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We propose a fully covariant model for smeared particle detectors in quantum field theory in curved spacetimes. We show how effects related to accelerated motion of the detector and the curvature of spacetime influence the way different observers assign an interaction Hamiltonian between the detector and the field. The fully covariant formulation explicitly leaves the physical predictions of the theory invariant under general coordinate transformations, hence providing a description of particle detector models (e.g., Unruh-DeWitt detectors, models for the light-matter interaction, etc) that is suitable for arbitrary trajectories in general spacetime backgrounds., Comment: 9 pages, 1 page of Appendix. RevTeX 4.1. V2: Updated to match published version

We demonstrate how one can use machine learning techniques to bypass the technical difficulties of designing an experiment and translating its outcomes into concrete claims about fundamental features of quantum fields. In practice, all measurements of quantum fields are carried out through local probes. Despite measuring only a small portion of the field, such local measurements have the capacity to reveal many of the field's global features. This is because, when in equilibrium with their environments, quantum fields store global information locally, albeit in a scrambled way. We show that neural networks can be trained to unscramble this information from data generated from a very simple one-size-fits-all local measurement protocol. To illustrate this general claim we will consider three non-trivial features of the field as case studies: a) how, as long as the field is in a stationary state, a particle detector can learn about the field's boundary conditions even before signals have time to propagate from the boundary to the detector, b) how detectors can determine the temperature of the quantum field even without thermalizing with it, and c) how detectors can distinguish between Fock states and coherent states even when the first and second moments of all their quadrature operators match. Each of these examples uses the exact same simple fixed local measurement protocol and machine-learning ansatz successfully. This supports the claim that the framework proposed here can be applied to nearly any kind of local measurement on a quantum field to reveal nearly any of the field's global properties in a one-size-fits-all manner., Comment: 29 pages (14 pages of appendices), 5 figures + 1 figure in Appendices, RevTeX 4.2. v3: Updated to match published version

We investigate the quantum channel consisting of two localized quantum systems that communicate through a scalar quantum field. We choose a scalar field rather than a tensor or vector field, such as the electromagnetic field, in order to isolate the situation where the qubits are carried by the field amplitudes themselves rather than, for example, by encoding qubits in the polarization of photons. We find that suitable protocols for this type of quantum channel require the careful navigation of several constraints, such as the no-cloning principle, the strong Huygens principle and the tendency of short field-matter couplings to be entanglement breaking. We non-perturbatively construct a protocol for such a quantum channel that possesses maximal quantum capacity., Comment: Minor typos and a couple of references are fixed

We calculate the gravitational wave power density emitted by quantum thermal sources. As particular cases, we calculate the emission of gravitational waves from the cosmic microwave background and from stellar sources. We study how treating gravity classically affects the prediction of the thermal emission. We find that the predicted emitted gravitational wave radiation does not exhibit an ultraviolet divergence, even if gravity is treated classically, as long as the fields describing the thermal source are quantum mechanical., Comment: 11 pages, 1 figure; RevTeX 4.1. v2: Minor changes to match published version (including title change)

We present new results on the causality violations introduced by the rotating wave approximation commonly used in quantum optics and high-energy physics. We find that the causality violations and faster-than-light signalling induced by the approximation have `fat tails', i.e., they are polynomially decaying with the distance from the light-cone of the emitter. Furthermore, we also show that the fundamental problems with the incompatibility between the approximation and relativity are not cured even in the long interaction time regime (where the approximation is often taken). This renders the approximation unsuitable for any regime where we are concerned about relativistic causality and information transmission via the electromagnetic field., Comment: 23 pages (13 pages + 10 pages of Appendices). RevTeX 4.1

We analyze the quantum mechanics of the friction experienced by a small system as it moves non-destructively with velocity $v$ over a surface. Specifically, we model the interactions between the system and the surface with a \textit{collision model}. We show that, under weak assumptions, the magnitude of the friction induced by this interaction decreases as $1/v$ for large velocities. Specifically, we predict that this phenomenon occurs in the Zeno regime, where each of the system's successive couplings to subsystems of the surface is very brief. In order to investigate the friction at low velocities and with velocity-dependent coupling strengths, we motivate and develop \textit{one-dimensional convex collision models}. Within these models, we obtain an analytic expression for the general friction-velocity dependence. We are thus able to determine exactly the conditions under which the usual friction-velocity dependency arises. Finally, we give examples that demonstrate the possibility, in principle, of anti-friction, in which case the system is accelerated by its interaction with the surface, a phenomenon associated with active materials and inverted level populations.

