We report the experimental observation of molecular unidirectional rotation (UDR) echoes and analyze their origin and behavior both classically and quantum mechanically. The molecules are excited by two time-delayed polarization-twisted ultrashort laser pulses and the echoes are measured by exploding the molecules and reconstructing their spatial orientation from the detected recoil ions momenta. Unlike alignment echoes which are induced by linearly polarized pulses, here the axial symmetry is broken by the twisted polarization, giving rise to molecular unidirectional rotation. We find that the rotation sense of the echo is governed by the twisting sense of the second pulse even when its intensity is much weaker than the intensity of the first pulse. In our theoretical study, we rely on classical phase-space analysis and on three-dimensional quantum simulations of the laser-driven molecular dynamics. Both approaches nicely reproduce the experimental results. Echoes in general and the unique UDR echoes in particular provide powerful tools for studies of relaxation processes in dense molecular gases.

One-dimensional quantum emitters with chiral couplings can exhibit nonreciprocal decay channels, along with light-induced dipole-dipole interactions mediated via an atom-waveguide interface. When the position disorders are introduced to such atomic array, we are able to identify the phase transition from excitation delocalization to localization, with an interplay between the directionality of decay rates and the strength of light-induced dipole-dipole interactions. Deep in the localization phase, its characteristic length decreases and saturates toward a reciprocal coupling regime, leading to a system dynamics whose ergodicity is strongly broken. We also find an interaction-driven reentrant behavior of the localization phase and a reduction of level repulsion under strong disorder. The former coincides with a drop in the exponent of power-law decaying von Neumann entropy, which gives insights to a close relation between the preservation of entanglement and nonequilibrium dynamics in open quantum systems, while the latter presents a distinct narrow distribution of gap ratios in this particular disordered system.

We study a system consisting of a superconducting flux qubit strongly coupled to a microwave cavity. Externally applied qubit driving is employed in order to manipulate the spectrum of dressed states. We observe resonance narrowing in the region where the splitting between the two dressed fundamental resonances is tuned to zero. The narrowing in this region of overlapping resonances can be exploited for long-time storage of quantum states. In addition, we measure the response to strong cavity mode driving, and find a qualitative deviation between the experimental results and the predictions of a semiclassical model. On the other hand, good agreement is obtained using theoretical predictions obtained by numerically integrating the master equation governing the system's dynamics. The observed response demonstrates a process of a coherent cancellation of two metastable dressed states.

We investigate Cherenkov radiation triggered by qubit acceleration, which can be simulated using superconducting circuits. By analyzing qubit dynamics, we confirm the existence of the Cherenkov speed threshold. A question immediately arises: What is the role of the Cherenkov speed threshold from the aspect of causation? More specifically, what is the effect of the threshold on the ability of the qubit to transmit quantum information? To address this question, we consider measurements of the quantum direct cause, which can be used to estimate channel capacity, based on a recently developed notion on temporal quantum correlations. When choosing proper values for qubit acceleration and qubit-field coupling in a single-mode model, we surprisingly discover that the Cherenkov threshold serves as the speed limit for quantum information propagation in the single-mode model. We further extend use of these measurements to a multimode model. The results indicate that introducing extra modes can lead to further suppression of the quantum direct cause. The suppression is further enhanced when the number of field modes involved in the system is increased.

Using semion substitution for spin variables we perform an ab initio derivation of effective action for an open quantum two-level system . For this purpose, we introduce, by using the Hubbard-Stratonovich transformation a two-time complex quantum field which average value plays the role of the Green's function for the spin variables. The field thus introduced allows us to develop a diagram technique in a standard way. The proposed formalism is used to study a spin embedded into an Ohmic reservoir as an example of the spin-boson model. Non-Markovian effects in this system are analyzed.

In the the double-slit experiment, nonclassical paths are Feynman paths that go through both slits. Prior work with atom cavities as which-way detectors in the double-slit experiment has shown these paths to be experimentally inaccessible. In this paper, we show how such a setup can indeed detect nonclassical paths with 1% probability if one considers a different type of nonclassical path than previously investigated. We also show how this setup can be used to erase and restore the coherence of the nonclassical paths. Finally, we also show how atom cavities may be used to implement a exact measure of Born-rule violation [Quach, Which-way double-slit experiments and Born-rule violation, Phys. Rev. A 95, 042129 (2017)], which up until now has only been a formal construct.

Nonstabilizer eigenstates of Clifford operators are natural candidates for endpoints of magic-state distillation routines. We provide an explicit bestiary of all inequivalent nonstabilizer Clifford eigenstates for qutrits and ququints. For qutrits, there are four nondegenerate eigenstates and two families of degenerate eigenstates. For ququints, there are eight nondegenerate eigenstates and three families of degenerate eigenstates. Of these states, a simultaneous eigenvector of all Clifford symplectic rotations known as the qutrit strange state is distinguished as both the most magic qutrit state and the most symmetric qutrit state. We show that no analog of the qutrit strange state (i.e., no simultaneous eigenvector of all symplectic rotations) exists for qudits of any odd-prime dimension $d>3$.

