Hydrodynamics provides a concise but powerful description of long-time and long-distance physics of correlated systems out of thermodynamic equilibrium. Here we construct hydrodynamic equations for nonrelativistic particles with a spacetime-dependent scattering length and show that it enters constitutive relations uniquely so as to represent the fluid expansion and contraction in both normal and superfluid phases. As a consequence, we find that a leading dissipative correction to the contact density due to the spacetime-dependent scattering length is proportional to the bulk viscosity (${\zeta }_2$ in the superfluid phase). Also, when the scattering length is slowly varied over time in a uniform system, the entropy density is found to be produced even without fluid flows in proportion to the bulk viscosity, which may be useful as a novel probe to measure the bulk viscosity in ultracold-atom experiments.

We report on the design, fabrication, and experimental characterization of reflective geometric phase optical elements enabling the generation of scalar and vectorial high-order Laguerre-Gauss optical beams over a broad spectral range. This is made possible by combining azimuthal and radial structuring of helix-based liquid-crystal films exhibiting the circular Bragg reflection phenomenon. By extending the previously introduced concept of Bragg-Berry optical elements to the polychromatic shaping of the radial degrees of freedom and showing their combination with azimuthal ones, this work suggests that spectrally broadband spin-orbit processing of optical information over multiple spatial degrees of freedom can be further considered on experimental grounds.

Photodetection plays a key role in basic science and technology, with exquisite performance having been achieved down to the single-photon level. Further improvements in photodetectors would open new possibilities across a broad range of scientific disciplines and enable new types of applications. However, it is still unclear what is possible in terms of ultimate performance and what properties are needed for a photodetector to achieve such performance. Here, we present a general modeling framework for photodetectors whereby the photon field, the absorption process, and the amplification process are all treated as one coupled quantum system. The formalism naturally handles field states with single or multiple photons as well as a variety of detector configurations and includes a mathematical definition of ideal photodetector performance. The framework reveals how specific photodetector architectures introduce limitations and tradeoffs for various performance metrics, providing guidance for optimization and design.

We investigate the rephasing efficiency of sequences of phased pulses for spin echoes and light storage by electromagnetically induced transparency (EIT). We derive a simple theoretical model and show that the rephasing efficiency is very sensitive to the phases of the imperfect rephasing pulses. The obtained efficiency differs substantially for spin echoes and EIT light storage, which is due to the spatially retarded coherence phases after EIT light storage. Similar behavior is also expected for other light-storage protocols with spatial retardation or for rephasing of collective quantum states with an unknown or undefined phase, e.g., as relevant in single-photon storage. We confirm the predictions of our theoretical model by experiments in a ${\mathrm{Pr}}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal.

A new class of laser, which harnesses coherence in both light and atoms, is possible with ultracold alkaline-earth-metal atoms trapped in an optical lattice inside an optical cavity. Different lasing regimes, including superradiance, superradiant, and conventional lasing, are distinguished by the relative coherence stored in the atoms and in the cavity mode. We analyze the physics in two different experimentally achievable regions of the superradiant lasing regime. Our calculations confirm the narrow linewidth of superradiant lasing for the doubly forbidden clock transition ${}^3{P}_0\to {}^1{S}_0$ of ${}^{87}\mathrm{Sr}$ atoms. Under strong driving of the dipole-forbidden transition ${}^3{P}_1\to {}^1{S}_0$ of ${}^{88}\mathrm{Sr}$ atoms, the superradiant linewidth narrows further due to the coherent excitation of the cavity field.

Mirrorless optical parametric oscillator (MOPO) is a consequence of intrinsic feedback provided by the nonlinearity in a medium due to the interaction of a pair of strong counterpropagating fields. As the name suggests, the device does not require a cavity for lasing other than the nonlinear medium. Here we report the demonstration of MOPO under the effect of an all-optical waveguide. The efficient four-wave mixing process due to counterpropagating pump and control fields interacting with a multilevel atomic system facilitates the generation of mirrorless Stokes and anti-Stokes fields counterpropagating to each other. The maximum generated laser power could rise up to mW with pump conversion efficiency more than 30%. Furthermore, the cross-phase modulation due to the strong fields creates all-optical waveguides for the generated fields and hence induces correlated spatial modes in the Stokes and the anti-Stokes fields.

