Optical microcavities have attracted increasing attention for high-performance sensing, where a laser scans the resonance, providing high-resolution and time-sensitive measurements. However, rapid resonance scanning can lead to readout distortions, affecting the accuracy of high-speed sensing. Traditional methods have attempted to alleviate this distortion by examining the relationship between the ringing behavior and the scanning speed or by reconstructing the steady-state spectra. Yet the complexity of distorted readouts often obscures their correlation with well-established steady-state spectra, presenting a significant challenge for transient sensing. In this study, we revisit electromagnetic theory in microcavities and discover that ringing distortion stems from quadratic phase interference between light emitted from the microcavity and the input. Remarkably, this quadratic phase solely arises from the spectral scanning speed, indicating a universality across microcavity types. As a result, the proposed method enables the recovery of a complex steady-state spectrum from a distorted readout at an arbitrary scanning speed after compensating for the introduced phases. This method bridges the gap between transient and steady-state spectra by harnessing the dimensions of spectral scanning speed, offering a promising avenue for high-speed sensing, precision measurement, and advanced scientific research.

The higher-order exceptional point (EP) and reconfigurable nonreciprocal transmission have been investigated in a four-mode optomechanical system, consisting of two passive cavities and two active mechanical modes. A phase diagram of multiple EPs including higher than third-order EP is divided into different regions with a certain EP pattern, the variation of which with the parameters are demonstrated by analyzing the eigenspectrum of the system. In particular, the emergence, coalescence, and separation of the multiple EPs are strongly dependent on the relative phase between different paths in the cavity optomechanics system. On the other hand, the optical nonreciprocity with a large isolation about 50 dB can be achieved by incorporating mechanical gain. It is shown that the optical nonreciprocity at the relative phase $\phi$ exhibits exchange symmetry with that in the case of the phase $2\pi -\phi$. Additionally, the system presents a special nonreciprocal spectra at different EPs, which can provide a means to identify the type of multiple EPs. Our scenario shows that the optical nonreciprocity associated with the multiple EPs may provide a useful platform for realizing optical communications and exploring the physics related to the higher-order EPs.

The study of atomic behaviors in a squeezed vacuum reservoir plays an important role in exploring quantum effect and quantum physics. We here propose a scheme to manipulate the dynamics of giant atoms coupled to a one-dimensional waveguide driven by a broadband squeezed field, which acts as a squeezed vacuum reservoir. By tuning the propagating phase of photons, the center-of-mass phase of the giant atom, and the squeezing parameter, both the dephasing dynamics and entanglement dynamics of giant atoms can be effectively controlled. In the single-giant-atom case, we obtain the expressions of the dephasing rates based on the Bloch equations derived from the quantum master equation of the giant atom. Adjusting these two phases enables the dephasing rates to be enhanced, suppressed, or reduced to zero, thereby changing the dephasing dynamics. For the two-giant-atom system, we find that the atomic individual and collective squeezing depend on these two phases simultaneously. The steady-state giant-atom population and entanglement are obtained by working in the collective state representation for the two atoms. Different from small atoms, the steady-state population and entanglement depend on not only the center-mass-of phase, but also the propagating phase. In addition, we identify the condition for achieving larger steady-state entanglement via analyzing the system parameters. This work will facilitate the study of light-matter interaction based on giant-atom waveguide QED systems driven by a squeezed vacuum reservoir and has potential applications in quantum information processing.

Ultranarrow scattering spectra have important applications in precision measurements. Here, we show that the dark state of two atoms coupled to a one-dimensional (1D) waveguide leads to an ultranarrow transparency window with a width significantly smaller than the spontaneous decay rate of the atoms. Specifically, the position of this transparency dip is linearly proportional to the atom-atom distance. This ultranarrow dip can measure the atom-atom distance with an accuracy of one percent of the resonant wavelength. Additionally, the dip depth is highly sensitive to the atom-atom detuning, which could be used to measure small magnetic-field gradients at the nanoscale. The sign of the detuning can be further determined by measuring the second-order correlation function of the reflected photons.