We study the work cost of processes in quantum fields without the need of projective measurements, which are always ill defined in quantum field theory. Inspired by interferometry schemes, we propose a work distribution that generalizes the two-point measurement scheme employed in quantum thermodynamics to the case of quantum fields and avoids the use of projective measurements. The distribution is calculated for local unitary processes performed on Kubo-Martin-Schwinger (thermal) states of scalar fields. Crooks theorem and the Jarzynski equality are shown to be satisfied for a family of spatio-temporally localized unitaries, and some features of the resulting distributions are studied as functions of temperature and the degree of localization of the unitary operation. We show how the work fluctuations become much larger than the average as the process becomes more localized in both time and space., Comment: 9 pages, 1 figure, 2 Appendices. RevTeX 4.1 (V2: Updated to match Phys. Rev. Lett. published version. Added non-perturbative results)

We examine when it is possible to locally extract energy from a bipartite quantum system in the presence of strong coupling and entanglement, a task which is expected to be restricted by entanglement in the low-energy eigenstates. We fully characterize this distinct notion of "passivity" by finding necessary and sufficient conditions for such extraction to be impossible, using techniques from semidefinite programming. This is the first time in which such techniques are used in the context of energy extraction, which opens a way of exploring further kinds of passivity in quantum thermodynamics. We also significantly strengthen a previous result of Frey et al., by showing a physically relevant quantitative bound on the threshold temperature at which this passivity appears. Furthermore, we show how this no-go result also holds for thermal states in the thermodynamic limit, provided that the spatial correlations decay sufficiently fast, and we give numerical examples., Comment: 4.5+9 Pages, 4 Figures, Published version

We introduce a classification scheme for the generators of open fermionic Gaussian dynamics. We simultaneously partition the dynamics along the following four lines: (1) unitary versus non-unitary, (2) active versus passive, (3) state-dependent versus state-independent, and (4) single-mode versus multi-mode. We find that only nine of these 16 types of dynamics are possible. Using this partition we discuss the consequences of imposing complete positivity on fermionic Gaussian dynamics. In particular, we show that completely positive dynamics must be either unitary (and so can be implemented without a quantized environment) or active (and so must involve particle exchange with an environment).

We present a scheme to produce shockwaves in quantum fields by means of pretimed emitters. We find that by suitably pre-entangeling the emitters, the shockwave's energy density can be locally modulated and amplified. When the large amplitudes in such a shockwave are used for communication, the channel capacity depends not only on the signal-to-noise ratio but also on the effect that the entanglement of the emitters has on the correlations in the signal and in the quantum noise at the receiver. As a consequence, by choosing the entanglement of the emitters, the flow of information in the shockwave can be modulated and spatially shaped to some extent independently of the flow of energy. We also find that there exists a finite optimal strength of the coupling between the receiver and the quantum field which optimizes the channel capacity by optimizing the tradeoff between sensitivity to the signal and sensitivity to the coupling-induced quantum noise., Comment: 5 pages, 3 figures

We show how two Unruh-DeWitt detectors that do not couple to the zero mode of a quantum field can exchange information faster than the speed of light. We analyze the specific cases of periodic and Neumann boundary conditions in flat spacetime with arbitrary spatial dimensions, and we show that the superluminal signal strength is only polynomially suppressed with the distance to the lightcone. Therefore, in any relativistic scenario modelling the light-matter interaction where a zero mode is present, particle detectors should explicitly couple to the zero mode., Comment: 14 pages, 7 figures. RevTeX 4.1 - V2: Updated to match published version

