We present a theoretical description of the Voigt and Faraday effect based optically pumped magnetometers using the Floquet expansion. Our analysis describes the spin-operator dynamics of the first- $\widehat{F}\left(t\right)$ and second-order ${\widehat{F}}^2\left(t\right)$ moments and takes into account different pumping profiles and decoherence effects. We find that the theoretical results are consistent with previous experimental demonstrations over a wide range of fields and pumping conditions. Finally, the theoretical analysis presented here is generalized and can be extended to different magnetometry schemes with arbitrary pumping profiles and multiple radio-frequency fields.

We propose and theoretically demonstrate a scheme to generate elliptically polarized attosecond pulses with mixed gases. As the harmonic radiations from different gases with opposite orbital parity interfere destructively for one helicity component and constructively for the other, the high-order harmonics from the mixture exhibit large ellipticity with the same helicity in a wide spectral range. Hence, highly elliptically polarized attosecond pulses can be generated from the mixed gases and the ellipticity can be tuned by controlling the mixing ratio and the molecular alignment angle. Furthermore, this interference effect is only related to the features of the mixed gases such as the structure and orbital symmetry of the molecules and atoms. The polarization control is independent of the temporal profile manipulation of the attosecond pulses by shaping the driving field. Consequently, our work paves an effective and convenient way to produce elliptically polarized isolated attosecond pulses with mixed gases by optimizing the polarization and temporal profile individually.

For two-electron diatomic molecules, we investigate magnetic field effects in nonsequential double ionization where recollisions prevail. We do so by formulating a three-dimensional semiclassical model that fully accounts for the Coulomb singularities and for magnetic field effects during time propagation. Using this model, we identify a prominent signature of nondipole effects. Namely, we demonstrate that the recolliding electron backscatters along the direction of light propagation. Hence, this electron escapes opposite to the direction of change in momentum due to the magnetic field. This is in striking contrast to strongly driven atoms where the recolliding electron forward scatters along the direction of light propagation. We attribute these distinct signatures to the different gate that the magnetic field creates jointly with a soft recollision in molecules compared to a hard recollision in atoms. These two different gates give rise, shortly before recollision, to different momenta and positions of the recolliding electron along the direction of light propagation. As a result, we show that the Coulomb forces from the nuclei act to backscatter the recolliding electron in molecules and forward scatter it in atoms along the direction of light propagation.

We theoretically study the propagation of an interacting Bose-Einstein condensate in a two-dimensional disorder potential, following the principle of an atom laser. The constructive interference between time-reversed scattering paths gives rise to coherent backscattering, which may be observed under the form of a sharp cone in the disorder-averaged angular backscattered current. As is found by the numerical integration of the Gross-Pitaevskii equation, this coherent backscattering cone is inversed when a nonvanishing interaction strength is present, indicating a crossover from constructive to destructive interferences. Numerical simulations based on the truncated Wigner method allow one to go beyond the mean-field approach and show that dephasing renders this signature of antilocalization hidden behind a structureless and dominant incoherent contribution as the interaction strength is increased and the injected density decreased, in a regime of parameters far away from the mean-field limit. However, despite a partial dephasing, we observe that this weak antilocalization scenario prevails for finite interaction strengths, opening the way for an experimental observation with ${}^{87}\mathrm{Rb}$ atoms.

We investigate the dimensional crossover from three to two dimensions in an ultracold Fermi gas across the whole BCS-BEC crossover. Of particular interest is the strongly interacting regime as strong correlations and pair fluctuations are more pronounced in reduced dimensions. Our results are obtained from first principles within the framework of the functional renormalization group (FRG). Here, the confinement of the transverse direction is imposed by means of periodic boundary conditions. We calculate the equation of state, the gap parameter at zero temperature, and the superfluid transition temperature across a wide range of transversal confinement length scales. Particular emphasis is put on the determination of the finite-temperature phase diagram for different confinement length scales. In the end, our results are compared with recent experimental observations and we discuss them in the context of other theoretical works.