We present an investigation of previously unidentified visible lines for ${\mathrm{W}}^{8+}$ and ${\mathrm{W}}^{9+}$ from the spectra observed using an electron-beam ion trap. The analysis is based on collisional-radiative (CR) modeling with fine-structure sublevel population kinetics. To ensure the identification of lines done by the CR model, we have also performed an accurate calculation of transition energies and transition probabilities within the multiconfigurational Dirac-Fock approach using grasp2018. In the spectrum, most of the observed lines are assigned as magnetic dipole transitions belonging to the first two lower-lying configurations of these ions.

Recently, the thulium atom was cooled down to the Bose-Einstein condensation temperature, thus opening a pathway to quantum simulation with this atom. However, successful simulations require instruments to control and read out states of the atom as well as the ability to control the interaction between either different species or different states of the same type of species. In this paper, we provide an experimental demonstration of high-fidelity (over 93%) manipulation of the ground-state magnetic levels of thulium, which utilizes a simple and efficient design of a microwave (MW) antenna. The coherence time and dephasing rate of the energetically highest hyperfine level of the ground state were also examined.

Anisotropic mirrors are used to form a laser resonator exhibiting non-Hermitian, parity-time (PT) symmetric, polarization states. The relative angle of the two mirrors’ principal axes is used to control the degree of nonhermiticity. A sharp symmetry-breaking transition is observed at a specific angle, called the exceptional point, where the two states coalesce into a single polarization state and the interference pattern produced by counterpropagating (CP) waves vanishes. At a smaller angle, in the unbroken PT symmetry regime, the polarization state experiencing higher losses is suppressed. In the broken-symmetry regime, the two polarization states coexist, but the orthogonality of the CP waves favors single longitudinal mode emission by suppressing the interference pattern of the standing wave. The two regimes meet at the exceptional point, where a unique polarization state exists in a resonator free from interference intensity pattern. Microchip PT-symmetric lasers operating at the exceptional point are thus an attractive solution to achieve single-mode operation from a miniature monolithic device without any intracavity element.

We consider theoretically light scattering by a resonant layer that periodically moves in real space. At small frequencies of motion the scattered light spectrum reveals the frequency shift that is governed by the Doppler effect. At higher motion frequencies, the scattered light spectra acquire sidebands stemming from the Raman effect. We investigate the crossover between these two regimes and propose a realistic quantum well structure for its observation.

We investigate the spin-orbit coupling of light in three-dimensional cylindrical and tubelike whispering gallery mode resonators. We show that its origin is the transverse confinement of light in the resonator walls, even in the absence of inhomogeneities or anisotropies. The spin-orbit interaction results in elliptical far-field polarization (spin) states and causes spatial separation of polarization handedness in the far field. The ellipticity and spatial separation are enhanced for whispering gallery modes with higher excitation numbers along the resonator height. We analyze the asymmetry of the ellipticity and the tilt of the polarization orientation in the far field of conelike microcavities. Furthermore, we find a direct relationship between the tilt of the polarization orientation in the far field and the local inclination of the resonator wall. Our findings are based on finite-difference time-domain simulations and are supported by three-dimensional diffraction theory.

We theoretically demonstrate the enhanced and dephasing-immune squeezing in the resonance fluorescence of a single quantum dot (QD) confined to a pillar microcavity and driven by a continuous wave laser. We employ a formalism based on polaron master equation theory for incorporating the influence of exciton-phonon coupling quite accurately in the dot-cavity system. We show a significant enhancement of squeezing due to cavity coupling of the QD as compared to that of an ideal single two-level system in free space. Particularly, we show a fourfold enhancement in squeezing as compared to that of a single QD without cavity coupling. We further demonstrate the persistence of squeezing even when the pure dephasing becomes greater than the radiative decay rate. These features are attributed to the cavity-enhanced coherence causing partial reduction of the deteriorating effects of phonon-induced incoherent rates. We also show that the deteriorating effects of phonon-induced incoherent rates on squeezing can be partially circumvented by properly adjusting the detunings.

We revisit quantum-classical hybrid systems of the Sudarshan type under the light of Galilean covariance. We show that these kinds of hybrids cannot be given as a unitary representation of the Galilei group and at the same time conserve the total linear momentum unless the interaction term only depends on the relative canonical velocities.

Entanglement of formation is a fundamental measure that quantifies the entanglement of bipartite quantum states. This measure has recently been extended into multipartite states, taking the name $\alpha$-entanglement of formation. In this work we follow an analogous multipartite extension for the Gaussian version of entanglement of formation, and focusing on the finest partition of a multipartite Gaussian state, we show that this measure is fully additive and computable for three-mode Gaussian states.

In this paper we explore the possibility of performing Heisenberg limited quantum metrology of a phase, without any prior, by employing only maximally entangled states. Starting from the estimator introduced by Higgins et al. [New J. Phys. 11, 073023 (2009)], the main result of this paper is to produce an analytical upper bound on the associated mean-squared error which is monotonically decreasing as a function of the square of the number of quantum probes used in the process. The analyzed protocol is nonadaptive and requires in principle (for distinguishable probes) only separable measurements. We explore also metrology in the presence of a limitation on the entanglement size and in the presence of loss.