Abstract Quantum coherence is a basis-dependent resource whose operational meaning becomes particularly subtle when quantum fields are observed from non-inertial reference frames or propagate through curved spacetime. In relativistic settings, the distinction between particles, modes, and accessible subsystems depends on the observer, while gravitational or accelerated horizons can restrict portions of the field from direct measurement. These effects imply that coherence assigned to one set of field modes need not coincide with that reconstructed by another observer. The problem becomes more intricate under non-stationary relativistic conditions, where acceleration, spacetime geometry, or mode structure changes with time and no single global decomposition into positive- and negative-frequency components remains universally preferred. This study develops a conceptual framework for analysing observer-dependent quantum coherence in such environments. The discussion combines quantum field theory in curved spacetime, the Unruh and Hawking effects, relativistic quantum information, and the resource-theoretic description of quantum coherence. Curved-spacetime field theory establishes that the concept of a particle is not generally observer independent [1]. Algebraic and geometric formulations further demonstrate that physically meaningful observables can remain well defined even when global particle interpretations become ambiguous [2]. Resource theory provides a complementary language for distinguishing coherence intrinsic to a chosen state representation from changes caused by observer-dependent mode decomposition [7]. Particular attention is given to accelerated observers, horizons, Bogoliubov mode mixing, time-dependent acceleration, dynamically changing accessibility of field sectors, and the distinction between genuine decoherence and redistribution of coherence into inaccessible degrees of freedom. Non-stationary relativistic motion is treated as a dynamical source of basis change rather than merely a perturbation of a stationary accelerated frame.
Abstract Quantum coherence is a basis-dependent resource whose operational meaning becomes particularly subtle when quantum fields are observed from non-inertial reference frames or propagate through curved spacetime. In relativistic settings, the distinction between particles, modes, and accessible subsystems depends on the observer, while gravitational or accelerated horizons can restrict portions of the field from direct measurement. These effects imply that coherence assigned to one set of field modes need not coincide with that reconstructed by another observer. The problem becomes more intricate under non-stationary relativistic conditions, where acceleration, spacetime geometry, or mode structure changes with time and no single global decomposition into positive- and negative-frequency components remains universally preferred. This study develops a conceptual framework for analysing observer-dependent quantum coherence in such environments. The discussion combines quantum field theory in curved spacetime, the Unruh and Hawking effects, relativistic quantum information, and the resource-theoretic description of quantum coherence. Curved-spacetime field theory establishes that the concept of a particle is not generally observer independent [1]. Algebraic and geometric formulations further demonstrate that physically meaningful observables can remain well defined even when global particle interpretations become ambiguous [2]. Resource theory provides a complementary language for distinguishing coherence intrinsic to a chosen state representation from changes caused by observer-dependent mode decomposition [7]. Particular attention is given to accelerated observers, horizons, Bogoliubov mode mixing, time-dependent acceleration, dynamically changing accessibility of field sectors, and the distinction between genuine decoherence and redistribution of coherence into inaccessible degrees of freedom. Non-stationary relativistic motion is treated as a dynamical source of basis change rather than merely a perturbation of a stationary accelerated frame.
Abstract Quantum coherence is a fundamental resource underlying interference, entanglement-assisted processes, quantum information transfer, and the operational distinction between classical and quantum states. In laboratory-scale systems, coherence is commonly analysed relative to approximately inertial observers and weak gravitational environments. A more complete description becomes necessary when quantum systems are considered in accelerated reference frames, strong gravitational fields, or curved spacetime geometries. Under such conditions, the definition of particles, field modes, temporal evolution, and observable quantum correlations can depend on the observer and on the spacetime structure used to describe the system. This article examines the conceptual and physical behaviour of relativistic quantum coherence in non-inertial frames and curved spacetime environments without relying on a formulation based on explicit mathematical expressions. Particular attention is given to accelerated observers, horizon-related mode restrictions, observer-dependent vacuum descriptions, gravitational frequency shifts, field localization, and the transformation of accessible quantum information. Coherence is treated not merely as an intrinsic property of an isolated microscopic object but as a relational quantity affected by the physical decomposition of a quantum field into operationally accessible degrees of freedom. The analysis distinguishes coherence degradation from fundamental destruction of quantum information and emphasizes the role of inaccessible field sectors, measurement limitations, and observer-dependent subsystem definitions. Curved spacetime is further considered as a structured environment capable of modifying mode propagation, phase relationships, and correlations without behaving as a conventional material reservoir. The discussion establishes a conceptual framework relevant to relativistic quantum information, quantum field theory in non-inertial settings, gravitational quantum sensing, and future experiments involving quantum systems operating across different gravitational potentials or accelerated trajectories. The resulting perspective indicates that quantum coherence remains meaningful in relativistic physics only when the observer, field decomposition, measurement procedure, and spacetime geometry are specified consistently.