We investigate entanglement harvesting in AdS$_4$. Applying the general results of [Phys. Rev. D 97, 125011 (2018)], we consider two scenarios: one where two particle detectors are geodesic, with equal redshift; and one where both are static, at unequal redshift. As expected, at large AdS length $L$, our results approximate flat space. However at smaller $L$ we observe non-trivial effects for various field boundary conditions. Furthermore, in the static case we observe a novel feature of the entanglement as a function of switching time delay, which we attribute to different (coordinate) frequencies of the detectors. We also find an island of separability in parameter space, analogous that that observed in AdS$_3$, to which we compare and contrast our other results. The variety of features observed in both cases suggest further study in other spacetimes., Comment: 21 pages 10 Figures RevTeX 4.1

We study the harvesting of entanglement and mutual information by Unruh-DeWitt particle detectors from thermal and squeezed coherent field states. We prove (for arbitrary spatial dimensions, switching profiles and detector smearings) that while the entanglement harvesting ability of detectors decreases monotonically with the field temperature $T$, harvested mutual information grows linearly with $T$. We also show that entanglement harvesting from a general squeezed coherent state is independent of the coherent amplitude, but depends strongly on the squeezing amplitude. Moreover, we find that highly squeezed states i) allow for detectors to harvest much more entanglement than from the vacuum, and ii) ensure that the entanglement harvested does not decay with their spatial separation. Finally we analyze the spatial inhomogeneity of squeezed states and its influence on harvesting, and investigate how much entanglement one can actually extract from squeezed states when the squeezing is bandlimited., Comment: RevTeX4.1. 16 pages, 6 figures

We analyze scenarios in which pairs of Unruh-DeWitt-like detectors interact with scalar field(s) with bi-linear and quadratic coupling. For all cases -- real scalar fields, charged scalar fields, and even for couplings mixing two different scalar fields -- we find that the detectors' dynamics depends on the Wightman function of a real scalar field with the same functional form up to a constant multiple. Consequently, entanglement harvesting exhibits low sensitivity to either the kind of scalar field or their charged nature. Furthermore, we show on general grounds that pairs of Unruh-DeWitt-like detectors exhibit persistent divergences when they interact with scalar field(s) with a quadratic or bi-linear coupling and discuss possible avenues to improve the models through the regularization of such divergences., Comment: 9 pages, 0 figures

We compare the predictions of the fundamentally motivated minimal coupling ($\hat{\boldsymbol{p}}\cdot\hat{\boldsymbol{A}}$) and the ubiquitous dipole coupling ($\hat{\boldsymbol{x}}\cdot\hat{\boldsymbol{E}}$) in the light-matter interaction. By studying the light-matter interaction for hydrogen-like atoms we find that the dipole approximation cannot be a-priori justified to analyze the physics of vacuum excitations (a very important phenomenon in relativistic quantum information) since a dominant wavelength is absent in those problems, no matter how small (as compared to any frequency scale) the atom is. Remarkably, we show that the dipole approximation in those regimes can still be valid as long as the interaction time is longer than the light-crossing time of the atoms, which is a very reasonable assumption. We also highlight some of the subtleties that one has to be careful with when working with the explicitly non-gauge invariant nature of the minimal coupling and we compare it with the explicitly gauge invariant dipole coupling., Comment: 19 pages: 11 pages + 8 pages of appendices. 7 figures. RevTeX 4.1

We analyze a quantum version of the weak equivalence principle, in which we compare the response of a static particle detector crossed by an accelerated cavity with the response of an accelerated detector crossing a static cavity in (1+1)-dimensional flat spacetime. We show, for both massive and massless scalar fields, that the non-locality of the field is enough for the detector to distinguish the two scenarios. We find this result holds for vacuum and excited field states of different kinds and we clarify the role of field mass in this setup., Comment: 18 pages, 18 figures. RevTeX 4.1. Updated to match published version

