All-microwave control of fixed-frequency superconducting quantum systems offers the potential to reduce control circuit complexity and increase system coherence. Nevertheless, due to the limited control flexibility in qubit parameters, one has to address several issues, such as quantum crosstalk and frequency crowding, for scaling up qubit architecture with nontunable elements. This paper proposes a microwave-activated two-qubit gate scheme for two fixed-frequency transmon qubits coupled via a fixed-frequency transmon coupler. The protocol relies on applying a microwave pulse exclusively to the coupler, enabling the implementation of a controlled-$Z$ gate. We show that the gate fidelity exceeding 0.999 can be achieved within 150 ns, excluding decoherence effects. Moreover, we also show that leakage from the computational subspace to noncomputational states can also be effectively suppressed.

We present a nonperturbative model to describe the stopping power by ionization of the $d$ electrons of transition metals. These metals are characterized by the filling of the $d$ subshell and the promotion of some of the electrons to the conduction band. The contribution of $d$ electrons at low-impact energies has been noted experimentally in the past as a break of the linear dependence of the stopping power with the ion velocity. In this paper we describe the response of these electrons considering the atomic inhomogeneous momentum distribution. We focus on the transition metals of groups 10 and 11 in the Periodic Table: Ni, Pd, Pt, Cu, Ag, and Au. Results are shown to describe the low-energy stopping power, with good agreement with the experimental data and available time-dependent density-functional theory results. By combining the present nonperturbative model for the $d$-subshell contribution with other approaches for the valence electrons and for the inner shells, we provide a coherent theoretical method capable of describing the stopping power of these transition metals from the very-low- to the high-energy region.

We present a direct lifetime measurement of the $5s5p{}^1{P}_1$ state of strontium using time-correlated single-photon counting of laser-induced fluorescence in a hot atomic beam. To achieve fast switch-off times and a high signal-to-noise ratio, we excite the strontium atoms with a femtosecond pulsed laser at approximately 461 nm and collect the fluorescence onto a hybrid single-photon detector. Analysis of the measured exponential decay gives a lifetime of the ${}^1{P}_1$ state of $\tau =(5.216\pm 0.{006}_{\text{stat}}\pm 0.{013}_{\text{sys}})$ ns, where all the systematic effects have been thoroughly considered.

We study how nonlinear strength affects topological pumping of edge solitons by using the Gross–Pitaevskii equation. For weak nonlinear strength, the introduction of nonlinearity breaks the symmetry of the energy spectrum, which makes the topological pumping from the left edge to the right edge differ from the inverse process. For moderate nonlinear strength, self-crossing structures appear in the spectrum, and the right-to-left adiabatic pumping channel is destroyed, only left-to-right topological pumping can be achieved under slow modulation. As the nonlinear strength further increases, although left-to-right topological pumping in one pumping cycle also breaks down, we find that a thin soliton, located at a single left edge, can be mixed with the bulk soliton, and hybridized topological pumping of edge and bulk solitons can be realized after multiple pumping cycles. For stronger nonlinear strength, edge solitons are self-trapped and all topological pumping channels are shut down. Our work could trigger further studies of the interplay between nonlinearity and topology.

We explore the decay rates of optical modes in asymmetric microcavities with mixed phase space across a wide range of wavelengths that extend deep into the semiclassical, i.e., short-wavelength, limit. Implementing an efficient numerical method, we computed ${10}^6$ eigenmodes and discovered that certain decay rates form sequential separate branches with increasing wave number that eventually merge into smooth curves. The analysis of the localization properties and Husimi distributions reveals that each branch corresponds to a periodic orbit in the closed classical system. Our findings show that these decay rates gradually resolve the structure of the islands in mixed phase space as we approach the short-wavelength limit. We present an effective semiclassical model incorporating wave-number-dependent localization, Fresnel reflection, and the Goos-Hänchen shift and demonstrate that these effects are crucial in accounting for the observed branches of decay rate curves.

Quantum routers determine the propagation of quantum signals in a quantum network. In a complex optical quantum network, the quantum signals should be routed to multilevel quantum channels, but how to modulate this quantum routing process is a challenge. We study the quantum routing of single photons with three-level quantum channels connected by two whispering-gallery resonators (WGRs). We find that single photons can be efficiently routed to a third level quantum channel, though the maximum probability is not higher than that to the second level quantum channel, which can reach $100\%$. Moreover, we also investigate how the phase shift parameter and the dissipation rates influence the routing probability in a third level quantum channel. Our work provides a robust proposal for harvesting the potential of quantum networks in the future.

