Recent advances in levitated optomechanics provide new perspectives for the use of rotational degrees of freedom for the development of quantum technologies as well as for testing fundamental physics. As for the translational case, their use, especially in the quantum regime, is limited by environmental noises, the characterization of which is fundamental in order to assess, control, and minimize their effect, in particular decoherence. Here, we present a general perturbative approach to compute decoherence for a quantum system in a superposition of its rotational degrees of freedom. The specific cases of the dipole-dipole and quadrupole-quadrupole interactions are solved explicitly, and we show that the rotational degrees of freedom decohere on a time scale that can be longer than the translational one.

An impurity atom immersed in an ultracold atomic Fermi gas can form a quasiparticle, so-called Fermi polaron, due to impurity-fermion interaction. We consider a three-dimensional homogeneous dipolar Fermi gas as a medium, where the interatomic dipole-dipole interaction (DDI) makes the Fermi surface deformed into a spheroidal shape, and, using a Chevy-type variational method, investigate the ground-state properties of the Fermi polaron: the effective mass, the momentum distribution of a particle-hole excitation, the drag parameter, and the medium-density modification around the impurity. These quantities are shown to exhibit spatial anisotropies in such a way as to reflect the momentum anisotropy of the background dipolar Fermi gas. We also give numerical results for the polaron properties at the unitarity limit of the impurity-fermion interaction in the case in which the impurity and fermion masses are equal. It has been found that the transverse effective mass and the transverse momentum drag parameter of the polaron both tend to decrease by $\sim 10\%$ when the DDI strength is raised from 0 up to around its critical value, while the longitudinal ones exhibit a very weak dependence on the DDI.

A general analytical model is established to study the change in the signal-to-noise ratio (SNR) of the spatially noisy optical signal after passing through a subwavelength-aperture system. The corresponding spatial resolution limit is redefined based on Shannon's theory of information, and the expression is analytically given derived from nonparaxial vectoral diffraction theory. Contrary to the conventional wisdom, it is demonstrated both analytically and numerically that the SNR of the optical signal will increase with the propagation distance due to the presence of the subwavelength aperture and the corresponding spatial resolution could, thus, exceed the traditional Abbe-Rayleigh diffraction limit even though when the input SNR is low. The theoretical result provides perspectives for optimizing optical resolution and is general, that can be extended to other sciences, such as x-ray imaging and quantum imaging.

Leggett-Garg (LG) tests for macrorealism were originally designed to explore quantum coherence on the macroscopic scale. Interference experiments and systems modeled by harmonic oscillators provide useful examples of situations in which macroscopicity has been approached experimentally and are readily turned into LG tests for a single dichotomic variable $Q$. Applying this approach to the double-slit experiment in which a noninvasive measurement at the slits is included, we exhibit LG violations. We find that these violations are always accompanied by destructive interference. The converse is not true in general and we find that there are nontrivial regimes in which there is destructive interference but the two-time LG inequalities are satisfied, which implies that it is in fact often possible to assign (indirectly determined) probabilities for the interferometer paths. Similar features have been observed in recent work involving a LG analysis of a Mach-Zehnder interferometer and we compare with those results. We extend the analysis to the triple-slit experiment again finding LG violations, and we also exhibit examples of some surprising relationships between LG inequalities and NSIT conditions that do not exist for dichotomic variables. For the simple harmonic oscillator, we find an analytically tractable example showing a two-time LG violation with a gaussian initial state, echoing in simpler form recent results of Bose et al. [Phys. Rev. Lett. 120, 210402 (2018)].

Quantum computers can be used for supervised learning by treating parametrized quantum circuits as models that map data inputs to predictions. While a lot of work has been done to investigate the practical implications of this approach, many important theoretical properties of these models remain unknown. Here, we investigate how the strategy with which data are encoded into the model influences the expressive power of parametrized quantum circuits as function approximators. We show that one can naturally write a quantum model as a partial Fourier series in the data, where the accessible frequencies are determined by the nature of the data-encoding gates in the circuit. By repeating simple data-encoding gates multiple times, quantum models can access increasingly rich frequency spectra. We show that there exist quantum models which can realize all possible sets of Fourier coefficients, and therefore, if the accessible frequency spectrum is asymptotically rich enough, such models are universal function approximators.

