We investigate theoretically the subradiance dynamics in a nonreciprocal chirally coupled atomic chain, in which infinite-range dipole-dipole interaction emerges in the dissipation. We find that super- and subradiance are both present in the dissipation process following single-photon excitation, and the decay dynamics shows burst emissions from uniform initial excitations, which reflects the influence of atomic ordering on the propagation of light-induced atom-atom correlations. By tuning the nonreciprocal couplings in the chirally coupled atomic system, we show that the subradiance dynamics can be greatly modified. We further study the effect of atomic local disorder and find the occurrence of plateaus on the decay curve dependent on the defect locations, as well as persistent localized excitations induced by disorder. We also discuss the effect of imperfections of systems on the subradiance dynamics. Our results show rich opportunities in the chirally coupled system toward photon storage and routing.

We study the ${}^{7}F_0\to {}^{5}D_1$ magnetic-dipole transition in the $4f$ shell of europium ions at a wavelength of 527.5 nm and demonstrate spectral hole burning, ion class selection, and optical pumping to specific ground hyperfine states. We then use the narrow lines of the selected ions to observe linear group delay of a weak probe pulse with a group velocity of $c/3800$. We also demonstrate electromagnetically induced transparency by establishing a $\mathrm{\Lambda }$ scheme between the ground hyperfine levels and driving the ions to a dark state.

In this paper, we propose a single integrated waveguide device to generate two different polarization-entangled Bell states involving nondegenerate photon pairs (with the signal photons being emitted in the telecom $C$ band) by utilizing type-0, type-I, and type-II quasi-phase-matching (QPM). In this context, we have studied ppKTP (periodically-poled potassium titanyl phosphate) in detail. To obtain polarization entanglement of the emitted photons, we have used two different situations, two cascaded QPM gratings (of the same length) to obtain $|{\psi }^{+}\rangle$, and two separate QPM gratings (of different lengths) to obtain $|{\varphi }^{+}\rangle$. In such a scenario, maximal entanglement can be quantified in terms of overlap bandwidths of the two phase-matched processes.

A trapped-ion quantum tunneling rotor (QTR) is in a quantum superposition of two different Wigner crystal orientations. In a QTR system, quantum tunneling drives the coherent transition between the two different Wigner crystal orientations. We theoretically study the quantum dynamics of a QTR, particularly when the spin state of one of the ions is flipped. We show that the quantum dynamics of an $\mathit{N}$-ion QTR can be described by continuous-time cyclic quantum walks. We also investigate the quantum dynamics of the QTR in a magnetic field. Flipping the spin state breaks the rotational symmetry of the QTR, making the quantum-tunneling-induced rotation distinguishable. This symmetry breaking creates coupling between the spin state of the ions and the rotational motion of the QTR, resulting in different quantum tunneling dynamics.

It is challenging to construct metrology schemes which harness quantum features such as entanglement and coherence to surpass the standard quantum limit. We propose an ansatz for devising an adaptive-feedback quantum metrology (AFQM) strategy which greatly reduces the searching space. Combined with the Markovian feedback assumption, the computational complexity for designing AFQM would be reduced from $N^7$ to $N^4$, for $N$ probing systems. The feedback scheme devising via machine learning such as particle-swarm optimization and differential evolution would thus require much less time and produce equally good imprecision scaling. We have thus devised an AFQM for the 207-partite system. The imprecision scaling would persist for $N>207$ in an admirable range when the parameter setting for the 207-partite system is employed without further training. Our ansatz indicates an built-in resilience of the feedback strategy against qubit loss. The feedback strategies designed for the noiseless scenarios have been tested against the qubit loss noise and the phase fluctuation noise. Our numerical result confirms great resilience of the feedback strategies against the two kinds of noise.

Quantum machine learning and optimization are exciting new areas that have been brought forward by the breakthrough quantum algorithm of Harrow, Hassidim, and Lloyd for solving systems of linear equations. The utility of classical linear system solvers extends beyond linear algebra as they can be leveraged to solve optimization problems using iterative methods like gradient descent. In this work, we provide a quantum method for performing gradient descent when the gradient is an affine function. Performing $\tau$ steps of the gradient descent requires time $O\left(\tau C_S\right)$ for weighted least-squares problems, where $C_S$ is the cost of performing one step of the gradient descent quantumly, which at times can be considerably smaller than the classical cost. We illustrate our method by providing two applications: first, for solving positive semidefinite linear systems, and, second, for performing stochastic gradient descent for the weighted least-squares problem with reduced quantum memory requirements. We also provide a quantum linear system solver in the QRAM data structure model that provides significant savings in cost for large families of matrices.