We compute exactly the average spatial density for $N$ spinless noninteracting fermions in a $2d$ harmonic trap rotating with a constant frequency $\mathrm{\Omega }$ in the presence of an additional repulsive central potential $\gamma /r^2$. We find that in the large-$N$ limit, the bulk density has a rich and nontrivial profile—with a hole at the center of the trap and surrounded by a multilayered “wedding cake” structure. The number of layers depends on $N$ and on the two parameters $\mathrm{\Omega }$ and $\gamma$ leading to a rich phase diagram. Zooming in on the edge of the $k\mathrm{th}$ layer, we find that the edge density profile exhibits $k$ kinks located at the zeros of the $k\mathrm{th}$ Hermite polynomial. Interestingly, in the large-$k$ limit, we show that the edge density profile approaches a limiting form, which resembles the shape of a propagating front, found in the unitary evolution of certain quantum spin chains. We also study how a newly formed droplet grows in size on top of the last layer as one changes the parameters.

Recent exploration in synthetic frequency lattices has re-formed the methods of manipulating the light spectrum by analogizing the diffraction management in real space, which can be created by employing electro-optic modulation (EOM) in dielectric waveguides or ring resonators. In the presence of effective gauge potential, that is, the accumulated phase of mode transfer between adjacent lattice sites, the frequency spectrum of incident light can be flexibly tailored. However, the finite bandwidth and modulation depth of EOM vastly restrain the frequency shift and efficiency of mode transfer. Here we experimentally demonstrate that the synthetic frequency lattice can be created with the nonlinear process of four-wave-mixing Bragg scattering. The bandwidth of frequency manipulation is expanded up to terahertz and the frequency shift can be larger than 200 GHz. Furthermore, we can realize effects such as negative refraction and perfect imaging in frequency dimension by changing the effective gauge potentials. The study provides a powerful and promising approach to precisely control light frequency in broadband and may benefit all-optical modulation in optical telecommunication systems.

Optical spin-Hall effect (SHE) exhibits many intriguing features as a linearly polarized (LP) light beam strikes an interface at incident angles around the Brewster angle, but the underlying physics remains obscure. Here, we elucidate the physics through reanalyzing this problem employing rigorous calculations and the Berry phase concept. As a circularly polarized (CP) light beam strikes an optical interface, the reflected light beam contains two components, a spin-flipped abnormal mode acquiring geometric phases (thus exhibiting a spin-Hall shift) and a spin-maintained normal mode without such phases. Strengths of these two modes are determined by the incident angle and the optical properties of the interface. Under the LP incidence, however, a spin component inside the reflected light beam must be the sum of normal and abnormal components of reflected light beams corresponding to CP incidences with different helicity, which thus sensitively depends on the incident angle. In particular, at incident angles near the Brewster one, reflection coefficients for two CP components exhibit opposite signs, leading to significant destructive interferences between normal and abnormal modes, finally generating highly deformed reflected light patterns with anomalously enhanced spin-Hall shifts. These findings can be extended to both reflected and transmitted cases with Brewster-like behaviors. Our analyses reinterpret previously discovered effects, providing an alternative understanding on the SHE of light.

We present a complete theory for laser cooling of a macroscopic radio-frequency $\mathit{LC}$ electrical circuit by means of an optoelectromechanical system, consisting of an optical cavity dispersively coupled to a nanomechanical oscillator, which is in turn capacitively coupled to the $\mathit{LC}$ circuit of interest. The driven optical cavity cools the mechanical resonator, which in turn sympathetically cools the $\mathit{LC}$ circuit. We determine the optimal parameter regime where the $\mathit{LC}$ resonator can be cooled down to its quantum ground state, which requires a large optomechanical cooperativity, and a larger electromechanical cooperativity. Moreover, comparable optomechanical and electromechanical coupling rates are preferable for reaching the quantum ground state.

This paper presents a theoretical investigation of the phase-sensitive modulation instability of a continuous or quasicontinuous probe beam in an asymmetric coupled quantum well (ACQW) system facilitated by electromagnetically induced transparency. The dispersive properties of the probe beam can be controlled not only by the Rabi frequency of the control field but by the relative phase between them as well. The ACQW system exhibits large Kerr and quintic nonlinearities which could be controlled by controlling the relative phase of the probe beam. The probe beam is modulation unstable, and the gain and the bandwidth of unstable frequencies could be controlled by the phase of the probe beam. The gain of the instability disappears at certain values of phase and at certain other values of the phase the gain of the instability, and the bandwidth of unstable frequencies could be made large. Similarly, at certain values of the probe phase, the interplay of fourth-order dispersion and nonlinearity leads to the creation of discrete sidebands of the modulation instability. Both the fourth-order dispersion and quintic nonlinearity considerably reduces the growth and bandwidth of unstable frequencies.

