We analyze the entanglement in a model Josephson photonics system in which a dc voltage-biased Josephson junction couples a collection of cavity modes and populates them with microwave photons. Using an approximate quadratic Hamiltonian model, we study the Gaussian entanglement that develops between the modes as the Josephson energy of the system is increased. We find that the modes in the system fall into a series of blocks, with bipartite entanglement generated between modes within a given block. Tripartite entanglement between modes within a given block is also widespread, though it is limited to certain ranges of the Josephson energy. The system could provide an alternative route to generating multimode microwave entanglement, an important resource in quantum technologies, without the need for ac excitation.

We investigate quantum Otto engines operating with a weakly interacting Bose gas confined in a cubic box with periodic boundary conditions. Our study focuses on two approaches to realizing adiabatic processes: varying the system volume and adjusting the interaction strength. We analyze the impact of Bose-Einstein condensation and interparticle interaction effects on engine performance, particularly power output and efficiency. Additionally, we explore the advantages of operating near the critical point of condensation, where quantum phenomena may enhance engine performance. Our findings underscore the significance of phase transitions and many-body effects in improving machine performance.

The four-body fragmentation of acetylene molecular ions (${{\mathrm{C}}_2\mathrm{H}}_2^{q+},4\le q\le 8$) upon impact with 1 MeV ${\mathrm{Ar}}^{8+}$ projectile ions with ${\mathrm{C}}_2{\mathrm{H}}_2$ molecules, using cold target recoil ion momentum spectrometer (COLTRIMS), is studied by measuring momenta of fragments in a recoil-ion projectile-ion coincidence measurement. Charge-symmetric (-asymmetric) channels in which carbon ions share the same (different) charge are observed. Within the fragmentation of a parent ion, where there is a possibility of both the symmetric and asymmetric charge distribution of carbon ion, charge-symmetric breakup channels are favored over charge-asymmetric ones. Kinetic energy release (KER) distributions are presented and probable electronic states are associated with particular features in the KER distribution by using high-level ab initio quantum chemical calculations. The ab initio calculations also indicated that the difference between KER values for linear and bent structures (cis- and trans-configurations) is of the order of 0.1 eV. The projectile's final charge state after the collision and the reaction windows calculation within the framework of the extended classical overbarrier model examine the role of various electron-capture-associated processes in the formation of the molecular ions in a particular charge state. We observed that as the acetylene ion's charge state increases, the contribution from capture stabilization also increases, indicating the population of captured electrons into the lower energy states of the projectile. The momentum correlation plots in fragments' momentum space were used to visualize the fragmentation mechanism, and the observation of the double distribution of momenta of carbon ions for charge-asymmetric channels showed the direct signatures of fragmentation from two different molecular orientations with respect to the spectrometer axis.

The behavior of an atomic system is influenced by introducing a metallic surface. This work explores how the decay landscape can be altered by the presence of sharp corners. We utilize analytical expressions for Hertz and vector potentials in the presence of a perfectly conducting wedge with arbitrary opening angle to obtain the relevant electrostatic Green's functions. We use these to examine two scenarios: the modified spontaneous decay of a single atom, which leads us to speculate about potential applications in microscopy, and the case of a more fundamental, theoretical interest, the behavior of an entangled pair of atoms near a corner. The latter, when two atoms are positioned “out of the line of sight,” opens up a possible line of investigation into devices that are able to “see around corners.”

We study time delays in atomic multiphoton ionization (MPI) using the phase-sensitive detection of photoelectron vortices. Photoelectron vortices are created by MPI with counterrotating circularly polarized (CRCP) ultrashort double-pulse sequences. The vortices are characterized by a spiral-shaped photoelectron momentum distribution (PMD) resulting from the Ramsey-type interference of the partial photoelectron wave packets created by each pulse. The slope of the spiral arms encodes the time delay between the partial wave packets with interferometric precision. We study the ionization time delay in the $1+2$ resonance-enhanced multiphoton ionization of potassium atoms using femtosecond CRCP white-light supercontinuum pulses. Pairwise interference of the partial wave packets with a reference wave packet gives rise to vortices of different rotational symmetry, which are superimposed to form the total PMD. The PMD is reconstructed tomographically and decomposed into the vortices using three-dimensional Fourier analysis. From the energy-dependent spectral phases of the vortices, we retrieve a resonant ionization delay on the order of 1 fs, much shorter than the pulse duration. Our results demonstrate the power of photoelectron vortices for the accurate determination of photoionization time delays and open up new perspectives for photoelectron chronoscopy including the measurement of ionization delays on the attosecond timescale.

