Obtaining tight bounds on higher-order interferences with a 5-path interferometer

Abstract

Within the established theoretical framework of quantum mechanics, interference always occurs between pairs of paths through an interferometer. Higher order interferences with multiple constituents are excluded by Born's rule and can only exist in generalized probabilistic theories. Thus, high-precision experiments searching for such higher order interferences are a powerful method to distinguish between quantum mechanics and more general theories. Here, we perform such a test in an optical multi-path interferometer, which avoids crucial systematic errors, has access to the entire phase space and is more stable than previous experiments. Our results are in accordance with quantum mechanics and rule out the existence of higher order interference terms in optical interferometry to an extent that is more than four orders of magnitude smaller than the expected pairwise interference, refining previous bounds by two orders of magnitude.

How to Measure the Quantum Measure

“…Beyond level 2, several researchers have been investigating the possibility of theories residing at level 3 or higher [2,15,14,16,22,19,21,29], but for the moment there has not been any evidence of higher-order interference from the experiments that have looked for it. See for example the three-slit experiments that have put increasingly stringent bounds on third order interference.…”

Hybrid waveguide-bulk multi-path interferometer with switchable amplitude and phase

“…[13][14][15][16]28 Here, the absence of ϵ 3 is implied by electrodynamics 21 and will be used for a consistency check of the interferometer. Note that ϵ 3 = 0 must hold also in the presence of non-vanishing reflected signals in the "off"-state of a mirror.…”

“…The only notable exception to this rule would be produced by correlations between the reflectivity and phase of a mirror with the settings of the other mirrors, as such correlations would lead to cross-talk between the paths of the interferometer and bias the second-order interference term. 16 The measured distribution of ϵ 3 is plotted in Fig. 4(a).…”

“…While being free of systematic errors on the current level of precision, that precision still falls short of the state of the art. 16 A large part of this imprecision arises from the fact that each amplitude switching by mirror translation also affects the relative phase between the paths. An improved version of the hybrid interferometer would, therefore, aim at decoupling transmission and phase switching from each other.…”

“…For many applications, however, it is also desirable to be able to switch and manipulate the interferometer. For example, multi-path interferometers with switchable transmission amplitudes are a promising platform for investigations on the foundations of quantum mechanics, [12][13][14][15][16] whereas tunable phases can be used to tailor the output state of the photons. 11 The two requirements of stability and capability for manipulation are often in conflict with each other.…”