We have performed microwave spectroscopy of sodium least-bound molecular states, improving the precision of the knowledge of their energies at zero magnetic field by almost three orders of magnitude. Our experimental observations give us access also to states submitted to predissociation, a phenomenon where a bound molecular state can naturally decay into the continuum. Our findings are compared to numerical calculations based on the latest interpolation of sodium interaction potentials and show good agreement, with slight discrepancies in the zero-field energy of the molecular states, suggesting a need for small adjustment of the interaction potentials.

We explore the paired superfluid phases of a Fermi gas in the presence of a continuous Rabi drive. We focus on the case where two components are Rabi coupled strongly enough that they form hybrid superpositions, which in turn interact with an uncoupled third component. Using a generalized Bardeen-Cooper-Schrieffer (BCS) ansatz, we show that there are two coupled superfluid order parameters, and we obtain the associated free energy and quasiparticle excitation spectrum. We find that we can drive BCS-BCS, BCS-Bose-Einstein condensate (BEC), and BEC-BEC crossovers purely by varying the detuning of the Rabi drive from the bare transition, with the precise crossover depending on the sign of the underlying interactions between the coupled and uncoupled components. We furthermore identify an exotic excited branch which features both normal to BCS superfluid transitions, as well as a BCS-BEC-BCS crossover. Introducing a generalized Thouless criterion, we show that this behavior is reflected in the critical temperature for superfluidity. Our Rabi-coupled scenario also possesses additional thermodynamic properties related to the pseudospin of the coupled components, which provide novel signatures of the state of the many-body system. The Rabi-driven Fermi gas thus emerges as a unique platform for engineering and probing a rich array of multiband superfluid phases.

We derive an extended version of the hierarchical equations of motion (HEOM) to compute output physical properties of a bosonic environment, which is allowed to be initially prepared at an earlier time in a non-Gaussian input state and then nonperturbatively interact with a quantum system with a linear environmental operator. While spectral assumptions analogous to those used in the regular HEOM are imposed to compute dynamical output bath observables, they are not required to model input states or output observables at a fixed time, in this case leading to time-dependent contributions to the equations. In the Markovian limit, we use this formalism to derive an input-output Lindblad equation which can be used to extend the applicability of the regular version. For a given desired input state and output observable, the range of the indexes extending the regular HEOM is, by construction, bounded. Overall, the aim of this formalism is to take advantage of the efficiency and generality of the HEOM framework to model non-Gaussian input states and the dynamics of environmental observables in bosonic, non-Markovian open quantum systems.

Understanding how to control phase transitions in quantum systems is at the forefront of research for the development of new quantum materials and technologies. Here, we study how the coupling of a quantum system to a non-Markovian environment, i.e., an environment with a frequency-dependent spectral density inducing memory effects, can be used to generate and reshape phase transitions and squeezing in matter phases. Focusing on a Lipkin-Meshkov-Glick model, we demonstrate that non-Markovian dissipation can be leveraged to engineer tricriticality via the fusion of second-order and first-order critical points. We identify phases that arise from different ways of breaking the single weak symmetry of our model, which led us to examine an alternative concept of directional spontaneous symmetry breaking (DSSB) as a general framework to understand this phenomenon. We show that signatures of DSSB can be seen in the emergence of spin squeezing along different directions, and that the latter is controllable via non-Markovian effects, opening up possibilities for applications in quantum metrology. Finally, we propose an experimental implementation of our non-Markovian model in cavity QED. Our work features non-Markovianity as a resource for controlling phase transitions in general systems, and highlights shortcomings of the Markovian limit in this context.

