The Dicke model can exhibit quantum phase transition between the normal and the superradiant phases when the strength of the light-matter coupling exceeds the ultrastrong-coupling regime. However, it is challenging to observe this phase transition in practical systems due to limited coupling strength or finite two-photon $A^2$ terms. Here we show that by applying a periodic modulation to the frequency of the two-level systems in a standard Dicke model in the strong-coupling regime, an anisotropic Dicke model with tunable rotating and counter-rotating terms in the ultrastrong-coupling regime can be achieved. We calculate the ground state and the excitation spectrum of this model in terms of the modulation parameters. Our result shows that the superradiant phases can be observed in cavity- or circuit-quantum electrodynamics systems with strong coupling.

Vacuum birefringence is one of the most fascinating properties of quantum electrodynamics. In laser-induced vacuum polarization signatures, the interaction length is usually limited by the pump laser's Rayleigh length and temporal length. Here, we show that a flying focus pump with focus velocity $–c$ can overcome the short interaction length of the tightly focused pump laser, providing high intensity and long interaction length at the same time, which may lead to the experimental detection of vacuum birefringence.

We show the controllable excitation transmission can be implemented based on the topologically protected edge channel in the modulated Su-Schrieffer-Heeger model with gain and loss. We find that the gain and loss open up a special edge channel with pure imaginary energy, by which the opening and closing of the topological excitation transmission between left and right edges can be controlled via tuning the amplitude of the imaginary potential. We reveal that the special edge transmission phenomenon can be comprehended using an effective lattice model only with the gain and loss at two edge sites, in which the gain or loss added on the right edge determines the opening or closing of the topological excitation transmission. Applying the concept of controllable excitation transmission into the lattice with interface, the controllable transmissions from interface site to right edge site or left edge site can be realized. If treating the interface site as the input port and treating two edge sites as output port, the present system is equivalent to a two-way topological switch. Especially, the transmission path of topological switch toward right edge or left edge can be chosen via tuning the amplitude of the imaginary potential. The controllable excitation transmission and topological switch further explore the potential applications of topological materials in quantum information processing, which may provide the enlightening suggestions for implementing the topological devices, including the topological triode and topological logical gate.

Variational quantum algorithms are a promising class of algorithms that can be performed on currently available quantum computers. In most settings, the free parameters of a variational circuit are optimized using a classical optimizer that updates parameters in Euclidean geometry. Since quantum circuits are elements of the special unitary group, we can consider an alternative optimization perspective that depends on the structure of this group. In this work, we investigate a Riemannian optimization scheme over the special unitary group and we discuss its implementation on a quantum computer. We illustrate that the resulting Riemannian gradient-flow algorithm has favorable optimization properties for deep circuits and that an approximate version of this algorithm can be performed on near-term hardware. We highlight the connections of our work with previously proposed heuristics like ADAPT-VQE and show that they can be understood as variants of our algorithm.

Recently, it was shown that when a single protuberance is introduced on an infinite flat conducting surface, an anisotropic particle, kept constrained to move on a plane above the surface, can feel not only a lateral van der Waals (vdW) force that leads it to the protuberance, but, when the protuberance is narrow enough, a lateral force that moves it away from the protuberance. Although this sign inversion in the lateral vdW force was found to be related to the increasing in the curvature of the protuberance, the role of the curvature was not directly isolated from the effect caused by the remaining infinite planar surface. In this paper, seeking to isolate the sign inversion of the lateral vdW force as a result of the increase in curvature of an object, we consider a perfectly conducting infinite cylinder and investigate the vdW interaction with a neutral polarizable point particle constrained to move in a plane near the cylinder. We show that, under the action of the lateral vdW force, an isotropic polarizable particle is always attracted to the point on the plane which is closest to the cylinder surface. On the other hand, when we have an anisotropic particle, for certain particle orientations, anisotropy, and sufficiently high curvature of the cylinder, this lateral vdW force can manifest a repulsive behavior, moving the particle away from the cylinder. Since, in the literature, the sign inversion in the lateral force was discussed only in the context of the vdW regime, here we also extend this inversion to the Casimir-Polder regime. In addition, we also show that there are classical counterparts of this sign inversion effect, involving a neutral point particle with a permanent electric dipole moment.