We study quantum friction and Casimir forces with a full-relativistic formalism for atoms modelled as Unruh-DeWitt detectors in the presence of arbitrary macroscopic objects. We consider the general case of atoms with arbitrary relativistic trajectories in arbitrary quantum states (including coherent superpositions) close to objects that impose arbitrary boundary conditions. Particularizing for conducting plates, we show that, for relative velocities close to the speed of light, the quantum friction diverges while the Casimir force is almost independent of the velocity. Since we include the effect of the finite size of the detector and the finite interaction time, we also obtain quantum friction when the detector is isolated but follows a non-inertial trajectory., Comment: 27 pages (includes 10 pages of appendices), 12 Figures. RevTeX 4.1

We analyze fast interaction cycles in bipartite quantum systems, showing that the statistics in one of the parties (the detector) can be used to determine the two-point correlator of the observable which mediates the coupling in the other (the target). We apply the results to the response of particle detectors coupled to quantum fields and subject to this kind of interactions. We show that, in principle, such a setup can be used to experimentally obtain a direct evaluation of the Wightman function (both its real and imaginary part) for any field state., Comment: 11 pages. RevTeX 4.1

We analyze the physics of accelerated particle detectors (such as atoms) crossing optical cavities. In particular we focus on the detector response as well as on the energy signature that the detectors imprint in the cavities. In doing so, we examine to what extent the usual approximations made in quantum optics in cavities (such as the single-mode approximation, or the dimensional reduction of 3+1D cavities to simplified 1+1D setups) are acceptable when the atoms move in relativistic trajectories. We also study the dependence of these approximations on the state of the atoms and the relativistic nature of the trajectory. We find that, on very general grounds, and already in the weak coupling limit, single- and few-mode approximations, as well as 1+1D dimensional reductions, yield incorrect results when relativistic scenarios are considered., Comment: 20 pages (7 pages of appendices), 10 figures and 6 tables. RevTeX 4.1. V2: updated to match published version and added journal reference

We study the impact of work cost fluctuations on optimal protocols for the creation of correlations in quantum systems. We analyze several notions of work fluctuations to show that even in the simplest case of two free qubits, protocols that are optimal in their work cost (such as the one developed by Huber et al. [NJP 17, 065008 (2015)]) suffer work cost fluctuations that can be much larger than the work cost. We discuss the implications of this fact in the application of such protocols and suggest that, depending on the implementation, protocols that are sub-optimal in their work cost could beat optimal protocols in some scenarios. This highlights the importance of assessing the dynamics of work fluctuations in quantum thermodynamic protocols., Comment: 12 pages, 5 figures. RevTeX 4.1. V2: Updated with latest content and also match published version

We discuss what kind of quantum channels can enable thermalization processes. We show that in order to determine a system's temperature, a thermometer needs to dynamically gain information about the system's local Hamiltonian and not just its state. We illustrate this showing that any temperature measurement protocol that does not resolve the system's local Hamiltonian (such as, e.g., full state tomography) is susceptible to being fooled into measuring any value for the temperature. We will establish necessary conditions for thermal contact for quantum systems. Furthermore, we will show that the intuitive idea of thermalization emerging out of quickly interacting with the microconstitutents of a thermal reservoir cannot be correct.

We present a new technique for computing entanglement harvesting with Unruh-DeWitt particle detectors. The method is particularly useful in cases where analytic solutions are rare and the Wightman function is known only via its mode expansion for which numerical integration can become very expensive. By exploiting the conjugate symmetry of the Wightman function, we may split the integral into parts dependent on the commutator and anti-commutator of the field. In cases where the commutator vanishes, such as spacelike separation, or timelike separation if the strong Huygens principle holds, we then show that the entangling term of the bipartite density matrix can be expressed in terms of the much simpler mutual information term. For the vacuum state, this can be translated into a simple Fourier transform, and thus a single sum over modes, simplifying the procurement of closed expressions. We demonstrate this for Minkowski space, finding an analytical solution where none was previously known., Comment: 8 pages, 4 figures. RevTeX 4.1. v2: Updated to match published version and small typo correction

