We present a manifestation of a Parrondo's effect in a continuous-time quantum walk (CTQW). In our protocol we consider a CTQW in the presence of a time-dependent transition defect. Our results show that the alternation between defects, that individually are detrimental to the wavepacket spreading, can paradoxically enhance overall wavepacket propagation. Our findings pave the way for the exploration of unconventional mechanisms that can potentially harness the adverse effects of defects to enhance quantum transport.

Density estimation is a central task in statistics and machine learning. This problem aims to determine the underlying probability density function that best aligns with an observed dataset. Some of its applications include statistical inference, unsupervised learning, and anomaly detection. Despite its relevance, few works have explored the application of quantum computing to density estimation. In this article, we present a quantum-classical density-matrix density estimation model, called Q-DEMDE, based on the expected values of density matrices and a quantum embedding called quantum Fourier features. The method uses quantum hardware to build probability distributions of training data via mixed quantum states. As a core subroutine, we propose an algorithm to estimate the expected value of a mixed density matrix from its spectral decomposition on a quantum computer. In addition, we present an application of the method for quantum-classical anomaly detection. We evaluated the density estimation model with quantum random and quantum adaptive Fourier features on different datasets on a quantum simulator and a real quantum computer. An important result of this work is to show that it is possible to perform density estimation and anomaly detection with high performance on present-day quantum computers.

Transferring quantum information between different types of quantum hardware is crucial for integrated quantum technology. In particular, converting information between continuous variable (CV) and discrete variable (DV) devices enables many applications in quantum networking, quantum sensing, quantum machine learning, and quantum computing. This paper addresses the transfer of CV-encoded information between CV and DV devices. We present a resource-efficient method for encoding CV states and implementing CV gates on DV devices, as well as two measurement-based protocols for transferring CV states between CV and DV devices. The success probability of the transfer protocols depends on the measurement outcome and can be increased to near-deterministic values by adding ancillary qubits to the DV devices.

Supersonic molecular beams are used in several scientific fields ranging from surface science to the spectroscopy of molecules, nanoparticles, and even viruses. It is therefore of interest to understand the fundamental properties of the expansion and resulting beam. Here, we present a study of the change in the most probable velocity with reservoir pressure, using a so-called Even-Lavie pulsed source with a 200-$\mu \text{m-diam}$ nozzle. Experiments were carried out in the throughput range $p_{0}d=400–2000\mathrm{mbar}\mathrm{cm}$, where $p_0$ is the reservoir stagnation pressure and $d$ is the nozzle diameter. Experiments were done on both neutral and metastable helium beams. A two-detector measurement setup was used to account for the valve opening delay. The measurements are modeled with theory which has hitherto only been applied to a $p_{0}d$ regime up to 190 $\text{mbar}\text{cm}$, showing good agreement.

We present an analysis of the sensitivity limits of a proposed experimental search for quantum gravity, using an approach based on optical magnetometry in the noble gas isotope ${}^{129}\mathrm{Xe}$. The analysis relies on a general uncertainty principle model that is consistent with most formulations of quantum gravity theory, where the canonical uncertainty relations are modified by a leading-order correction term that is linear in momentum. In turn, this correction modifies the magnetic moment of the spin-polarized ${}^{129}\mathrm{Xe}$ atoms that are immersed in a magnetic field in the proposed experiment, which results in a velocity-dependent variation of their Larmor frequency, that is detected via two-photon laser spectroscopy. The thermal distribution of atomic velocities, in conjunction with the Doppler effect, is used to scan the interrogating laser over different atomic velocities and search for a corresponding variation in their Larmor frequencies. We show that the existing bounds on the leading-order quantum gravity correction can be improved by ${10}^7$ with existing technology, where another factor of ${10}^2$ is possible with near-future technical capabilities.

