arXiv subjects

T. Rick Perche

Publications and source records attributed to T. Rick Perche.

At least 19 recordsLinked to original sources

arXiv2026

Non-perturbative measurements of two-point functions in quantum field theory

We present a non-perturbative method through which local probes can access the two-point function of a quantum field within a region of spacetime. By considering a lattice of gapless particle detectors, we identified the probe observables that encode the field's two-point function. We quantify the discrepancies introduced by physical finite-sized interaction regions by performing a spacetime multipole expansion of the smeared two-point function. Our protocol expresses the two-point function entirely in terms of measurable detectors correlations, providing an operational notion of states in QFT.

quant-ph↗

arXiv2026

Optimization of entanglement harvesting with arbitrary temporal profiles: the limit of second order perturbation theory

We study the protocol of entanglement harvesting when two local probes couple to the vacuum of a real scalar quantum field with arbitrary temporal profiles. We use a Hermite expansion to efficiently compute smeared field propagators in closed-form, recasting the negativity between the probes as a matrix product. We then optimize the protocol under different signalling conditions, enhancing entanglement harvesting by several orders of magnitude. This optimization would take current experimental proposals beyond the regime of second order perturbation theory.

quant-ph↗

arXiv2026

When does entanglement through gravity imply gravitons?

Detection of entanglement through the Newtonian potential has been claimed to support the existence of gravitons, by extrapolating to a thought experiment which demonstrates that complementarity and causality would be in conflict unless quantum fluctuations exist. We critically assess this consistency argument using scalar field models. We show that whether complementarity or no-signalling is violated when quantum fluctuations are neglected, depends on how this approximation is taken, while in both cases entanglement is generated locally in spacetime. We clarify that the correct reading of the paradox requires making a clear distinction between two notions of causality violation: Newtonian action-at-a-distance and the quantum mechanical no-signalling; the latter is pertinent while the former is not. We conclude that the thought experiment (a) does not add to the epistemological relevance of entanglement through Newtonian potentials (b) lends support for the existence of gravitons, if retardation effects are detected in entanglement through gravity.

gr-qc↗

arXiv2025

(PhD Thesis) The Information Locally Stored in Quantum Fields: From Entanglement to Gravity

This is an updated version of my PhD thesis, defended at the University of Waterloo on the 2nd of April 2025, uploaded to the ArXiv with the goal of reaching a wider audience. The thesis is divided into 5 chapters, respectively containing (I) a brief introduction to local quantum field theory (QFT), (II) a description of local probes in QFT, (III) a discussion of entanglement in QFT and how to probe it, (IV) a description of the regimes where QFT interactions can be approximated by direct interactions, and (V) a discussion the information about the geometry of spacetime contained in quantum fields. The partial goal of this thesis is to serve as a guide for students aiming to tackle these different research programs. If the reader is interested in pursuing one or more research projects detailed here, they are encouraged to contact me for collaboration in these topics.

quant-ph↗

arXiv2025

Bose polarons as relativistic Unruh-DeWitt detectors: Entanglement harvesting from Bose-Einstein condensates

We show that a bound impurity in a Bose-Einstein condensate can be directly mapped to an Unruh-DeWitt detector interacting with a relativistic quantum field. We provide explicit experimental parameters for an implementation using ${}^{41}\text{K}$ impurities coupled to a ${}^{87}\text{Rb}$ condensate via finite-time Feshbach tuning. As an application, we study the extraction of vacuum entanglement from distant regions of the condensate and find viable parameters for the implementation of entanglement harvesting.

quant-ph↗

arXiv2025

Subsystem entanglement and separability in quantum reference frames

We find a necessary condition for subsystems to become entangled after a quantum reference frame transformation. We then distinguish between subsystems that admit separable descriptions relative to a quantum reference frame, and those that do not. On the one hand, we show that separable descriptions maximize the entanglement internal to the subsystem, and relate our results with the conservation of entanglement and coherence under quantum reference frame transformations. On the other hand, systems that do not admit a separable description relative to any subsystem display a form of entanglement that is genuine to the quantum reference frame formalism.