We consider a bound state of two discrete solitons in a two-dimensional waveguide array. Using numerical and variational calculations, we investigate the effect of binding on the mobility of the two solitons, which we found to be marginal. Considering anisotropic waveguides, where coupling in one direction is stronger than in the other, we show that mobility is enhanced considerably along the weaker-coupling direction. We show also that a stable bound state of two solitons exists in such a setup where each one of the two solitons is located at a different waveguide. The stability of the resulting soliton molecule is provided by the Peierls-Nabarro potential and the mobility of the individual solitons is facilitated by the anisotropy. Considering a combination of two out-of-phase solitons, we find that they form a metastable state of a single soliton that suddenly splits into two solitons propagating away from each other.

We theoretically investigate the enhanced charge-dependent generation of the optical second-order sidebands (OSS) in a gain-cavity-assisted optomechanical (GCAOM) system coupled to a charged object. The hybrid optomechanical system is coherently driven by an external two-tone laser field which consists of a continuous-wave pump field and a pulsed probe field. Beyond the conventional linearized description of optomechanical interactions, the nonlinear optomechanical interactions are included in the Heisenberg-Langevin equations and are treated analytically by means of the perturbation method. It is shown that an assisted gain cavity can significantly enhance the OSS generation and can also lead to higher charge dependence of the output OSS spectrum than that achieved from a lossy cavity optomechanical system. Subsequently we discuss the application of such a GCAOM system as a family of high-sensitivity sensor for measuring the charges. Using experimentally achievable parameters, we identify the conditions under which the assisted gain cavity allows us to enhance the OSS generation and improve sensitivity of the sensor beyond what is achievable in a lossy cavity optomechanical system. The present investigation may provide a route toward modulating the nonlinear optical properties of the electro-optic hybrid system, as well as to guide the design of sensitive devices.

We consider an array of dual-core waveguides, which represent an optical realization of a chain of dimers, with an active (gain-loss) coupling between the cores, opposite signs of discrete diffraction in the parallel arrays, and a phase-velocity mismatch between them (which is necessary for the stability of the system). The array provides an optical emulation of the charge-parity ($\mathcal{CP}$) symmetry. The addition of the intracore cubic nonlinearity gives rise to several species of fundamental discrete solitons, which exist in continuous families, although the system is non-Hermitian. The existence and stability of the soliton families are explored by means of analytical and numerical methods. An asymptotic analysis is presented for the case of weak intersite coupling (i.e., near the anticontinuum limit), as well as weak coupling between cores in each dimer. Several families of fundamental discrete solitons are found in the semi-infinite gap of the system's spectrum, which have no counterparts in the continuum limit, as well as a branch which belongs to the finite band gap and carries over into a family of stable gap solitons in that limit. One branch develops an oscillatory instability above a critical strength of the intersite coupling, others being stable in their entire existence regions. Unlike solitons in conservative lattices, which are controlled solely by the strength of the intersite coupling, here fundamental-soliton families have several control parameters, one of which, viz., the coefficient of the intercore coupling in the active host medium, may be readily adjusted in the experiment by varying the gain applied to the medium.

We theoretically and experimentally investigate the optimal conditions for the Bell experiment using spontaneous parametric down-conversion (SPDC) sources. In theory, we show that a relatively large average photon number (typically $\sim 0.5$) is desirable to observe the maximum violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality. Moreover, we show that the violation converges to 0.27 in the limit of large average photon number. In experiment, we perform the Bell experiment without postselection using polarization-entangled photon pairs at 1550-nm telecommunication wavelength generated from SPDC sources. While the violation of the CHSH inequality is not directly observed due to the overall detection efficiencies of our system, the experimental values agree well with those obtained by the theory with experimental imperfections. Furthermore, in the range of small average photon numbers ($\le 0.1$), we propose and demonstrate a method to estimate the ideal CHSH value intrinsically contained in the tested state from the lossy experimental data without assuming the input quantum state.

