Fastness and robustness are both critical in the implementation of high-fidelity gates for quantum computation, but in practice a trade-off has to be made between them. In this paper, we investigate the robust time-optimal control problem that aims at the best balance. Based on the Taylor expansion of the system in terms of uncertainty parameters, we formulate the design problem as the optimal control of an augmented finite-dimensional system at its quantum speed limit (QSL), where the robustness is graded by the order of series truncation. The gradient-descent algorithm is then introduced to sequentially seek QSLs corresponding to different orders of robustness. Numerical simulations are carried out for single-qubit systems with frequency and field amplitude uncertainties, and the obtained time-optimal control pulses can effectively suppress gate errors to the prescribed robustness order. These results provide a practical guide for selecting pulse lengths in the pulse-level compilation of quantum circuits.

We demonstrate an approach for producing a high-flux beam of ${}^7\mathrm{Li}{}^4\mathrm{He}$ dimers by performing postnozzle seeding of a lithium beam into a supersonic helium expansion. The molecular beam has a longitudinal temperature of 13(6) mK and a continuous flux on the order of ${10}^{11}$ dimers/s confined within a solid angle of 0.16 sr. Monte Carlo simulations of molecular formation based on $\mathit{ab}$ initio quantum scattering calculations are carried out and compared to the experimentally observed molecular flux. Extensions of this work could lead towards a more quantitative understanding of few-body collision processes of alkali and helium atoms.

Using Doppler-free two-photon spectroscopy of the Rb $5S_{1/2}$ to $6S_{1/2}$ transition in a temperature-controlled vapor cell, for both naturally occurring isotopes, we measure to high accuracy the hyperfine splittings and constants, as well as the isotope shift of the $6S_{1/2}$ state. We lock a tunable microwave-driven electro-optic modulator sideband of the $993\text{−}\mathrm{n}\mathrm{m}$ laser to an ultrastable high-finesse cavity, thus achieving microwave frequency accuracy for the relative laser tuning. The line shapes are fit with a Voigt profile to extract line centers in order to calculate the hyperfine splittings, magnetic dipole hyperfine constants, isotope shift, and hyperfine anomaly. For the hyperfine splittings of the $6S_{1/2}$ state in ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$, respectively, we find $717.195\left(3\right)\mathrm{M}\mathrm{Hz}$ and $1614.709\left(3\right)\mathrm{M}\mathrm{Hz}$. For the hyperfine constants $A$ for the $6S_{1/2}$ states, we find $239.065\left(2\right)\mathrm{M}\mathrm{Hz}$ and $807.355\left(2\right)\mathrm{M}\mathrm{Hz}$ for ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$, respectively, and $-99.189\left(3\right)\mathrm{M}\mathrm{Hz}$ for the isotope shift (${}^{85}\mathrm{Rb}$ minus ${}^{87}\mathrm{Rb}$). These hyperfine splittings and constants are 10 to 25 times more accurate than previously published results. We measure the hyperfine anomaly ${}^{85}\mathrm{\Delta }^{87}$ of the $6S_{1/2}$ state to be $-0.00350\left(1\right)$, which is about 20 times more accurate than previously published results.

A relativistic analytical theory of strong-field ionization applicable across the regimes of deep tunneling up to over-the-barrier ionization (OTBI) is developed, accounting also for the bound-state polarization and the Stark shift beyond perturbation theory. The latter improvement with respect to the state-of-the-art quasiclassical theory of Perelomov-Popov-Terent'ev (PPT) for strong-field ionization is essential to describe analytically the ionization in the OTBI regime and to resolve the order-of-magnitude discrepancy of the ionization yield in the relativistic regime with respect to PPT theory that has remained unexplained since the numerical result using the Klein-Gordon equation of Hafizi et al. [Phys. Rev. Lett. 118, 133201 (2017)]. The predictions of the present relativistic model, in deviation to PPT theory, are shown to be observable using ultrashort laser pulses of relativistic intensities.

We introduce a noninvasive imaging technique based on spin-dependent off-axis holography for spin-1 Bose-Einstein condensates. Utilizing a dual reference beam strategy, this method records two orthogonal circular polarization components of a single probe beam. The circular birefringence of spin-polarized atoms induces differing complex phase shifts in the polarization components of the light, which are reconstructed from the interference patterns captured on camera. Our approach enables spin- and density-resolved imaging of both phase and amplitude information in situ on a submillisecond timescale with minimal disturbance to the condensate. We explore the technique's efficacy under various background static fields, demonstrating its sensitivity to the quantization axis of the atoms and confirming its effectiveness.

We demonstrate the controllable generation of distinct types of dispersive shock waves emerging in a quantum droplet bearing environment with the aid of steplike initial conditions. Dispersive regularization of the ensuing hydrodynamic singularities occurs due to the competition between mean-field repulsion and attractive quantum fluctuations. This interplay delineates the dominance of defocusing (hyperbolic) and focusing (elliptic) hydrodynamic phenomena being designated by the real and the imaginary speed of sound, respectively. Specifically, the symmetries of the extended Gross-Pitaevskii model lead to a three-parameter family, encompassing two densities and a relative velocity, of the underlying Riemann problem utilized herein. Surprisingly, dispersive shock waves persist across the hyperbolic-to-elliptic threshold, while a plethora of additional wave patterns arise, such as rarefaction waves, traveling dispersive shock waves, (anti)kinks, and droplet wave trains. The classification and characterization of these features are achieved by deploying Whitham modulation theory. Our results pave the way for unveiling a multitude of unexplored coherently propagating waveforms in such attractively interacting mixtures and should be detectable by current experiments.