Violation of centrosymmetry (VOC) in time-resolved diffraction patterns has been predicted in ultrafast-electron and x-ray diffraction from electronic and molecular motions. Extending from our x-ray studies, we theoretically investigate the VOC in time-resolved ultrafast-electron diffraction from rovibrational motion of oriented diatomic molecules lithium hydride and hydrogen. We simulate and compare the electron and x-ray diffraction images, especially focused on the differences in the VOC and molecular interference fringes. In addition, using Newton diagrams, we provide an intuitive semiclassical interpretation to explain how the VOC is arisen from and related to the particles' motion and why is there an anticorrelation between the VOC and molecular interference fringes.

The trapped ${}^{171}\mathrm{Yb}^{+}$ ion is a promising candidate for portable atomic clock applications. However, with buffer-gas cooled ytterbium ions, the ions can be pumped into a low-lying ${}^{2}F_{7/2}$ state or form ${\mathrm{YbH}}^{+}$ molecules. These dark states reduce the fluorescence signal from the ions and can degrade the clock stability. In this work, we study the dynamics of the populations of the ${}^{2}F_{7/2}$ state and ${\mathrm{YbH}}^{+}$ molecules under different operating conditions of our ${}^{171}\mathrm{Yb}^{+}$ ion system. Our study indicates that ${}^{2}F_{7/2}$-state ions can form ${\mathrm{YbH}}^{+}$ molecules through interactions with hydrogen gas. As observed previously, dissociation of ${\mathrm{YbH}}^{+}$ is observed at wavelengths around 369 nm. We also demonstrate ${\mathrm{YbH}}^{+}$ dissociation using 405 nm light. Moreover, we show that the population in the dark states can be limited by using a single repump laser at 935 nm. Our study provides insights into the molecular formation in a trapped ion system.

A theoretical optimization method of high-order harmonic generation (HHG) is developed in the framework of optimal control theory (OCT). The target of optimization is the emission radiation of a particular frequency. The OCT formulation includes restrictions on the frequency band of the driving pulse, the permanent ionization probability, and the total energy of the driving pulse. The optimization task requires a highly accurate simulation of the dynamics. Absorbing boundary conditions are employed, where a complex absorbing potential is constructed by an optimization scheme for maximization of the absorption. A highly accurate propagation scheme is employed, which can address explicit time dependence of the driving as well as a non-Hermitian Hamiltonian. The optimization process is performed by a second-order gradient scheme. The method is applied to a simple one-dimensional model system. The results demonstrate a significant enhancement of selected harmonics, with minimized total energy of the driving pulse and controlled permanent ionization probability. A successful enhancement of an even harmonic emission is also demonstrated.

The dynamics of photoabsorption, photoionization, and the associated Auger decay have been investigated at the carbon $K$ edge in methyl iodide ($\mathrm{C}{\mathrm{H}}_3\mathrm{I}$) using linearly polarized synchrotron radiation. Ion yield measurements were used to investigate transitions in the pre-edge region due to excitations into either unoccupied valence or Rydberg states. The assignment of these transitions was achieved through comparison with theoretical x-ray absorption spectra calculated using time-dependent density functional theory, within the Tamm-Dancoff approximation. Several of the Rydberg states belonging to series converging onto the $\mathrm{C}1s$ ionization limit exhibit significant vibrational structure that is also interpreted using theoretical calculations. The $\mathrm{C}1s$ in $\mathrm{C}{\mathrm{H}}_3\mathrm{I}$ photoelectron spectrum was measured, and the observed vibrational structure was assigned with the aid of theoretical predictions. Polarization dependent, resonantly excited, valence shell photoelectron spectra were recorded at photon energies coinciding with the $\mathrm{C}1s\to {\sigma }^{*}, \mathrm{C}1s\to 6s{a}_1$ and $\mathrm{C}1s\to 6pe$ transitions in $\mathrm{C}{\mathrm{H}}_3\mathrm{I}$, thereby allowing photoelectron angular distributions to be determined. The nonresonantly excited C(KVV) Auger electron spectrum was measured and some of the features observed at high kinetic energies were attributed to transitions into valence orbitals possessing significant iodine character. The contributions of participator and spectator Auger decay to the resonantly excited photoelectron spectra have been assessed. The influence of participator decay appears minor whereas spectator decay results in the enhanced population of satellite states.