In this paper we study the phase sensitivity of generic linear interferometric schemes using Gaussian resources and measurements. Our formalism is based on the Fisher information. This allows us to separate the contributions of the measurement scheme, the experimental imperfections, and auxiliary systems. We demonstrate the strength of this formalism using a broad class of multimode Gaussian states that includes well-known results from single- and two-mode metrology scenarios. Using this, we prove that input coherent states or squeezing improve upon the nonclassical states proposed in preceding boson-sampling-inspired phase-estimation schemes. We also develop a polychromatic interferometric protocol, demonstrating an enhanced sensitivity with respect to two-mode squeezed-vacuum states, for which the ideal homodyne detection is formally shown to be optimal.

We report the results of an ab initio study of resonant vibrational excitation of molecular oxygen by electron impact at low energies where the lowest-lying ${}^2{\mathrm{\Pi }}_g$ resonant state of ${\mathrm{O}}_2^{-}$ dominantly contributes to the cross sections. The contribution of this resonance to the dissociative electron attachment cross section is also determined and the origin of its unusual oscillations is discussed. Calculations were performed within the nonlocal resonance model describing the nuclear dynamics of ${\mathrm{O}}_2^{-}$ after electron capture into the resonant state. The model was constructed using potential-energy curves obtained with standard quantum-chemical methods and eigenphase sums from the fixed-nuclei $R$-matrix calculations of electron scattering off ${\mathrm{O}}_2$. The effect of the spin-orbit interaction is taken into account when determining the potential-energy curves of the molecular negative ion. The vibrational excitation cross sections are compared with other available theoretical and experimental results.

We investigate the generation of high-order harmonics with arbitrary ellipticity using synthesized two-color (800-nm + 400-nm) cross-linearly-polarized laser fields. It is shown that one can realize control over harmonic ellipticity by finely adjusting the relative phase and the crossing angle of such two-color fields. Through analyzing the time-frequency distributions and the quantum orbits of harmonic radiations, we show that the harmonic helicity is originated from the unsymmetrical contribution between different quantum orbits released in one 800-nm cycle. And, we further reveal that the controllability of harmonic ellipticity in such two-color fields is closely related to the modulation of crossing angle and relative phase on the driving light's spin angular momentum. Finally, we show the probability of producing and modulating isolated helical attosecond pulses with few-cycle two-color cross-linearly-polarized laser fields.

We carry out a combined experimental and numerical investigation on the population distribution between the ground state and the excited state of molecular nitrogen ions in an 800-nm laser field. Efficient population transfers between the ground state ${\mathrm{N}}_2^{+}(X{}^2{\mathrm{\Sigma }}_g^{+},\nu =0)$ and the excited state ${\mathrm{N}}_2^{+}(B{}^2{\mathrm{\Sigma }}_u^{+},{\nu }^{\prime }=0)$ occur with a 400-nm time-delayed pulse, resulting in an enhanced fluorescence at 391 and 428 nm. The population distribution between the ground state ${\mathrm{N}}_2^{+}(X{}^2{\mathrm{\Sigma }}_g^{+},\nu =0)$ and the excited state ${\mathrm{N}}_2^{+}(B{}^2{\mathrm{\Sigma }}_u^{+},{\nu }^{\prime }=0)$ is extracted from the fluorescence enhancement factor. The results show that the population of ${\mathrm{N}}_2^{+}(X{}^2{\mathrm{\Sigma }}_g^{+},\nu =0)$ decreases when the laser exceeds 2.1 mJ in our experiment. Our study provides a method to check the population information of both excited and ground states experimentally in a strong-field-induced plasma of ionized nitrogen molecules.

We study a minimal model that has a driven-dissipative quantum phase transition, namely, a Kerr nonlinear oscillator subject to driving and dissipation. Using mean-field theory, exact diagonalization, and the Keldysh formalism, we analyze the critical phenomena in this system, showing which aspects can be captured by each approach and how the approaches complement each other. Then critical scaling and finite-size scaling are calculated analytically using the quantum Langevin equation. The physics contained in this simple model is surprisingly rich: it includes a continuous phase transition, $Z_2$ symmetry breaking, $\mathcal{PT}$ symmetry, state squeezing, and critical fluctuations. Due to its simplicity and solvability, this model can serve as a paradigm for exploration of open quantum many-body physics.