We consider the cooperative spontaneous emission of a system of two identical atoms interacting with the electromagnetic field in the vacuum state and in the presence of an oscillating mirror. We assume that the two atoms, one in the ground state and the other in the excited state, are prepared in a correlated (symmetric or antisymmetric) Bell-type state. We also suppose that the perfectly reflecting plate oscillates adiabatically, with the field modes satisfying the boundary conditions at the mirror surface at any given instant, so that the time dependence of the interaction Hamiltonian is entirely enclosed in the instantaneous atom-wall distance. Using time-dependent perturbation theory, we investigate the spectrum of the radiation emitted by the two-atom system, showing how the oscillation of the boundary modifies the features of the emitted spectrum, which exhibits two lateral peaks not present in the case of a static boundary. We also evaluate the transition rate to the collective ground state of the two-atom system in both cases of the superradiant (symmetric) and subradiant (antisymmetric) state. We show that it is modulated in time and that the presence of the oscillating mirror can enhance or inhibit the decay rate compared to the case of atoms in vacuum space or near a static boundary. Our results thus suggest that a dynamical (i.e., time-modulated) environment can offer further possibilities to control and manipulate radiative processes of atoms or molecules nearby, such as the cooperative decay, and strongly indicate a similar possibility for other radiative processes, for example, the resonance interaction and the energy transfer between atoms or molecules.

We present a simple and efficient method for computing single and double ionization cross sections in ion-atom and ion-molecule collisions using $L^2$ Gaussian basis sets. Gaussian functions are widely employed to compute bound states of ions, atoms, and molecules. However, the description of continuum states, and therefore ionization phenomena, remains a theoretical challenge. Our approach is tested on the benchmark system antiproton-helium collisions in the so-called intermediate energy range. A good agreement with numerically exact calculations is observed. The proposed method is general and can thus be employed in any collisional systems in the challenging nonperturbative regime. Our work opens the way to investigate multiple ionization processes by ion impact in multicenter polyelectronic systems.

The precise-timing community is currently working on the task of synchronizing moving clocks in the field to an accuracy of femtoseconds. We argue that its recent claims of partial success must be examined in the context of relativity, and show that one must establish the context very carefully when claiming any sort of synchronization at this level. In particular, irrespective of the sense of the synchronization, we show that realistic movements of the clocks will destroy the synchronization moments after it is established.

Quantum entanglement shared by remote network nodes serves as a valuable resource for promising applications in distributed computing, cryptography, and sensing. However, distributing high-quality entanglement via fiber-optic routes could be challenging due to the various decoherence mechanisms in fibers. In particular, one of the primary polarization decoherence mechanisms in optical fibers is polarization mode dispersion (PMD), which is the distortion of optical pulses by randomly varying birefringences. To mitigate the effect of decoherence in entangled particles, quantum entanglement distillation (QED) algorithms have been proposed. One particular class, the recurrence QED algorithms, stands out because it has relatively relaxed requirements both on the size of the quantum circuits involved and on the initial quality of entanglement between particles. However, because the number of required particles grows exponentially with the number of distillation rounds, an efficient recurrence algorithm needs to converge quickly. We present a recurrence QED algorithm designed for photonic qubit pairs affected by PMD-degraded channels. Our proposed algorithm achieves the optimal fidelity as well as the optimal success probability (conditioned on the fact that optimal fidelity is achieved) in every round of distillation. The attainment of the maximal fidelity improves the convergence speed of fidelity with respect to the number of distillation rounds from linear to quadratic and, hence, significantly reduces the number of rounds. Combined with the fact that the optimal success probability is achieved, the proposed algorithm provides an efficient method to distribute entangled states with a high fidelity via optical fibers.

A simple expression to compute the quantum discord between two orbitals in fermion systems is derived using the parity superselection rule. As the correlation between orbitals depends on the basis chosen, we discuss a special orbital basis, the natural one. We show that quantum correlations between natural orbital pairs disappear when the pairing tensor is zero, i.e., the particle number symmetry is preserved. The Hartree-Fock orbitals within a Slater determinant state, Hartree-Fock-Bogoliubov quasiparticle orbitals in a quasiparticle vacuum, or the ground state of a Hamiltonian with particle symmetry and their corresponding natural orbitals are some relevant examples of natural bases and their corresponding states. Since natural orbitals have that special property, we seek the quantum discord in non-natural orbital bases. We analyze our findings in the context of the Lipkin-Meshkov-Glick and Agassi models.

