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Finite-time quantum heat engines operating with working substances of quantum nature are of practical relevance as they can generate finite-power. However, they encounter energy losses due to quantum friction, which is particularly pronounced in many-body systems with non-trivial coherences in their density operator. Strategies such as shortcuts to adiabaticity and fast routes to thermalization have been developed although the associated cost requirements remain uncertain. In this study, we theoretically investigate the finite-time operation of a trapped-atom Otto engine with light-induced dipole–dipole interactions and projection measurements in one of the isochoric processes. The investigation reveals that when atoms are sufficiently close to each other and their dipoles are oriented perpendicularly, light-induced dipole–dipole interactions generate strong coherent interactions. This has enhanced engine efficiency to near unity and accelerate the thermalization process by sixtyfold. The interactions also boost engine performance during finite-unitary strokes despite the significant quantum friction induced by the time-dependent driving field. Furthermore, the projection measurement protocol effectively erases quantum coherences developed during both the finite-unitary expansion and finite thermalization stages and allows finite-time engine operation with an output power. This setup presents a compelling avenue for further investigation of finite-time many-body quantum heat engines and provides an opportunity to explore the full potential of photon-mediated dipole–dipole interactions.

Manifestations of dipole–dipole interactions in dilute thermal gases are difficult to sense because of strong inhomogeneous broadening. Recent experiments reported signatures of such interactions in fluorescence detection-based measurements of multiple quantum coherence (MQC) signals, with many characteristic features hitherto unexplained. We develop an original open quantum systems theory of MQC in dilute thermal gases, which allows us to resolve this conundrum. Our theory accounts for the vector character of the atomic dipoles as well as for driving laser pulses of arbitrary strength, includes the far-field coupling between the dipoles, which prevails in dilute ensembles, and effectively incorporates atomic motion via a disorder average. We show that collective decay processes—which were ignored in previous treatments employing the electrostatic form of dipolar interactions—play a key role in the emergence of MQC signals.

We consider a pair of dipoles such as Rydberg atoms for which direct electrostatic dipole-dipole interactions may be significantly larger than the coupling to transverse radiation. We derive a master equation using the Coulomb gauge, which naturally enables us to include the inter-dipole Coulomb energy within the system Hamiltonian rather than the interaction. In contrast, the standard master equation for a two-dipole system, which depends entirely on well-known gauge-invariant S-matrix elements, is usually derived using the multipolar gauge, wherein there is no explicit inter-dipole Coulomb interaction. We show using a generalised arbitrary-gauge light-matter Hamiltonian that this master equation is obtained in other gauges only if the inter-dipole Coulomb interaction is kept within the interaction Hamiltonian rather than the unperturbed part as in our derivation. Thus, our master equation depends on different S-matrix elements, which give separation-dependent corrections to the standard matrix elements describing resonant energy transfer and collective decay. The two master equations coincide in the large separation limit where static couplings are negligible. We provide an application of our master equation by finding separation-dependent corrections to the natural emission spectrum of the two-dipole system.

Ultracold collisions of the polyatomic species CaOH are considered, in internal states where the collisions should be dominated by long-range dipole-dipole interactions. The computed rate constants suggest that evaporative cooling can be quite efficient for these species, provided they start at temperatures achievable by laser cooling. The rate constants are shown to become more favorable for evaporative cooling as the electric field increases. Moreover, long-range dimer states (CaOH) 2 * are predicated to occur, having lifetimes on the order of microseconds.

We investigate the spectral shift known as the collective Lamb shift in forward scattering for a cold dense atomic cloud. The shift results from resonant dipole–dipole interaction mediated by real and virtual photon exchange, forming many-body states displaying various super- and subradiant behaviour. However, the scattering spectrum reflects the overall contributions from these states but also averages out the radiative details associated with the underlying spin orders, causing ambiguity in determination and raising controversy on the scaling property of this shift. We employ a Monte–Carlo simulation to study how the collective states contribute to emission. We thus distinguish two kinds of collective shift that follow different scaling laws. One results from dominant occupation of the near-resonant collective states. This shift is usually small and insensitive to the density or the number of participating atoms. The other comes from large spatial correlation of dipoles, associated with the states of higher degree of emission. This corresponds to larger collective shift that is approximately linearly dependent on the optical depth. We further demonstrate that the spatial spin order plays an essential role in superradiant emission. Our analysis provides a novel perspective for understanding collective scattering and cooperative effects.