Coherence is a cornerstone of quantum theory and a prerequisite for the advantage of quantum technologies. In recent work, the notion of coherence with respect to a general quantum measurement, i.e., positive operator-valued measure (POVM), was introduced and embedded into a resource-theoretic framework that generalizes the standard resource theory of coherence. In particular, POVM-incoherent (free) states and operations were established. In this work, we explore features of this framework which arise due to the rich structure of POVMs compared to projective measurements. Moreover, we introduce a rigorous, probabilisitic framework for POVM-based coherence measures and free operations. This leads to the introduction of strongly monotonic resource measures that neatly generalize well-known standard coherence measures. Finally, we show that the relative entropy of POVM coherence is equal to the cryptographic randomness gain, providing an important operational meaning to the concept of coherence with respect to a general measurement.

We have employed the theory of harmonically trapped dipolar Bose-Einstein condensates to examine the influence of a uniform magnetic field that rotates at an arbitrary angle to its own orientation. This is achieved by semi-analytically solving the dipolar superfluid hydrodynamics of this system within the Thomas-Fermi approximation and by allowing the body frame of the condensate's density profile to be tilted with respect to the symmetry axes of the nonrotating harmonic trap. This additional degree of freedom manifests itself in the presence of previously unknown stationary solution branches for any given dipole tilt angle. We also find that the tilt angle of the stationary state's body frame with respect to the rotation axis is a nontrivial function of the trapping geometry, rotation frequency, and dipole tilt angle. For rotation frequencies of at least an order of magnitude higher than the in-plane trapping frequency, the stationary state density profile is almost perfectly equivalent to the profile expected in a time-averaged dipolar potential that effectively vanishes when the dipoles are tilted along the “magic angle” 54.7 deg. However, by linearizing the fully time-dependent superfluid hydrodynamics about these stationary states, we find that they are dynamically unstable against the formation of collective modes, which we expect would result in turbulent decay.

We investigate the nonequilibrium dynamics of an impurity coupled to a Bose-Einstein condensate, systematically compared with recent experimental results [M. G. Skou et al., Nat. Phys. (2021)]. The dynamics of the impurity is tracked down by using a time-dependent variational coherent ansatz. For weak coupling between the impurity and the bath, analytical expressions for the time-dependent contrast are derived, matching quite well with previous findings obtained within a master equation approach. For strong coupling instead, the variational ansatz provides a good quantitative description of the polaron dynamics, in particular, in signaling the transition from the few- to the many-body correlated regime where polarons are expected to form.

The propagation of light in a pair of waveguides (WGs) with a complex index of refraction is studied, when the WG parameters are adjusted in such a way as to bring the system extremely close to an exceptional point (EP) while slightly breaking $\mathcal{P}\mathcal{T}$ symmetry. We find that this breaking of $\mathcal{P}\mathcal{T}$ symmetry does not affect the structure of the output signals, although the corresponding propagation constants become slightly complex. Only for sufficiently strong breaking of the $\mathcal{P}\mathcal{T}$ symmetry the signal is enhanced or suppressed in a substantial manner. Moreover, due to the EP it is impossible to tap light into another WG and thus read the transmitted signal without destroying completely the nodal structure and changing dramatically the intensity of the signal.

We investigate the possibilities for a deterministic conversion between two important types of maximally entangled multiqubit states, namely, $W$ and Greenberger-Horne-Zeilinger (GHZ) states, in the Rydberg-blockade regime of a neutral-atom system where each atom is subject to four external laser pulses. Such interconversions between $W$ states and their GHZ counterparts have quite recently been addressed using the method of shortcuts to adiabaticity, more precisely techniques based on Lewis-Riesenfeld invariants [R.-H. Zheng et al., Phys. Rev. A 101, 012345 (2020)]. Motivated in part by this recent work, we revisit the $W$ to GHZ state-conversion problem using a fundamentally different approach, which is based on the dynamical symmetries of the system and a Lie-algebraic parametrization of its permissible evolutions. In contrast to the previously used invariant-based approach, which leads to a state-conversion protocol characterized by strongly time-dependent Rabi frequencies of external lasers, ours can also yield one with time-independent Rabi frequencies. This feature makes our protocol more easily applicable experimentally, with the added advantage that it allows the desired state conversion to be carried out in a significantly shorter time with the same total laser pulse energy used.