Vibrational Cooper minima due to weak dipole coupling were previously observed in strong-field dissociation of ${\mathrm{H}}_2{}^{+}$ as minima in the kinetic-energy-release (KER) spectrum [McKenna et al., Phys. Rev. Lett. 103, 103006 (2009)]. We demonstrate in two independent experiments that these vibrational Cooper minima occur more generally and can be used as “fingerprints” to identify the final electronic state in the dissociation pathway. In the first experiment, we show the wavelength dependence of the vibrational Cooper minima observed in the KER spectrum of ${\mathrm{O}}_2{}^{+}$ dissociation induced by 396- and 264-nm photons. These minima appear at the locations predicted by our first-order perturbation-theory calculations. In the second experiment, a delay-dependent KER spectrum is obtained from ${\mathrm{O}}_2$ using an ionizing pump and a dissociating probe, then Fourier-transformed to generate a spin-rotation quantum beat spectrum of ${\mathrm{O}}_2{}^{+}$. This spectrum shows vibrational Cooper minima at the expected locations, confirming their presence in another observable. In this case, these minima are used to identify the dissociation pathway.

Light-matter interaction involves the transfer of both energy and linear momentum to matter. In the context of strong-field tunneling ionization, they are manifested as nonadiabatic and nondipole effects, respectively, which have been shown to interact on the subcycle scale. In this work we propose two experimentally feasible protocols for direct visualization of the subcycle interplay between nonadiabatic and nondipole tunneling effects. These protocols hinge on the intersection of the average linear momentum transferred to the released photoelectron at a specific photoemission angle, with the magnitude of this intersection serving as an indicator of the coupling strength. In addition, by defining the instantaneous effective angular frequency of the laser field, we have formulated analytical expressions that provide a quantitative description of the coupling effect. Our results provide a clear and intuitive approach to visualizing the impact of the subcycle nonadiabatic-nondipole coupling effects, thereby enhancing our comprehension of the time-resolved tunneling process through which light imparts both energy and linear momentum to electrons.

We investigate defect formation and the universal critical dynamics of two-dimensional two-component Bose-Einstein condensates (BECs) confined in both box and harmonic traps. Our focus is on the nonequilibrium universal dynamics of the miscible-immiscible phase transition, considering both linear and nonlinear quenching protocols. The general condition for a phase transition, $g_{11}{g}_{22}=g_{12}^2$, depends exclusively on inter- and intraspecies interactions. In this system, simultaneous phase transitions occur in both the box and harmonic traps. Consequently, the nonequilibrium critical dynamics can still be described using the homogeneous Kibble-Zurek mechanism. Through an analysis of Bogoliubov excitations, we establish a power-law relationship linking the domain correlation length, phase transition delay, and quench time. Additionally, real-time simulations of phase transition dynamics demonstrate the formation of domain defects and the delay of phase transitions in nonequilibrium conditions. These simulations also reveal the universal scaling of the correlation length and phase transition delay across various quench times and nonlinear coefficients, showing well agreement with our analytical predictions. Our study confirms that the universality class of two-component BECs is independent of dimensionality. Additionally, larger nonlinear coefficients effectively suppress the growth of nonadiabatic excitations, offering a fresh perspective on achieving quasiadiabatic evolution.

Measuring the effect of gravity on antimatter is a longstanding problem in physics that has significant implications for our understanding of the fundamental nature of the universe. Here, we present a technique to measure the gravitational mass of atoms, motivated by a recent measurement of antimatter atoms at CERN [Nature (London) 621, 716 (2023)]. We demonstrate our technique on cold atoms by measuring the fraction of atoms that survive in the trap after gradually softening a quadrupole magnetic trap in a gravitational potential. We compare our measurements with a Monte Carlo simulation to extract the value of the gravitational acceleration. The difference between the accepted value for $g$, the local acceleration due to gravity, and the measured value is $(-1.9\pm {12}^{\text{stat}}\pm {40}^{\text{syst}})\times {10}^{-4}g$. We demonstrate the importance of various design parameters in the experiment setup and estimate their contribution to the achievable accuracy in future experiments. Our method demonstrates simplicity, precision, and reliability, facilitating future precision studies of the gravitational force on antimatter. It can also be used to precisely calibrate atom traps based on the known gravitational attraction of ordinary matter to Earth.

