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Eduardo Martín-Martínez

Publications and source records attributed to Eduardo Martín-Martínez.

At least 19 recordsLinked to original sources
arXiv2026

Towards an experimental implementation of entanglement harvesting in superconducting circuits: effect of detector gap variation on entanglement harvesting

Motivated by the prospect of experimental implementations of entanglement harvesting in superconducting circuits, we propose a model of variable-gap particle detector that aims to bridge some of the gaps between Unruh-DeWitt (UDW) models and realistic implementations. Using parameters tailored to potential experimental setups, we investigate entanglement harvesting in both spacelike-separated and causally connected scenarios. Our findings reveal that while variations in the energy gap reduce the ability to harvest entanglement for spacelike-separated detectors, detectors in causal contact can still become entangled through their interaction with the field. Notably, our analysis shows that (due to the derivative coupling nature of the model) even for causally connected detectors, the entanglement primarily originates from the field's correlations. This demonstrates the potential for genuine entanglement harvesting in the lab and opens the door to near-future entanglement harvesting experiments in superconducting circuits.

quant-ph↗
arXiv2026

Nonlinear particle detectors across the Rindler firewall

We investigate Unruh-DeWitt detectors coupled to composite observables of a quantum scalar field, including quadratic coupling to the field momentum and coupling to the local energy density. We develop a distributional framework for evaluating the corresponding detector response functions and apply it to detectors crossing the Rindler firewall. While we recover the finite response of the derivative-coupling model, we show that quadratic momentum coupling leads to products of distributions that do not admit a canonical, regulator-independent definition in the sharp-firewall idealization. The vacuum correspondence between the local energy-density and quadratic momentum responses motivates the conjecture that analogous difficulties may arise for energy-density coupling in the sharp firewall model. Within the sharp, pointlike composite-coupling model considered here, our analysis further suggests that these pathologies originate from the discontinuous severing of correlations across the Rindler horizon.

quant-ph↗
arXiv2026

Partner-mode overlap as a symplectic-invariant measure of correlations in Gaussian quantum field theories

We introduce a locally symplectic-invariant quantifier of correlations between arbitrary bosonic Gaussian modes, with particular emphasis on quantum field theory. The quantity, denoted by~$\mathcal{D}^{\mathrm{sym}}$, admits a simple geometric interpretation as the symmetric overlap between each mode and the purification partner of the other, providing a direct geometric characterization of how correlations are distributed between modes. We derive a necessary and sufficient criterion for two-mode Gaussian entanglement in terms of $\mathcal{D}^{\mathrm{sym}}$, placing on firm quantitative footing the intuition that the entanglement of a localized mode is encoded in the spatial support of its purification partner. We demonstrate the framework for wavepacket modes of a scalar quantum field in Minkowski spacetime (illustrating how the geometry of partner modes reveals the spatial structure of quantum correlations) and discuss extensions to multimode systems and mixed Gaussian states.

quant-ph↗
arXiv2026

Correlation and Entanglement partners in Gaussian systems

We introduce a framework to identify where the total correlations and entanglement with a chosen degree of freedom reside within the rest of a system, in the context of bosonic many-body Gaussian quantum systems. Our results are organized into two main propositions. First, for pure Gaussian states, we show that every correlated mode possesses a unique single-degree-of-freedom partner that fully captures its correlations (consisting of entanglement), and we provide an explicit construction of this partner from the complex structure of the system's state. Second, for mixed Gaussian states, we constructively demonstrate that the notion of a partner subsystem splits into two: a correlation partner, which contains all classical and quantum correlations and need not correspond to a single degree of freedom, and an entanglement partner, which is always at most single-mode. Finally, we extend the construction of partners to multi-mode subsystems. Together, these results provide conceptual practical tools to study how bipartite correlations and entanglement are structured and where they can be found in complex Gaussian many-body systems.

quant-ph↗
arXiv2026

Ultraviolet structure of entanglement harvesting from energy density and other quadratic couplings

We study entanglement harvesting with particle detectors coupled to the renormalized energy density of a massless scalar field. Our analysis identifies the distributional mechanism underlying the persistent ultraviolet divergences previously observed for quadratic detector couplings and discusses that the energy density coupling exhibits the same underlying structure. We show that these divergences are entirely controlled by the switching correlation at coincident interaction times. Consequently, pointlike detectors are UV finite whenever the switchings do not overlap. For Gaussian-smeared detectors, the harvesting correlations are automatically finite in 1+1 and 2+1 dimensions, while in higher dimensions the remaining divergences are removed by non-overlapping switchings. Finally, we derive general expressions for arbitrary zero-mean Gaussian states and illustrate the formalism with a thermal field state.

quant-ph↗
arXiv2026

Cosmological Expansion Induces Interference Between Communication and Entanglement Harvesting