Under the Franck-Condon approximation, we systematically validated the performance of density functional theory (DFT) and the effects of anharmonicity in simulating C, N, and O $K$-edge vibrationally resolved x-ray spectra of common diatomic molecules and cations. To get “transparent” validations, vibronic fine structures of only the lowest $1s$ excited or ionized state in x-ray absorption spectroscopy (XAS) or x-ray photoelectron spectroscopy (XPS) were investigated. All six systems $({\mathrm{N}}_2, {\mathrm{N}}_2^{+}, \mathrm{NO}, {\mathrm{NO}}^{+}, \mathrm{CO}, \text{and} {\mathrm{CO}}^{+})$ were studied within the harmonic oscillator (HO) approximation using DFT with four functionals (BLYP, BP86, B3LYP, and M06-2X) for ten XAS and four XPS spectra, and excellent agreement between theoretical and experimental spectra was found in most systems, except O $1s$ XAS of NO, CO, and ${\mathrm{NO}}^{+}$. We analyzed and established a connection between their complex vibronic structures (many weak oscillating features within a broad peak) and the significant geometrical changes induced by the O $1s$ hole. The three spectra were well reproduced with anharmonic (AH) calculations by using quantum wave-packet dynamics based on potential energy curves (PECs) generated by DFT methods or multiconfigurational levels, highlighting sensitivity to the anharmonic effect and the PEC quality. In other examples of XAS $({\mathrm{CO}}^{+}, \mathrm{C} 1s \text{and} \mathrm{O} 1s; \mathrm{NO}, \mathrm{N} 1s)$ corresponding to smaller structural changes, HO and AH approaches lead to similar fine structures, which are dominated by 0-0 and 0-1 transitions. This study highlights the use of DFT with selected functionals for such diatomic calculations due to its easy execution and generally reliable accuracy. Functional dependence in diatomic systems is generally more pronounced than in polyatomic ones. We found that the BLYP, BP86, and B3LYP functionals consistently exhibited high accuracy in predicting spectral profiles, bond lengths, and vibrational frequencies, which slightly outperformed M06-2X.

We present a quantum-Monte-Carlo–based density functional to describe droplet formation and supersolidity in dipolar systems. The usual Lee-Huang-Yang term, accounting for quantum correlations in the conventional extended Gross-Pitaevskii equation (eGPE), has been substituted by the correlation energy evaluated with quantum Monte Carlo. We demonstrate the ability of our functional to reproduce existing experimental data for the minimum critical number of atoms $N_{\mathrm{c}}$ required for droplet formation. $N_{\mathrm{c}}$ is a challenging quantity for theoretical predictions, and the eGPE provides only a qualitative description of it, mainly when it is applied to dysprosium. We also use our approach to characterize the BEC-supersolid transition. The quantum-Monte-Carlo–based functional can be easily implemented in any existing eGPE code, improving the description of dipolar systems without increasing the computational cost.

The number of atoms in Bose-Einstein condensate determines the scale of experiments that can be performed, making it crucial for quantum simulations. Optimization of the number of atoms in the condensate is a complex problem which could be efficiently solved using the machine learning technique. Nevertheless, this approach usually does not give any insight in the underlying physics. Here we demonstrate the possibility to learn physics from machine learning on an example of condensation of thulium atoms at a 1064-nm dipole trap. Optimization of the number of condensed atoms revealed a saturation, which was explained as a limitation imposed by a three-body recombination process. This limitation was successfully overcome by leveraging Fano-Feshbach resonances.

A generalized Bose-Hubbard model in a two-mode approximation is applied to study the rotational dynamics of a direct-current atomtronic quantum interference device. Modified values of on-site interaction and pair-tunneling parameters of the Hamiltonian, derived from the small-oscillation periods of the Josephson modes, are shown to provide an excellent agreement to the Gross-Pitaevskii simulation results for the whole rotational frequency range, reaching also the critical values of imbalance and current. This amounts to a full validation of the semiclassical approximation of the modified Hamiltonian, whose quantization is employed to investigate the quantum features of the stationary states. Focusing on the frequency interval where the potential energy presents two minima, it is shown that the central frequency, at which such minima are symmetric, yields an atom number parity-protected qubit with a maximum entanglement of both persistent-current states, similar to those of superconducting circuits threaded by a half quantum of applied flux. Such a parity protection scheme survives within a small interval around the central frequency, which sets the minimum rotational frequency precision that should be required to implement the qubit. It is found that such a maximum admissible error in the frequency determination turns out to be inversely proportional to the qubit quality factor that measures the gap between the qubit energy levels and the following levels. It is shown that the chemical potential or condensate particle number could be employed as suitable control parameters to achieve the best trade-off between such qubit characteristics.

We perform a diagrammatic analysis of the energy of a mobile impurity immersed in a strongly interacting two-component Fermi gas to second order in the impurity-bath interaction. These corrections demonstrate divergent behavior in the limit of large impurity momentum. We show the fundamental processes responsible for these logarithmically divergent terms. We study the problem in the general case without any assumptions regarding the fermion-fermion interactions in the bath. We show that the divergent term can be summed up to all orders in the Fermi-Fermi interaction and that the resulting expression is equivalent to the one obtained in the few-body calculation. Finally, we provide a perturbative calculation to the second order in the Fermi-Fermi interaction, and we show the diagrams responsible for these terms.