We show that uniformly accelerated detectors can display genuinely thermal features even if the Kubo-Martin-Schwinger (KMS) condition fails to hold. These features include satisfying thermal detailed balance and having a Planckian response identical to cases in which the KMS condition is satisfied. In this context, we discuss that satisfying the KMS condition for accelerated trajectories is just sufficient but not necessary for the Unruh effect to be present in a given quantum field theory. Furthermore, we extract the necessary and sufficient conditions for the response function of an accelerated detector to be thermal in the infinitely adiabatic limit. This analysis provides new insights about the interplay between the KMS condition and the Unruh effect, and a solid framework in which the robustness of the Unruh effect against deformations of quantum field theories (perhaps Lorentz-violating) can be answered unambiguously., Comment: 6 pages. no figures. RevTeX 4.1

We study under what circumstances a separable bipartite system A-B can or cannot become entangled through local interactions with a bi-local entangled source $\text{S}_1$-$\text{S}_2$. We obtain constraints on the general forms of the interaction Hamiltonians coupling A with $\text{S}_1$ and B with $\text{S}_2$ necessary for A and B to become entangled. We are able to generalize and provide non-perturbative insight on several previous no-go theorems of entanglement harvesting from quantum fields using these general results. We also discuss the role of communication in the process of entanglement extraction, establishing a distinction between genuine entanglement extraction and communication-assisted entanglement generation., Comment: 16 pages, 5 Figures, two appendices. RevTeX 4.1. Minor typos fixed

In this note we discuss the invariance under general changes of reference frame of all the physical predictions of particle detector models in quantum field theory in general and, in particular, of those used in quantum optics to model atoms interacting with light. We find explicitly how the light-matter interaction Hamiltonians change under general coordinate transformations, and analyze the subtleties of the Hamiltonians commonly used to describe the light-matter interaction when relativistic motion is taken into account., Comment: 11 pages. ReVteX 4.1. V2: Updated to match published version

We analyze in full detail the time evolution of an open Gaussian quantum system rapidly bombarded by Gaussian ancillae. As a particular case this analysis covers the thermalization (or not) of a harmonic oscillator coupled to a thermal reservoir made of harmonic oscillators. We derive general results for this scenario and apply them to the problem of thermalization. We show that only a particular family of system-environment couplings will cause the system to thermalize to the temperature of its environment. We discuss that if we want to understand thermalization as ensuing from the Markovian interaction of a system with the individual microconstituents of its (thermal) environment then the process of thermalization is not as robust as one might expect.

We show that introducing a small uncertainty in the parameters of quantum systems can make the dynamics of these systems robust against perturbations. Concretely, for the case where a system is subject to perturbations due to an environment, we derive a lower bound on the fidelity decay, which increases with increasing uncertainty in the state of the environment. Remarkably, this robustness in fidelity can be achieved even in fragile chaotic systems. We show that non-Markovianity is necessary for attaining robustness in the fidelity., Comment: 5 pages, 2 figures

We study how quantum randomness generation based on unbiased measurements on a hydrogen-like atom can get compromised by virtue of the unavoidable coupling of the atom with the electromagnetic field. Concretely, we show that an adversary with access to the quantum EM field, but not the atom, can perform an attack on the randomness of a set of unbiased quantum measurements. We analyze the light-atom interaction in 3+1 dimensions with no single-mode or rotating-wave approximations. In our study, we also take into account the non-pointlike nature of the atom and the exchanges of angular momentum between atom and field and compare with previous results obtained under scalar approximations. We show that preparing the atom in the ground state in the presence of no field excitations is, in general, not the safest state to generate randomness in atomic systems (such as trapped ions or optical lattices)., Comment: 22 pages (11 pages main text + 11 pages of appendices and bibliography). 6 Figures. RevTeX 4.1. V2: Updated to match published version