We derive a generalized quantum Langevin equation and its fluctuation-dissipation relation describing the quantum dynamics of a tagged particle interacting with a medium (environment), where both the particle and the environment are driven by an external time-dependent (e.g. oscillating) field. We specialize on the case of a charged tagged particle interacting with a bath of charged oscillators, under an external ac electric field, although the results are much more general and can be applied to any type of external time-dependent fields. We derive the corresponding quantum Langevin equation, which obeys a modified fluctuation-dissipation relation (FDR) where the ac field plays an explicit role. The modified FDR is non-Markovian even if the undriven particle-bath system is Markovian without the external field. We provide an illustration of the usefulness of these results and derive a new form of the quantum Nyquist noise for the voltage fluctuations in electrical circuits under ac conditions (finite frequency), which is the most general since it also accounts for the response of the heat bath (e.g. lattice ions) to the applied ac electric field in the GHz–THz region, of relevance for 5G/6G wireless technologies. This generalized quantum fluctuation-dissipation relation for driven systems can also find other applications ranging from quantum noise in quantum optics to quantum computing with trapped ions.

We report on the realization and characterization of a novel magic-wavelength nanofiber-based two-color optical dipole trap for cesium that allows us to generate two diametral periodic one-dimensional arrays of trapping sites with a spacing significantly smaller than half the resonant free-space wavelength of the cesium $D_2$ transition. This is achieved by launching a blue-detuned partial standing wave and two red-detuned light fields through the nanofiber. We trap and optically interface the atoms in the resulting periodic optical potential and characterize the trap by measuring the lifetime of the trapped atoms, the atom-light coupling strength, the filling factor, and the trap frequencies in the radial and axial directions. The implementation of this nanofiber-based optical interface with magic trapping wavelengths and sub-$\lambda /2$ spacing is an important step towards the exploration of novel collective radiative effects, such as selective radiance.

We present a class of generalized entropic quantum speed limits based on $\alpha \text{−}z$-Rényi relative entropy, a real-valued, contractive, two-parameter family of distinguishability measures. The quantum speed limit (QSL) falls into the class of Mandelstam-Tamm bounds, and it applies to finite-dimensional quantum systems that undergo a general physical process, i.e., their effective dynamics can be modeled by unitary or nonunitary evolutions. The results cover pure or mixed, separable, and entangled probe quantum states. The QSL time depends on the smallest and largest eigenvalues of the probe and instantaneous states of the system, and its evaluation requires low computational cost. In addition, it is inversely proportional to the time-average of the Schatten speed of the instantaneous state, which in turn is fully characterized by the considered dynamics. We specialize our results to the case of unitary and nonunitary evolutions. In the former case, the QSL scales with the inverse of the energy fluctuations, while the latter depends on the Schatten 1-norm of the rate of change of the quantum channel Kraus operators. We illustrate our findings for single-qubit and two-qubit states, and unitary and nonunitary evolutions. Our results may find applications in the study of entropic uncertainty relations, quantum metrology, and also entanglement entropies signaled by generalized entropies.

Ultracold dipolar hard-core bosons in optical ladders provide exciting possibilities for the quantum simulation of anisotropic spin ladders. We show that introducing a tilt along the rungs results in a rich phase diagram at unit rung filling. In particular, for a sufficiently strong dipolar strength, the interplay between the long-range tail of the dipolar interactions and the tilting leads to the emergence of a quantum floating phase, a critical phase with incommensurate density-density correlations. Interestingly, the floating phase is topological, constituting an intermediate gapless stage in the melting of a crystal into a gapped topological Haldane phase. This novel scenario for topological floating phases in dipolar spin ladders can be investigated in ongoing experiments.

Reconstruction of attosecond beatings by two-photon interferences (RABBIT) has amply been used to retrieve photoionization delays from photoelectron spectra measured in atoms and, more recently, in molecules. For the latter, the molecular orientation and multicenter nature of the potential felt by the escaping electron often makes the extraction of these delays problematic. One of the complications comes from the fact that the energy separation between the states of the resulting molecular cation is of the order of the photon energy of the driving infrared (IR) field, so that, during the ionization process, the IR field may induce resonant transitions between such states. This adds a third ionization path to the standard two-path ionization scheme of RABBIT. Here we present a detailed theoretical study of RABBIT in the acetylene molecule in which we have explicitly evaluated time-resolved photoelectron spectra as those recorded in actual experiments. We do so for both random and well-defined orientations of the molecule, and for photoelectron spectra integrated over and resolved in the photoelectron emission angle. We show that the presence of this additional third path can strongly affect the values and physical meaning of photoionization delays extracted from fits based on the commonly used two-path interference scheme. This is indeed the case for acetylene molecules parallel to the light polarization direction, where the extracted delays strongly depend on the IR frequency, while for molecules perpendicular to the polarization direction the standard two-path RABBIT conditions are fulfilled.