In this paper, the propagation characteristics of fundamental and third harmonic waves within the designed hyperstructure are examined using the transfer matrix method. At the same time, the efficiency of third harmonic generation is quantified. The transfer matrix method provides a framework that takes into account the interference phenomena and multiple reflections occurring within each layer, thereby facilitating a precise emulation of the generation and propagation dynamics of the harmonic wave field within the hyperstructure. Moreover, the conspicuous phenomenon of strong field localization, coupled with the lower group delay observed at the edges of the photonic band gap, confers an augmented response time on the process of third harmonic conversion. Consequently, electromagnetic waves situated in close proximity to the band-gap edge experience a heightened conversion efficiency. Comparatively, when juxtaposed against those designed in the other works hinged on conventional quasiphase matching techniques and endowed with similar lengths, the distinctive advantages offered by layered periodic structures manifest in their propensity to establish more precise phase matching conditions and yield higher conversion efficiencies. The crucial role played by third harmonic generation in advancing optical imaging, spectroscopy, biomedical applications, and laser technology is due to its distinct optical characteristics and energy conversion capabilities.

Plasmonic chirality garners considerable attention owing to its distinctive optical characteristics and wide-ranging applications. Understanding the optical activity of single particles holds paramount significance for both theoretical frameworks and experimental measurements of chirality. When probing the optical response of chiral particles, high numerical aperture (NA) objectives, adept at concentrating light into exceedingly small regions, are frequently employed. With escalating NA, the incident light no longer satisfies the paraxial approximation, engendering electric field components perpendicular to the focal plane. However, this electric field component is usually neglected in research. Here, we utilize angular spectrum theory to explore the optical response and circular dichroism of single chiral nanoparticles subjected to tightly focused circularly polarized light. Remarkably, we find that the electric field component perpendicular to the focal plane exhibits a pronounced influence on chirality, potentially inducing a reversed circular dichroism response when the particle deviates from the optical axis. Simultaneously, variations in the position of particles within the light field yield significant discrepancies in higher-order modes, particularly electric quadrupole moments. Our findings hold profound implications for the measurement and analysis of chiral particles, offering invaluable guidance for the future investigations of chirality.

Microwave frequency combs have attracted particular attention due to their great application prospects in microwave communication and measurement. In recent years, the scheme of generating microwave frequency combs based on an optical method has been widely concerned; however, the investigation of obtaining microwave frequency combs based on a magnetic method has rarely been reported. Here, an efficient mechanism for the generation of microwave frequency combs induced by the rotational enhanced magnetostrictive effect is theoretically proposed. The results show that the magnetostrictive interaction can be greatly enhanced by introducing the Sagnac effect, and then a robust microwave frequency comb can be observed even at low power. Furthermore, when we change the incidence direction of the driving field, the generation of microwave frequency combs has strong nonreciprocity. Our propose thus provides a pathway for the generation of a flat microwave frequency comb based on magnonic platforms that may be beneficial for microwave frequency metrology and spectroscopy, as well as an in-depth understanding of magnomechanical nonlinearity.

We present a loop-based optical processor enabling time-domain programmable beam-splitter (BS) operations for a phase-sensitive non-Gaussian state. The loop itself is of high quality, allowing for storage of a non-Gaussian state for up to seven round trips while preserving its Wigner negativity and phase coherence. We perform various BS operations on a non-Gaussian state and evaluate them as transformations of the state's waveforms. Our work integrates non-Gaussian states with time multiplexing, laying the foundation for large-scale universal quantum information processing.

We propose a mechanism to significantly enhance the photon-phonon entanglement twice by coupling an auxiliary magnetic sphere to a standard cavity magnomechanical system. The deformed sphere acts as a cold reservoir that can simultaneously cool two hybrid modes superposed over the optical cavity and the mechanical oscillator by modulating the relationship between the coupling strengths, which enables us to obtain large steady-state cavity-oscillator entanglement. On the basis of the double-mode cooling effect, the steady-state photon-phonon entanglement can be significantly improved by opening the single-coupled auxiliary-sphere-assisted channel due to the fact that the nonsuperposed optical mode can be efficiently cooled via the auxiliary sphere. More importantly, the photon-phonon entanglement can be significantly enhanced again by switching the dual-coupled auxiliary-sphere-assisted (DC-ASA) passage, where the auxiliary sphere serves as an additional cold bath to simultaneously cool both the nonsuperposed optical mode and the reservoir mode. Moreover, in the DC-ASA regime, the steady-state entanglement is more robust against the mechanical thermal noise. This DC-ASA scheme can be potentially applied to accelerate magnomechanical cooling and enhance magnomechanical squeezing.

