Studies on high-order harmonic generation (HHG) in nonliquid media driven by ultrafast lasers have made extensive progress. However, the liquid HHG remains in its infancy. Here, the correlation length of liquids is extracted from the radial distribution function, which gives a link to the maximum harmonic photon energy ${\mathrm{\Omega }}_{\max }$. The predicted ${\mathrm{\Omega }}_{\max }$ agrees with the numerical one- and two-dimensional model simulations and the real experimental measurements. It provides a powerful tool in order to select optimal liquid candidates to achieve higher harmonic photon energy.

We investigate the precision of estimating the interaction strength between a two-level system (TLS) and a single-photon pulse when the latter is subject to chirping. We consider linear, quadratic, and sinusoidal temporal phases applied to Gaussian and exponential temporal profiles. At the asymptotic time, when the TLS has fully decayed to its ground state, the fundamental precision depends solely on the magnitude of its spectral amplitude. For quadratically phase-modulated Gaussian pulses, this is entirely determined by the spectral bandwidth. We provide expressions for evaluating the fundamental precision for general temporal profiles and phase modulations. Finally, we show that experimentally feasible mode-resolved measurements are optimal, or close to it, for chirped, pulsed single-photon spectroscopy.

It is well known that in the quantum Rabi model, a three-photon resonance occurs when the cavity field bare frequency is about 1/3 of the atomic transition frequency. In this manuscript, we show that the resonance can also be generated in the absence of the “1/3 condition” by employing an artificial atom with tunable transition frequency. To realize the protocol, the modulation frequency should be comparable to the cavity frequency in order to induce a counter-rotating interaction in the effective Hamiltonian. In this way, three-photon Rabi oscillations can be observed in a small-detuning regime, avoiding the excitation of high-energy states of the atom. We derive an effective Hamiltonian (equivalent to the anisotropic Rabi model Hamiltonian) to determine the magnitude of the energy splitting and the resonance position. Numerical simulation results show that the protocol not only generates a three-photon resonance, but also has a detectable output photon flux. We hope the protocol can be exploited for the realization of Fock-state sources and the generation of multiparticle entanglement.

The quantum state discrimination problem has Alice sending a quantum state to Bob who wins if he correctly identifies the state. The pretty good measurement, also known as the square root measurement, performs pretty well at this task. We study the version of this problem where Bob tries to lose with the greatest probability possible (which is harder than it sounds). We define the pretty bad measurement, which performs pretty well at this task, or in other words, pretty poorly for the original task. We show that both the pretty good measurement and the pretty bad measurement are always no worse than blind guessing at their respective tasks. As an application, we apply the pretty bad measurement to the quantum state anomaly detection problem and show how to avoid pretty bad qubits.

In adiabatic quantum computation the required run-time to reach a given ground-state fidelity is dictated by the size of the minimum gap that appears between the ground and first-excited state in the annealing spectrum. In general the presence of avoided level crossings demands an exponential increase in the annealing time with the system size which has consequences both for the efficiency of the algorithm and the required qubit coherence times. One promising avenue being explored to produce more favorable gap scaling is the introduction of nonstoquastic $XX$ couplings in the form of a catalyst—of particular interest are catalysts which utilize accessible information about the optimization problem in their construction. Here we show extreme sensitivity of the effect of an $XX$ catalyst to subtle changes in the encoding of the optimization problem. In particular, we observe that a targeted catalyst containing a single coupling can significantly reduce the gap closing with system size at an avoided level crossing. For slightly different encodings of the same problems however, these same catalysts result in closing gaps in the annealing spectrum. To understand the origin of these closing gaps, we study how the evolution of the ground-state vector is altered by the presence of the catalyst and find that the negative components of the ground-state vector are key to understanding the response of the gap spectrum. We also consider how and when these closing gaps could be utilized in diabatic quantum annealing protocols—a promising alternative to adiabatic quantum annealing in which transitions to higher energy levels are exploited to reduce the run time of the algorithm.