The optical gradient force trap lies at the heart of optical tweezers, with applications in a wide spectrum of sciences. It is conventionally perceived that the trap is either bright or dark, capturing small particles towards the intensity maxima or minima. Here, we demonstrate that there may exist an intermediate trapping state, which we refer to as the full-gray optical trapping, to permit distinct equilibrium positions where the light intensity incident on each part of the particle is neither maximized nor minimized. The appearance of this third-type trap is attributed to the excitation of high-order multipole resonances, which strongly couples nonlocal intensity inhomogeneity to the gradient force, yielding a unique force-vector field that converges on the gray region. By expanding the classification of optical traps and generalizing the gradient force theory to arbitrary-order multipoles, our findings underscore the impact of Mie responses on optomechanics and will facilitate the development of nanoparticle cooling, patterning, and ultrasensitive sorting.

Mutual scattering arises when multiple waves intersect within a finite scattering object, resulting in cross interference between the incident and scattered waves. By measuring mutual scattering, we determine the complex-valued scattering amplitude $f$—both amplitude and phase—of the finite object, which holds information on its scattering properties by linking incident and outgoing waves from any arbitrary direction. Mutual scattering is present for any coherent wave: acoustic, electromagnetic, particle, and we demonstrate the effect here using optical experiments. We propose an experimental technique for characterization that utilizes mutual scattering and we present our results for four distinct finite objects: a polystyrene sphere (diameter $59\mu \mathrm{m}$), a single black human hair (diameter $92\mu \mathrm{m}$), a strip of pultruded carbon (edge length $140\mu \mathrm{m}$), and a block of ${\mathrm{ZnO}}_2$ (edge length $64\mu \mathrm{m}$). Our measurements exhibit qualitative agreement with Mie scattering calculations where the model is applicable. Deviations from the model indicate the complexity of the objects, both in terms of their geometrical structure and scattering properties. Our results offer new insights into mutual scattering and have significant implications for future applications of sample characterization in fields such as metrology, microscopy, and nanofabrication.

Klyshko's advanced-wave picture (AWP) is mainly interpreted by replacing the nonlinear crystal producing biphotons via spontaneous parametric down-conversion (SPDC) by a mirror in quantum imaging protocols with thin crystals, where the biphotons are perfectly correlated in position at the crystal. To better explain the biphoton spatial states produced by arbitrary crystals and pump beams, we develop a formal theory of AWP with monochromatic lights that the conditional wave function of one photon is calculated by propagation, multiplication, and another propagation. The case of more general photon postselection or no detection and the inclusion of polarization are studied. Then, we explain the form of the biphoton state from SPDC with a bulk crystal and its free-space propagation. By treating the biphoton wave function as an impulse response function of a classical optical setup, we analyze quantum imaging with undetected photons and quantum holography with polarization entanglement, where properties like the spatial resolution can be concisely deduced. This method can be employed to design nonlinear materials or novel quantum imaging techniques. Finally, we discuss Klyshko's original proposal beyond monochromatic lights with the Hong-Ou-Mandel effect as an example.

We propose a scheme to achieve nonreciprocal unconventional magnon blockade (NUMB) in an optomagnonic system composed of a spinning resonator with ${\chi }^{\left(2\right)}$ nonlinearity and one yttrium iron garnet sphere. When two-photon driving is input from different ends of the resonator, complete quantum destructive interference appears and disappears between different paths for two-magnon excitation due to the opposite Sagnac-Fizeau shifts induced by Fizeau drag, which is the physical origin for achieving NUMB. To intuitively understand the nonreciprocal phenomenon, the analytical solution of the equal-time second-order correlation function is approximately given through the Schrödinger equation and compared with the full numerical solution obtained from the Lindblad master equation. From the two almost identical solutions, we show that a giant NUMB can be observed in the absence of probe detuning. We also show that a switchable NUMB can be achieved by tuning the relative phase between the external laser fields. In addition, increasing the amplitude of the probe field appropriately can reduce the impact of ambient thermal noise on NUMB. Our work may have potential applications in the design of nonreciprocal single-magnon devices.