A chip-based continous-variable quantum-key-distribution (CVQKD) system with a high practical confidentiality performance is crucial for constructing quantum metropolitan communication networks, but imperfections in the chip-based modulation will threaten the practical security of the chip-based CVQKD system. In this paper, we combine the plasma dispersion effect of free carriers to model the carrier fluctuations and reveal the essential mechanism of carrier fluctuations' influence on the system. The simulations show that the chip-based CVQKD system may face potential loophole threats or its performance will dramatically decrease under different carrier fluctuations. Moreover, two preliminary defense strategies are proposed to completely solve the practical security problems commonly induced by modulators in general chip-based CVQKD systems. This work proposes a set of modeling and analysis methods for general chip-based CVQKD systems' modulators, which provides constructive methods to build the chip-based CVQKD system with more rigorous practical security.

The magnetic hyperfine-structure constants have been calculated for low-lying levels in a neutral gold atom and a goldlike ion of mercury taking into account the Bohr-Weisskopf (BW) effect. The BW effect is represented as a product of atomic and nuclear ($d_{\mathrm{nuc}}$) factors. We have calculated the atomic factors, which enable one to extract BW-correction values for far from stability gold nuclei from the experimental data. The possible uncertainty of our atomic calculations have been estimated by the comparison with the available experimental data. It has been shown that the standard single-particle approach in $d_{\mathrm{nuc}}$ calculation reasonably well describes experimental data for $11/2^{-}$ gold isomers and the $3/2^{+}$ ground state of ${}^{199}\mathrm{Au}$. At the same time, it fails to describe the hyperfine constant in ${}^{197}\mathrm{Au}$. This indicates the more pronounced configuration mixing in ${}^{197}\mathrm{Au}$ than in ${}^{199}\mathrm{Au}$.

The transmission rate of a low-energy ${\mathrm{Ar}}^{q+}$ ion beam through a macroscopic glass tube of large aspect ratio is simulated. Secondary electron (SE) emission, induced by ion impacts with the inner surface of the capillary, are taken explicitly into account by adding a SE source term to the charge dynamics equation. We find that the additional SE channel significantly alters the distribution of the deposited charge in the capillary wall. Compared to the case without the SE channel, the electric field generated by the self-organized charge patches is generally weaker, yielding drastically different transmission rates, especially for higher beam intensities. The effect of SEs on the patch formation and resulting transmission rate is found to be significant for SE yields as low as 1 SE per ion impact, in the case of ${\mathrm{Ar}}^{+}$ ions. We propose a numerical experiment that can be tested experimentally, potentially allowing us to conclude if the SE channel is indeed crucial for accurately simulating the guiding of an ion beam through insulating capillaries. In the long run, our simulations may provide theoretical support for measuring the SE yield of low-energy ions impacting insulating surfaces at grazing angles.

We numerically simulate the photoionization of a hydrogen molecular ion exposed to a monochromatic extreme ultraviolet laser field beyond the dipole approximation. In such a diatomic system, the laser field reaches the two nuclei at different instants, adding extra phase on the ionization events from two nuclei and resulting in the shift of the angular maxima in the photoelectron momentum distribution. In the limit of the infinite internuclear distance, the double-slit interference scenario can be adopted to explain the simulated photoelectron momentum distribution. However, when the internuclear distance is as short as 1 Å, the electron distributes more in the center of the molecule, and the photoelectron momentum distribution carries the single-slit diffraction character. The extracted birth time delay of photoemission from two nuclei is about a few hundred of zeptoseconds, which is similar to the experimental measurement in ${\mathrm{H}}_2$ [Grundmann et al., Science 370, 339 (2020)].

We give a detailed account of a stationary impurity in an ideal Bose-Einstein condensate, which we call the ideal Bose polaron, at both zero and nonzero temperatures and arbitrary strength of the impurity-boson coupling. The time evolution is solved exactly and it is found that, surprisingly, many of the features that have been predicted for the real BEC are already present in this simpler setting and can be understood analytically therein. We obtain explicit formulas for the time evolution of the condensate wave function at $T=0$ and of the one-particle density matrix at $T>0$. For negative scattering length, the system is found to thermalize even though the dynamics are perfectly coherent. The time evolution and thermal values of the Tan contact are derived and compared to a recent experiment. We find that contrary to the Fermi polaron, the contact is not bounded at unitarity as long as a condensate exists. An explicit formula for the dynamical overlap at $T=0$ allows us to compute the rf spectrum which can be understood in detail by relating it to the two-body problem of one boson and the impurity.