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arXiv:2408.03259 · Physical Review Letters

Single-Photon Interference over 8.4 km Urban Atmosphere: Toward Testing Quantum Effects in Curved Spacetime with Photons

Hui-Nan Wu, Yuhuai Li, Bo Li, Xiang You, Runze Liu, Ji-Gang Ren, Juan Yin, Chao‐Yang Lu, Yuan Wen Cao, Cheng-Zhi Peng, Jian-Wei Pan

Published Jul 12, 2024• 18 citations
arXivHTMLPDF

Abstract

The emergence of quantum mechanics and general relativity has transformed our understanding of the natural world significantly. However, integrating these two theories presents immense challenges, and their interplay remains untested. Recent theoretical studies suggest that the single-photon interference covering huge space can effectively probe the interface between quantum mechanics and general relativity. We developed an alternative design using unbalanced Michelson interferometers to address this and validated its feasibility over an 8.4 km free-space channel. Using a high-brightness single-photon source based on quantum dots, we demonstrated single-photon interference along this long-distance baseline. We achieved a phase measurement precision of 16.2 mrad, which satisfied the measurement requirements for a gravitational redshift at the geosynchronous orbit by 5 times the standard deviation. Our results confirm the feasibility of the single-photon version of the Colella-Overhauser-Werner experiment for testing the quantum effects in curved spacetime.

DOI
10.1103/physrevlett.133.020201

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The emergence of quantum mechanics and general relativity has transformed our understanding of the natural world significantly. However, integrating these two theories presents immense challenges, and their interplay remains untested. Recent theoretical studies suggest that the single-photon interference covering huge space can effectively probe the interface between quantum mechanics and general relativity. We developed an alternative design using unbalanced Michelson interferometers to address this and validated its feasibility over an 8.4 km free-space channel. Using a high-brightness single-photon source based on quantum dots, we demonstrated single-photon interference along this long-distance baseline. We achieved a phase measurement precision of 16.2 mrad, which satisfied the measurement requirements for a gravitational redshift at the geosynchronous orbit by 5 times the standard deviation. Our results confirm the feasibility of the single-photon version of the Colella-Overhauser-Werner experiment for testing the quantum effects in curved spacetime.
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.
Quantum technologies present new opportunities for fundamental tests of nature. One potential application is to probe the interplay between quantum physics and general relativity—a field of physics with no empirical evidence yet. Here we show that quantum networks open a new window to test this interface. We demonstrate how photon mediated entanglement between atomic or atomlike systems can be used to probe time-dilation-induced entanglement and interference modulation. Key are nonlocal measurements between clocks in a gravitational field, which can be achieved either through direct photon interference or by using auxiliary entanglement. The resulting observable depends on the interference between different proper times, and can only be explained if both quantum theory and general relativity are taken into account. The proposed protocol enables clock interferometry on kilometer-scale separations and beyond. Our work thus shows a realistic experimental route for a first test of quantum theory on curved spacetime, opening up new scientific opportunities for quantum networks.
Quantum mechanics and general relativity are the foundational pillars of modern physics, yet experimental tests that combine the two frameworks remain rare. Measuring optical phase shifts of massless photons in a gravitational potential provides a unique quantum platform to probe gravity beyond Newtonian descriptions, but laboratory-based interferometers have not yet reached the sensitivity needed to access this regime. Here, we report the realization of a 50-km table-top Mach-Zehnder fiber interferometer operating at the single-photon level, achieving a phase sensitivity of 4.42×10−64.42\times10^{-6}4.42×10−6 rad root-mean-square (RMS) within the frequency range of 0.01 Hz to 5 Hz. We demonstrate that this sensitivity is sufficient to resolve a phase-shift signal of (6.18±0.44)×10−5(6.18 \pm 0.44)\times10^{-5}(6.18±0.44)×10−5 rad RMS at 0.1 Hz, associated with a modulated gravity-induced signal. Our results establish a milestone for quantum sensing with large-scale optical interferometry, demonstrating the capability to detect gravitational redshifts in a local laboratory, thereby paving the way for testing quantum phenomena within general relativistic frameworks.
In this paper we propose an experiment designed to observe a general-relativistic effect on single photon interference. The experiment consists of a folded Mach-Zehnder interferometer, with the arms distributed between a single Earth orbiter and a ground station. By compensating for other degrees of freedom and the motion of the orbiter, this setup aims to detect the influence of general relativistic time dilation on a spatially superposed single photon. The proposal details a payload to measure the required effect, along with an extensive feasibility analysis given current technological capabilities.
n n n The precision of physical parameters is fundamentally limited by irreducible levelsof noise set by quantum mechanics. Quantum metrology is the study of reaching theselimits of noise by employing optimal schemes for parameter estimation. Techniquesin quantum metrology can assist in developing devices to measure the fundamentalinterplay between quantum mechanics and general relativity at state-of-the-artprecision. An example of this is the recent detection of gravitational waves bythe LIGO interferometer. In this thesis, we focus on using quantum metrologyfor estimating space-time parameters. We show the optimal quantum resourcesthat are needed for estimating the gravitational redshift of light propagating in theSchwarzschild space-time of Earth including the inevitable losses due to atmosphericdistortion. We also propose a quantum interferometer using higher order Kerrnon-linearities to improve the sensitivity of estimating gravitational time dilation.In principle, we would be able to downsize interferometers and probe gravity over asmall scale potentially making it practical for measuring gravitational gradients. Wethen study the interesting features of the metric around a rotating massive bodyknown as the Kerr metric and propose implementing a stationary interferometer tomeasure the effect of frame dragging. Finally, we consider loss in the visibility ofquantum interference of single photons in rotating reference frames, and analogouslyin the Kerr metric. In essence, the quantum interference of photons will be affectedby the relativistic effect of rotation. We find experimentally feasible parametersrequiring long optical fibre for long coherence lengths of photons. Our results willhopefully contribute to the efforts of building future quantum technologies that willenter a new regime where general relativistic effects can be measured.
The focus of this work is discussing the realization of space based experiments testing quantum mechanics, specifically quantum optics. The main experimental work is the practical realization of the earthbound part of a possible interferometric experiment that can be realized with a delayed-choice operational mode. The full Earth-satellite realization of this experimental scheme could set a new record on the distance to which delayed-choice schemes have been experimentally realized. General relativistic effects on Earth-satellite experiments is also discussed, in the case of two different setups. One of those is essentially the ground-space implementation of the interferometer realized in the laboratory, while the other represents a different possibility that could in principle measure gravitational redshift to single photons to greater extent. Which-path behavior and interference were observed to a very good degree on the interferometer realized in the lab. Specifically, the assessment of interference visibility, which was done after careful alignement of the setup, yields a value of approximately 95%, and a comparable value of approx 93% was obtained with an alternative setup which was also experimentally realized in the laboratory. As far as theoretical computations are concerned, a good formalism to deal with space-based interferometric measurements was pointed out, which allows to take into account both special relativistic contributes and general relativistic corrections to the amplitude describing the photons involved with this kind of experiments. Two different setups, which are candidates for the measurement of single photon gravitational redshift, were analyzed with this formalism. The setup with one interferometer on the ground and the other on the satellite, in principle, constitutes a good way to observe this effect by means of a phase measurement, to the first order in the gravitational potentials.