Schemes for creation of $N$-particle entangled Greenberger-Horne-Zeilinger (GHZ) states are important for understanding multiparticle nonclassical correlations. Here, a theoretical scheme for creation of a multiparticle GHZ state implemented on a target ensemble of N, $\mathrm{\Lambda }$ three-level Rydberg atoms and a single Rydberg atom as a control using stimulated Raman adiabatic passage (STIRAP) is presented. We work in the Rydberg blockade regime for the ensemble atoms induced due to excitation of the control atom to a high-lying Rydberg level. It is shown that using STIRAP, atoms from one ground state of the ensemble can be adiabatically transferred with high fidelity to the other ground state, depending on the state of the control atom. Measurement of the control atom in a specific basis after this conditional transfer facilitates one-step creation of an $N$-particle GHZ state. A thorough analysis of adiabatic conditions for this scheme and the influence of radiative decay from the excited Rydberg levels is presented. We show that this scheme is immune to the decay rate of the excited level in ensemble atoms and provides a robust way of creating GHZ states.

The ability to entangle quantum systems is crucial for many applications in quantum technology, including quantum communication and quantum computing. Here, we propose a simple and versatile setup for deterministically creating Bell and Greenberger-Horne-Zeilinger states between photons of different frequencies in a two-step protocol. The setup consists of a quantum bit (qubit) coupled ultrastrongly to three photonic resonator modes. The only operations needed in our protocol are to put the qubit in a superposition state and then tune its frequency in and out of resonance with sums of the resonator-mode frequencies. By choosing which frequency we tune the qubit to, we select which entangled state we create. We show that our protocol can be implemented with high fidelity using feasible experimental parameters in state-of-the-art circuit quantum electrodynamics. One possible application of our setup is as a node distributing entanglement in a quantum network.

In multipole-bound anions, the excess electron is attached by a short-range multipole potential of a neutral molecule. Such anions are prototypical marginally bound open quantum systems. In particular, around the critical multipole moment required to attach the valence electron, multipole-bound anions exhibit critical behavior associated with a transition from bound states dominated by low-$\ell$ partial waves to the electron continuum. In this work, multipole-bound anions are described using a nonadiabatic electron-plus-rotor model. The electron-molecule pseudopotential is represented by a short-range multipole field with a Gaussian form factor. The resulting coupled-channel Schrödinger equation is solved by means of the Berggren expansion method, in which the electron's wave function is decomposed into bound states, narrow resonances, and the nonresonant scattering continuum. We show that the Gaussian model predicts the critical transition at the detachment threshold. Resonant states, including bound states, decaying resonances, subthreshold resonances, and antibound states are studied, and exceptional points where two resonant states coalesce are predicted. We discuss the transition of rotational band structures around the threshold and study the effects of channel coupling on the decay width of resonant poles.

Properties of persistent spectral holes (SHs) relevant for frequency metrology have been investigated in the system ${\mathrm{Eu}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5(0.5\%)$ at crystallographic site 1 and a temperature of 1.2 K. Hole linewidths as small as 0.6 kHz have been reliably achieved. The theoretically predicted $T^4$ dependence of the frequency shift with temperature has been confirmed with high precision. The thermal hysteresis of the SH frequency between 1.15 and 4.1 K was measured to be less than $6\times {10}^{-3}$ fractionally. After initially burning a large ensemble of SHs, their properties were studied on long time scales by probing different subsets at different times. SHs could still be observed 49 days after burning if not interrogated in the meantime. During this time, the SH linewidth increased by 1.5 kHz, and the absorption contrast decreased from 35% to 15%. During a 14-day interval the absolute optical frequencies of previously unperturbed spectral holes were measured with respect to a Global-Positioning-System-monitored active H maser, using a femtosecond frequency comb. The fractional frequency drift rate exhibited an upper limit of $2.3\times {10}^{-19}{\mathrm{s}}^{-1}$, 65 times smaller than the most stringent previous limit.

