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arXiv:2103.15838 · Physical Review Letters

Acceleration-Induced Effects in Stimulated Light-Matter Interactions

Barbara Šoda, Vivishek Sudhir, Achim Kempf

Published Apr 21, 2022• 29 citations
arXivHTMLPDF

Abstract

We show that, in addition to the Unruh effect, there exist two new phenomena that are due to acceleration in the quantum theory of the light-matter interaction. The first is the phenomenon of acceleration-induced transparency which arises since acceleration impacts not only the counter-rotating terms in the light-matter interaction (the cause of the conventional Unruh effect) but also the rotating wave terms. The second new phenomenon is that the Unruh effect can be stimulated, a phenomenon that arises since not only rotating-wave terms can be stimulated (as in conventional stimulated emission) but also counter-rotating terms. The new phenomena are potentially strong enough to be experimentally observable.

DOI
10.1103/physrevlett.128.163603

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•Sep 6, 2024•1 citation
We show that, in addition to the Unruh effect, there exist two new phenomena that are due to acceleration in the quantum theory of the light-matter interaction. The first is the phenomenon of acceleration-induced transparency which arises since acceleration impacts not only the counter-rotating terms in the light-matter interaction (the cause of the conventional Unruh effect) but also the rotating wave terms. The second new phenomenon is that the Unruh effect can be stimulated, a phenomenon that arises since not only rotating-wave terms can be stimulated (as in conventional stimulated emission) but also counter-rotating terms. The new phenomena are potentially strong enough to be experimentally observable.
We investigate the transition of a two-level atom as the Unruh-Dewitt detector accelerated in the electromagnetic field in this paper. The enhancement of the transition probability is found for different field states under the conditions that the anti-Unruh effect appears. In particular, the enhancement is the most prominent when the field state takes the squeezed state. We also construct a new atomic trajectory to realize the effect of acceleration-induced transparency. Based on this, we show that the anti-Unruh effect still exists even at the specific atomic energy difference where acceleration-induced transparency happens.
Stimulated transitions are among the most fundamental processes in light-matter interaction, underlying resonant absorption and emission in atomic systems. Here we show that uniform acceleration can convert this familiar response into a frequency-selective absence of response. Specifically, when an incident photon has a nonzero momentum component transverse to the acceleration, the stimulated transition probability vanishes at a discrete set of frequencies fixed by the acceleration, the atomic transition frequency, and the photon propagation angle. At these spectral blind spots, both ordinary stimulated absorption and acceleration-induced excitation are simultaneously suppressed, rendering the atom effectively unresponsive to the incident radiation. The effect arises from the nontrivial response of accelerated atoms to quantum vacuum fluctuations and provides a distinctive signature of the Unruh effect through the absence, rather than the enhancement, of stimulated transitions. We further provide an order-of-magnitude estimate showing that an electron-based implementation with spin splitting in combined electric and magnetic fields could access the required parameter regime. These results reveal an unexplored form of acceleration-modified light-matter interaction and identify spectral blind spots as a new manifestation of the Unruh effect.
Electromagnetism and light-matter interaction in rotating systems is a rich area of ongoing research. We study the interaction of light with a gas of non-interacting two-level atoms confined to a rotating disk. We numerically solve the optical Bloch equations to investigate the how relativistic rotation affects the atoms' polarisation and inversion. The results are used to predict the steady-state stimulated emission seen by an observer at rest with the optical source in the laboratory frame. Competing physical effects due to time dilation and motion-induced detuning strongly modify solutions to the Bloch equations when the gas's velocity becomes relativistic. We account for the non-inertial motion by including acceleration-dependent excitation and emission rates, arising from a generalised Unruh effect. The effective thermal vacuum resulting from large accelerations de-polarises the gas while driving it towards population inversion, negating coherent driving due to the external light source. The results illustrate the intuitive, special-relativistic approach of assigning instantaneously comoving frames to understand non-inertial motion's influence when only local fields are physically significant.
A concept termed “stimulated dipole–dipole interaction” as a framework is proposed to describe coherence-enhanced light–matter interactions. Unlike conventional dipole–dipole interactions, excitation-induced dipoles, generated through external perturbations, coherently stimulate and couple with each other, leading to reinforced transition pathways and modified physical characteristics. Theoretically, the phenomenon of stimulated dipole–dipole interaction can be well understood and calculated based on the additional interference terms between transition dipole matrix elements of perturbed Hamiltonians in Fermi’s golden rule, where coherent coupling between excitation-induced and intrinsic dipoles alters the overall transition probability. This framework can be applied to many platforms, including wrinkled graphene, quantum dots, plasmonic dipoles, exciton–polaritons in stacked two-dimensional materials, and electric and magnetic dipoles in spin resonance, offering a versatile approach for engineering light–matter interactions. It is pointed out that the underlying mechanism of the originally proposed stimulated emission can also be understood well based on stimulated dipole–dipole interaction. Therefore, the proposed stimulated dipole–dipole interaction paves a milestone to advance the further exploration of not-yet realized quantum optics phenomena.
The entanglement generated for a uniformly accelerated two-atom system in vacuum during its evolution may increase with acceleration, while that for a static one in a thermal bath always decreases monotonically with temperature. This phenomenon is named as the anti-Unruh effect in terms of the entanglement generated. In this paper, we study the effects of the interatomic interaction induced by the electromagnetic vacuum fluctuations on the entanglement dynamics of two uniformly accelerated atoms. We show that the anti-Unruh phenomenon may exist or disappear depending critically on the configuration of the orientation of the atomic polarization and the directions of the uniform acceleration and the interatomic separation. This is in sharp contrast to the scalar-field case, in which the anti-Unruh phenomenon is always lost when the environment-induced interatomic interaction is considered.
Abstract The Unruh effect predicts an astonishing phenomenon that an accelerated detector would detect counts despite being in a quantum field vacuum in the rest frame. Since the required detector acceleration for its direct observation is prohibitively large, recent analog studies in quantum simulation platforms help in revealing various properties of the Unruh effect and thus to explore the not-yet-understood physics of quantum gravity. To further reveal the quantum aspect of the Unruh effect, analogous experimental exploration of the correlation with respect to the detector and the field, and the consequences for coherent quantum trajectories of the detector that without classical counterparts, are essential steps but currently missing. Here, we utilize a laser-controlled trapped ion to experimentally simulate an oscillating detector coupled with a cavity field. We observe joint excitation of both the detector and the field in the detector’s frame, matching with the coordinated dynamics predicted by the Unruh effect. Particularly, we simulate the detector moving in single and superposed quantum trajectories, where the latter case shows coherent interference of excitation. Our demonstration reveals properties of quantum coherent superposition of accelerating trajectories associated with quantum gravity theories that have no classical counterparts, and may offer a new avenue to investigate phenomena in quantum field theory and quantum gravity. We also show how a generalization of the method and results in this work may be beneficial for direct observation of the Unruh effect.