We investigate the interplay between genuine entanglement harvesting and communication mediated correlations for local particle detectors in expanding cosmological spacetimes. Focusing on a conformally coupled scalar field in de Sitter spacetime, we analyze how spacetime expansion induces interference between these two sources of entanglement when the detectors are in causal contact. We compare two physically distinct detector models: detectors whose spatial profile expands with the Universe, and detectors whose proper size remains fixed despite cosmological expansion. We find that the lack of time-reversal symmetry in cosmological settings generically leads to constructive or destructive interference between communication mediated correlations and harvested field correlations, dramatically affecting the entanglement that detectors can acquire. In particular, rapid expansion can suppress entanglement entirely for expanding detectors through destructive interference, even when both communication and field correlations are individually large, whereas detectors that maintain a fixed proper size remain capable of acquiring significant entanglement. Our results show that cosmological expansion qualitatively reshapes the balance between communication and harvesting, and that the detector internal cohesion (whether it expands with the Universe or not) plays a crucial role in determining whether detectors' entanglement can survive in rapidly expanding universes.

quant-ph↗
arXiv2026

Entanglement harvesting in conformal field theory

We study entanglement harvesting in general $d$-dimensional conformal field theories using pointlike Unruh-DeWitt detectors coupled to scalar primary operators. This extends standard harvesting protocols beyond free fields to interacting conformal theories and arbitrary spatial dimensions. We find that increasing the operator scaling dimension suppresses both negativity and mutual information, reflecting the faster decay of correlations. For holographic CFTs, we show that bulk effective field theory enables a separation between field-harvested and communication-mediated entanglement. We also derive asymptotic, closed-form approximations that agree well with numerical results.

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↗
arXiv2025

Localizing quantum fields with time-dependent potentials

In this paper we study the effect of localizing quantum field degrees of freedom by dynamically growing cavity walls through a time-dependent potential. We use our results to show that it is possible to do this without introducing non-negligible mixedness in localized modes of the field. We discuss how this addresses the concerns, raised in previous literature, that the high degree of entanglement of regular states in QFT may hinder relativistic quantum information protocols that make use of localized relativistic probes.

quant-ph↗
arXiv2025

A review of applications of Quantum Energy Teleportation: from experimental tests to thermodynamics and spacetime engineering

Quantum energy teleportation (QET) exploits the existence of correlations to enable remote energy transfer without the need for physical energy carriers between emitter and receiver. This paper presents a review of the thermodynamic foundations of QET and reviews its first experimental demonstration (performed using Nuclear Magnetic Resonance), along with its implementation on publicly available superconducting quantum hardware. Additionally, we review an application of QET in the field of quantum thermodynamics as an efficient algorithmic cooling technique to cool down individual parts of interacting systems. Finally, we will review how QET can be employed to optimally generate exotic quantum states characterized by negative average stress-energy densities, offering a new operational approach to engineering such states which are promising in the context of semiclassical gravity.

quant-ph↗
arXiv2025

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 $\hat{\boldsymbolσ}\cdot \boldsymbol{B}$) 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↗
arXiv2025

Phase space analysis of Bell inequalities for mixed Gaussian states

We present a phase space formalism to evaluate Bell inequality violations in continuous variable systems. By doing so we can generalize previous analyses (which have dealt only with pure states) to arbitrary mixed states. We leverage these results to analyze the effect of temperature on violations of Bell inequalities in a two-mode squeezed thermal state, which can become useful in tests of local realism in the presence of thermal noise. We also explore the non-monotonic relationship between the violations of Bell inequalities and the amount of entanglement present in this family of mixed states. Additionally, we discuss the optimal choices of pseudospin operators for states beyond the two-mode squeezed vacuum.

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

Derivative coupling enables genuine entanglement harvesting in causal communication

We show that particle detectors coupled to a massless quantum field through its derivative can genuinely harvest entanglement form the field even when they are in causal contact in flat spacetime. This is particularly relevant since the derivative coupling model captures some interesting experimentally realizable systems and since the harvested entanglement peaks at full light contact.

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↗
arXiv2024

No black holes from light

We show that it is not possible to concentrate enough light to precipitate the formation of an event horizon. We argue that the dissipative quantum effects coming from the self-interaction of light (such as vacuum polarization) are enough to prevent any meaningful buildup of energy that could create a black hole in any realistic scenario.

gr-qc↗
arXiv2024

Particle detectors under chronological hazard

We analyze how the presence of closed timelike curves (CTCs) characterizing a time machine can be discerned by placing a local particle detector in a region of spacetime which is causally disconnected from the CTCs. Our study shows that not only can the detector tell if there are CTCs, but also that the detector can separate topological from geometrical information and distinguish periodic spacetimes without CTCs (like the Einstein cylinder), curvature, and spacetimes with topological identifications that enable time-machines.

quant-ph↗