In this study, we reexamine a recent optimal control simulation targeting the preparation of a superposition of two excited electronic states in the ultraviolet (UV) range in a complex molecular system. We revisit this control from the perspective of reinforcement learning, offering an efficient alternative to conventional quantum control methods. The two excited states are addressable by orthogonal polarizations and their superposition corresponds to a right or left localization of the electronic density. The pulse duration spans tens of femtoseconds to prevent excitation of higher excited bright states which leads to a strong perturbation by the nuclear motions. We modify an open source software by Giannelli et al. [L. Giannelli et al., Phys. Lett. A 434, 128054 (2022)] that implements reinforcement learning with Lindblad dynamics, to introduce non-Markovianity of the surrounding reservoir either by time-dependent rates, or more exactly, by using the hierarchical equations of motion with the QuTip-BoFiN package. This extension opens the way to wider applications for non-Markovian environments, in particular when the active system interacts with a highly structured noise.

Air lasing from the singly ionized nitrogen molecule ${{\text{N}}_2}^{+}$ in a femtosecond laser filament under low gas pressures has been of intense interest in the field of ultrafast optics in the past decade. Several theoretical models have been proposed to explain the origin of ${{\text{N}}_2}^{+}$ lasing, but none of them have considered the competition between spontaneous emission and gain-swept lasing from ${{\text{N}}_2}^{+}$ in the filamentation process. In this work, we experimentally investigate such competition by measuring the dynamic fluorescence distribution along the filament in the presence of an external probe pulse. The results show that the occurrence of lasing in the middle of the filament reduces the side fluorescence and the absorption of lasing in the rear part of the filament increases the fluorescence. By solving a time-dependent nonlinear Schrödinger equation and applying the transient ionization injection model, population distributions along the filament are numerically simulated and well explain our experimental observations.

We explore ultradilute Bose-Bose liquid droplets squeezed by an external harmonic potential in one spatial direction. Our theoretical study is based on a functional that is built using quantum Monte Carlo results of the bulk phase and incorporates finite-range effects. A characteristic feature of these drops is the existence of a critical atom number, which is the minimum number of particles to have a many-body bound state. We report results on the critical atom numbers for different magnetic fields and applying confinement strengths towards a quasi-two-dimensional setup. In the regime where the local-density approximation is expected to be valid, we find that the critical atom number decreases linearly with the harmonic oscillator length of the confining potential. With the largest squeezing explored in our work, we predict stable drops containing from approximately 1000 to 2000 atoms for magnetic fields from 56.230 to 56.574 G, respectively. Our functional reduces the critical numbers for any confinement and applied magnetic field with respect to the estimations based on the Lee-Huang-Yang model. We observe saturated drops when the number of atoms in the drop is much larger than the critical value, their central density being higher for the quantum Monte Carlo functional than for the Lee-Huang-Yang one.

We analyze the nonlinear dynamics of Fabry-Perot cavities of arbitrary finesse filled by a dispersive Kerr medium, pumped by a continuous-wave laser or a synchronous train of flat-top pulses. The combined action of feedback, group velocity dispersion, and Kerr nonlinearity leads to temporal instability with respect to perturbations at specified frequencies. We characterize the generation of new spectral bands by deriving the exact dispersion relation, and we find approximate analytical expressions for the instability threshold and gain spectrum of modulation instability (MI). We show that, in contrast to ring resonators, both the stationary solutions and the gain spectrum are significantly affected by the duration of the pump pulse. We derive the extended Lugiato-Lefever equation for the Fabry-Perot resonator (FP-LLE) starting from coupled nonlinear Schrödinger equations, and we compare the outcome of the stability analysis of the two models. While FP-LLE gives overall good results, we show regimes that are not captured by the mean-field limit, namely, the period-two modulation instability, which may appear in highly detuned or nonlinear regimes. We report numerical simulations of the generation of MI-induced Kerr combs by solving FP-LLE and the coupled Schrödinger equations.