We study Heisenberg's uncertainty relation relative to a quantum reference frame (QRF). We introduce the QRF as a covariant phase space observable, show that when described relative to it, position and momentum appear compatible, and derive novel, frame-relative uncertainty relations. This is achieved by constructing a joint observable for position and momentum, and calculating the variances of its margins. We then verify that in the classical limit of the QRF, the standard uncertainty relations are recovered, fortifying claims that standard quantum theory must be understood relative to an external, classical frame. These results may open up new research directions at the interface between QRFs and incompatibility.

We investigate coherent multiple scattering effects in the random quantum kicked rotor model. By changing the starting time of the Floquet period, two classes of models can be introduced that exhibit similar interference structures. For one of the two classes, these structures appear on top of a nontrivial background, which we describe in detail. Its origin is identified and an approximate analytical formula is proposed. The asymptotic contrast profile, as well as the height of the coherent backscattering and the coherent forward scattering peaks, are also analyzed. Our findings are relevant to provide an interpretation of cold atom experiments aimed at observing such interference effects with matter waves.

Bose-Einstein condensates (BECs) of neutral atoms constitute an important quantum system for fundamental research and precision metrology. Many applications require short preparation times of BECs, for example, for optimized data acquisition rates in scientific applications, and reduced dead times and improved bandwidths for atomic quantum sensors. Here, we report on the generation of rubidium BECs with a repetition rate of more than $2\mathrm{Hz}$. The system relies on forced evaporation in a dynamically adjusted optical potential, which is created by the spatial modulation of laser beams. Our system provides a versatile source of the ubiquitous rubidium BECs, and promotes their exploitation for high-precision atom interferometers.

Electron emission (EE) from insulating polystyrene nanoparticles induced by a single collision with swift ions is investigated. The ions electronic stopping powers explored range from 35 ev/nm to 2.3 kev/nm. A cross-beam experiment is developed to measure charge-state distributions of nanoparticles of 100 nanometer radius. The proportionality between the EE and the stopping power observed when measured using grounded conductive foils is no longer fulfilled. We perform a complementary analysis of nanoparticle charging by using Monte Carlo simulations. We emphasize the role of particle charging by taking into account the effect of the holes on the dynamics of electron emission. The resulting charging effect can be as large as two orders of magnitude for the highest stopping power. This explains rather well the experimentally observed strong sublinearity of electron emission with stopping power. As the nanoparticle size decreases, the simulation suggests that the charging effect diminishes, but remains significant for large stopping power.

Characterizing the set of distributions that can be realized in the triangle network is a notoriously difficult problem. In this work, we investigate inner approximations of the set of local (classical) distributions of the triangle network. A quantum distribution that appears to be nonlocal is the elegant joint measurement (EJM) [Entropy 21, 325 (2019)], which motivates us to study distributions having the same symmetries as the EJM. We compare analytical and neural-network-based inner approximations and find a remarkable agreement between the two methods. Using neural network tools, we also conjecture network Bell inequalities that give a trade-off between the levels of correlation and symmetry that a local distribution may feature. Our results considerably strengthen the conjecture that the EJM is nonlocal.

