometric Test of Macrorealism Using Heralded Single Photons

Kaushik Joarder, Debashis Saha, Dipankar Home, Urbasi Sinha

Published Jan 12, 2022• 21 citations
arXivHTMLPDF

Abstract

Experiment shows an unambiguous violation of macro-realism for single photons using different versions of Leggett-Garg inequalities with non-invasive measurements, also providing a powerful platform for harnessing the non-classicality of photons in quantum technologies.
DOI
10.1103/prxquantum.3.010307

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According to the world view of macrorealism, the properties of a given system exist prior to and independent of measurement, which is incompatible with quantum mechanics. Leggett and Garg put forward a practical criterion capable of identifying violations of macrorealism, and so far experiments performed on microscopic and mesoscopic systems have always agreed with quantum mechanics. However, a macrorealist can always assign the cause of such violations to the perturbation that measurements effect on such small systems, and hence a definitive test would require using noninvasive measurements, preferably on macroscopic objects, where such measurements seem more plausible. However, the generation of truly macroscopic quantum superposition states capable of violating macrorealism remains a big challenge. In this work we propose a setup that makes use of measurements on the polarization of light, a property that has been extensively manipulated both in classical and quantum contexts, hence establishing the perfect link between the microscopic and macroscopic worlds. In particular, we use Leggett-Garg inequalities and the criterion of no signaling in time to study the macrorealistic character of light polarization for different kinds of measurements, in particular with different degrees of coarse graining. Our proposal is noninvasive for coherent input states by construction. We show for states with well-defined photon number in two orthogonal polarization modes, that there always exists a way of making the measurement sufficiently coarse grained so that a violation of macrorealism becomes arbitrarily small, while sufficiently sharp measurements can always lead to a significant violation.
Macrorealism is a characteristic feature of many, but not all, classical systems. It is known, for example, that classical light can violate a Leggett–Garg inequality and, hence, reject a macrorealist interpretation. A recent experiment has used entangled light and negative measurements to demonstrate a loophole-free test of macrorealism [Joarder et al., PRX Quantum 3, 010307 (2022)]. This paper shows that such an experiment, while soundly rejecting macrorealism, may nevertheless be open to a classical interpretation. This is done by offering an explicit classical model of heralded photon detection in an optical interferometer with beam blockers. A numerical analysis of the model shows good agreement with experimental observations and consistency with both local realism and a rejection of macrorealism.
The rise of quantum information theory has lent new relevance to experimental tests for non-classicality, particularly in controversial cases such as adiabatic quantum computing superconducting circuits. The Leggett-Garg inequality is a "Bell inequality in time" designed to indicate whether a single quantum system behaves in a macrorealistic fashion. Unfortunately, a violation of the inequality can only show that the system is either (i) non-macrorealistic or (ii) macrorealistic but subjected to a measurement technique that happens to disturb the system. The "clumsiness" loophole (ii) provides reliable refuge for the stubborn macrorealist, who can invoke it to brand recent experimental and theoretical work on the Leggett-Garg test inconclusive. Here, we present a revised Leggett-Garg protocol that permits one to conclude that a system is either (i) non-macrorealistic or (ii) macrorealistic but with the property that two seemingly non-invasive measurements can somehow collude and strongly disturb the system. By providing an explicit check of the invasiveness of the measurements, the protocol replaces the clumsiness loophole with a significantly smaller "collusion" loophole. © 2011 Springer Science+Business Media, LLC.