We report that an anomalous optical whispering-gallery mode (WGM) emerges with self-focusing field patterns in a nearly spherical cavity of ultrahigh-$Q$ factor, driven by rotational symmetry breaking and positive surface curvature. The positive Gaussian curvature modulates the evolution of light fields in the vicinity of a microsized surface, which is revealed by transformation optics theory. The geometry-induced self-focusing effect is resolved by the strongly confined field patterns of ultrahigh-$Q$ modes. Furthermore, the nondiffracting interferometric field patterns with twist-chain-like profiles are observed by leveraging the degenerate-mode excitations in the surface of the microsphere cavity. The morphology-dependent WGMs can have potential applications in generating multiple orbital angular momentum beams when integrated with grating structures at the cavity surface. This work not only bridges the transformation optics and microcavity photonics in curved geometries but also provides a new route to on-chip optical field manipulation and light-matter interactions.

Semiconductor quantum dots are among the best sources of on-demand entangled photon pairs. The degree of entanglement, however, is generally limited by the fine structure splitting of exciton states. In this paper, we theoretically investigate the generation of polarization-entangled photon pairs under two-photon excitation and cavity-assisted two-photon emission, both in the weak and strong cavity coupling regimes. We demonstrate and clarify that cavity coupling together with an excitation pulse reduces the degree of entanglement in three different ways. First, in a strong coupling regime, the cavity introduces the unequal ac-Stark shift of horizontally and vertically polarized exciton states, which results in the effective splitting of exciton states. Second, it induces the cross-coupling between the exciton states even in the weak coupling regime, causing the creation of unfavorable two-photon states. Finally, higher excited states of the cavity modes also contribute to the reduction of entanglement. Therefore, in the setting considered here, cavity coupling, which is generally required for the efficient collection of emitted photons, degrades the entanglement both in weak and strong coupling regimes.

We analyze the properties across steady-state phase transitions of two all-to-all driven-dissipative spin models that describe possible dynamics of $N$ two-level systems inside an optical cavity: squeezed superradiance and driven superradiance. We show that the finite size behavior around the critical points can be captured correctly by carefully identifying the relevant nonlinearities in the Holstein-Primakoff representation of spin operators in terms of bosonic variables. With these tools, we calculate analytically various observables across the phase transitions and obtain their finite size scalings, including numerical prefactors. In particular, we look at the amount of spin squeezing carried by the steady states, of relevance for quantum metrology applications, and describe in analytical detail the mechanism by which the optimal spin squeezing acquires logarithmic corrections that depend on the system size. We also demonstrate that the logarithmic nature of these corrections is difficult to characterize through numerical procedures for any experimentally realistic and/or simulable values of particle number. We complement all of our analytical arguments with numerical benchmarks.

Interactions between Rydberg atoms can result in a dipole blockade for which the probability $P_n$ to have $n$ atomic excitations is reduced significantly when $n\ge 2$. Ideally, this allows one to create a single collective Rydberg excitation in an atomic ensemble with $P_1=1$. A single-photon source is realized by mapping this atomic excitation into a propagating light field. Even if $P_n\ne 0$ for $n\ge 2$, a single-photon source can be approximated if interaction-induced dephasing damps the contributions from multiply excited collective states into the preferred spatial field mode. Two quantities that can be used as figures of merit for these single-photon sources are the second-order correlation function $g^{\left(2\right)}$ associated with the phase-matched field emitted by the sample and the second-order correlation function $g^{\left(A\right)}$ associated with the atoms. Here we demonstrate that interaction-induced dephasing can lead to significant differences between $g^{\left(2\right)}$ and $g^{\left(A\right)}$ even if $P_2\ll P_1\approx 1$. Theoretical expressions are derived for these quantities and it is shown that $g^{\left(2\right)}\le g^{\left(A\right)}$. It is also shown that there is a distinct advantage for minimizing $g^{\left(2\right)}$ and $g^{\left(A\right)}$ by using adiabatic pulsed fields rather than constant amplitude fields to excite the ensemble. These results maybe useful for optimizing Rydberg single-photon sources.

