Emission of pairs of Minkowski photons through the lens of the Unruh effect

Abstract How long does a uniformly rotating observer need to interact with a quantum field in order to register an approximately thermal response due to the circular motion Unruh effect? We address this question for a massless scalar field in 2+1 dimensions, defining the effective temperature via the ratio of excitation and de-excitation rates of an Unruh-DeWitt detector in the long interaction time limit. In this system, the effective temperature is known to be significantly smaller than the linear motion Unruh effect prediction when the detector's energy gap is small: the effective temperature tends to zero in the small gap limit, linearly in the gap. We show that a positive small gap temperature at long interaction times can be regained via a controlled long-time-small-gap double limit, provided the detector's coupling to the field is allowed to change sign. The resulting small gap temperature depends on the parameters of the circular motion but not on the details of the detector's switching. The results broaden the energy range for pursuing an experimental verification of the circular motion Unruh effect in analogue spacetime experiments. As a mathematical tool, we provide a new implementation of the long interaction time limit that controls in a precise way the asymptotics of both the switching function and its Fourier transform.

We identify low-acceleration conditions under which the Unruh effect manifests as an early superradiant burst in a collection of excited atoms. The resulting amplified Unruh signal is resolved from the inertial signal both in time and intensity. We demonstrate theoretically that these conditions are realized inside a subresonant cavity that highly suppresses the response of an inertial atom, while allowing significant response from an accelerated atom as, owing to the acceleration-induced spectral broadening, it can still couple to the available field modes. The setup thus selectively amplifies the modified field fluctuations underlying the Unruh effect into an early superradiant burst. In comparison, the field fluctuations perceived inertially would cause a superradiant burst much later. In this way, we simultaneously address the extreme acceleration requirement, the weak Unruh signal, and the dominance of the inertial signal, all within a single experimental arrangement.

The Unruh Effect and Theory Interpretation in an Effective Framework

Ultracold fermionic atoms in an optical lattice, with a sudden position-dependent change (a quench) in the effective dispersion relation, have been proposed by Rodríguez-Laguna as an analog spacetime test of the Unruh effect. We provide new support for this analog by analyzing the entanglement of a scalar field in a (1+1)-dimensional continuum spacetime with a similar quench, and the harvesting of this entanglement by a pair of Unruh-DeWitt detectors. We present numerical evidence that the concurrence and mutual information harvested by the detectors are qualitatively similar to those in Rindler spacetime, but they exhibit a small yet noticeable variation when the energy pulse created by the quench crosses the detectors. These findings provide further motivation to implement the experimental proposal of Rodríguez-Laguna

We study the response of an accelerated Unruh-DeWitt detector to a one-parameter family of “kappa Rindler” vacua, which generalize the standard Unruh effect. These states, parametrized by κ, continuously interpolate between the Rindler (κ→0) and Minkowski (κ=1) vacua. We find the detector registers a perfect thermal bath at a tunable temperature Tκ=κTU. This result establishes a framework for environments perceived as both “hotter” (κ>1) and “colder” (κ<1) than the standard Unruh temperature. We establish this thermality by demonstrating the Kubo-Martin-Schwinger condition for the Wightman function and by analyzing the associated particle creation process. Furthermore, we visualize the spacetime structure of the created field quanta, revealing an intuitive link between the κ-controlled symmetry of the modes and the perceived temperature. Our work provides a comprehensive framework for a modulated Unruh effect, bridging formal quantum field theory with clear visual intuition.

We discuss the emission of radiation from general sources in quantum scalar, electromagnetic, and gravitational fields using the Rindler coordinate frame, which is suitable for uniformly accelerated observers, in the Minkowski vacuum. In particular, we point out that to recover, from the point of view of uniformly accelerated observers in the interaction picture, the usual Larmor radiation, which is independent of the choice of the vacuum state, it is necessary to incorporate the Unruh effect assuming the Minkowski vacuum. Thus, the observation of classical Larmor radiation in the Minkowski vacuum could be seen as vindicating the Unruh effect in the sense that it is not correctly recovered in the uniformly accelerated frame unless the Unruh effect is taken into account.

Abstract I found an extended duality (triality) between Dirac fermions in periodic spacetime metrics, nonrelativistic fermions in gauge fields (e.g. Harper–Hofstadter model), and in periodic scalar fields on a lattice (e.g. Aubry–André model). This indicates an unexpected equivalence between spacetime metrics, gauge fields, and scalar fields on the lattice, which are understood as different physical representations of the same mathematical object, the quantum group $\mathcal {U}_q(\mathfrak {sl}_2)$. This quantum group is generated by the exponentiation of two canonical conjugate operators, namely a linear combination of position and momentum (periodic spacetime metrics), the two components of the gauge-invariant momentum (gauge fields), and position and momentum (periodic scalar fields). Hence, on a lattice, Dirac fermions in a periodic spacetime metric are equivalent to nonrelativistic fermions in a periodic scalar field after a proper canonical transformation. The three lattice Hamiltonians (periodic spacetime metric, Harper–Hofstadter, and Aubry–André) share the same properties, namely fractal phase diagrams, self-similarity, S-duality, topological invariants, flat bands, and topologically quantized current in the incommensurate regimes. In essence, this work unveils an unexpected link between gravity and gauge fields, opens new avenues for studying analog gravity—e.g. the Unruh effect and universe expansions/contractions, suggests the existence of an S-duality of spacetime curvatures, and hints at novel pathways to quantized gravity theories.

