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arXiv:2006.04221 · Physical Review Research

Frame dragging and the Hong-Ou-Mandel dip: Gravitational effects in multiphoton interference

Anthony J. Brady, Stav Haldar

Published Apr 8, 2021• 19 citations
arXivHTMLPDF

Abstract

Applying quantum field theory on a classical, curved space-time has led to the enthralling theoretical predictions of, e.g., black hole evaporation and particle creation in the early, expanding universe.However, the prospects of observing such in the near to mid future seem far from practical.This is discouraging because extrapolation of general relativity "down to the Hilbert space level," though expected, carries no empirical support as of yet.On the other hand, fueled by the continuing developments in quantum technology, there has been growing interest in the prospect of observing more modest effects of general relativity at the level of quantum interference phenomena.Motivated by these recent developments, in this work, we investigate the Hong-Ou-Mandel (HOM) effect-a two-photon quantum-interference effect-in the space-time of a rotating spherical mass.In particular, we propose and analyze a common-path setup restricted to the surface of the Earth and show that, in principle, gravitational frame dragging induces observable shifts in the two-photon HOM dip.For completeness and correspondence with current literature, we also analyze the emergence of gravitational time dilation, for a dual-arm interferometer.The formalism thus presented establishes a basis for encoding general-relativistic effects into local, multiphoton, quantum-interference experiments.We also consider signatures of noninertial reference-frame effects on two-photon interference such as, e.g., the Sagnac effect induced by Earth's rotation, which seems detectable with current quantum-optical technology.This can be viewed as a stepping-stone towards the more ambitious goal of observing gravitational frame dragging at the level of individual photons.