We demonstrate the generation of photons within a fiber-cavity quantum memory, followed by later on-demand readout. Signal photons are generated by spontaneous four-wave mixing in a fiber cavity comprising a birefringent fiber with dichroic reflective end facets. The detection of the partner herald photon indicates the creation of the stored signal photon. After a delay, the signal photon is switched out of resonance with the fiber cavity by intracavity frequency translation using Bragg scattering four-wave mixing, driven by ancillary control pulses. We measure sub-Poissonian statistics in the output signal mode, with $g_{\text{AC}}^{\left(2\right)}=0.54\left(1\right)$ in the first readout bin and a readout frequency translation efficiency of $\approx 80\%$. The $1/e$ memory lifetime is $\approx 67$ cavity cycles, or 1.68 $\mu \mathrm{s}$. In an alternate fiber cavity, we show a strategy for noise reduction and measure $g_{\text{AC}}^{\left(2\right)}=0.068\left(10\right)$ after one cavity cycle.

We propose a transformation for spin and charge degrees of freedom in one-dimensional lattice systems, constrained to have no doubly occupied sites, that allows direct access to the dynamical correlations of the system. The transformation delivers particle creation and annihilation operators in a form of a spinless particle and a nonlocal operator acting on the space of states of a spin-$1/2$ chain. This permits a decomposition of dynamical correlation functions as a convolution of those for impenetrable anyons together with those of a spin chain. Further analysis can be done by methods tailored for each part of the convolution, greatly increasing the impact and flexibility of the approach.

The problem of establishing Bell and Greenberger-Horne-Zeilinger states between faraway places or distant nodes of a circuit is a difficult and an extremely important one, and a strategy which addresses it is entanglement percolation. We provide a method for attaining the end through a quantum measurement strategy involving three-, two-, and single-qubit measurements on a single-layer honeycomb lattice of partially entangled pure bipartite entangled states. We then move over to a double-layered lattice, and introduce entanglement percolation on that lattice under a realistic restriction on local quantum operations and classical communication allowed on the nodes of the lattice. When applied to a single-layered honeycomb lattice, our strategy would call for less noise effects in an actual realization than when the same phenomenon is attained via existing methods. Moreover, for the double-layered honeycomb lattice, we report advantage of quantum entanglement percolation over classical entanglement percolation under the realistic restriction.

This paper addresses the question of thermodynamic entropy production in the context of the dynamical Casimir effect. Specifically, we study a scalar quantum field confined within a one-dimensional ideal cavity subject to time-varying boundary conditions dictated by an externally prescribed trajectory of one of the cavity mirrors. The central question is how the thermodynamic entropy of the field evolves over time. Utilizing an effective Hamiltonian approach, we compute the entropy production and reveal that it exhibits scaling behavior concerning the number of particles created in the short-time limit. Furthermore, this approach elucidates the direct connection between this entropy and the emergence of quantum coherence within the mode basis of the field. In addition, by considering a distinct approach based on the time evolution of Gaussian states we examine the long-time limit of entropy production within a single mode of the field. This approach results in establishing a connection between the thermodynamic entropy production in a single field mode and the entanglement between that particular mode and all other modes. Consequently, by employing two distinct approaches, we comprehensively address both the short-term and long-term dynamics of the system. Our results thus link the irreversible dynamics of the field, as measured by entropy production and induced by the dynamical Casimir effect, to two fundamental aspects of quantum mechanics: coherence and entanglement.

We explore the feasibility of extracting electroweak observables from a measurement of atomic parity violation in hydrogen. Our proposed quantum-control scheme focuses on the $2s-3s$ or $2s-4s$ transitions in hydrogen. This work is motivated by the recently observed anomaly in the W-boson mass, which may substantially modify the Standard Model value of the proton weak charge. We also study the accuracy of the previously employed approximations in computing parity-violating effects in hydrogen.