Nonclassical mechanical states, as vital quantum resources for exploring macroscopic quantum behavior, have wide applications in the study of the fundamental quantum mechanics and modern quantum technology. In this work, we propose a scheme for deterministically generating nonclassical mechanical states in cavity optomechanical systems. By working in the eigenrepresentation of the nonlinear optomechanical systems, we identify the carrier-wave resonance conditions and seek optimal driving pulses for state preparations. Concretely, we employ the reinforcement learning method to optimize the pulsed driving fields, effectively suppressing the undesired transitions induced by both the pulsed driving fields and dissipations. This approach enables the high-fidelity preparation of phononic Fock states and superposed Fock states in the single-resonator optomechanical systems, as well as two-mode entangled states in the two-resonator optomechanical systems. The statistical properties of the generated states are also examined. Our results create an opportunity for quantum state engineering in quantum optics and quantum information science via reinforcement learning.

The study of the long-time dynamics of quantum systems can be a real challenge, especially in systems such as ultracold gases, where the required timescales may be longer than the lifetime of the system itself. In this Letter, we show that it is possible to access the long-time spin dynamics of a strongly repulsive atomic gas mixture in shorter times. The shortcut-to-dynamics protocol that we propose does not modify the fate of the observables, but effectively jumps ahead in time without changing the system's inherent evolution. Just as the next-chapter button in a movie player that allows to quickly reach the part of the movie one wants to watch, it is a leap into the future.

We solve the problem of a dimer moving on a spherical surface and find that its binding energy and wave function are sensitive to the total angular momentum. The dimer gets squeezed in the direction orthogonal to the center-of-mass motion and can qualitatively change its geometry from two dimensional to one dimensional. These results suggest that combining the curved geometry with finite angular momentum may give rise to qualitatively new many-body phenomena in ultracold shell-shaped gases.

Macroscopic QED (MQED) is the field theory for computing quantum electromagnetic effects in dispersive media. Here, we extend MQED to treat time-varying, dispersive media. For a time-dependent Drude model, we find that the expected replacement $\varepsilon \left(\omega \right)\to \varepsilon (t,\omega )$ within standard MQED does not lead to a permittivity that is similar to that of the classical time-dependent Drude model and can lead to large polarization currents, becoming singular in the limit of a step change in carrier density. We show this unwanted behavior can be removed through instead modifying the reservoir dynamics, quantizing the resulting theory and finding the nonequilibrium, time-varying noise currents, which exhibit extra correlations due to temporal reflections within the material dynamics.

Alkali-metal–noble-gas comagnetometers are precision probes well suited for tests of fundamental physics and inertial rotation sensing, combining the high sensitivity of the spin-exchange-relaxation-free magnetometers with inherent suppression of magnetic field noise. Past versions of the device utilizing continuous-wave optical pumping are sensitive to rotation and anomalous spin couplings along a single axis perpendicular to the plane spanned by the orthogonal pump and probe laser beams. These devices are susceptible to light shifts in the alkali atoms, and to power and beam pointing fluctuations of both the probe and pump lasers, the latter of which is a dominant source of $1/f$ noise. In this work, we model and implement an approach to alkali-metal–noble-gas comagnetometers using pulsed optical pumping. After each pump laser pulse, an off-resonance probe beam measures the precession of noble-gas-spins-coupled alkali spins via optical rotation in the dark, thus eliminating effects from pump laser light shift and power fluctuations. Performing nonlinear fitting on the sinusoidal transient signal with a proper phase enables independent and simultaneous measurement of signals along two orthogonal axes in the plane perpendicular to the pump beam. Effects from beam pointing fluctuations of the probe beam in the pump-probe plane are fundamentally eliminated, and signal response to pump beam pointing fluctuations is suppressed by compensation from noble-gas nuclear spins.

We study superfluid drag in the two-component Bose-Hubbard model with infinitely strong repulsive interactions. In this system, all transport is mediated by the motion of empty sites, or “holes,” and it is hard to move one component without moving the other. We demonstrate, with a combination of analytic and numeric techniques, that the motion of holes leads to strong dissipationless coupling between currents in the two components. This behavior is attributable to polaronic correlations that emerge in the presence of spin currents, which can be observed in experiments. We derive a closed-form expression for the coupling on various lattices in arbitrary spatial dimensions, which we verify through numerical simulations on two-dimensional lattices.