Abstract In this work, we propose to investigate the information behavior of quantum systems through accelerated detectors quadratically coupled with a massless scalar field. In addition, we made detailed comparisons with the case of linear coupling. The perturbative method was used to evolve the density matrix that describes the interaction of the detector-field system during a finite time. The systems studied were: accelerated single-qubit, quantum interferometric circuit, and the which-path distinguishability circuit. The results on the probability transition rates show that quadratic coupling amplifies the Unruh effect. This is due to the modification of the interaction structure, allowing the simultaneous absorption of multiple quanta. Our findings showed that the information is degraded more quickly in the case of quadratic coupling, when compared to the linear case. Furthermore, this change is mainly given by the coupling constant and by an additional factor that arises in the case of quadratic coupling. Therefore, these results indicate that the nature of the coupling between the detector and the field plays a fundamental role in the behavior of quantum information in high acceleration regimes.

Molecular entanglement as a signature of the Unruh effect

Abstract We investigate tripartite quantum steering dynamics in a three-Unruh–DeWitt-detector system, examining both $$1\rightarrow 2$$ 1 → 2 (i.e., from Alice to Bob and Charlie) and $$2\rightarrow 1$$ 2 → 1 steering (i.e., from Alice and Bob to Charlie) configurations under two paradigmatic entangled initial states: the Greenberger–Horne–Zeilinger (GHZ) and W states. Our analysis demonstrates that $$1\rightarrow 2$$ 1 → 2 steering exhibits better resistance to Unruh decoherence than $$2\rightarrow 1$$ 2 → 1 steering. Notably, the $$1\rightarrow 2$$ 1 → 2 steering of the W state displays a non-monotonic dependence on the acceleration parameter, indicating that the Unruh effect can, in certain regimes, enhance quantum steering. These results suggest that the Unruh effect plays a dual role in tripartite quantum systems, as it can both enhance and degrade quantum steering. Consequently, moving beyond the conventional interpretation of the Unruh effect as merely a source of decoherence, our study provides a new insight by exploring tripartite quantum steering in a relativistic setting.

The Unruh effect is a well-understood phenomenon in which a quantum field in a vacuum state within Minkowski spacetime appears thermally populated for a uniformly accelerating Rindler observer. In this article, we derive a variant of the Unruh effect involving two distinct accelerating observers and aim to address the following questions: (i) Is it possible to have a particle excitation for a selective subset of momentum modes for the case of massless scalar fields, and (ii) Is it possible to excite only the left-chiral massless fermions while keeping right-chiral fermionic fields in an unexcited state or vice versa? To this end, we consider a Rindler wedge R1 constructed from a class of accelerating observers and another Rindler wedge R2 (with R2⊂R1) constructed from another class of accelerating observers such that the wedge R2 is displaced along a null direction with respect to R1 by a parameter Δ. By first considering a massless scalar field in the R1 vacuum, we demonstrate that if we choose the displacement Δ along one null direction, the positive momentum modes are thermally populated, whereas negative momentum modes remain unaffected (and vice versa if we choose the displacement along the other null direction). We then consider a massless fermionic field in a vacuum state in R1 and show that the reduced state in R2 is such that the left-chiral fermions are excited and are thermal for large frequencies. In contrast, the right-chiral fermions have negligible particle density (and vice versa). We argue that the null-shifted Rindler spacetime considered in this article is a good toy model for understanding a class of evolving horizons. Additionally, based on our work, we hypothesize that massless fermions underwent selective chiral excitations during the radiation-dominated era of cosmology. Published by the American Physical Society 2025

Unruh Effect and Minimum Viscosity

We demonstrate that a quantum field theory (QFT) in general two-dimensional curved spacetimes can be realized by a system of quantum spins or qubits. We consider a spin-1/2 model on a one-dimensional ring with spatially and temporally varying exchange couplings and magnetic fields. This model reduces to a QFT of Majorana fermions in the continuum limit. From this correspondence, we establish a dictionary for translating between the spacetime-dependent parameters of the spin model and the general metric on which the QFT is defined. After addressing the general case, we consider the Friedmann-Lemaître-Robertson-Walker (FLRW) metric as a simple example. According to the dictionary, the QFT of Majorana fermions on the FLRW metric corresponds to the Ising model with a time-dependent transverse magnetic field. We demonstrate that the production of Majorana particles in the expanding universe can be simulated with the transverse-field Ising model by increasing the strength of the magnetic field. Furthermore, we examine the Unruh effect through the spin system by using our prescription and show the direct relation between the entanglement (or modular) Hamiltonian in the spin system and the Rindler Hamiltonian. This approach provides an experimentally viable system for probing various phenomena in QFT within curved spacetime, while also opening the door to uncovering nontrivial phenomena in spin systems inspired by curved spacetime physics. It offers fresh perspectives on both QFT in curved spacetimes and quantum many-body spin systems, revealing profound connections between these fields. Published by the American Physical Society 2025

