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arXiv:2103.09165 · Physical review. D/Physical review. D.

Generation and catalysis of coherence with scalar fields

Nikolaos K. Kollas, Dimitris Moustos

Published Jan 6, 2022• 10 citations
arXivHTMLPDF

Abstract

Employing time-dependent perturbation theory, we discuss the conditions under which a spatially extended two level Unruh-DeWitt detector, coupled to the proper time derivative of a scalar field, can drive itself into a coherent superposition of its energy basis. It is shown that this process, which is assisted due to the energy cost required for the implementation of the interaction, is only possible for a field with a nonzero coherent amplitude distribution. When the detector interacts instantaneously with the field through a delta coupling, the latter acts as a catalyst resulting in the extraction of the same amount of coherence each time at the cost of a positive amount of work. For a Gaussian smeared detector and a field in a coherent state the amount generated to lowest order in the coupling constant depends on the phase of the amplitude distribution, the initial energy of the field, the mean radius of the detector and the mean interaction duration between the two. We observe that for a detector moving at a constant velocity and with a mean radius of the same order as its transition wavelength, coherence swelling effects are present the intensity of which depends on the dimension of the underlying Minkowski spacetime.

DOI
10.1103/physrevd.105.025006

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Employing time-dependent perturbation theory, we discuss the conditions under which a spatially extended two level Unruh-DeWitt detector, coupled to the proper time derivative of a scalar field, can drive itself into a coherent superposition of its energy basis. It is shown that this process, which is assisted due to the energy cost required for the implementation of the interaction, is only possible for a field with a nonzero coherent amplitude distribution. When the detector interacts instantaneously with the field through a delta coupling, the latter acts as a catalyst resulting in the extraction of the same amount of coherence each time at the cost of a positive amount of work. For a Gaussian smeared detector and a field in a coherent state the amount generated to lowest order in the coupling constant depends on the phase of the amplitude distribution, the initial energy of the field, the mean radius of the detector and the mean interaction duration between the two. We observe that for a detector moving at a constant velocity and with a mean radius of the same order as its transition wavelength, coherence swelling effects are present the intensity of which depends on the dimension of the underlying Minkowski spacetime.
We study the effects of motion, on the amount of coherence extracted onto an Unruh-DeWitt detector, modeled as a two-level system, when it interacts with a massless scalar coherent field in 1+11+11+1 Minkowski spacetime. We observe that compared to a detector at rest, for certain values of the initial energy of the field and the duration of the interaction, the amount extracted is larger for a detector moving with constant speed or uniform acceleration. This ``swelling'' of coherence becomes more intense for increasing values of its velocity and acceleration, and is mostly observed for short interaction durations, when the energy of the field is larger compared to that of the detector, and for longer interaction durations but smaller field energies. As a consequence, the rate at which coherence is lost, is sometimes slower for a moving detector than for a detector at rest.
Employing a nonperturbative approach based on an instantaneous interaction between a two-level Unruh-DeWitt detector and a massive scalar field, we investigate the ability of the latter to generate or destroy coherence in the detector by deriving the cohering and decohering power of the induced quantum evolution channel. For a field in a coherent state a previously unobserved effect is reported in which the amount of coherence that the field generates displays a revival pattern with respect to the size of the detector. Extending previous results into the nonperturbative regime of an arbitrary coupling strength, it is demonstrated that in the case of a thermal field with a positive mass a detector initialized in a maximally coherent state experiences a smaller degree of decoherence after its interaction with the field, compared to the massless case. In both examples for a suitable choice of detector radius, field energy, and coupling strength it is possible to infer the mass of the field by either measuring the coherence present in the detector in the case of an interaction with a coherent field or the corresponding decoherence of a maximally coherent state in the case of a thermal field. In view of recent advances in the study of Proca metamaterials, these results suggest the possibility of utilizing the theory of massive electromagnetism for the construction of novel applications for use in quantum technologies.
Abstract We explore the quantum coherence between a pair of entangled Unruh–DeWitt detectors, interacting with a quantum field, using a nonperturbative approach in a (3+1)-dimensional Minkowski spacetime with instantaneous switching ( δ\delta δ δ -switching). It is intriguing to observe that for a maximally entangled state, increasing the coupling strength enhances the detectors’ initial quantum coherence while simultaneously causing a monotonic decrease in their initial entanglement. This reveals a remarkable phenomenon: through nonperturbative interactions, entangled Unruh–DeWitt detectors can exhibit a dual effect-amplifying quantum coherence while degrading quantum entanglement. This finding stands in stark contrast to previous studies based on perturbative methods or Gaussian switching functions, which generally concluded that interactions between detectors and the field lead to a simultaneous degradation of quantum coherence and entanglement due to environmental decoherence. Notably, while initially separable detectors successfully harvest quantum coherence from the vacuum, entanglement extraction remains fundamentally prohibited. These contrasting behaviors underscore the fundamental distinction between coherence and entanglement as quantum resources, and highlight their complementary roles in field-detector interactions.
We investigate the decoherence of an Unruh-DeWitt detector coupled to scalar, electromagnetic, and spinor fields in four-dimensional Minkowski spacetime. By employing the Schwinger-Keldysh influence functional formalism, we derive a universal scaling law relating the decoherence rate to the proper acceleration aaa and the scaling dimension ΔΔΔ of the environmental field operator. By analyzing both sharp (top-hat) and smooth Gaussian switching functions, it is shown that the decoherence rate in the asymptotic long-time limit scales as a2Δ−1a^{2Δ-1}a2Δ−1. This scaling indicates that increasing scaling dimension of the coupling field operators can significantly enhance the decoherence, thereby providing a more sensitive probe of the Unruh effect.
A bstract We investigate the decoherence of an Unruh-DeWitt detector coupled to scalar, electromagnetic, and spinor fields in four-dimensional Minkowski spacetime. By employing the Schwinger-Keldysh influence functional formalism, we derive a universal scaling law relating the decoherence rate to the proper acceleration a and the scaling dimension ∆ of the environmental field operator. By analyzing both sharp (top-hat) and smooth Gaussian switching functions, it is shown that the decoherence rate in the asymptotic long-time limit scales as a 2∆−1 . This scaling indicates that increasing scaling dimension of the coupling field operators can significantly enhance the decoherence, thereby providing a more sensitive probe of the Unruh effect.
Recently, Danielson, Satishchandran, and Wald (DSW) have proposed a novel source of decoherence for uniformly accelerated charges and masses in spatial superposition in spacetimes containing a bifurcating Killing horizon. Such an effect can be traced back to the emission and absorption of soft photons and gravitons, which effectively act as "which-path'' information probes. This results in the decoherence of any such superposition in a finite proper time. With this in mind, we study the DSW mechanism using a gapless finite-time detector prepared in a spatial superposition of uniformly accelerated paths in Minkowski spacetime that interacts with a massive scalar field. The calculation is nonperturbative. Such a model will enable us to analyze the decoherence process in a more controlled manner, highlighting the main factors that give rise to this interesting mechanism.