Abstract Standard diagnostics of quantum non-Markovianity are commonly formulated in terms of dynamical maps defined on a preferred time foliation. This becomes conceptually and operationally ambiguous in curved spacetime, where no global time coordinate exists and causal structure is primary. We develop a covariant framework for open quantum dynamics along arbitrary timelike worldlines by constructing multi-time processes (process tensors) from overlapping causal diamonds. For an Unruh–DeWitt detector weakly coupled to a scalar field in a Hadamard state, we quantify memory by the operational distance between the resulting process tensor and the convex set of Markovian (CP-divisible) combs, yielding a foliation-independent measure of non-Markovianity. Numerical benchmarks in ( 1 + 1 ) dimensions compare inertial and uniformly accelerated motion, as well as static and infalling trajectories in Schwarzschild spacetime. Inertial motion is nearly Markovian, whereas acceleration, curvature, and horizons generate long-range temporal correlations and strong multi-time memory, including an activation gap: effects that remain weak in single-step diagnostics become detectable and can be enhanced by genuinely multi-time protocols. Our results provide an operational route to quantifying quantum memory in relativistic settings and identify acceleration, curvature, and horizons as controllable ingredients for relativistic quantum-information tasks.
Quantum effects in accelerated frames and gravitational fields have been studied for decades. One of the most influential outcomes is the discovery of thermal radiation from a black hole by Hawking in 1975. Other important discoveries include the Unruh effect, dynamical Casimir effect etc.. Although these discoveries are very exciting, experimental verification of them is extremely challenging. Even in the theoretical aspect, not all the issues have been resolved, e.g., the well known black hole information paradox. Quantum information science was developed rapidly during the last thirty years. The well established concepts and tools in quantum information science have been used to explore the quantum effects in gravitational fields and relativistic frames, giving birth to a new research field named relativistic quantum information. This thesis studies quantum effects in accelerated frames and gravitational fields by exploiting the concepts and techniques in quantum information science. The Unruh effect implies that the state of the fields confined within part of the Minkowski spacetime can appear thermal, and entanglement exists between different spacetime regions. We show that the particle number distribution of the field modes confined within a finite diamond region is also thermal in the Minkowski vacuum, an analogue to the Unruh effect; and there exists entanglement between different diamonds. The vacuum entanglement can be extracted and utilized for some quantum information protocols, e.g., quantum key distribution. Furthermore, we show that the presence of a horizon and the Unruh thermal noise has important consequences to the quantum communication protocols where one of the parties is a uniformly accelerated observer. Interactions between uniformly accelerated objects and quantum fields are traditionally studied using perturbation theory. The quantum circuit model, a crucial tool in quantum communication and computation, can be exploited to calculate radiations from the uniformly accelerated objects non-perturbatively. By further combining field detection scheme in quantum optics, e.g., homodyne detection, the output field from the uniformly accelerated objects can be fully studied. These techniques help to study decoherence effect in non-inertial frames, which may provide important insights for the black hole information paradox. Dynamical spacetimes generally create quantum particles. Gravitational perturbations around a black hole oscillate and decay, due to the emission of gravitational waves to spatial infinity and into the black hole. We show that they play the role as a multimode squeezer, squeezing the state of the quantum fields and creating particles.
