Numerical simulations of the rotational contribution of oxygen and nitrogen molecules to the Kerr refractive index change in air are performed for ultrashort laser pulses in the near- and mid-infrared wavelength region. The calculated molecular response is parametrized by means of a damped harmonic oscillator model that is easily tractable in numerical simulations of long-distance propagation of ultrashort laser pulses. Our simulations show that pulses from available mid-infrared laser systems are long enough to be dramatically affected by the molecular response of air during propagation.

The studies of multimagnon excitations will extend our understandings of quantum magnetism and strongly correlated matters. Here, by using the time-evolving block decimation algorithm, we investigate the Bloch oscillations of two-magnon excitations under a gradient magnetic field. Through analyzing the symmetry of our Hamiltonian, we derive a rigorous and general relation between ferromagnetic and antiferromagnetic systems. Under strong interactions, in addition to free-magnon Bloch oscillations, fractional bounded-magnon Bloch oscillations appear which can be understood by an effective single-particle model. To extract the frequencies of Bloch oscillations and determine the gradient of the magnetic field, we analyze the fidelity and the substandard deviation in both time and frequency domains. Our study not only explores the interaction-induced Bloch oscillations of multimagnon excitations, but also provides an alternative approach to determine the gradient of the magnetic field via ultracold atoms in optical lattices.

Chains of coupled two-level atoms behave as one-dimensional quantum spin systems, exhibiting free magnons and magnon bound states. While these excitations are well studied for closed systems, little consideration has been given to how they are altered by the presence of an environment. This will be especially important in systems that exhibit nonlocal dissipation, e.g., systems in which the magnons decay due to optical emission. In this paper, we consider free magnon excitations and two-magnon bound states in an XXZ chain with nonlocal dissipation. We prove that while the energy of the bound state can lie outside the two-magnon continuum of energies, the decay rate of the bound state has to always lie within the two-magnon continuum of decay rates. We then derive analytically the bound-state solutions for a system where both the XY interaction and nonlocal dissipation are nearest neighbor or next-nearest neighbor, finding that the inclusion of nonlocal dissipation allows more freedom in engineering the energy and decay rate dispersions for the bound states. Finally, we numerically study a model of an experimental setup that should allow the realization of dissipative bound states by using Rydberg-dressed atoms coupled to a photonic crystal waveguide. We demonstrate that this model can exhibit many key features of our simpler models.

We calculate the transmission spectrum of a superconducting circuit realization of the Dicke model and identify spectroscopic features that can serve as signatures of the superradiant phase. In particular, we calculate the resonance frequencies of the system as functions of the bias term, which is usually absent in studies on the Dicke model but is commonly present in superconducting qubit circuits. To avoid over-complicating the proposed circuit, we assume a fixed coupling strength. This situation precludes the possibility of observing signatures of the phase transition by varying the coupling strength across the critical point. We show that the spectrum obtained by varying the bias point under fixed coupling strength can contain signatures of the normal and superradiant phases: in the normal phase one expects to observe two spectral lines, while in the superradiant phase four spectral lines are expected to exist close to the qubits' symmetry point. Provided that parameter fluctuations and decoherence rates are sufficiently small, the four spectral lines should be observable and can serve as a signature of the superradiant phase.

We address quantum state engineering of single- and two-mode states by means of nondeterministic noiseless linear amplifiers (NLAs) acting on Gaussian states. In particular, we consider the optimal implementation of a NLA proposed by Pandey et al. [Phys. Rev. A 88, 033852 (2013)] and we show that it provides an effective scheme to generate highly non-Gaussian and nonclassical states, when used outside the regime of high-fidelity amplification. Additionally, we show that in this regime also the amplification of a two-mode squeezed vacuum state (twin-beam) highly increases entanglement.

The non-Markovian dynamics of a charged harmonic oscillator linearly coupled to a neutral bosonic heat bath is investigated in external uniform magnetic field. The analytical expression is derived for the asymptotic angular momentum. The orbital diamagnetism of quantum system in a dissipative environment is studied.

In a recent series of works [Ebler et al., Phys. Rev. Lett. 120, 120502 (2018); arXiv:1809.06655v2; arXiv:1810.10457v2], it has been shown that the quantum superposition of causal order—the quantum switch—offers an enhancement of classical and quantum channel capacity through noisy channels, a phenomenon that was coined “causal activation.” In this paper, we attempt to clarify the nature of the advantage, by comparing the quantum switch to a class of processes that can be interpreted as quantum superposition of processes with the same causal order. We show that some of these processes can match or even outperform the quantum switch at enhancing classical and quantum channel capacity, and argue that they require the same resources as the switch. We conclude, in agreement with Abbott et al. [arXiv:1810.09826v1], that the aforementioned advantages appear to be attributable to the ability to coherently control quantum operations, and not to indefinite causal order per se.