This work discusses the problem of optimal excitation of a three-level atom of ladder configuration by light in the two-photon state and coherent light carrying an average of two photons. The applied atom-light interaction model is based on the Wigner-Weisskopf approximation. We characterize the properties of the optimal two-photon state that excites an atom perfectly, i.e., with probability equal to one. We find that the spectrotemporal shape of the optimal state of light is determined by the lifetimes of the atomic states, with the degree of photonic entanglement in the optimal state depending on the lifetime ratio. In consequence, two distinct interaction regimes can be identified in which the entanglement of the input state of light has a qualitatively different impact. As the optimal states may be challenging to prepare in general, we compare the results with those obtained for photon pairs of selected experimentally relevant pulse shapes. As these shapes are optimized for maximal atomic excitation probability, the results can be interpreted in terms of the overlap between the optimal and investigated pulse shapes.

Continuous-time quantum algorithms for combinatorial optimization problems, such as quantum annealing, have previously been motivated by the adiabatic principle. A number of continuous-time approaches exploit dynamics, however, and therefore are no longer physically motivated by the adiabatic principle. In this work, we take Planck's principle as the underlying physical motivation for continuous-time quantum algorithms. Planck's principle states that the energy of an isolated system cannot decrease as the result of a cyclic process. We use this principle to justify monotonic schedules in quantum annealing, which are not adiabatic. This approach also highlights the limitations of reverse quantum annealing in an isolated system.

Collective Einstein-Podolsky-Rosen (EPR) steering is a unique multipartite resource that enables the realization of more secure communication protocols. However, due to the lack of suitable experimental criteria, research on collective EPR steering has so far been limited to continuous variable systems. Here, we employ the coherence-based collective EPR steering criteria proposed by Mondal et al. [Phys. Rev. A 99, 012312 (2019)] and develop a semidefinite programming method to find the optimal measurement settings. Experimentally, we take the three-qubit generalized Greenberger-Horne-Zeilinger states and generalized $W$ states as examples, demonstrating that collective EPR steering can be observed by detecting the average $l_1$ norm of coherence of the steered subsystem. We also explored the effect of measurement settings on complementarity relations between different collective steering criteria, and established a connection between collective steering and coherence complementarity. Our paper fills a gap in the verification of collective steerability in discrete variable systems and reveals a profound connection between EPR steering and quantum coherence.

The recombination of an incoming electron with an atomic cation $A^{+}$, accompanied by energy transfer to a neighboring atom $B$, which becomes electronically excited and subsequently relaxes radiatively, is investigated. The theory of this process, known as two-center dielectronic recombination (2CDR), is significantly extended to account for the interatomic motion between the two atoms involved. Additionally, the electron transfer mechanism, due to orbital overlap between the atoms, is incorporated into the model. Using the example reaction $e^{-}+{\text{Li}}^{+}\text{He}\to {\text{LiHe}}^{*}\to \text{LiHe}+\gamma$, we demonstrate that both interatomic motion and electron transfer have a substantial impact on the predicted efficiency of 2CDR. Our findings suggest that 2CDR can dominate over photorecombination across a much broader energy range of the incoming electron than previously estimated. Moreover, 2CDR is particularly efficient when the dimer dissociates during recombination, as this suppresses the energy transfer back to the captured electron, preventing its re-emission.

We present comprehensive determinations of the diffusion coefficients $D$ at $T=24{}^{\circ }\mathrm{C}$ for trace amounts of naturally abundant Rb atoms in inert, naturally abundant He, Ne, ${\mathrm{N}}_2$, Ar, Kr, and Xe buffer gases using a single measurement technique. They have been measured by establishing a spatially periodic population grating in the Rb sample using two laser beams that intersect at a small angle $\theta$ of a few milliradians. The atomic population grating decays exponentially in time due to diffusive motion induced by momentum-changing elastic collisions between Rb and buffer gas atoms or molecules. This decay is monitored by observing the scattered field from a readout beam aligned along the direction of one of the excitation beams. We distinguish the contribution of diffusion from other collisional processes by measuring the characteristic ${\theta }^2$ dependence of the decay rate. We also measure the systematic dependence of the decay rate on the buffer gas pressure over a range of 7000 to $90000$ Pa. In this manner, we obtain diffusion coefficients at standard atmospheric pressure of $101325$ Pa and at a temperature of $24.0\left(5\right){}^{\circ }\mathrm{C}$. We use two models to correct for systematic effects due to the transit time, one assuming a rectangular profile of the population distribution and a rectangular readout beam profile, and a second using Gaussian profiles. We obtain weighted averages of 0.33(5), 0.214(14), 0.132(7), 0.123(9), 0.093(9), and $0.073\left(4\right){\mathrm{cm}}^2$/s for Rb in He, Ne, ${\mathrm{N}}_2$, Ar, Kr, and Xe, respectively. The number in parentheses represents one standard deviation of the combined statistical and systematic uncertainty. We also compare these data with diffusion coefficients obtained using quantum, classical, and semiclassical theoretical methods based on the most accurate interatomic interaction potentials from the literature. Near room temperature, simulations of $D$ using classical and quantum methods agree within their intrinsic, sub-1% standard uncertainties. We find that the semiclassical model only gives the correct orders of magnitude for $D$. Our computed diffusion coefficients based on the quantum theory agree with the experimental determinations when systematic effects are taken into account. Our measurements and modeling are relevant to the optimization of magnetometers, biomedical imaging using spin-polarized noble gases, tests of collision models based on interatomic potentials, and the development of pressure sensors.