DOI
10.1103/physrevresearch.3.023024

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Applying quantum field theory on a classical, curved space-time has led to the enthralling theoretical predictions of, e.g., black hole evaporation and particle creation in the early, expanding universe.However, the prospects of observing such in the near to mid future seem far from practical.This is discouraging because extrapolation of general relativity "down to the Hilbert space level," though expected, carries no empirical support as of yet.On the other hand, fueled by the continuing developments in quantum technology, there has been growing interest in the prospect of observing more modest effects of general relativity at the level of quantum interference phenomena.Motivated by these recent developments, in this work, we investigate the Hong-Ou-Mandel (HOM) effect-a two-photon quantum-interference effect-in the space-time of a rotating spherical mass.In particular, we propose and analyze a common-path setup restricted to the surface of the Earth and show that, in principle, gravitational frame dragging induces observable shifts in the two-photon HOM dip.For completeness and correspondence with current literature, we also analyze the emergence of gravitational time dilation, for a dual-arm interferometer.The formalism thus presented establishes a basis for encoding general-relativistic effects into local, multiphoton, quantum-interference experiments.We also consider signatures of noninertial reference-frame effects on two-photon interference such as, e.g., the Sagnac effect induced by Earth's rotation, which seems detectable with current quantum-optical technology.This can be viewed as a stepping-stone towards the more ambitious goal of observing gravitational frame dragging at the level of individual photons.
In this study, we investigate how Earth's gravitational field affects Hong-Ou-Mandel (HOM) interference experiments conducted in a terrestrial laboratory. To second order, we calculate the relativistic time delay from the null geodesic equation (particle perspective), while the phase shift and the associated effective time delay are derived from the Klein-Gordon equation (wave perspective). Since gravity influences both the temporal and spatial parts of the phase shift, these two time delays differ and lead to different coincidence probabilities. The previous HOM experiment conducted on a rotating platform suggests that the wave perspective can explain the experimental results. We further explore the frame dragging and redshift effects in an arbitrarily oriented rectangular interferometer under two distinct scenarios with different photon paths, measuring one effect in each scenario. We find that both effects can be amplified by increasing the number of light loops. Additionally, we emphasize that the next-to-leading order Sagnac effect, arising from gravitational acceleration, is comparable to the Thomas precession, the geodetic effect, and the Lense-Thirring effect. To detect the leading order Sagnac effect and the redshift effect caused by gravitational acceleration, we estimate the number of loops that photons should travel in the interferometer. Furthermore, we propose that the difference between two HOM patterns can be used as a probe to detect gravitational effects on quantum systems.
Quantum mechanics and general relativity have been extensively and independently confirmed in many experiments. However, the interplay of the two theories has never been tested: all experiments that measured the influence of gravity on quantum systems are consistent with non-relativistic, Newtonian gravity. On the other hand, all tests of general relativity can be described within the framework of classical physics. Here we discuss a quantum interference experiment with single photons that can probe quantum mechanics in curved space-time. We consider a single photon traveling in superposition along two paths in an interferometer, with each arm experiencing a different gravitational time dilation. If the difference in the time dilations is comparable with the photon's coherence time, the visibility of the quantum interference is predicted to drop, while for shorter time dilations the effect of gravity will result only in a relative phase shift between the two arms. We discuss what aspects of the interplay between quantum mechanics and general relativity are probed in such experiments and analyze the experimental feasibility.
Two-photon interference effects arise because photons are indistinguishable particles. In the wellknown Hong-Ou-Mandel (HOM) effect, the transmission of two photons at a beam splitter interferes destructively with the reflection of both photons, requiring both photons to "bunch up" by leaving the beam splitter on the same side. Here, we show that the interference between locally propagating photons and photons exchanged by a mode swap can be implemented by post-selecting spatially separated photon outputs of a four-path interferometer. Even though the photons detected at spatially separated locations must have travelled along paths that never met up at the same beam splitter, the Hong-Ou-Mandel effect can be observed in correlations between the output ports that originate from the association of detection events with non-local output modes defined by the two single photon inputs. Local phase shifts can be used to map out non-classical correlations between the photons detected at different output locations, clarifying the role of linear optics in generating entanglement between spatially separated photons. Our work thus establishes a fundamental relation between multiphoton interference and entanglement, opening the door to new possibilities in optical quantum technologies.
We consider the time delay of interfering single photons oppositely traveling in the Kerr metric of a rotating massive object. Classically, the time delay shows up as a phase difference between coherent sources of light. In quantum mechanics, the loss in visibility due to the indistinguishability of interfering photons is directly related to the time delay. We can thus observe the Kerr frame dragging effect using the Hong–Ou–Mandel dip, a purely quantum mechanical effect. By Einstein's equivalence principle, we can analogously consider a rotating turntable to simulate the Kerr metric. We look at the feasibility of such an experiment using optical fiber and note a cancellation in the second order dispersion but a direction dependent difference in group velocity. However, for the chosen experimental parameters, we can effectively assume light propagating through a vacuum.
Although quantum physics is well understood in inertial reference frames (flat spacetime), a current challenge is the search for experimental evidence of nontrivial or unexpected behavior of quantum systems in noninertial frames. Here, we present a novel test of quantum mechanics in a noninertial reference frame: we consider Hong-Ou-Mandel (HOM) interference on a rotating platform and study the effect of uniform rotation on the distinguishability of the photons. Both theory and experiments show that the rotational motion induces a relative delay in the photon arrival times at the exit beam splitter and that this delay is observed as a shift in the position of the HOM dip. This experiment can be extended to a full general relativistic test of quantum physics using satellites in Earth's orbit and indicates a new route toward the use of photonic technologies for investigating quantum mechanics at the interface with relativity.
We investigate the effect of entanglement dynamics due to gravity – the basis of a mechanism of universal decoherence – for photonic states and quantum memories in Mach-Zehnder and Hong-Ou-Mandel interferometry setups in the gravitational field of the earth. We show that chances are good to witness the effect with near-future technology in Hong-Ou-Mandel interferometry. This would represent an experimental test of theoretical modeling combining a multi-particle effect predicted by the quantum theory of light and an effect predicted by general relativity. Our article represents the first analysis of relativistic gravitational effects on space-based quantum memories which are expected to be an important ingredient for global quantum communication networks.