It is well known that the notions of spatial locality are often lost in quantum systems with long-range interactions, as exhibited by the emergence of phases with exotic long-range order and faster propagation of quantum correlations. We demonstrate here that such induced “quasinonlocal” effects do not necessarily translate to growth of global entanglement in the quantum system. By investigating the ground and quenched states of the variable-range, spin-1/2 Heisenberg Hamiltonian, we observe that the genuine multiparty entanglement in the system can either increase or counterintuitively diminish with a growing range of interactions. The behavior is reflective of the underlying phase structure of the quantum system and provides key insights for generation of multipartite entanglement in experimental atomic, molecular, and optical physics where such variable-range interactions have been implemented.

Quantum memories are an important building block for quantum information processing. Ideally, these memories preserve the quantum properties of the input. We present general criteria for measures to evaluate the quality of quantum memories. Then we introduce a quality measure based on coherence satisfying these criteria, which we characterize in detail for the qubit case. The measure can be estimated from sparse experimental data and may be generalized to characterize other building blocks, such as quantum gates and teleportation schemes.

Suppressing undesired nonunitary effects is a major challenge in quantum computation and quantum control. In this paper, by considering the adiabatic dynamics in the presence of a surrounding environment, we theoretically and experimentally analyze the robustness of adiabaticity in open quantum systems. More specifically, by considering a decohering scenario, we exploit the validity conditions of the adiabatic approximation as well as its sensitiveness to the resonance situation, which typically harm adiabaticity in closed systems. As an illustration, we implement an oscillating Landau-Zener Hamiltonian, which shows that decoherence may drive the resonant system with high fidelities to the adiabatic behavior of open systems. Moreover we also implement the adiabatic quantum algorithm for the Deutsch problem, where a distinction is established between the open system adiabatic density operator and the target pure state expected in the computation process. Preferred time windows for obtaining the desired outcomes are then analyzed. We experimentally realize these systems through a single trapped ytterbium ion ${}^{171}\mathrm{Yb}^{+}$, where the ion hyperfine energy levels are used as degrees of freedom of a two-level system, with both driven field and decohering strength efficiently controllable.

Atomic lanthanum anion ${\mathrm{La}}^{-}$ is the best candidate for laser cooling of any atomic anions found so far. The bound-bound electric dipole transitions of ${\mathrm{La}}^{-}$ have been identified and measured. Theoretical studies have shown that ${\mathrm{La}}^{-}$ has a very complicated electronic structure, which cannot be well resolved by the traditional photoelectron spectroscopy. In the present work, we report high-resolution photoelectron spectroscopy of ${\mathrm{La}}^{-}$ via the slow-electron velocity-map imaging method in combination with an ion trap. The electron affinity of La was determined to be $4496.97\left(20\right){\mathrm{cm}}^{-1}$ or 0.557 553(25) eV. In addition, the energy levels of ${\mathrm{La}}^{-}$, ${}^{3}F_3^e$, ${}^{3}F_4^e$, ${}^{1}D_2^e$, ${}^{1}D_2^o$, ${}^{3}P_0^e$, ${}^{3}P_1^e$, and ${}^{3}P_2^e$, were also determined.

Sub-Doppler spectroscopy in alkali-metal vapor cells using two counterpropagating dual-frequency laser beams allows the detection of high-contrast sign-reversed natural-linewidth sub-Doppler resonances. Previously, a qualitative theory based on a simplified Λ-scheme model has been reported to explain underlying physics of this phenomenon. In this paper, an extended theoretical model of dual-frequency sub-Doppler spectroscopy (DFSDS) for the Cs $D_1$ line is reported. Taking into account the real atomic energy structure, main relaxation processes, and various nonlinear effects, this model describes quantitatively the respective contributions of involved physical processes and predicts main properties (height and linewidth) of the sub-Doppler resonances. Experimental tests are performed with a Cs vapor microfabricated cell and results are found to be in correct agreement with theoretical predictions. Spatial oscillations of the sub-Doppler resonance amplitude with translation of the reflection mirror are highlighted. A beat note between two laser systems, including one stabilized with DFSDS on a Cs vapor microcell, yields a fractional frequency stability of $2\times {10}^{-12}{\tau }^{-1/2}$ until 10-s averaging time. These results demonstrate that DFSDS could be an interesting approach for the development of a high-performance microcell-based optical frequency reference, with applications in various compact quantum devices.

We predict the existence of a torque acting on an isotropic neutral nanosphere activated by a static magnetic field when the particle temperature differs from the surrounding vacuum. This phenomenon originates in time-reversal symmetry breaking of the particle interaction with the vacuum electromagnetic field. We present a rigorous quantum treatment of photons and particle excitations that leads to a nonzero torque even in a motionless particle. We also find that the dynamical evolution of the particle temperature and rotation frequency follow an exotic dynamics, including spontaneous changes in the rotation direction. Magnetically activated thermal vacuum torques open a unique avenue for the investigation of the effect of time-reversal symmetry breaking in thermal and Casimir physics.