Rapid separation of linear crystals of trapped ions into different subsets is critical for realizing trapped ion quantum computing architectures where ions are rearranged in trap arrays to achieve all-to-all connectivity between qubits. We introduce a general theoretical framework that can be used to describe the separation of same-species and mixed-species crystals into smaller subsets. The framework relies on an efficient description of the evolution of Gaussian motional states under quadratic Hamiltonians that only requires a special solution of the classical equations of motion of the ions to describe their quantum evolution under the influence of a time-dependent applied potential and the ions' mutual Coulomb repulsion. We provide time-dependent applied potentials suitable for separation of a mixed-species three-ion crystal on timescales similar to that of free expansion driven by Coulomb repulsion, with all modes along the crystal axis starting and ending close to their ground states. Three separately confined mixed-species ions can be combined into a crystal held in a single well without energy gain by time-reversal of this separation process.

Quantum speed limits are the bounds that define how quickly one quantum state can transform into another. Instead of focusing on the transformation between pairs of states, we provide bounds on the speed limit of quantum evolution by unitary operators in arbitrary dimensions. These do not depend on the initial and final state but depend only on the trace of the unitary operator that is to be implemented and the gross characteristics (average and variance) of the energy spectrum of the Hamiltonian which generates this unitary evolution. The bounds that we find can be thought of as the generalization of the Mandelstam-Tamm (MT) and the Margolus-Levitin (ML) bound for state transformations to implementations of unitary operators. We will discuss the application of these bounds in several classes of transformations that are of interest in quantum information processing.

Through nonadiabatic vibrational dynamics, conical intersection (CI) couples electronic states and opens ultrafast nonradiative reaction channels. To reveal and track the unique pathways of CI using ultrashort x-ray pulses, several time-resolved x-ray spectroscopies (such as photoelectron, absorption, and Raman) have been proposed. In this work, we investigate time-resolved resonant Auger scattering, utilizing femtosecond to attosecond x-ray probe pulses, to detect ultrafast nonadiabatic electron-nuclear dynamics through the CI triggered by an ultrashort UV pump pulse. Our simulations indicate that the electronic coherence signal from the wave packet passing through the CI pathways is influenced by the core-excited potential and its lifetime broadening due to the dynamical contribution from bound nuclear degrees of freedom. Given the prevalence of CIs in polyatomic molecules and their wide range of applications, this study offers a new perspective on monitoring nonadiabatic dynamics in photochemistry and photophysics using ultrashort x-ray pulses.

In the Fano resonant ionization process, a decaying autoionizing state (AIS) is imbedded in the continuum resulting in a characteristic asymmetric photoelectron line shape. When the ionizing XUV pump is aided by a weak IR probe, the resonant path of the Fano process is transferred to the adjacent sidebands. As the XUV-IR delay grows, this resonant part of the sideband becomes decoupled from the embedding continuum. In result, the sideband line shape becomes symmetric and its magnitude decreases exponentially with the timing constant of the AIS decay. In the present work we simulate this process to extract the lifetime of several AISs in the He atom and the ${\mathrm{Li}}^{+}$ ion with sufficient numerical accuracy. The proposed method can be realized experimentally in diverse quantum systems ranging from nuclei to nanofabricated solids.

The role of the fine-structure intramanifold resonances to the Zeeman shifts caused by the blackbody radiation (BBRz shifts) in optical clock transitions is analyzed. The clock frequency measurement in the ${}^{1}S_0-{}^{3}P_0$ clock transition of the singly charged aluminium ion (${\mathrm{Al}}^{+}$) has already reached the ${10}^{-19}$ level at which the BBRz effect was considered significant in determining the uncertainty. In view of this, we probe first the BBRz shift in this transition rigorously and demonstrate the importance of the contributions from the intramanifold resonances explicitly. To carry out the analysis, we determine the dynamic magnetic dipole ($M1$) polarizabilities of the clock states over a wide range of angular frequencies by employing two variants of relativistic many-body methods. It is found that the BBR spectrum at 300 K exhibits a blue detuning with respect to the ${}^{3}P_0-{}^{3}P_1$ fine-structure intramanifold resonance in ${\mathrm{Al}}^{+}$. This produces a large deviation from the static $M1$ polarizability limit, which cannot be used to estimate the BBRz shift since the temperature exceeds the fine-structure splitting ($k_{B}T>\mathrm{\Delta }E$). The resonance also leads to a reversal behavior of the temperature dependence and a cancellation in the shift. Extending our analyses to other optical clocks, we show that these shifts are typically on the order of microhertz leading to fractional shifts in the clock transitions at the ${10}^{-20}$ level or below.