Introducing spatially tunable interactions to atomic Fermi gases makes it feasible to study two phenomena, the proximity effect and spatial Kibble-Zurek mechanism (KZM), in a unified platform. While the proximity effect of a superconductor adjacent to a normal metal corresponds to a step-function quench of the pairing interaction in real space, the spatial KZM is based on a linear drop of the interaction that can be modeled as a spatial quench. After formulating the Fermi gases with spatially varying pairing interactions by the Bogoliubov–de Gennes equation, we obtain the profiles of the pair wave function and its correlation function to study their penetration into the noninteracting region. For the step-function quench, both correlation lengths from the pair wave function and its correlation function follow the BCS coherence length and exhibit the same scaling behavior. In contrast, the scaling behavior of the two correlation lengths are different in the spatial quench, which then allows more refined analyses of the correlation lengths from different physical quantities. Moreover, adding a weakly interacting bosonic background does not change the scaling behavior. We also discuss relevant experimental techniques that may realize and verify the inhomogeneous phenomena.

The scattered and guided radiation excited by a point charge moving in the vicinity of an infinite, dielectric cylinder is considered. It is shown that guided waves constitute the dominant contribution to the emitted radiation for short distances between the particle and the cylinder, low yet relativistic particle energies (roughly ${10}^{-1}<\gamma -1<{10}^1$, where $\gamma$ is the Lorentz factor), and large cylinder permittivity. In all the complementary cases, radiation by scattered waves dominates the spectrum. Analytic, approximate expressions for the spectrum of the scattered radiation are presented, with the spectrum for high energies resembling that of plane-wave scattering. The relative contribution of the various cylinder eigenmodes to the guided radiation is also investigated. The effect of a finite-size bunch on the emitted radiation is considered. Finally, the number of emitted photons via both scattered and guided radiation is examined, with the scattered photon number observed to scale for high energies as $ln\left(b\gamma \right)$.

Parametric conversion of ultrashort pulses is a versatile tool in nonlinear and quantum optics. Here, we present a detailed theory and numerical investigation of the phenomenon of the signal and idler trapping by the ultrashort pump pulse in a material with second- and third-order nonlinearities. The trapping regime becomes possible by balancing the characteristic length scales associated with the group-velocity mismatch and second-order nonlinearity, ensuring that signal, idler, and pump pulses propagate with the pump group velocity. We have derived analytical solutions for the trapped states and found that the transition between the broadband coherent and in-coherent signal and idler spectra accompanies the changeover between the trapping and no-trapping regimes. We have also demonstrated the higher-order trapped states, reported a significant boost to conversion efficiency in the trapping regime and considered in detail a particular case of the pump pulse being an optical soliton.

We propose to integrate dark-state based localization techniques into a neutral atom quantum computing architecture and numerically investigate two specific schemes. The first scheme implements state-selective projective measurement by scattering photons from a specific qubit with very little cross talk on the other atoms in the ensemble. The second scheme performs a single-qubit phase gate on the target atom with an incoherent spontaneous emission probability as low as $0.01$. Our numerical simulations in rubidium (Rb) atoms show that for both of these schemes a spatial resolution at the level of tens of nanometers using near-infrared light can be achieved with experimentally realistic parameters.

We propose methods for the exact synthesis of single-qubit unitaries with high success probability and gate fidelity, considering both time-bin and frequency-bin encodings. The proposed schemes are experimentally implementable with a spectral linear-optical quantum computation (S-LOQC) platform, composed of electro-optic phase modulators and phase-only programmable filters (pulse shapers). We assess the performances in terms of fidelity and probability of the two simplest three-component configurations for arbitrary gate generation in both encodings and give an exact analytical solution for the synthesis of an arbitrary single-qubit unitary in the time-bin encoding, using a single-tone rf driving of the electro-optic modulators. We further investigate the parallelization of arbitrary single-qubit gates over multiple qubits with a compact experimental setup, both for spectral and temporal encodings. We systematically evaluate and discuss the impact of the rf bandwidth, which conditions the number of tones driving the modulators, and of the choice of encoding for different targeted gates. We moreover quantify the number of high-fidelity Hadamard gates that can be synthesized in parallel, with minimal and increasing resources in terms of driving rf tones in a realistic system. Our analysis positions spectral S-LOQC as a promising platform to conduct massively parallel single-qubit operations, with potential applications to quantum metrology and quantum tomography.