This study investigates the influence of quantum effects on Coulomb explosion dynamics using time-dependent density-functional theory (TDDFT) simulations in comparison to classical simulations. The goal is to elucidate how electron dynamics affect the kinetic energy, angular distribution, and final velocities of ejected ions. The results indicate that quantum effects result in lower kinetic energies of all ions, deviating from classical predictions. Furthermore, TDDFT simulations exhibit broader angular distributions and more diverse ion trajectories, aligning closely with experimental observations. The research also highlights the role of laser intensity and the resultant ionization in enhancing quantum effects, particularly in modifying ion velocities and distributions. These findings provide a deeper understanding of the role of electron dynamics in Coulomb explosions, offering valuable insights for both experimental and theoretical studies of molecular fragmentation.

We analyze the tunneling of two bosons in a double-well potential, for contact, soft-, and hard-core Coulomb interaction of tunable strength. Transitions from correlated to uncorrelated tunneling of the left well's two-particle ground state are due to resonances with states with one particle in either well. Their abundance and dependence on the interaction strength is indicative of the interaction type.

The interplay of non-Hermiticity and spin-orbit (SO) coupling has attracted great interest in condensed-matter physics, ultracold atomic physics, optics, and other fields. Here we study the ground states, collective modes, and free-expansion dynamics of non-Hermitian SO-coupled Bose-Einstein condensates (BECs) with momentum-dependent gain and loss in a harmonic trap. An efficient analytical method for revealing the impact of non-Hermiticity on the ground states and dynamics of SO-coupled BECs is proposed. A dispersion relation with effective mass $m_2$ that can be larger than 1, less than 1, or negative is engineered by the momentum-dependent gain and loss, which is distinct from the ordinary SO-coupled BECs, where $m_2>1$ always holds. Negative effective mass can lead to the self-interference effect of condensates, while $m_2<1$ means that condensates can gain a larger acceleration when they are subjected to a constant force. Interestingly, the high-order zero-point kinetic energies are derived analytically and confirmed numerically, establishing a link between the zero-point kinetic energy and dispersion relation. In particular, the second-order zero-point kinetic energy can be controlled by the effective mass $m_2$, which has an important impact on many phenomena of ultracold atomic physics. In addition, another key result is that the frequencies of dipole and breathing modes can exceed the values of traditional BECs without SO coupling when the effective mass $m_2<1$, showing that gain (loss) is a powerful tool for probing and controlling quantum systems. Moreover, the asymmetric free expansion and self-interference effect of the condensates caused by negative effective masses are observed, and it is found that the rate of expansion is controlled by the effective mass $m_2$. Our results show that the controllable gain-loss engineering in SO-coupled BECs provides a prospect to explore novel quantum many-body physics.

We experimentally demonstrated and thoroughly investigated the up-conversion of midinfrared signals from nonlinear spectroscopy in a molecular gas to the regime of shorter wavelength to improve the features and applicability of such sensitive techniques for the detection of molecular species. In particular, we apply sum-frequency mixing (SFM) of weak signals from resonantly enhanced third-harmonic-generation (THG) spectroscopy with strong nanosecond pump pulses, implemented in a conventional bulk nonlinear crystal. The conversion efficiency reaches 26%, i.e., a substantial fraction of the THG signal photons from the molecular sample at long midinfrared wavelength are converted to a SFM signal pulse at much shorter wavelength in the near-ultraviolet regime. This permits detection of the SFM signal beam on a standard photomultiplier tube with large amplification and aperture rather than detection of the midinfrared THG signal on an avalanche photodiode with much lower amplification and smaller aperture. The measure also considerably improves the signal-to-noise ratio by more than a factor of 20, which in turn pushes the particle detection limit by an order of magnitude towards much lower gas pressures.