We discuss the possibility to enhance the sensitivity of optical interferometric devices by increasing their open area using an external field gradient that acts differently on the two arms of the interferometers. The use of a combined electric and magnetic field cancels nonlinear terms that dephase the interferometer. This is possible using well-defined (typically with $n\sim 20$ Rydberg) states, a magnetic field of a few Tesla, and an electric field gradient of $\sim 10\mathrm{V}/{\mathrm{cm}}^2$. However, this allows for interaction times only on the order of tens of microseconds, leading to a reachable accuracy of only 1 or 2 orders of magnitude higher than standard light-pulse atom interferometers. Furthermore, the control of fields and states and three-dimensional (3D) trajectories puts severe limits to the reachable accuracy. This idea is therefore not suitable for precision measurement but might eventually be used for gravity or neutrality in antimatter studies.

We consider the problem of finding the bound-state spectrum of an impurity immersed in a weakly interacting two-dimensional Bose-Einstein condensate supporting a single vortex. We obtain approximate expressions for the energy levels and show that, due to the finite size of the condensate, the impurity can access only a finite number of physical bound states. By virtue of the topological quantization of the vorticity and of the emergence of the Tkachenko lattice, this system is promising as a robust and scalable platform for the realization of qubits. Moreover, it provides a potentially new paradigm for polaron physics in Bose-Einstein condensates and a glimpse towards the study of quantum turbulence in low-dimensionality systems.

Lévy flights for light have been demonstrated in disordered systems with and without optical gain and remained unobserved in ordered ones. In the present article, we investigate, numerically and experimentally, Lévy flights for light in ordered systems due to an ordered (conventional) laser. The statistical analysis was performed on the intensity fluctuations of the output spectra upon repeated identical experimental realizations. We found out that the optical gain and the mirror reflectivity are critical parameters governing the fluctuation statistics. We identified Lévy regimes for gain around the laser threshold, and Gaussian-Lévy-Gaussian crossovers were unveiled when increasing the gain from below to above threshold. The experimental results were corroborated by Monte Carlo simulations, and the fluctuations were associated to a Langevin noise source that takes into account the randomness of the spontaneous emission, which seeds the laser emission and can cause large fluctuations of the output spectra from shot to shot under identical experimental realizations.

We propose a scheme to generate structured light using a medium consisting of semiconductor quantum dots via four-wave mixing (FWM). We consider a four-level quantum-dot structure where two strong electromagnetic fields couple upper-level biexciton transitions, whereas a weak field is applied to one of the ground-level exciton transitions. A weak signal field is generated, corresponding to a second ground-level exciton transition; thus the overall configuration transforms into a closed-loop diamond-type shape. To realize a spatially dependent modulating quantum-dot medium, we consider strong coupling fields as structured lights and investigate different regions of spatially structured electromagnetically induced transparency via absorption of the generated signal field. The azimuthal phase-dependent modulation of the absorption profile of the generated signal field is the key factor behind the creation of structured light. In addition, we also demonstrate transfer of orbital angular momentum (OAM) of the control beam to the generated beam through the FWM process and study the effect of phase mismatch on efficiency of the OAM transfer.

In this paper, based on the idea of reverse engineering, we propose a scheme to find analytical expressions of a laser pulse that allows for controlling the evolution of two-level systems as desired, regardless of the time-dependent phase of the system. The pulse relates to particular functions that predefine the evolution of a system. For illustration, we present concrete examples of driving the system from an arbitrary initial state to a final state along the user-prescribed particular way in both closed and open quantum systems. Simulation results indicate that the desired evolutionary path is accurately obtained when the rotating wave approximation (RWA) is satisfied. Most importantly, the scheme allows us to maintain system coherence and populations even in the presence of certain dephasing and thermal noises. Thus it may naturally find applications in quantum computing and quantum memories.