Helium atoms in pulsed supersonic beams have been prepared in the circular Rydberg state with principal quantum number $n=55$ using the crossed-fields method. High-resolution microwave spectroscopy of the transition from this state to the $n=56$ circular state, at frequencies close to 38.5 GHz, was performed to measure static magnetic and electric fields along the axis of propagation of the beams with quantum-state-selective detection by pulsed-electric-field ionization. Magnetic fields of between 1.3 and 1.6 mT were measured to a relative precision of $\pm 900\mathrm{nT}$ by rf spectroscopy and $\pm 1.3\mu \mathrm{T}$ by microwave spectroscopy, with absolute calibration, accounting for Doppler shifts and effects of weak stray electric fields to $\pm 2.0\mu \mathrm{T}$ and a spatial resolution of $\pm 0.87$ mm. Magnetic-field gradients could be determined to a precision of $\pm 1.49\mu \mathrm{T}/\mathrm{mm}$ ($\pm 53$ nT/mm) over a baseline of 1.74 mm (35 mm). To perform these measurements, static electric fields and contributions from the motional Stark effect were minimized, and residual electric fields in each of the three spatial dimensions in the apparatus were measured to an absolute precision of between $\pm 85$ and $\pm 750\mu \mathrm{V}/\mathrm{cm}$. The methods used in this work can be transferred to experiments with other atoms or molecules. They are therefore well suited for applications in minimally invasive, absolute static-field magnetometry and electrometry, for example, at hybrid interfaces between Rydberg atoms and superconducting circuits; in tests of bound-state QED or the weak equivalence principle with atomic or molecular hydrogen, antihydrogen, or positronium; and in measurements of the absolute neutrino mass by cyclotron radiation emission spectroscopy.

We report on the observation of a high-density, band insulating state in a three-dimensional optical lattice clock. Filled with a nuclear-spin-polarized degenerate Fermi gas of ${}^{87}\mathrm{Sr}$, the three-dimensional (3D) lattice has one atom per site in the ground motional state, thus guarding against frequency shifts due to contact interactions. At this high density where the average distance between atoms is comparable to the probe wavelength, conventional imaging techniques at saturation intensity suffer from large systematic errors. To spatially probe frequency shifts in the clock and measure thermodynamic properties of this system, accurate imaging techniques at high optical depths are required. Using a combination of highly saturated fluorescence and absorption imaging, we confirm the density distribution in our 3D optical lattice in agreement with a single spin band insulating state. Combining our clock platform with this high filling fraction opens the door to studying new classes of long-lived, many-body states arising from dipolar interactions.

We investigate the emission of surface polaritons (SPs) by a charged particle moving inside a dielectric cylinder parallel to its axis. It is assumed that the cylinder is immersed into a homogeneous medium with negative dielectric permittivity in the spectral range under consideration. The SP modes are present for the positive dielectric function of the cylinder material. In order to find the electromagnetic fields corresponding to SPs, the respective contributions to the components of the Green tensor are separated. The expressions for scalar and vector potentials and for electromagnetic-field strengths are provided inside and outside the cylinder. Those fields are expressed in terms of the SP eigenmodes of the waveguide and we give detailed analysis for their properties. The SP energy fluxes through the plane perpendicular to the cylinder axis are evaluated in the interior and exterior media. The energy flux is directed towards the charge motion inside the cylinder and towards the opposite direction in the exterior region. The relativistic effects may essentially increase the radiated energy. Important features of relativistic effects include the possibility of essential increase of the radiated energy, the narrowing of the confinement region of the SP fields near the cylinder surface in the exterior region, the enlarging of the frequency range for radiated SPs, and the decrease of the cutoff factor for radiation at small wavelengths compared with the waveguide radius. The general results are specified for the Drude dispersion in the exterior medium. By using the Green tensor we also evaluate the total-energy losses of the charged particle for the general case of the interior and exterior dielectric functions. The corresponding results are compared with those previously discussed in the literature. The numerical data are presented in terms of scale-invariant quantities that allow us to clarify the features of the SP radiation for different values of the waveguide radius.