In this study, we explore a form of quantum circuit complexity that extends to open systems. To illustrate our methodology, we focus on a basic model where the projective Hilbert space of states is depicted by the set of orientations in the Euclidean plane. Specifically, we investigate the dynamics of mixed quantum states as they undergo interactions with a sequence of gates. The latter aim to accurately adjust the path from referent to target, aligning it as closely as possible with the path we have chosen. Our approach involves the analysis of sequences of real $2\times 2$ density matrices. This mathematical model is physically exemplified by the Stokes density matrices, which delineate the linear polarization of a quasimonochromatic light beam, and the gates, which are viewed as quantum polarizers, whose states are also real $2\times 2$ density matrices. The interaction between polarizer-linearly polarized light is construed within the context of this quantum formalism. Each density matrix for the light evolves in an approach analogous to a Gorini-Kossakowski-Lindblad-Sudarshan (GKLS) process during the time interval between consecutive gates. Notably, when considering an upper limit for the tolerance or accuracy, we unearth that the number of gates follows a power-law relationship which gives an upper bound of the complexity.

Excited-state quantum phase transitions extend the quantum phase transition concept beyond the ground state and offer insights into the complex behavior of quantum systems. In the present work we assess the use of the multiple quantum coherence spectrum as a valid tool to probe excited-state quantum phase transitions within the framework of the Lipkin-Meshkov-Glick model. The time dependence and the long-time average of the multiple quantum coherence spectrum reveal the intricate dynamics that stems from the existence of singularities in the excited-state spectrum of a quantum many-body system.

For Dirac particles interacting with external fields, we use the exact operator of the Foldy-Wouthuysen transformation obtained by Eriksen and rigorously derive a leading correction in the weak-field approximation to the known relativistic Foldy-Wouthuysen Hamiltonian. For this purpose, we carry out the operator extraction of a square root in the Eriksen operator. The derived correction is important for the scattering of relativistic particles. Since the description of this scattering by a relativistic wave equation of the second order is more convenient, we determine a general connection between relativistic wave equations of the first and second orders. For Dirac particles, the relativistic wave equation of the second order is obtained with a correction similar to that of the Foldy-Wouthuysen Hamiltonian.

Advances in quantum algorithms suggest a tentative scaling advantage on certain combinatorial optimization problems. Recent work, however, has also reinforced the idea that barren plateaus render variational algorithms ineffective on large Hilbert spaces. Hence, finding annealing protocols by variation ultimately appears to be difficult. Similarly, the adiabatic theorem fails on hard problem instances with first-order quantum phase transitions. Here we show how to use the spin coherent-state path integral to shape the geometry of quantum adiabatic evolution, leading to annealing protocols at polynomial overhead that provide orders-of-magnitude improvements in the probability to measure optimal solutions, relative to linear protocols. These improvements are not obtained on a controllable toy problem but on randomly generated hard instances (Sherrington-Kirkpatrick and maximum 2-satisfiability), making them generic and robust. Our method works for large systems and may thus be used to improve the performance of state-of-the-art quantum devices.

We develop a random sampling method for calculating the time evolution of the Rényi entanglement entropy after a quantum quench from an insulating state in free boson systems. Because of the non-Gaussian nature of the initial state, calculating the Rényi entanglement entropy calls for the exponential cost of computing a matrix permanent. We numerically demonstrate that a simple random sampling method reduces the computational cost of a permanent; for an $N_{\mathrm{s}}\times N_{\mathrm{s}}$ matrix corresponding to $N_{\mathrm{s}}$ sites at half filling, the sampling cost becomes $O\left(2^{\alpha N_{\mathrm{s}}}\right)$ with a constant $\alpha \ll 1$, in contrast to the conventional algorithm with the $O\left(2^{N_{\mathrm{s}}}\right)$ number of summations requiring the exponential time cost. Although the computational cost is still exponential, this improvement allows us to obtain the entanglement entropy dynamics in free boson systems for more than 100 sites. We present several examples of the entanglement entropy dynamics in low-dimensional free boson systems.