We discuss the rotational cooling of diatomic molecules in a Bose-Einstein condensate (BEC) of ultracold atoms by emission of phonons with orbital angular momentum. Despite the superfluidity of the BEC there is no frictionless rotation for typical molecules since the dominant cooling occurs via emission of particle-like phonons. Only for macrodimers, whose size becomes comparable to or larger than the condensate healing length, a Landau-like, critical angular momentum exists below which phonon emission is suppressed. We find that the rotational relaxation of typical molecules is, in general, faster than the cooling of the linear motion of impurities in a BEC. This also leads to a finite lifetime of angulons, quasiparticles of rotating molecules coupled to phonons with orbital angular-momentum. We analyze the dynamics of rotational cooling for homonuclear diatomic molecules based on a quantum Boltzmann equation including single- and two-phonon scattering and discuss the effect of thermal phonons.

Alkali-metal-vapor magnetometers, using coherent precession of polarized atomic spins for magnetic-field measurement, have become one of the most sensitive magnetic-field detectors. Their application areas range from practical uses such as NMR signal detection to fundamental physics research such as searches for permanent electric dipole moments. One of the main noise sources of atomic magnetometers comes from the light shift that depends on the frequency of the pump laser. In this work, we theoretically study the light shift, taking into account the relaxation due to the optical pumping and the collision between alkali-metal atoms and between alkali-metal atoms and the buffer gas. Starting from a full master equation containing both the ground and excited states, we adiabatically eliminate the excited states and obtain an effective master equation in the ground-state subspace that shows an intuitive picture and dramatically accelerates the numerical simulation. Solving this effective master equation, we find that in the light-narrowing regime, where the linewidth is reduced while the coherent precession signal is enhanced, the frequency dependence of the light shift is largely reduced, which agrees with experimental observations in cesium magnetometers. Since this effective master equation is general and is easily solved, it can be applied to an extensive parameter regime, and also to study other physical problems in alkali-metal-vapor magnetometers, such as heading errors.

A three-state system subjected to a time-dependent Hamiltonian whose bare energies undergo one or more crossings, depending on the relevant parameters, is considered, also taking into account the role of dissipation in the adiabatic following of the Hamiltonian eigenstates. Depending on whether or not the bare energies are equidistant, the relevant population transfer turns out to be very sensitive to the environmental interaction or relatively robust. The physical mechanisms on the basis of this behavior are discussed in detail.

Wigner time delay in photodetachment from the $3p_{3/2}$ and $3p_{1/2}$ subshells of ${\mathrm{Cl}}^{-}$ has been studied in the vicinity of the $2p_{3/2}$ and $2p_{1/2}$ thresholds, using the relativistic random phase approximation (RRPA). The results show time delay spectra dominated by many-body correlations along with very complicated dependence on the energy over a broad spectral range. In addition, the time delay spectra of the two spin-orbit split $3p$ subshells differ significantly from one another, thereby revealing the importance of relativistic effects even in the case of a low-$Z$ system.

We report on sawtooth-wave adiabatic-passage (SWAP) slowing of bosonic and fermionic dysprosium isotopes by using a 136-kHz-wide transition at 626 nm. A beam of precooled atoms is further decelerated in one dimension by the SWAP force and the amount of atoms at near zero velocity is measured. We demonstrate that the SWAP slowing can be twice as fast as in a conventional optical molasses operated on the same transition. In addition, we investigate the parameter range for which the SWAP force is efficiently usable in our setup and relate the results to the adiabaticity condition. Furthermore, we add losses to the hyperfine ground-state population of fermionic dysprosium during deceleration and observe more robust slowing with SWAP compared to slowing with the radiation pressure force.

Manifestations of and possibilities related to vortex electrons in strong-field physics are discussed. We present a theory which extends the foundation of a powerful method of target structure and dynamics imaging to vortex electrons. The theory enables one to extract the differential cross section (DCS) for elastic scattering of a vortex electron on the parent ion—a collision property introduced here—from the observable photoelectron momentum distribution (PEMD). We illustrate this by considering strong-field ionization from $\pi$ orbitals in two atoms, Xe and ${\mathrm{He}}^{+}$, and a molecule, ${\mathrm{O}}_2$. The vortex DCS is shown to be sensitive to the target structure. The PEMDs formed by vortex electrons are predicted to be sensitive to the chirality of the target. Extracting vortex DCSs from experimental PEMDs may open a new avenue for rescattering photoelectron spectroscopy.

We report on measurements of the binding energies of several weakly bound vibrational states of the paramagnetic ${}^{174}\mathrm{Yb}{}^6\mathrm{Li}$ molecule in the electronic ground state using two-photon spectroscopy in an ultracold atomic mixture confined in an optical dipole trap. We theoretically analyze the experimental spectrum to obtain an accurate description of the long-range potential of the ground-state molecule. Based on the measured binding energies, we arrive at an improved value of the interspecies $s$-wave scattering length $a_{s0}=30a_0$. Employing coherent two-photon spectroscopy we also observe the creation of “dark” atom-molecule superposition states in the heteronuclear Yb-Li system. This paper is an important step towards the efficient production of ultracold YbLi molecules via association from an ultracold atomic mixture.