Phase-dependent photoelectron spectra in resonance-enhanced multiphoton ionization (REMPI) of Ar are obtained by scanning the relative phase of two-color femtosecond lasers. Five REMPI channels are resolved and the relative time delays in the attosecond-timescale are extracted. The channel-resolved time delays can be assigned as the contribution of the resonant phase shift of electrons trapped in the specific Rydberg states and also influenced by the phase shift during the continuum-continuum transition. Our observations highlight the important influences of Freeman resonances on photoelectron emission dynamics in strong laser fields and will enable a subfemtosecond photoelectron wavepacket reconstruction in quantum mechanics.

We study the out-of-equilibrium dynamics of noninteracting atoms confined within a one-dimensional harmonic trap triggered by dragging an external long-range potential through the system. The symmetry-breaking nature of this moving potential couples adjacent eigenstates in the atoms' effective potential, leading to an energy landscape reminiscent of systems exhibiting trap-induced shape resonances. These couplings may be exploited to selectively excite the atoms into higher vibrational states of the harmonic trap by controlling the motion of the dragged potential. To this end, we consider two protocols designs: The first protocol strives to maintain adiabaticity at critical points during the atoms' dynamics, while the second protocol utilizes the fast tunneling of the atoms within their effective double-well potential. These protocols take place in the few to many millisecond regime and achieve high-fidelity excitation of the atoms into pure vibrational states and superpositions thereof. Overall, our study highlights the significance of dragged potentials for controlling and manipulating atom dynamics and offers intuitive protocols for achieving desired excitations.

We address the possibility of realizing Bose-Einstein condensation as a first-order phase transition by admixture of particles of different species. To this aim we perform a comprehensive analysis of phase diagrams of two-component mixtures of bosons at finite temperatures. As a prototype model, we analyze a binary mixture of Bose particles interacting via an infinite-range (Kac-scaled) two-body potential. We obtain a rich phase diagram, where the transition between the normal and Bose-Einstein-condensed phases may be either continuous or first order. The phase diagram hosts lines of triple points, tricritical points, and quadruple points. We address the structure of the phase diagram depending on the relative magnitudes of the inter- and intraspecies interaction couplings. In addition, even for purely repulsive interactions, we identify a first-order liquid-gas-type transition between noncondensed phases characterized by different particle concentrations. In the obtained phase diagram, a surface of such first-order transitions terminates with a line of critical points.

We present a technique to estimate the proportion of coherent emission in the light emitted by a practical laser source without spectral filtering. The technique is based on measuring interferometric photon correlations between the output ports of an asymmetric Mach-Zehnder interferometer. With this, we characterize the fraction of coherent emission in the light emitted by a laser diode when transiting through the lasing threshold.

Gaussian boson sampling (GBS) is a promising candidate for an experimental demonstration of quantum advantage using photons. However, sufficiently large noise might hinder a GBS implementation from entering the regime where quantum speedup is achievable. Here, we investigate how thermal noise affects the classical intractability of generic quantum optical sampling experiments, GBS being a particular instance of the latter. We do so by establishing sufficient conditions for an efficient simulation to be feasible, expressed in the form of inequalities between the relevant parameters that characterize the system and its imperfections. We demonstrate that the addition of thermal noise—modeled by (passive) linear optical interaction between the system and a Markovian thermal bath—has the effect of tightening the constraints on the remaining noise parameters, required to show quantum advantage. Furthermore, we show that there exists a threshold temperature, under the assumption of a uniform loss rate, at which quantum sampling experiments become classically simulable, and provide an intuitive physical interpretation by relating this occurrence with the disappearance of the quantum state's nonclassical properties.

Tommey and Hogan [Phys. Rev. A 104, 033305 (2021)] have reported a matter-wave interference experiment using Rydberg atoms traveling through inhomogeneous electric fields at approximately $2000\mathrm{m}/\mathrm{s}$. Using a simplified model containing the essential physics of their experiment, we show that the phase difference measured by their observed interference fringes does not depend, in any significant way, on the acceleration of the Rydberg atoms, but instead simply on the uniform motion of the atoms through the inhomogeneous electric field.