Previous studies have shown that the Unruh effect completely destroys quantum entanglement and coherence of bipartite states, as modeled by entangled Unruh-DeWitt detectors. But does the Unruh effect have a different impact on quantum entanglement of multipartite states within this framework? In this paper, we investigate the influence of the Unruh effect on 1 − 3 entanglement in the context of entangled tetrapartite Unruh-DeWitt detectors. We find that quantum entanglement of tetrapartite W state first decreases to a minimum value and then increases to a fixed value with the growth of the acceleration. This indicates that the Unruh effect can, under certain conditions, enhance quantum entanglement. In other words, the Unruh effect plays a dual role in the behavior of quantum entanglement-both diminishing and enhancing it. This discovery challenges and overturns the traditional view that the Unruh effect is solely detrimental to quantum entanglement and coherence in entangled Unruh-DeWitt detectors, offering a fresh and profound perspective on its impact.

The physical phenomena seen by an observer are defined for a local inertial system that is subjective to the observer. Such a coordinate system is called a “moving frame” because it changes from time to time. However, unlike the Thomas precession, the Unruh-DeWitt detector has been discussed for a fixed frame. We discuss the Unruh-DeWitt detector by defining the vacuum for the moving frame, showing that the problem of the Stokes phenomenon can be solved by using the vierbeins and the exact WKB, to find factor 2 discrepancy from the standard result. Differential geometry is constructed in such a way that local calculations can be performed rigorously. If one expects Markov property, the calculation is expected to be local. The final piece that was missing was a local non-perturbative calculation, which is now complemented by the exact WKB. Our analysis defines a serious problem regarding the relationship between entanglement of the Unruh effect and differential geometry.

The purpose of this review is to provide a pedagogical development of the Unruh effect and the thermofield double state. In Sec. 2 , we construct Rindler spacetime and analyze the perspective of an observer undergoing constant acceleration in Minkowski spacetime, which motivates the establishment of the relationship between the Fourier modes in both geometries using the Bogoliubov–Valatin transformation. In Sec. 3 , we explore the underlying physics leading to the Unruh effect, its analogy with the thermal radiation observed around a Schwarzschild black hole, and its manifestation through the coupling of a particle detector to the scalar field. Finally, in Sec. 4 , we derive the thermofield double state by conducting a Euclidean analysis of the field and geometry.

We investigated the influence of the massive scalar field on the information degradation concerning the Unruh-DeWitt (UDW) detectors. In this conjecture, we adopted a system with a finite and large interaction time. To accomplish our purpose, one examines the quantum coherence of a uniformly accelerated qubit and the probability of finding the detector in the ground state. In this framework, we consider a quantum interferometric circuit to obtain the probability, visibility, and coherence. Naturally, these measurements provide us with wave-like information. Besides, one modifies the circuit to describe the path distinguishability and the particle-like information. These results are promising, as they allow us to understand the influence of the Unruh effect on the wave-particle duality. Thus, our findings announce that the increase in the scalar field mass induces a decrease in information degradation. Finally, we noted that the information concerning the Unruh effect remains preserved when m ≥ Ω. Therefore, the detector cannot absorb particles with mass equal to or greater than its energy gap. These results indicate that the scalar field mass is a protective factor against information degradation for systems under high acceleration conditions.

Modern physics proposals present deep tensions between seemingly contradictory descriptions of reality. Views of wave-particle duality, black hole complementarity, and the Unruh effect demand explanations that shift depending on how a system is observed. However, traditional models of scientific explanation impose a fixed structure that fails to account for varying observational contexts. This paper introduces context-dependent mapping, a framework that reorganizes physical laws into self-consistent subsets structured around what can actually be observed in a given context. By doing so, it provides a principled way to integrate complementarity into the philosophy of explanation.

Fulling–Davies–Unruh effect contains great amount of theoretical importance in various branches of physics. Requirement of very high acceleration hinders its experimental evidence. We put forward an idea to experimentally probe this effect by utilizing the Pancharatnam–Berry phase of an accelerated atom in presence of mirrors. We show that for much lower accelerations, the phase gets significantly enhanced in the presence of mirrors. We propose a schematic design of an interferometric set-up to experimentally capture this effect by utilizing the phase difference between an accelerated and an inertial atoms. For the choice of hydrogen atoms and suitable separation between atoms and mirrors, the required acceleration can be very low.