quant-ph↗

arXiv2025

Finite-size clocks in quantum field theory and the twin paradox

Vacuum fluctuations in quantum field theory impose fundamental limitations on our ability to measure time at arbitrarily short scales. To investigate the impact of universal quantum field theory effects on observer-dependent time measurements, we introduce a clock model based on the vacuum decay probability of a finite-sized quantum system. This model defines an effective notion of proper time that depends on the microscopic properties of the clock and on how it samples vacuum fluctuations along its trajectory. We show that, in the long-time regime, this notion of time reduces to the usual proper time of special relativity. However, by studying a microscopic twin-paradox scenario, we find that, in general, time is not determined solely by the trajectory connecting two events, but also by how vacuum fluctuations interact with the internal structure of the clocks.

quant-ph↗

arXiv2025

Trajectory-Protected Quantum Computing

We introduce a novel method that simultaneously isolates a quantum computer from decoherence and enables the controlled implementation of computational gates. We demonstrate a quantum computing model that utilizes a qubit's motion to protect it from decoherence. We model a qubit interacting with a quantum field via the standard light-matter interaction model: an Unruh-DeWitt detector, i.e., the qubit, follows a prescribed classical trajectory while interacting with a scalar quantum field. We switch off the rotating-wave terms, i.e., the resonant transitions, using the technique of acceleration-induced transparency which eliminates the dominant decoherence channels by controlling the qubit's trajectory. We are able to perform one-qubit gates by stimulating the counter-rotating wave terms (i.e., the non-resonant transitions) and two-qubit gates by extracting the entanglement from the quantum field prepared in a squeezed state. Finally, we discuss the fundamental limits on quantum error protection: on the trade-off between isolating a quantum computer from decoherence, and the speed with which entangling gates may be applied, comparable to the Eastin-Knill theorem for quantum error correction.

quant-ph↗

arXiv2025

State updates and useful qubits in relativistic quantum information

We address the longstanding challenge of consistently updating quantum states after selective measurements in a relativistic spacetime. Standard updates along the future lightcones preserve causality but break correlations between causally disconnected parties, whereas updates along the past lightcone either imply retrocausality or do not respect the causal propagation of information. We introduce a minimal extension of multipartite states to encode subsystem-specific contextual information. This "polyperspective" formalism ensures causally consistent covariant state updates, preserves multipartite correlations, and respects conservation laws.

quant-ph↗

arXiv2024

Causality in relativistic quantum interactions without mediators

We analyse the interaction between two quantum systems in spacetime and we compare two possible models to describe it: 1) a fully quantum field theoretical (QFT) description of the coupling of two quantum systems mediated by a quantum field and 2) a quantum-controlled model (qc-model), which is an effectively relativistic direct-coupling in which the interaction of two quantum systems is not mediated by a field with local quantum degrees of freedom. We show that while there are regimes where the qc-model can approximate QFT arbitrarily well, it can suffer from retrocausal effects. We discuss in what regimes those retrocausal predictions of the qc-model are non-negligible and whether they can be used to argue that gravity induced entanglement experiments can reveal genuinely quantum aspects of the gravitational interaction or not.

quant-ph↗

arXiv2024

The effect of curvature on local observables in quantum field theory

We compute the leading order corrections to the expected value of the squared field amplitude of a massless real scalar quantum field due to curvature in a localized region of spacetime. We use Riemann normal coordinates to define localized field operators in a curved spacetime that are analogous to their flat space counterparts, and the Hadamard condition to find the leading order curvature corrections to the field correlations. We then apply our results to particle detector models, quantifying the effect of spacetime curvature in localized field probes.

quant-ph↗

arXiv2024

A relativistic QFT description for the interaction of a spin with a magnetic field

We analyze how non-relativistic effective models for the magnetic coupling of a spin to the electromagnetic field (proportional to $\hat{\boldsymbol{\sigma}}\cdot \boldsymbol{B}$) emerge from a full quantum field theoretical description of charged fermionic fields with the quantum electromagnetic field. This allows us to keep track of relativistic corrections to the models commonly used in experimental spin physics. We discuss how this interaction compares to the usual simplified models used in relativistic quantum information.