The superradiant laser, based on the clock transition between the electric ground state ${}^1{S}_0$ and the metastable state ${}^3{P}_0$ of fermionic alkaline-earth-metal-like atoms, has been proposed to be a new promising light source with linewidth of the order of millihertz. However, due to the small ${}^1{S}_0$ to ${}^3{P}_0$ transition strength, the steady-state power in that system is relatively low (approximately ${10}^{-12}\mathrm{W}$). In this work we propose an alternative superradiant laser scheme based on a Raman-transition-induced coupling between the ${}^3{P}_0$ and ${}^3{P}_1$ states in bosonic alkaline-earth-metal-like atoms and achieve a laser with narrow linewidth (less than approximately $2\pi \times 1\mathrm{Hz}$) and high power (greater than approximately ${10}^{-10}\mathrm{W}$, approximately ${10}^3$ photons in steady state) at a small pumping cost (less than approximately $2\pi \times 10\mathrm{kHz}$). The Raman beams play two significant roles in our scheme. First, the coherence between the dark and bright states induced by the Raman beams produces a new local minimum in the pumping-linewidth curve with pumping rate lower than $2\pi \times 10\mathrm{kHz}$, which is beneficial for continuous output. Second, the Raman beams mix the long-lived ${}^3{P}_0$ state into the lasing state and thus make the laser linewidth tunable. Our work greatly improves the output performance of the superradiant laser system with coherence induced by Raman transitions and may offer a firm foundation for its practical use in the future.

We detail the derivation of nanolaser models that include coherent and incoherent variables and predict the existence of a laser threshold, irrespective of cavity size and emitter number, for both single- and multielectron systems. The growth in photon number in the lasing mode is driven by an increase in correlation between absorption and emission processes, leading to the onset of self-sustained stimulated emission (laser threshold), followed in turn by a correlation decrease and ending with the dominance of coherent emission. The first-order coherence $g^{\left(1\right)}$ steadily increases, as the pump grows towards the laser threshold value, and reaches unity at or beyond threshold. The transition toward coherent emission becomes increasingly sharp as the numbers of emitters and coupled electromagnetic cavity modes increase, continuously connecting, in the thermodynamic limit, the physics of nano- and macroscopic lasers at threshold. Our predictions are in remarkable agreement with experiments whose first-order coherence measurements have so far been explained only phenomenologically. A consistent evaluation of different threshold indicators provides a tool for a correct interpretation of experimental measurements at the onset of laser action.

The dominant decay pathways of argon $2p^{-2}$ double-core-hole states have been investigated using synchrotron radiation and a magnetic-bottle-type spectrometer coupled with an ion time-of-flight spectrometer. This experiment allows for efficient multi-electron-ion coincidence measurements, and thus for following the Auger cascade step by step in detail. Dominant decay pathways leading to ${\mathrm{Ar}}^{4+}$ final states via ${\mathrm{Ar}}^{3+}$ intermediate states have been assigned with the help of theoretical ab initio calculations. The weak correlated decay of the two core holes by emission of a single Auger electron, leading to ${\mathrm{Ar}}^{3+}$ final states, has been observed at 458.5-eV kinetic energy. Compared to the total decay of the $2p^{-2}$ double core vacancies, this two-electron–one-electron process was measured to have a branching ratio of $1.9\times {10}^{-3}\pm 1.0\times {10}^{-3}$. Furthermore, the remaining decay paths of the ${\mathrm{Ar}}^{1+}\left(1s^{-1}\right)$ core hole to higher charge states and their respective contributions to the total yield have been analyzed and show very good agreement with theoretical results.

Adiabatic rapid passage (ARP) is extensively used to achieve efficient transfer or inversion of populations in quantum systems. Landau and Zener accurately estimated the transfer probability of ARP for a closed system and showed that this probability improved with higher drive amplitude. Recently, we found that, in open quantum systems, applying a strong drive can give rise to significant drive-induced dissipation (DID). Here, we investigate the effect of DID on the performance of ARP that is implemented using a linearly chirped pulse on a two-level system. From the Landau-Zener formula, the population transfer was known to be enhanced with increasing drive amplitude. However, here we show that, beyond a threshold value of the drive amplitude, the transfer probability is reduced because of the detrimental effect of DID. We show that the competition between the two processes results in an optimal behavior of the population transfer. We also propose a phenomenological model that helps explain such nonmonotonic behavior of the transfer. Using this model, we estimate the optimum time at which the maximum population transfer occurs. We extend the analysis for rectangular as well as Gaussian pulse profiles and conclude that a Gaussian pulse outperforms a rectangular pulse.