In the quest toward realizing novel quantum matter in ultracold molecular gases, we perform a numerical study of evaporative cooling in ultracold gases of microwave-shielded polar fermionic molecules. Our Monte Carlo simulations incorporate accurate two-body elastic and inelastic scattering cross sections, realistic modeling of the optical dipole trap, and the influence of Pauli blocking at low temperatures. The simulations are benchmarked against data from evaporation studies performed with ultracold NaK molecules, showing excellent agreement. We further explore the prospects for optimizing the evaporation efficiency by varying the ramp rate and duration of the evaporation trajectory. Our simulation shows that it is possible to reach $<10\%$ of the Fermi temperature under optimal conditions even in the presence of two-body molecular losses.

We perform a theoretical study of the coupled dynamics of two species of Bose-Einstein condensates (BECs) in a double-well potential where both the tunneling and the interatomic interactions are driven periodically in time. The population difference between the wells of each species gives rise to a two-dimensional lattice in Fock space with dimensions given by the number of atoms in each BEC. We use a Floquet analysis to derive an effective Hamiltonian that acts in this Fock space and find that it contains an artificial gauge field. This system simulates noninteracting particles in a tight-binding lattice subject to an additional harmonic potential and vector potential. When the intraspecies interactions are attractive there is a critical value at which the ground state undergoes a transition from a Gaussian state to a quantized vortex state in Fock space. The transition can be quantified in terms of the angular momentum and the entanglement entropy of the ground state with both showing sudden jumps as the intraspecies interactions become stronger. The stability of the vortex state vanishes in the thermodynamic limit.

Non-Hermiticity and synthetic gauge fields play two fundamental roles in engineering exotic phases and dynamics in artificial quantum systems. Here, we explore the mean-field dynamics of interacting bosons in a two-leg ladder with synthetic magnetic flux and nonreciprocal hopping under the open boundary condition. In the Hermitian limit, we showcase the breakdown of the flux-driven chiral dynamics due to the nonlinear self-trapping effect. We further find that the nonreciprocity can drive the transition between chiral dynamics and antichiral dynamics. The antichiral motion is manifested as the non-Hermitian skin dynamics along the same direction on two legs that are not suppressed by the magnetic flux, while the chiral-antichiral transition is flux tunable. We also reveal the trap-skin dynamics with the coexistence of the self-tapping and skin dynamics in the ladder. Dynamical phase diagrams with respect to the chiral-antichiral dynamics, skin dynamics, self-trapping dynamics, and trap-skin dynamics are presented. Our results shed light on intriguing dynamical phenomena under the interplay among non-Hermiticity, nonlinearity, and artificial gauge fields.

The interaction between the electric dipole moments of a quantum emitter and a metal nanoparticle gives rise to unique optical properties, such as interference-induced photon correlations, that could be useful for enhanced intensity-based sensing. Using the quantum theory of photodetection, we propose a nanosensor system comprising a quantum emitter and a metal nanoparticle that explores the possibility of utilizing higher-order photon correlations for refractive index sensing. Both the refractive index sensitivity and resolution of the nanosensor, whose scattering spectrum lies within the visible region, are predicted. The sensor is supported by a substrate and driven weakly by a coherent field. By calculating the mean photocount and its second factorial moment resulting from the scattered field of the system, the sensing performance of the intensity and intensity-intensity correlation $g^{\left(2\right)}\left(0\right)$ are compared at optimal driving wavelengths. The mean photocount was found to be inherently low, inhibiting the role of interference-induced photon antibunching in minimizing the sensor's intensity shot noise. However, a regime in which the noise could be reduced below the shot noise limit is identified, leading to a quantum enhancement in the sensing performance.

Linear-optics gates, the enabling tool of photonic quantum information processing, depend on indistinguishable photons, as they harness quantum interference to achieve nonlinear operations. Traditionally, meeting this criterion involves generating pure identical photons, a task that remains a significant challenge in the field. Yet, the required indistinguishability is linked to the spatial exchange symmetry of the multiphoton wave function and does not strictly necessitate identical photons. Here, we show that the ideal operation of two-photon gates, particularly fusion gates, can be recovered from distinguishable photons by ensuring the exchange symmetry of the input photonic state. To this end, we introduce a temporal quantum eraser between a pair of modally impure single-photon sources, which heralds the symmetry of the generated two-photon state. We demonstrate this mechanism in two relevant platforms: parametric photon pair generation and single-photon extraction by a single quantum emitter. The ability to lift the requirement for identical photons bears considerable potential in linear-optics quantum information processing.