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arXiv:2003.05014 · Physical Review Research

Decoherence as a detector of the Unruh effect

Alexander I. Nesterov, Manuel A. Rodríguez Fernández, Gennady P. Berman, Xidi Wang

Published Nov 13, 2020• 19 citations
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Abstract

We propose a type of the Unruh-DeWitt detector which measures the decoherence of the reduced density matrix of the detector interacting with the massless quantum scalar field.We find that the decoherence decay rates are different in the inertial and accelerated reference frames.We show that the exponential phase decay can be observed for relatively low accelerations, that can significantly improve the conditions for measuring the Unruh effect.

DOI
10.1103/physrevresearch.2.043230

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We propose a type of the Unruh-DeWitt detector which measures the decoherence of the reduced density matrix of the detector interacting with the massless quantum scalar field.We find that the decoherence decay rates are different in the inertial and accelerated reference frames.We show that the exponential phase decay can be observed for relatively low accelerations, that can significantly improve the conditions for measuring the Unruh effect.
The Unruh effect remains a central topic in quantum field theory, although its direct experimental verification continues to be challenging. Recent efforts have therefore focused on indirect detection strategies in which the Unruh effect emerges through measurable physical processes. In this work, we extend a previously introduced detector model, originally formulated for a massless scalar field, to the electromagnetic field. We show that the decoherence decay rates differ between inertial and accelerated frames. Furthermore, we demonstrate that the characteristic exponential decay associated with the Unruh effect can be observed at lower accelerations than those considered in earlier studies.
Detecting the Unruh effect is currently an important experimental goal, but it is difficult to realize under laboratory conditions because of the extreme acceleration required for observable signals. Therefore, various different schemes for indirect detection have been proposed. Recently, Nesterov and his collaborators discovered that the exponential phase decay in the decoherence of a uniformly accelerated particle detector can significantly improve the conditions for measuring the Unruh effect, and the acceleration required for observable signals can be considerably reduced to ∼1013m/s2. We notice that this exponential phase decay is consistent with its counterpart of the dephasing channel in quantum information theory. In this paper, we use the IBM quantum cloud platform to construct a quantum circuit that can realize the dephasing channel, successfully simulate the Unruh signal, and test the correctness of the method.
While the Unruh effect has traditionally been studied under the assumption of uniform acceleration, a simplification motivated by experimental considerations, it is not necessarily true for all non-inertial motions. We propose a novel approach for the indirect detection of the Unruh effect without relying on the former restriction. Previous studies have shown that probing the decoherence of an Unruh-DeWitt detector can significantly reduce the acceleration required for observing the effect by several orders of magnitude compared to earlier proposals. Building on this idea, we develop a theoretical framework describing a non-inertial observer equipped with a detector undergoing non-uniform, time-dependent acceleration. We show that, in a non-uniformly accelerated Rindler spacetime, the particle distribution perceived in the Minkowski vacuum acquires a time-dependent modification of the standard Unruh spectrum. Furthermore, we demonstrate that the inclusion of quantum entanglement leads to a deformation of the Minkowski vacuum into squeezed states.
Vacuum fluctuations of quantum fields provide an unavoidable environment for any quantum system coupled to it. We study the interplay between boundary conditions and acceleration in determining decoherence of a two-level Unruh-DeWitt detector coupled to a scalar field in a cylindrical cavity. We show that the decoherence rate closely follows the emission profile, and exhibits {\it Purcell-like} enhancement for both inertial and uniformly accelerated detectors. The acceleration induces an effective smearing of the resonant density of states, diluting the resonance enhancement for large accelerations while replacing the inertial off-resonant decay with an oscillatory behavior for small accelerations. For moderate accelerations, this interplay between cavity-induced and acceleration-assisted effects results in an extended region of cavity parameters where decoherence is strongly suppressed, particularly in regimes where the inertial detector otherwise experience strong decoherence. Thus, contrary to naive expectations, the Unruh thermality in a suitably engineered cavity can enhance rather than degrade quantum coherence, providing a very uncharacteristic feature of quantum fields in non-inertial frames.
We consider the influence of continuous decoherence on the excitation of an accelerated detector, an effect which must be taken into account in physically realistic realizations of the Unruh effect. As a model we use a quantum two-level system (as the detector) coupled to a massless scalar quantum field. This composite quantum system is treated as an open system which interacts with an environment so that continuous decoherence results. We develop a formalism that models the interaction with an environment approximately as a continuous measurement, represented by a sequence of state reductions. This leads to a description in terms of ``histories'' of the detector. Evolution operators corresponding to different histories of the detector can then be considered separately. The density matrix for the mixed state which results after a sequence of nonselective measurements is worked out explicitly. Based on the same formalism, it is also possible to correlate the time of the transition of the detector with the trajectories of the Minkowski particles which are emitted during this process, thus localizing the space-time region where they are created.
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.