quant-ph↗

arXiv2024

The stress-energy tensor of an Unruh-DeWitt detector

We propose a model for a finite-size particle detector, which allows us to derive its stress-energy tensor. This tensor is obtained from a covariant Lagrangian that describes not only the quantum field that models the detector, $\phi_{\text{d}}$, but also the systems responsible for its localization: a complex scalar field, $\psi_{\text{c}}$, and a perfect fluid. The local interaction between the detector and the complex field ensures the square integrability of the detector modes, while the fluid serves to define the spatial profile of $\psi_{\text{c}}$, localizing it in space. We then demonstrate that, under very general conditions, the resulting energy tensor -- incorporating all components of the system -- is physically reasonable and satisfies the energy conditions.

quant-ph↗

arXiv2024

A Universal Quantum Computer From Relativistic Motion

We present an explicit construction of a relativistic quantum computing architecture using a variational quantum circuit approach that is shown to allow for universal quantum computing. The variational quantum circuit consists of tunable single-qubit rotations and entangling gates that are implemented successively. The single qubit rotations are parameterized by the proper time intervals of the qubits' trajectories and can be tuned by varying their relativistic motion in spacetime. The entangling layer is mediated by a relativistic quantum field instead of through direct coupling between the qubits. Within this setting, we give a prescription for how to use quantum field-mediated entanglement and manipulation of the relativistic motion of qubits to obtain a universal gate set, for which compact non-perturbative expressions that are valid for general spacetimes are also obtained. We also derive a lower bound on the channel fidelity that shows the existence of parameter regimes in which all entangling operations are effectively unitary, despite the noise generated from the presence of a mediating quantum field. Finally, we consider an explicit implementation of the quantum Fourier transform with relativistic qubits.

quant-ph↗

arXiv2024

The multimode nature of spacetime entanglement in QFT

We demonstrate the presence of multimode entanglement in the vacuum state of a free, massless scalar quantum field in four-dimensional flat spacetime between two sets of field modes, each contained within a spacetime region that is causally disconnected from the other. This is true despite the fact that entanglement between pairs of individual field modes is sparse and appears only when the two individual modes are carefully selected. Our results reveal that, while entanglement between individual modes is limited, bipartite multimode entanglement in quantum field theory is ubiquitous. We further argue that such multimode entanglement is operationally extractable, and it forms the basis of the entanglement commonly discussed in entanglement harvesting protocols.

quant-ph↗

arXiv2023

Closed-form expressions for smeared bi-distributions of a massless scalar field: non-perturbative and asymptotic results in relativistic quantum information

Using spacetime Gaussian test functions, we find closed-form expressions for the smeared Wightman function, Feynman propagator, retarded and advanced Green's functions, causal propagator and symmetric propagator of a massless scalar field in the vacuum of Minkowski spacetime. We apply our results to localized quantum systems which interact with a quantum field in Gaussian spacetime regions and study different relativistic quantum information protocols. In the protocol of entanglement harvesting, we find a closed-form expression for the entanglement that can be acquired by probes which interact in Gaussian spacetime regions and obtain asymptotic results for the protocol. We also revisit the case of two gapless detectors and show that the detectors can become entangled if there is two-way signalling between their interaction regions, providing closed-form expressions for the detectors' final state.

quant-ph↗

arXiv2023

Fully Relativistic Entanglement Harvesting

We study the protocol of entanglement harvesting when the particle detectors that harvest entanglement from the field are replaced by fully relativistic quantum field theories. We show that two localized modes of the quantum field theories are able to harvest the same amount of leading order entanglement as two non-relativistic particle detectors, thus implying that QFT probes can generally harvest more entanglement than particle detectors. These results legitimize the use of particle detectors to study entanglement harvesting regardless of their internally non-relativistic nature.

quant-ph↗

arXiv2023

Ambient temperature versus ambient acceleration in the circular motion Unruh effect

It is well known that the experience of a linearly accelerated observer with acceleration $a$, interacting with a massless scalar field in its vacuum state in $3+1$ Minkowski spacetime, is identical to that of a static observer interacting with a massless scalar field in a thermal state of temperature $a/2\pi$ in $3+1$ Minkowski spacetime. We study the robustness of this duality by comparing an observer undergoing circular motion in a thermal bath with an observer that undergoes circular motion around a linearly accelerated trajectory. We find that in most regimes, observers in these two cases experience the field in different ways, and are generally able to tell the difference between the two cases by measuring observables localized along their trajectories.

gr-qc↗