We investigate the validity of two common assumptions in the modelling of superconducting circuits: first, that the superconducting qubits are pointlike, and second, that the UV behaviour of the transmission line is not relevant to the qubit dynamics. We show that in the experimentally accessible ultra-strong coupling regime and for short (but attainable) times, the use of an inaccurate cutoff model (such as sharp, or none at all) could introduce very significant inaccuracies in the model's predictions., Comment: 10 pages, 5 figures. RevTeX 4.1

We introduce a classification scheme for the generators of bosonic open Gaussian dynamics, providing instructive diagrams description for each type of dynamics. Using this classification, we discuss the consequences of imposing complete positivity on Gaussian dynamics. In particular, we show that non-symplectic operations must be active to allow for complete positivity. In addition, non-symplectic operations can, in fact, conserve the volume of phase space only if the restriction of complete positivity is lifted. We then discuss the implications for the relationship between information and energy flows in open quantum mechanics.

Wireless communication derives its power from the simultaneous emission of signals in multiple directions. However, in the context of quantum communication, this phenomenon must be reconciled carefully with the no-cloning principle. In this context, we here study how wireless communication of quantum information can be realized via relativistic fields. To this end, we extend existing frameworks to allow for a non-perturbative description of, e.g., quantum state transfer. We consider, in particular, the case of 1+1 spacetime dimensions, which already allows a number of interesting scenarios, pointing to, for example, new methods for tasks similar to quantum secret sharing., Comment: v2: published version

The signaling between two observers in 3+1 dimensional flat spacetime coupled locally to a non-local field is considered. We show that in the case where two observers are purely timelike related -- so that an exchange of on-shell massless quanta cannot occur -- signaling is still possible because of a violation of Huygens' principle. In particular, we show that the signaling is exponentially suppressed by the non-locality scale. Furthermore, we consider the case in which the two observers are light-like related and show that the non-local modification to the local result is polynomially suppressed in the non-locality scale. This may have implications for phenomenological tests of non-local theories., Comment: 9 pages, 3 Figures. RevTeX 4.1

We study (non-perturbatively) the interactions of delta-coupled detectors with coherent states of a scalar field. We show that the time-evolved density matrix spectra of i) a single detector, ii) two detectors, and iii) the partial transpose of the latter, are all independent of which coherent state the field was in. Furthermore, we find that the eigenvalues in iii) are non-negative, implying that a delta-coupled detector pair cannot harvest entanglement from the vacuum, or any other coherent field state., Comment: 20 pages. RevTeX 4.1

We show that a particle detector can distinguish the $\mathbb{RP}^3$ geon from the Schwarzschild black hole, even though they differ only by a topological identification beyond the event horizon. This shows that the detector can read out information about the non-local structure even when separated from the non-locality by an event horizon. Our analysis of the dependence of the transition on the detector gap is novel, and in principle presents an interesting observational signal., Comment: 10 pages, 8 figures. RevTeX 4.1

We analyze the interactions of particle detectors with coherent states of a free scalar field. We find that the eigenvalues of the post-interaction density matrices of i) a single detector, ii) two detectors, and iii) the partial transpose of the latter, are all independent of which coherent state the field was in. A consequence of these results is that a detector pair can harvest the same amount of entanglement from any coherent field state as from the vacuum., Comment: 11 pages. RevTeX 4.1. Updated to fix minor typos

We analyze correlations between pairs of particle detectors quadratically coupled to a real scalar field. We find that, while a single quadratically coupled detector presents no divergences, when one considers pairs of detectors there emerge unanticipated persistent divergences (not regularizable via smooth switching or smearing) in the entanglement they acquire from the field. We have characterized such divergences, discussed whether a suitable regularization can allow for fair comparison of the entanglement harvesting ability of the quadratic and the linear couplings, and finally we have found a UV-safe quantifier of harvested correlations. Our results are relevant to future studies of the entanglement structure of the fermionic vacuum., Comment: 17 pages, 4 figures. RevTeX 4.1