We investigate the wave-packet dynamics in an interacting Floquet system described by the Gross-Pitaevskii equation with a ratchet potential. Under the quantum resonance condition, we thoroughly examine the exotic dynamics of directed current, mean energy, and quantum scrambling based on the exact expression of a time-evolving wave packet. The directed current is controlled by the phase of the ratchet potential. Interestingly, the phase modulation induced by self-interaction dominates the quadratic growth of both mean energy and out-of-time-ordered correlators (OTOCs). Under the quantum nonresonance condition, the disorder in momentum space, induced by the pseudorandom feature of the free evolution operator, suppresses the directed current at all times. Meanwhile, the disorder also leads to the dynamical localization of the mean energy and the freezing of quantum scrambling for an initially finite-time interval. The dynamical localization can be effectively manipulated by the phase, with underlying physics rooted in the different quasieigenenergy spectrum modulated by ratchet potential. Both the mean energy and OTOCs exponentially increase after long-time evolution, which is governed by the classically chaotic dynamics dependent on the self-interaction. Possible applications of our findings for quantum control are discussed.

Resonators with weak single-photon self-Kerr nonlinearities can theoretically be used to prepare Fock states in the presence of a loss much larger than their nonlinearities. Two necessary ingredients are large displacements and a two-photon (parametric) drive. Here, we find that these systems can be controlled to achieve any desired gate operation in a finite-dimensional subspace (whose dimensionality can be chosen at will). Moreover, we show that the two-photon driving requirement can be relaxed and that full controllability is achievable with only one-photon (linear) drives. We make use of both Trotter-Suzuki decompositions and gradient-based optimization to find control pulses for a desired gate, which reduces the computational overhead by using a small blockaded subspace. We also discuss the infidelity arising from input power limitations in realistic settings, as well as from corrections to the rotating-wave approximation. Our universal control protocol opens the possibility for quantum information processing using a wide range of lossy systems with weak nonlinearities.

Subspace methods are powerful, noise-resilient methods that can effectively prepare ground states on quantum computers. The challenge is to get a subspace with a small condition number that spans the states of interest using minimal quantum resources. In this work, we will use eigenvector continuation to build a subspace from the low-lying states of a set of Hamiltonians. The basis vectors are prepared using truncated versions of standard state preparation methods such as imaginary-time evolution (ITE), adiabatic state preparation (ASP), and variational quantum eigensolver. By using these truncated methods combined with eigenvector continuation, we can directly improve upon them, obtaining more accurate ground-state energies at a reduced cost. We use several spin systems to demonstrate convergence even when methods like ITE and ASP fail, such as ASP in the presence of level crossings and ITE with vanishing energy gaps. We also showcase the noise resilience of this approach beyond the gains already made by having shallower quantum circuits. Our findings suggest that eigenvector continuation can be used to improve existing state preparation methods in the near term.

This paper presents an enhanced interfering-or-not-interfering quantum key distribution (INI-QKD) protocol incorporating a scheme without intensity modulation (IM), which circumvents the impulse correlation issue associated with IM. Based on the minimum smooth entropy, we derive the key rate for finite code lengths and estimate the phase bit error rate using a no-decoy state method. Simulation results show that the protocol achieves the maximum communication distance of 337 km with quantum bit error rate of approximately $0.36\%$. Moreover, the feasibility of using fewer phase plates ($M=6,8$) to realize discrete random phases in INI-QKD is also verified. Notably, the INI-QKD without IM exhibits significant advantages under polarization misalignment and phase mismatch conditions, reflecting greater adaptability to complex environments. By simplifying experimental setups and reducing the technical challenges, the INI-QKD without IM provides an alternative realization idea in the field of quantum communication.

We probe the production of single and double $K$-shell vacancies in xenon atoms during symmetric collisions with ${\mathrm{Xe}}^{54+}$ ions at 95, 146, and 197 MeV/u at the heavy-ion storage ring equipped with an internal gas-jet target. For each collision energy, the xenon $K\alpha$ x-ray spectra are measured at five observation angles, ranging from ${35}^{\circ }$ to ${145}^{\circ }$ with respect to the projectile beam. To resolve the measured Xe target $K\alpha$ x-ray spectra into four Gaussian peaks, including the hypersatellites and satellites of the $K{\alpha }_{2,1}$ lines, we develop a data-analysis method that reduces the fitting parameters from 12 to 6. The determined intensities of the hypersatellites and satellites, which shows no significant radiation anisotropy, are subsequently used to calculate the cross-section ratio $R_{21}$ for double-to-single $K$-shell vacancy production in the Xe target atoms. The obtained $R_{21}$ values decrease from 0.286 $\pm$ 0.036 to 0.176 $\pm$ 0.021 as the collision energy increases. We compare these experimental results to relativistic time-dependent two-center calculations and find reasonable agreement between the theoretical predictions and the experimental data.

The reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) setup is theoretically studied for various combinations of extreme ultraviolet and infrared (IR) field components polarization: “$\mathrm{linear}+\mathrm{linear}$,” “$\mathrm{linear}+\mathrm{circular}$” with crossed propagation directions, and “$\mathrm{circular}+\mathrm{circular}$” with parallel propagation directions. We examine the general properties of photoelectron angular distributions and their response to the variation of the IR pulse delay. Numerical simulations are performed for the neon valence shell ionization into the structureless continuum using two approaches: time-dependent perturbation theory and the solution of amplitude rate equations. To distinguish between “geometrical” governed by fields polarization and spectroscopic features, we provide an additional analysis for the case of $s$-shell ionization.

Recent studies have shown that both Rabi oscillations (ROs) and dynamic interference (DI) lead to pronounced multipeak patterns in the spectrum of ionized photoelectrons. These two prominent dynamical effects—characteristic of intense, short, and (near-)resonant laser pulses interacting with matter—usually coexist, and are thus hard to distinguish. Here, we revisit the multipeak pattern of the Autler-Townes doublet to uncover its physical origin, by calculating the photoelectron spectrum accurately. Our joint analytical and numerical analysis reveals that selectively populating the two emerging dynamically dressed states is key to an efficient balancing between ROs and DI. We demonstrate physical situations where either ROs or DI dictate the spectrum shape, and explore the transition between these regimes. The present work provides further insight into our results reported in a previous work [Phys. Rev. A 108, L061101 (2023)].

A repulsively interacting Bose-Einstein condensate on a ring is well known to show persistent currents. For attractive interactions, however, a bound state may form that renders the rotation classical. Here we show that in a multiply connected confinement, the strong in-plane attraction of an antidipolar condensate can form stacks of ring-shaped density lumps which may coherently overlap to form a supersolid along the azimuthal symmetry axis of the system. Intriguingly, the functional behavior of the energy–angular-momentum dispersion of the antidipolar ring condensate differs from that of a usual repulsive superfluid. The periodic maxima between persistent flow and the nonrotating ground state flatten significantly and the typical pronounced cusps in the energy dispersion also occur in the rotationally symmetric supersolid state. A weak link results in the reduction of this minimum, shifting it to smaller angular momenta. With an asymmetric link potential one can selectively induce superfluid and rigid-body rotation in different layers within the same system. This intriguing setup offers new perspectives for atomtronics applications.

A dual-atom-interferometer gyroscope has high sensitivity for accurate rotation measurements because of its immunity to common-mode noise. However, a measurement error is induced as the common phase noise, e.g., vibration noise, increases. In this work, we demonstrated a real-time compensation (RTC) scheme to improve the accuracy of measurements of the differential phase in dual atom interferometers. The vibration noise was measured by two seismometers and its induced phase shift was fed back to the Raman lasers after this shift was calculated based on the sensitivity function of the atom interferometers. After the RTC module was applied, the interference signals obtained via the square modulation method were stabilized at their midfringe points, which caused the vibration-induced phase noise for each of the atom interferometers to decrease from 495 mrad to 165 mrad. The differential phase error obtained by the dual atom interferometers and two seismometers was reduced from 114.8 mrad to 49.6 mrad, which implies that the rotation measurement error was improved by a factor of 2.3. This work paves the way to accurately measuring the rotation signal and has promising applications in fields of precision measurements.

We give an exact application of a recently developed, operator-based theory of wave propagation in dispersive, time-varying media. Using this theory, we find that the usual symmetry of complex conjugation plus changing the sign of the frequency, required for real-valued fields, implies that the allowed propagation constants in the medium are either real valued or come in conjugate pairs. The real-valued wave numbers are only present in time-varying media, implying that time variation leads to modes that are free from dissipation, even in a lossy medium. Moreover, these symmetry-unbroken waves lack a defined propagation direction. This can lead to a divergent transmission coefficient when waves are incident onto a finite, time-varying slab. The techniques used in this paper present a route toward further analytic applications of this operator formalism.

Broadband temporal modes of pulsed optical fields have been recently recognized as very promising for photonic information processing and time-frequency metrology. Exploiting their full potential demands efficient and flexible tools for their manipulation. Among the tools demonstrated, surprisingly the most basic, a single-mode temporal filter, is missing. In this paper, we propose an experimentally feasible approach to realize a genuine single-mode temporal filter that is based on the concept of temporal cavity, a device with temporal mode-dependent resonances in the basis of temporal broadband modes. This functionality is achieved as the temporal-domain analogy of spatial-mode cleaner cavities. This device will enable robust temporal mode filtering and detection, opening new prospects in time-frequency metrology and multidimensional classical and quantum information processing.

We propose simple quantitative criteria, based on counting statistics in resonant harmonic detectors, that probe the quantum mechanical character of radiation fields. They provide, in particular, practical means to test the null hypothesis that a given field is “maximally classical,” i.e., accurately described by a coherent state. We suggest circumstances in which that hypothesis plausibly fails, notably including gravitational radiation involving nonlinear or stochastic sourcing.