The interplay of three light-matter interaction processes, i.e., rotating and counterrotating interactions and two-photon relaxation of the light field, is a topic of interest in quantum optics and quantum information processing. In this work we investigate theoretically the three light-matter interaction processes using the anisotropic Rabi model, which accounts for different strengths of rotating and counterrotating interactions and the unique occurrence of photon escape exclusively in pairs. By numerically solving the Lindblad master equation, we analyze the excitation-relaxation dynamics and derive a non-Hermitian effective Hamiltonian to gain further physical insights. To explore the individual effects of these interactions, we examine three analytically tractable limits of the effective Hamiltonian. Our analysis reveals that the three competitive light-matter interaction processes exhibit sensitivity to parity, leading to intriguing phenomena in both transient and steady states. Particularly interesting dynamical patterns resembling quantum phase transitions emerge when these three interaction terms compete. This work deepens the understanding of ultrastrong light-matter interaction in open quantum systems and offers valuable insights into cavity-based quantum computations.

We analytically and numerically study collective emission from an ensemble of atoms in the weak-excitation regime with the initial condition of uniform excitation of the ensemble. We show that under certain conditions, subradiance at the later stages of the collective decay does not necessarily need to be accompanied by early-time superradiance. We analyze the conditions where such subradiance-only decay occurs for ordered and disordered ensembles in one-, two-, and three-dimensional spatial configurations.

We prove fundamental rigorous bounds on the speed of quantum evolution for a quantum system coupled to a thermal bath. The bounds are formulated in terms of expectation values of few-body observables derived from the system-bath Hamiltonian. They do not rely on the Markov approximation and, as a consequence, are applicable beyond the limit of weak system-bath coupling.

We present a theoretical description of charged particles with nonzero projection of the orbital angular momentum (OAM) in a uniform magnetic field with broken axial symmetry. The wave functions we find naturally account for the asymmetry of the magnetic field at the entrance of the solenoid through the continuous parameter and are a generalization of the Laguerre-Gauss states commonly used to describe twisted charged particles. We analyze the asymmetric Hamiltonian from an algebraic point of view and show how the OAM projection of the twisted state is modified by symmetry breaking. We provide analytical frameworks for properties of the asymmetric states, such as energy, rms size, and Casimir invariant, and discuss advantages of the proposed description.

The quantum brachistochrone problem addresses the fundamental challenge of achieving the quantum speed limit in applications aiming to realize a given unitary operation in a quantum system. Specifically, it looks into optimization of the transformation of quantum states through controlled Hamiltonians, which form a small subset in the space of the system's observables. Here we introduce a broad family of completely integrable brachistochrone protocols, which arise from a judicious choice of the control Hamiltonian subset. Furthermore, we demonstrate how the inherent stability of the completely integrable protocols makes them numerically tractable and therefore practicable as opposed to their nonintegrable counterparts.

Among various quantum machine learning (QML) algorithms, the quantum kernel method has especially attracted attention due to its compatibility with noisy intermediate-scale quantum devices and its potential to achieve quantum advantage. This method performs classification and regression by nonlinearly mapping data into quantum states in a higher-dimensional Hilbert space. Thus far, the quantum kernel method has been implemented only on qubit-based systems, but continuous-variable (CV) systems can potentially offer superior computational power by utilizing its infinite-dimensional Hilbert space. Here, we demonstrate the implementation of the classification task with the CV quantum kernel method on a programmable photonic quantum processor. We experimentally prove that the CV quantum kernel method successfully classifies several datasets robustly even under the experimental imperfections, with high accuracies comparable to the classical kernel. This demonstration sheds light on the utility of CV quantum systems for QML and should stimulate further study in other CV QML algorithms.

Recently, the entanglement dynamics of two harmonic oscillators initially prepared in a separable-coherent state was demonstrated to offer a pathway for prime number identification. This article presents a generalized approach and outlines a deterministic algorithm making possible the implementation of this theoretical concept on scalable fault-tolerant qubit-based quantum computers. We prove that the diagonal unitary operations employed in our algorithm exhibit a polynomial-time complexity of degree two, contrasting with the previously reported exponential complexity of general diagonal unitaries.

In quantum networks an important goal is to reduce resource requirements for the transport and communication of quantum information. Quantum network coding presents a way to do this by distributing entangled states over a network that would ordinarily exhibit contention. In this work, we study measurement-based quantum network coding (MQNC), which is a protocol particularly suitable for noisy intermediate-scale quantum devices. In particular, we develop techniques to adapt MQNC to state-of-the-art superconducting processors and subsequently demonstrate successful teleportation of quantum information, giving new insight into MQNC in this context after a previous study was not able to produce a useful degree of entanglement. The teleportation in our demonstration is shown to occur with fidelity higher than could be achieved via classical means, made possible by considering qubits from a polar cap of the Bloch sphere. We also present a generalization of MQNC with a simple mapping onto the heavy-hex processor layout and a direct mapping onto a proposed logical error-corrected layout. Our work provides some useful techniques to test and successfully carry out quantum network coding.