We show how access to sufficiently flexible trapping potentials could be exploited in the generation of three-dimensional atomic bright matter-wave solitons. Our proposal provides a route towards producing bright solitonic states with good fidelity, in contrast to, for example, a nonadiabatic sweeping of an applied magnetic field through a Feshbach resonance.

In this paper, we have developed a method for describing the dynamics of an arbitrary quantum system under a bidirectional time-delayed feedback loop. For this purpose, we have described the evolution in terms of the time propagation of the quantum system of interest without feedback together with several identical systems, which represent the history of the quantum system under study. This technique provides a numerically efficient solution for describing a system's dynamics in the case of significant time delays in which direct investigation of the state of the reservoirs becomes numerically intractable. Using this method, we have studied two scenarios of multiple scatterings of photons incident on a cavity with a two-level atom positioned inside it, coupled to two waveguides that are connected at their ends. In the first scenario, two photons impinge on the cavity through separate waveguides with a delay between them. We have demonstrated that the maximum difference between the two output photon numbers occurs when the delay between the incident photons becomes close to the inverse of their linewidth. In the second scenario, multiple photons impinge on the cavity through the same waveguide and go through multiple interactions. We have shown that, for a fixed atom-cavity coupling rate, the transmission rate enhances as the number of photons increases and have quantified this enhancement. The developed method enables us to study a broad range of nonlinear dynamics in complex quantum networks.

Near-concentric optical cavities of spherical mirrors can provide technical advantages over the conventional near-planar cavities in applications requiring strong atom-light interaction, as they concentrate light in a very small region of space. However, such cavities barely support stable optical modes, and thus impose practical challenges. Here, we present an experiment where we maintain a near-concentric cavity at its last resonant length for laser light at 780 nm resonant with an atomic transition. At this point, the spacing of two spherical mirror surfaces is $207\left(13\right)\mathrm{nm}$ shorter than the critical concentric point, corresponding to a stability parameter $g=-0.999962\left(2\right)$ and a cavity beam waist of $2.4\mu \mathrm{m}$.

Quantum key distribution protocol requires true random numbers to prepare and measure quantum states, but the randomness may be weakened by applying the imperfect input random numbers or imperfect quantum devices. In this work, we propose the randomness attack model in the practical quantum key distribution system, and we analyze the final secret key rate under the randomness attack. The security analysis result demonstrates that the imperfect randomness can be amplified by considering the signal loss, thus the quantum key distribution system may be attacked even if the randomness has very small part of imperfection. Combining the practical experimental parameters with the small nonrandom probability parameters $0.1\%$ and $1\%$, we prove that the maximal transmission distance will be sharply reduced from 140 km to 60 and 12 km, respectively.

Most implementations of Coulomb-distorted strong-field approaches that contain features such as tunneling and quantum interference use real trajectories in continuum propagation, while a fully consistent approach must use complex trajectories throughout. A key difficulty in the latter case are singularities of the Coulomb potential in the complex time plane. These singularities have the form of branch points which generate corresponding branch cuts. We present a method for treating branch cuts in quantum-trajectory models, which is subsequently applied to photoelectron holography. Our method is not numerically intensive and is applicable to Coulomb-free and Coulomb-distorted trajectories. We show that the presence of branch cuts leads to discontinuities and caustics in the holographic fringes in above-threshold ionization (ATI) photoelectron momentum distributions. These artifacts are removed with our method, provided no hard recollision takes place during the interaction. A comparison with the full solution of the time-dependent Schrödinger equation is also performed, and a discussion of the applicability range of the present approach is provided.