We demonstrate the robustness of the recently established Floquet-assisted superradiant phase (FSP) of the coherently driven dissipative Dicke model, inspired by light-induced dynamics in graphene. In particular, we show the robustness of this state against key imperfections and argue for the feasibility of utilizing it for unconventional laser operation. We consider the effect of a finite linewidth of the driving field, modeled via phase diffusion. We find that the linewidth of the light field in the cavity narrows drastically across the FSP transition, reminiscent of a line narrowing at the laser transition. We then demonstrate that the FSP is robust against inhomogeneous broadening, while displaying a reduction of light intensity. We show that the depleted population inversion of near-resonant Floquet states leads to hole burning in the inhomogeneously broadened Floquet spectra. Finally, we show that the FSP is robust against dissipation, with coefficients up to values that are experimentally available. We conclude that the FSP presents a robust mechanism that is capable of laser operation.

We present an optical configuration for the generation of degenerate counterpropagating photon pairs via spontaneous-parametric down-conversion in subwavelength thin waveguides with $\overline{4}3m$ symmetry. The generated photon pairs exhibit orthogonal polarizations and excite TE- and TM-like modes. Compensation of the momentum mismatch between the excited modes by pump tilting enables tunable degenerate emission across the $S$, $C$, and $L$ bands (1400 to 1600 nm). Experimental verification confirms an efficiency of $\approx {10}^{-5}$ photon pairs per pump photon with an expected linewidth of 0.3 nm in 2-mm-long ${\mathrm{Ga}}_{0.51}{\mathrm{In}}_{0.49}\mathrm{P}$ waveguides.

The jump unraveling of a quantum master equation decomposes the dynamics of an open quantum system into abrupt jumps, interspersed by periods of coherent dynamics when no jumps occur. Such open quantum systems are ubiquitous in quantum optics and mesoscopic physics, hence the need for efficient techniques for their stochastic simulation. Numerical simulation techniques fall into two main categories. The first splits the evolution into small time steps and determines stochastically for each step if a jump occurs or not. The second, known as Monte Carlo wave-function simulation, is based on the reduction of the norm of an initially pure state in the conditional no-jump evolution. It exploits the fact that the purity of the state is preserved by the finest unraveling of the master equation. In this paper, we present an alternative method for the simulation of the quantum jump unraveling, inspired by the classical Gillespie algorithm. The method is particularly well suited for situations in which a large number of trajectories is required for relatively small systems. It allows for non-purity-preserving dynamics, such as the ones generated by partial monitoring and channel merging. We describe the algorithm in detail and discuss relevant limiting cases. To illustrate it, we include four example applications of increasing physical complexity and discuss the performance of the algorithm across regimes of interest for open quantum systems simulation. Publicly available implementations of our code are provided in julia and mathematica.

From the interaction of the permanent magnetic dipole moment of a neutral particle with an electric field inside a long nonconducting cylindrical shell of inner radius $r_a$ and outer radius $r_b$, we show that a geometric quantum phase stems from the missing electric charge per unit length. Thus, we discuss the possibility of existing Aharonov-Bohm-type effects with regard to this geometric quantum phase. Further, we discuss the persistent spin currents.

Split transmons have been widely used in making quantum processors for building scalable and fault-tolerant superconducting quantum computers. With tunable external flux threading the superconducting quantum interference devices (SQUIDs) of qubits, split transmons have a degree of freedom for tuning frequencies but introduce flux noise as an important dephasing factor at the same time. Here, an improved version of a transmon is proposed and is called a “triple-Josephson-junction transmon” (tri-JJ transmon). A tri-JJ transmon is obtained by shunting another Josephson junction to the SQUID of a split transmon, which can produce much more different local minima and maxima of qubit frequencies tuned by external flux as “sweet spots” than those of split transmons. The first-order effect of the flux noise at the sweet spots can be eliminated and the corresponding pure-dephasing times are estimated as about $200\mu \mathrm{s}$ by considering only critical current noise. Tri-JJ transmons are compatible with the process and the protocol of control and measurement of split transmons and can be competitive candidates for constructing a high-performance superconducting quantum processor.