Quantum effects in non-inertial frames and curved space-time have been studied for decades. One of the most intriguing discoveries is the existence of virtual particles in the quantum vacuum. One example in which such particles can be observed is through the Unruh effect, which predicts that a uniformly accelerated observer would see a thermal bath of (virtual) particles in the quantum vacuum. Initial doubts on the existence of these virtual particles were allayed by the Unruh De-Witt (UDW) detector, which promoted these virtual particles to real excitations of the detector. Ever since, semiclassical approaches to relativistic quantum field theory (QFT) in flat and curved space-time have been predominantly analysed through a two-level Unruh De-Witt detector utilizing perturbation theory. It is noted, however, that the change of relativistic frame preserves Gaussian properties (e.g. Gaussian distribution of the Wigner function) of the system. In this thesis, we specifically explore the effects of relativistic (non-inertial) reference frames on QFT by considering relativistic QFT in flat-space time. These systems are analysed from the perspective of Gaussian quantum information, via homodyne detection and UDW detectors with a harmonic oscillator degree of freedom.Our project starts by reviewing and developing bipartite/multi-partite entanglement measures (specifically, entanglement of formation), in the hope that we can utilize these measures to understand the entanglement properties of the quantum vacuum. In general, these measure have infinite degrees of freedom and are incomputable. In this project the multi-partite entanglement measure is applied to the Gaussian regime, which reduces the number of degrees of freedom to a finite one, hence making it a computable measure.In our next project, we attempt to understand how relativistic reference frames affect the communication between different observers. To obtain an intuition of its effect, a passive (time-delay) signal sent from a uniformly accelerated observer to a stationary observer was analysed. When we naively implemented the self-homodyne detection scheme, the signal appeared noisy. This effect was referred to as apparent decoherence, and its effect could be traced back to our naive assumption that all of the signal’s information would be stored in the time-delayed mode; the vacuum entanglement that pre-existed before the (time-delay) signal was created must be accounted for. We follow by analysing the effects that emerge due to communication between different relativistic reference frames. We then develop a communication technique referred to as ideal-homodyne which is a homodyne detection scheme robust to these effects. We used this detection scheme to analyse the correlation/entanglement properties of an accelerated signal (created via accelerated mirror, squeezer and phase shifter) sent to an inertial observer.In our last project, we utilize the knowledge that was gained through the prior projects to analyse (subcycle) electro-optic sampling. In the literature, there were doubts as to whether electro-optic sampling were truly detecting virtual particles in the quantum vacuum, or particle excitations that were created as a by-product of the detection mechanism. We were able to pin-point some similarities between electro-optic sampling and subcycle probe via the UDW detector. The regime in which the electro-optic sampling directly maps virtual particles from the vacuum into real excitations of the probe field was identified by establishing an equivalence to the UDW detector in certain parameter regimes.
Abstract Quantum mechanics and relativity are two of the most successful theoretical frameworks in modern physics, yet their conceptual foundations remain difficult to reconcile at the deepest level. Quantum theory describes physical systems through states, observables, probabilities, measurement contexts, superposition, entanglement, and nonclassical correlations. Relativity describes physical processes through space-time structure, causal order, invariant physical law, observer-dependent decomposition of time and space, and constraints on signal propagation. This article examines the conceptual relationship between quantum measurement, relativistic causality, and space-time description. The purpose is not to present a mathematical unification but to analyze how the two frameworks organize physical explanation differently and where their interpretive tensions arise. Special attention is given to the measurement problem, the status of quantum states, the role of observers and reference frames, the meaning of locality, the distinction between correlation and causal influence, and the challenge of describing quantum processes without privileging a single global time. The article argues that many apparent conflicts between quantum theory and relativity result from importing classical intuitions into domains where they no longer apply. Quantum entanglement does not permit controllable faster-than-light communication, yet it challenges classical separability. Relativity forbids superluminal signaling, yet it does not require that all physically meaningful correlations be reducible to local classical properties. Measurement introduces another difficulty because standard quantum theory often describes state update in a way that appears instantaneous, while relativistic space-time does not provide a universal present. The discussion develops a conceptual framework in which quantum measurement is treated as a physical interaction, relativistic causality as a constraint on operational influence, and space-time description as an organizing structure for events rather than a simple background stage. This analysis is relevant for quantum field theory, quantum information, relativistic quantum foundations, and future approaches to quantum gravity.
In general relativity, the description of spacetime relies on idealised rods and clocks, which identify a reference frame. In any concrete scenario, reference frames are associated to physical systems, which are ultimately quantum in nature. A relativistic description of the laws of physics hence needs to take into account such quantum reference frames (QRFs), through which spacetime can be given an operational meaning. Here, we introduce the notion of a spacetime quantum reference frame, associated to a quantum particle in spacetime. Such formulation has the advantage of treating space and time on equal footing, and of allowing us to describe the dynamical evolution of a set of quantum systems from the perspective of another quantum system, where the parameter in which the rest of the physical systems evolves coincides with the proper time of the particle taken as the QRF. Crucially, the proper times in two different QRFs are not related by a standard transformation, but they might be in a quantum superposition one with respect to the other.Concretely, we consider a system of N relativistic quantum particles in a weak gravitational field, and introduce a timeless formulation in which the global state of the N particles appears "frozen", but the dynamical evolution is recovered in terms of relational quantities. The position and momentum Hilbert space of the particles is used to fix the QRF via a transformation to the local frame of the particle such that the metric is locally inertial at the origin of the QRF. The internal Hilbert space corresponds to the clock space, which keeps the proper time in the local frame of the particle. Thanks to this fully relational construction we show how the remaining particles evolve dynamically in the relational variables from the perspective of the QRF. The construction proposed here includes the Page-Wootters mechanism for non interacting clocks when the external degrees of freedom are neglected. Finally, we find that a quantum superposition of gravitational redshifts and a quantum superposition of special-relativistic time dilations can be observed in the QRF.