We propose a method for increasing purity of interacting quantum systems that takes advantage of correlations present due to the internal interaction. In particular we show that by using the system's quantum correlations one can achieve cooling beyond established limits of previous conventional algorithmic cooling proposals which assume no interaction., Comment: 5 pages, 4 figures. RevTeX 4.1

We show, under a very general set of assumptions, that pairs of identical particle detectors in spacelike separation, such as atomic probes, can only harvest entanglement from the vacuum state of a quantum field when they have a non-zero energy gap. Furthermore, we show that degenerate probes are strongly challenged to become entangled through their interaction through scalar and electromagnetic fields even in full light-contact. We relate these results to previous literature on remote entanglement generation and entanglement harvesting, giving insight into the energy gap's protective role against local noise, which prevents the detectors from getting entangled through the interaction with the field., Comment: 10 pages. RevTeX 4.1. V2: Updated to match published version

We use quantum energy teleportation in the light-matter interaction as an operational means to create quantum field states that violate energy conditions and have negative local stress-energy densities. We show that the protocol is optimal in the sense that it scales in a way that saturates the quantum interest conjecture., Comment: 16 pages, 5 figures. RevTeX 4.1

We investigate the open dynamics of a quantum system when it is rapidly repeatedly updated by a quantum channel. Specifically, we analyze when this dynamics can purify the system. We develop a necessary and sufficient condition for such purification effects to occur and characterize their strength. We thoroughly analyze the specific scenario of a quantum system undergoing rapid unitary interactions with a sequence of ancillary quantum systems. We find that while the purification effects are generally present, in order for these effects to be strong compared to the decoherence effects the interaction Hamiltonian must have a minimum degree of complexity. Specifically, a tensor product interaction $\hat{Q}_\text{S}\otimes\hat{R}_\text{A}$, as well as many common light-matter interactions cannot purify efficiently., Comment: 14 Pages. RevTeX 4.1

We analyze the exchange of information in different cosmological backgrounds when sender and receiver are timelike separated and communicate through massless fields (without the exchange of light-signals). Remarkably, we show that the dominance of a cosmological constant makes the amount of recoverable information imprinted in the field by the sender extremely resilient: it does not decay in time or with the spatial separation of sender and receiver, and it actually increases with the rate of expansion of the Universe. This is in stark contrast with the information carried by conventional light-signals and with previous results on timelike communication through massless fields in matter dominated cosmologies., Comment: 9 pages, 3 figures. RevTeX 4.1. v2: Minor typos corrected and updated to match published version (including minor title change)

We study the Anti-Unruh effect in general stationary scenarios. We find that, for accelerated trajectories, a particle detector coupled to a KMS state of a quantum field can cool down (click less often) as the KMS temperature increases. Remarkably, this is so even when the detector is switched on adiabatically for infinitely long times. We also show that the Anti-Unruh effect is characteristic of accelerated detectors, and cannot appear for inertially moving detectors (e.g., in a thermal bath)., Comment: 11 pages, 3 Figures. RevTeX 4.1. V2: Added new section IV, corrected minor typos and updated to match published version. Notice the title change to match published version

We show that a particle detector can distinguish the interior of a hollow shell from flat space for switching times much shorter than the light-crossing time of the shell, even though the local metrics are indistinguishable. This shows that a particle detector can read out information about the non-local structure of spacetime even when switched on for scales much shorter than the characteristic scale of the non-locality., Comment: 9 pages, 5 figures. ReVTeX 4.1

We study how two fully-featured hydrogenlike atoms harvest entanglement from the electromagnetic field vacuum, even when the atoms are spacelike separated. We compare the electromagnetic case ---qualitatively and quantitatively--- with previous results that used scalar fields and featureless, idealized atomic models. Our study reveals the new traits that emerge when we relax these idealizations, such as anisotropies in entanglement harvesting and the effect of exchange of angular momentum. We show that, under certain circumstances, relaxing previous idealizations makes vacuum entanglement harvesting more efficient., Comment: 28 pages (12 pages manuscript + 16 pages of appendices), 6 figures. RevTeX 4.1. V4: A few minor typos corrected