The entanglement of assistance quantifies the amount of entanglement that can be concentrated among a group of spatially separated parties using the assistance of some auxiliary system. In this paper we study the entanglement of assistance in the simplest scenario of three qubits. We consider how much entanglement is lost when the helper party becomes uncoupled by local measurement and classical communication. For three-qubit pure states, it is found that lossless decoupling is always possible when entanglement is quantified by the maximum probability of locally generating a Bell state. However, with respect to most other entanglement measures, such as concurrence and entanglement entropy, a lossless decoupling of the helper is possible only for a special family of states that we fully characterize.

We consider a range of unconventional modifications to quantum annealing (QA), applied to an artificial trial problem with continuously tunable difficulty. In this problem, inspired by “transverse field chaos” in larger systems, classical and quantum methods are steered toward a false local minimum. To go from this local minimum to the global minimum, all $N$ spins must flip, making this problem exponentially difficult to solve. We numerically study this problem by using a variety of new methods from the literature: inhomogeneous driving, adding transverse couplers, and other types of coherent oscillations in the transverse field terms (collectively known as RFQA, which stands for Random Field Quantum Annealing or Radio Frequency Quantum Annealing). We show that all of these methods improve the scaling of the time to solution (relative to the standard uniform sweep evolution) in at least some regimes. Comparison of these methods could help identify promising paths towards a demonstrable quantum speedup over classical algorithms in solving some realistic problems with near-term quantum annealing hardware.

The hyperfine constants of the $4f^{14}6s6p{}^{3}P_2^o$ state in neutral Yb have been measured using three different dipole transitions. This state was recently shown to have a comparatively large hyperfine magnetic octupole splitting, and thus a puzzlingly large magnetic octupole moment. The measurement is performed using collinear laser spectroscopy on a fast atomic beam, which provides a straightforward route to probing long-lived metastable atomic states with high resolution. From the combined analysis of all three lines we find no significant evidence for a nonzero octupole moment in ${}^{173}\mathrm{Yb}$.

A combined experimental and theoretical study on single capture in 15–$50\mathrm{keV}/\mathrm{u}{\mathrm{C}}^{4+} +$ He collisions was performed. State-selective single-electron capture cross sections and projectile scattering angle distributions were obtained by using a reaction microscope. A comparison of the state-selective cross sections with theoretical calculations based on the two-active-electron semiclassical atomic-orbital close-coupling method showed an excellent agreement for the considered impact energies. For the angular differential cross sections, an overall agreement was also obtained between the present experimental and theoretical results. Simulations performed using an extended Fraunhofer-type diffraction model suggest that the undulatory structures observed at small scattering angles for capture to ${\mathrm{C}}^{3+}(1s^{2}2s$) and ${\mathrm{C}}^{3+}(1s^{2}2p_0$) can be interpreted as the diffraction pattern of the incident projectile de Broglie wave confined by an aperture defined as the region around the target where the considered capture process is likely. The strong competition between single-electron capture to ${\mathrm{C}}^{3+}(1s^{2}2p_0$) and ${\mathrm{C}}^{3+}(1s^{2}2p_1$) is also discussed to interpret the differences observed between their respective differential cross sections.

The Auger decay process of the $3d_{5/2}$ hole is studied experimentally and theoretically along the isoelectronic Br, ${\mathrm{Kr}}^{+}$, and ${\mathrm{Rb}}^{2+}$ series with electron configuration [Ar] $3d^{9}4s^{2}4p^6$. The experimental results consist of multielectron coincidence data measured from all three elements and conventional high-resolution Auger spectrum for the Br case. Theoretical interpretation was done by using multiconfiguration Dirac-Fock calculations. It is found that the decay rate of two-Auger-electron emission increases along the series in the direction of decreasing nuclear charge. Also, complexity of the Auger spectrum follows the same trend, requiring a drastic increase to the size of the configuration space for describing the observed spectra.