Macroscopic ensembles of radiating dipoles are ubiquitous in the physical and natural sciences. In the classical limit the dipoles can be described as damped-driven oscillators, which are able to spontaneously synchronize and collectively lock their phases in the presence of nonlinear coupling. Here we investigate the corresponding phenomenon with arrays of quantized two-level systems coupled via long-range and anisotropic dipolar interactions. Our calculations demonstrate that by incoherently driving dense packed arrays of strongly interacting dipoles, the dipoles can overcome the decoherence induced by quantum fluctuations and inhomogeneous coupling and reach a synchronized steady-state characterized by a macroscopic phase coherence. This steady-state bears much similarity to that observed in classical systems, and yet also exhibits genuine quantum properties such as quantum correlations and quantum phase diffusion (reminiscent of lasing). Our predictions could be relevant for the development of better atomic clocks and a variety of noise tolerant quantum devices.

One of the most pertinent problems in the debate on non-trivial quantum effects in biology concerns natural photosynthesis. Since sunlight is composed of thermal photons, it was argued to be unable to induce quantum coherence in matter, and that quantum mechanics is therefore irrelevant for the dynamical processes following photoabsorption. Our present analysis of a toy 'molecular aggregate'-composed of two dipole-dipole interacting two-level atoms treated as an open quantum system-however shows that incoherent excitations indeed can trigger persistent, coherent dynamics in both the site and the exciton bases: we demonstrate that collective decay processes induced by the dipole-dipole interactions create coherent intermolecular transport-regardless of the coherence properties of the incoming radiation. Our analysis shows that the steady state coherence is mediated by the population imbalance between the molecules and, therefore, increases with the energy difference between the two-level atoms. Our results establish the importance of collective decay processes in the study of ultrafast photophysics, and especially their potential role to generate stationary coherence in incoherently driven quantum transport.

To facilitate the transition of quantum effects from the controlled laboratory environment to practical real-world applications, there is a pressing need for scalable platforms. One promising strategy involves integrating thermal vapors with nanostructures designed to manipulate atomic interactions. In this tutorial, we aim to gain deeper insights into this by examining the behavior of thermal vapors that are confined within nanocavities or waveguides and exposed to near-resonant light. We explore the interactions between atoms in confined dense thermal vapors. Our investigation reveals deviations from the predictions of continuous electrodynamics models, including density-dependent line shifts and broadening effects. In particular, our results demonstrate that by carefully controlling the saturation of single atoms and the interactions among multiple atoms using nanostructures, along with controlling the geometry of the atomic cloud, it becomes possible to manipulate the effective optical nonlinearity of the entire atomic ensemble. This capability renders the hybrid thermal atom-nanophotonic platform a distinctive and valuable one for manipulating the collective effect and achieving substantial optical nonlinearities.

We show that a single photon pulse incident on two interacting two-level atoms induces a transient entanglement force between them. After absorption of a multi-mode Fock state pulse, the time-dependent atomic interaction mediated by the vacuum fluctuations changes from the van der Waals interaction to the resonant dipole-dipole interaction (RDDI). We explicitly show that the RDDI force induced by the single photon pulse fundamentally arises from the two-body transient entanglement between the atoms. This single photon pulse induced entanglement force can be continuously tuned from being repulsive to attractive by varying the polarization of the pulse. We further demonstrate that the entanglement force can be enhanced by more than three orders of magnitude if the atomic interactions are mediated by graphene plasmons. These results demonstrate the potential of shaped single photon pulses as a powerful tool to manipulate this entanglement force and also provides a new approach to witness transient atom-atom entanglement.

The subwavelength array of quantum emitters provides an ideal platform for exploring rich many-body dynamics, such as super- and subradiance. In this paper, we explore the dynamics of spin wave exchange between two dipole-coupled atomic ring arrays. Subradiant spin waves lead to low-loss and high efficiency of ring-to-ring transfer. The optimal subradiant spin wave exchange occurs at appropriate separations between coplanar rings, despite the fact that the energy transfer efficiency is monotonically enhanced (in the regime ⩽ λ 0 / 2 ) as the rings’ separation decreases. However, the spin wave will scatter due to the dephasing mechanism of close-by atom pairs, as the separation of two rings is too small. With the increase in the number of atoms on the ring, the subradiant shielding effect also strengthens, leading to a shorter distance for the transfer of spin waves. We investigate the rotation of one of the rings and find that the optimal spin wave exchange corresponds to the scenario where the line connecting the two nearest atoms of the two rings aligns with the center of the circle. Moreover, we study the influence of transition dipole moment orientations on the effective interaction between two atomic rings. We observe that there is a critical point where the effective interaction strength changes dramatically owing to the cooperative effect of the subwavelength atomic array. We believe that our results could be important for quantum information processing based on atomic arrays.