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.
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.
A test of quantum gravity Quantum mechanics and the general theory of relativity represent two pillars of modern physics, but unification of the two theories remains an open problem. Theories of quantum gravity abound, but they tend to lack an experimental foundation. One such proposed theory, event formalism, predicts that a pair of entangled particles decorrelate as they pass through different regions of the gravitational well of a planetary object. Xu et al. present results of a quantum optical test of this proposal using the quantum satellite Micius. Using entangled photon pairs, one sent to the satellite and the other retained on Earth, they find no evidence for the predicted decorrelation effects. The results may help shed light on the interplay between quantum theory and gravity. Science , this issue p. 132
Gravitational time dilation implies that clocks held at different heights accumulate different proper times. We analyze a memory-assisted quantum clock interferometer in which a frequency-bin photonic clock is stored in two vertically separated quantum memories for a controllable duration, such that the joint state evolves in a quantum superposition of two proper times. After retrieval, the photonic modes interfere in a Hong-Ou-Mandel (HOM) interferometer, for which we derive analytic expressions for the resulting multiphoton detection statistics. Extending this HOM-based scheme from entangled photon pairs to frequency-entangled 2N-photon inputs, we show that the proper-time dependent phase is amplified by a factor N, leading to an N-times faster collapse and revival of the interference signal compared with the two-photon case. Incorporating finite memory efficiency and lifetime, we identify regimes where this modulation remains observable. For parameters compatible with demonstrated Rb and Cs memories and achievable optical frequency separations, the first collapse occurs for height differences in the order of 10-100 m with subsecond to few-second storage times, while suitable rare-earth ion and alkali memory combinations can reduce the required height to the few-metre scale. These results establish near-term laboratory conditions for observing entanglement dynamics driven by gravitational time dilation in a photonic platform.
Relativistic quantum information investigates the interplay between quantum mechanics and relativity and is becoming increasingly relevant for future quantum technologies operating over large distances or in gravitational fields. This dissertation studies how relativistic effects influence the storage, transmission, and interference of quantum states of light, with particular emphasis on quantum memories and frequency-entangled photons. A theoretical framework is developed to describe relativistic transformations of photonic quantum states and their interaction with realistic quantum memories. Special attention is given to the influence of gravitational redshift and relativistic motion on quantum coherence, entanglement, and Hong–Ou–Mandel interference. The resulting models provide quantitative predictions for experimentally accessible scenarios and identify conditions under which relativistic effects become relevant for quantum communication and distributed quantum information processing. The presented results contribute to the emerging field of relativistic quantum information by connecting concepts from quantum optics, quantum communication, and general relativity. Beyond their fundamental significance, they provide theoretical tools for the development of future space-based quantum networks and relativistic quantum technologies. Beyond the results presented in this dissertation, the developed framework points toward a broader research program centered on the **Nonlinear Separability Problem**. This problem is proposed as a unifying mathematical perspective for investigating the limits of distinguishability, classification, and information processing in both classical and quantum systems. Its formulation naturally connects to a wide range of contemporary research areas, including computational complexity theory (e.g., P versus NP), quantum information processing, machine learning, optimization, information theory, quantum foundations, and mathematical physics. In particular, the framework suggests new approaches to studying the role of nonlinearity in quantum state discrimination, quantum communication protocols, and the mathematical structure of high-dimensional information spaces. While many of these directions remain to be explored, the present work establishes a conceptual and mathematical basis for future investigations. It is anticipated that further development of the Nonlinear Separability Problem may contribute to a deeper understanding of fundamental questions across mathematics, computer science, and quantum technologies, thereby opening new interdisciplinary avenues of research.
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.
Quantum Physics
Quantum mechanics and the general theory of relativity represent two pillars of modern physics, but unification of the two theories remains an open problem. Theories of quantum gravity abound, but they tend to lack an experimental foundation. One such proposed theory, event formalism, predicts that a pair of entangled particles decorrelate as they pass through different regions of the gravitational well of a planetary object. Xu et al. present results of a quantum optical test of this proposal using the quantum satellite Micius. Using entangled photon pairs, one sent to the satellite and the other retained on Earth, they find no evidence for the predicted decorrelation effects. The results may help shed light on the interplay between quantum theory and gravity.
Science , this issue p. [132][1]
[1]: /lookup/doi/10.1126/science.aay5820
We present a fully quantum field-theoretic framework for gravitational wave (GW) detection, describing the interaction as inelastic photon-graviton scattering. We demonstrate that for \textit{a coherent graviton state}, this microscopic model explicitly reproduces the standard classical geometric-optics time delay, providing a rigorous consistency test of the correspondence limit. We propose a detection scheme based on Hong-Ou-Mandel interference, where the GW-induced spectral shifts render entangled photon pairs distinguishable. This encodes the gravitational signal into the modulation of photon coincidence rates --- a quantum observable robust against laser intensity noise. Furthermore, we show that non-classical graviton states, such as squeezed states, induce unique decoherence signatures, establishing theoretical bounds on gravitational decoherence and defining the operational limits for probing the quantum nature of gravity.