We present a peculiar transition triggered by infinitesimal dissipation in the interpolating Dicke-Tavis-Cummings model. The model describes a ubiquitous light-matter setting using a collection of two-level systems interacting with quantum light trapped in an optical cavity. In a previous work [Phys. Rev. Lett. 120, 183603 (2018)], dissipation was shown to extend a normal phase (dark state) into new regions of the model's parameter space. Harnessing Keldysh's action formalism to compute the response function of the light, we show that the normal phase does not merely spread but encompasses a transition between the old and the dissipation-stabilized regimes of the normal phase. This transition, however, solely manifests in the dynamical fluctuations atop the empty cavity through stabilization of an excited state of the closed system. Consequently, we reveal that the fluctuations flip from being particlelike to holelike across this transition. This inversion is also accompanied by the behavior of the Liouvillian eigenvalues akin to exceptional points. Our work forges the way to discovering transitions in a wide variety of driven-dissipative systems and is highly pertinent for current experiments.

Continuous variable quantum state tomography is one of the most powerful techniques to study the properties of light fields in quantum optics. However, the need for a fixed phase reference has so far prevented widespread usage in other fields such as semiconductor spectroscopy. Here, we introduce nonstationary quantum state tomography, which adapts the technique to the special requirements of ultrafast spectroscopy. In detail, we gain access to the amplitude and phase of light fields with a temporal resolution of about 100 fs without the need for a fixed phase reference. Furthermore, we show how our technique allows us to perform conditional studies of stochastic dynamics that are inaccessible experimentally by conventional means and demonstrate the capabilities experimentally by monitoring the stochastic dynamics of a thermal light field on the sub-ps scale. Finally, we discuss differences and similarities to more standard Hanbury Brown–Twiss photon correlation experiments, which may be considered as the discrete variable analogs of our technique.

In order to engineer an open quantum system and its evolution, it is essential to identify and control the memory effects. These are formally attributed to the non-Markovianity of dynamics that manifests itself by the evolution being indivisible in time, a property which can be witnessed by a nonmonotonic behavior of contractive functions or correlation measures. We show that by monitoring directly the entanglement behavior of a system in a tripartite setting it is possible to witness all invertible non-Markovian dynamics, as well as all (also noninvertible) qubit evolutions. This is achieved by using negativity, a computable measure of entanglement, which in the usual bipartite setting is not a universal non-Markovianity witness. We emphasize further the importance of multipartite states by showing that non-Markovianity cannot be faithfully witnessed by any contractive function of single qubits. We support our statements by an explicit example of eternally non-Markovian qubit dynamics, for which negativity can witness non-Markovianity at arbitrary timescales.

We investigate the longitudinal photon-momentum transfer in nonsequential double ionization of argon atoms exposed to a $2.0-\mu \mathrm{m}$ linearly polarized strong laser field. The zero momentum of the photoelectron is precisely calibrated using the photoelectron-photoion coincidence spectrum. The sum-longitudinal momentum of two photoelectrons from the double ionization noticeably deflects along the laser propagation direction, which is opposite to and much larger than that of single ionization. By tracking the electron trajectories for double ionization in semiclassical simulations, we find that the Coulomb focusing of the ionic core with the assistance of the Lorentz pushing of the laser field dominates the observed longitudinal momentum deflection. Our results open the possibility to control the photon-momentum transfer by manipulating the waveform of the laser fields and thus the Coulomb and magnetic effects experienced by the photoelectrons.

Beyond the metrology and computational challenges associated with molecular hydrogen, key data are expected to assess the physics of simple molecular systems, and even the new physics beyond the standard model. To assist the deciphering of Doppler-free spectra obtained at very high accuracy ($\sim {10}^{-9})$, we report on hyperfine transitions of HD in the lowest vibrational levels of the ground electronic state. Using the spin-rotation, nuclear spin-spin, and quadrupolar hyperfine couplings determined by means of high-level quantum-chemical calculations, the hyperfine energy levels and the associated line intensity have been obtained by using tensorial momentum algebra. To illustrate our purpose, the hyperfine line intensity of two specific transitions ($P_1$ and $R_1$) belonging to the first overtone of HD is reported and commented on. The calculated stick spectra emphasize the experimental challenge (in terms of sensitivity and of spectral resolution) associated with the spectral analysis, because the lines can be apart by less than 10 kHz.