We investigate the emergent open dynamics of a quantum system that undergoes rapid repeated unitary interactions with a sequence of ancillary systems. We study in detail how decoherence appears as a subleading effect when a quantum system is 'bombarded' by a quick succession of ancillas. In the most general case, these ancillas are a) taken from an ensemble of quantum systems of different dimensions, b) prepared in different states, and c) interacting with the system through different Hamiltonians. We derive an upper bound on decoherence rates in this regime, and show how a rich variety of phenomena in open dynamics (such as projection, thermalization, purification, and dephasing) can emerge out of our general model of repeated interaction. Furthermore, we show a fundamental link between the strength of the leading order dissipation and the intrinsic "unpredictability" in the system-ancilla interaction. We also discuss how these results encompass and extend results obtained with other earlier models of repeated interaction., Comment: 28 pages, 1 figure, RevTeX 4.1. V2: Updated to match published version

The response of inertial particle detectors coupled to a scalar field satisfying nonlocal dynamics described by nonanalytic functions of the d'Alembertian operator $\Box$ is studied. We show that spontaneous emission processes of a low energy particle detector are very sensitive to high-energy nonlocality scales. This allows us to suggest a nuclear physics experiment ($\sim$ MeV energy scales) that outperforms the sensitivity of LHC experiments by many orders of magnitude. This may have implications for the falsifiability of theoretical proposals of quantum gravity., Comment: 5 pages, 1 figure. RevTeX 4.1. This version matches the published one in PRD Rapid Communications

Randomness is an indispensable resource in modern science and information technology. Fortunately, an experimentally simple procedure exists to generate randomness with well-characterized devices: measuring a quantum system in a basis complementary to its preparation. Towards realizing this goal one may consider using atoms or superconducting qubits, promising candidates for quantum information processing. However, their unavoidable interaction with the electromagnetic field affects their dynamics. At large time scales, this can result in decoherence. Smaller time scales in principle avoid this problem, but may not be well analysed under the usual rotating wave and single-mode approximation (RWA and SMA) which break the relativistic nature of quantum field theory. Here, we use a fully relativistic analysis to quantify the information that an adversary with access to the field could get on the result of an atomic measurement. Surprisingly, we find that the adversary's guessing probability is not minimized for atoms initially prepared in the ground state (an intuition derived from the RWA and SMA model)., Comment: 11 pages. 8 figures. RevTeX 4.1

We propose a generalization of the superconducting circuit simulation of the dynamical Casimir effect where we consider relativistically moving boundary conditions following different trajectories. We study the feasibility of the setup used in the past to simulate the dynamical Casimir effect to reproduce richer relativistic trajectories differing from purely sinusoidal ones. We show how different relativistic oscillatory trajectories of the boundaries of the same period and similar shape produce a rather different spectrum of particles characteristic of their respective motions., Comment: 11 pages, 8 figures. RevTeX 4.1

We study the transmission of information and correlations through quantum fields in cosmological backgrounds. With this aim, we make use of quantum information tools to quantify the classical and quantum correlations induced by a quantum massless scalar field in two particle detectors, one located in the early universe (Alice's) and the other located at a later time (Bob's). In particular, we focus on two phenomena: a) the consequences on the transmission of information of the violations of the strong Huygens principle for quantum fields, and b) the analysis of the field vacuum correlations via correlation harvesting from Alice to Bob. We will study a standard cosmological model first and then assess whether these results also hold if we use other than the general relativistic dynamics. As a particular example, we will study the transmission of information through the Big Bounce, that replaces the Big Bang, in the effective dynamics of Loop Quantum Cosmology., Comment: 16 pages, 10 figures. RevTeX 4.1. V2: Updated to match published version