We report an unusual collision-induced collective annihilation of solitons at 2 $\mu \mathrm{m}$ in a figure-8 Tm-doped fiber laser. The solitons are generated in a unique bidirectional soliton rain state, where the interaction of heterogeneous vector solitons stimulates deep inelastic collisions. The experiment shows that the solitons activate four collisional scenarios and eventually are annihilated after an average of three to four collisions, adhering to a geometric distribution. The collision-annihilation state can be maintained even for certain energy perturbations, unveiling an extreme soliton characteristic in a large anomalous-dispersion regime.

In recent studies on ultrastrong coupling between matter and light in cavities, the significance of gauge choice when employing the widely used two-level approximation has been highlighted. Expanding upon these investigations, we extend the analysis to waveguide QED, where we demonstrate that truncations performed in the dipole gauge also yield accurate results. To illustrate this point, we consider the case of a dipole coupled to a cavity array. Various numerical and analytical techniques have been employed to investigate the low-energy dynamics of the system. Leveraging these theoretical tools, we argue that single-photon scattering is an ideal method for investigating gauge-related issues. Our findings reveal two effects in the scattering spectra, which cannot be reproduced in a truncated model using the Coulomb gauge. First, the primary resonance is modified due to a Lamb shift contribution. Second, we observe asymmetric transmission amplitudes surrounding this resonance, reflecting the asymmetry of the spectral density in this model. Additionally, we explore other features in the scattering spectra resulting from ultrastrong couplings, such as the emergence of Fano resonances and inelastic channels. Finally, we propose an experimental test of our ideas in the context of circuit QED.

The Tavis-Cummings model is a paradigmatic central-mode model in which a set of two-level quantum emitters (spins) are coupled to a collective cavity mode. Here we study the eigenstate spectrum, its localization properties, and the effect on dynamics, focusing on the two-excitation sector relevant for nonlinear photonics. These models admit two sources of disorder: in the coupling between the spins and the cavity, and in the energy shifts of the individual spins. While this model was known to be exactly solvable in the limit of a homogeneous coupling and inhomogeneous energy shifts, we establish here the solvability in the opposite limit of a homogeneous energy shift and inhomogeneous coupling, presenting the exact solution and corresponding conserved quantities. We identify three different classes of eigenstates, exhibiting different degrees of multifractality and semilocalization closely tied to the integrable points, and we study their stability to perturbations away from these solvable points. The dynamics of the cavity occupation number away from equilibrium, exhibiting boson bunching and a two-photon blockade, is explicitly related to the localization properties of the eigenstates, and it illustrates how these models support a collective spin description despite the presence of disorder.

Fabry-Perot microcavities can strongly enhance interactions between light and molecules, leading to the formation of hybrid light-matter states known as polaritons. Polaritons possess much smaller effective masses and much larger group velocities when the molecules are resonant with cavity modes that have finite (nonzero) in-plane wave vectors, giving rise to the possibilities of long-range and ultrafast ballistic transport. In this paper, we present the results of numerical simulations of the ultrafast ballistic transport phenomenon in real space and time during and after initialization with a short, spatially localized pulse. We address this problem with two approaches: the standard transfer-matrix method (TMM) for planar structures and a second simulation based on the numerical solution of the Maxwell-Bloch equations that can be used for general configurations (complex metasurfaces, for instance) and excitation modes. The agreement between the TMM and the full numerical calculation with the Maxwell-Bloch equations when applied to multilayer planar configurations provides proof of the validity of both approaches for the present analyses. Overall, we find that the transport of the molecular excitons inside the cavity synchronize with the evolution of the enhanced electromagnetic field inside the cavity. Moreover, the synchronized transport rate is in good agreement with the group velocities predicted from a calculated dispersion relation across a wide range of frequencies. Finally, we relate the group velocity to the Hopfield coefficient obtained from quantum modeling and suggest that the dependence of light-matter coupling on the in-plane wave vector can be an important but overlooked factor for understanding the transport behavior of polaritons. These simulations provide an intuitive tool for understanding the collective motion of light and excitons and helps us to better understand how experimental observations of polaritons should be interpreted.

Synchronization has recently been explored deep in the quantum regime with elementary few-level quantum oscillators such as qudits and weakly pumped quantum van der Pol oscillators. To engineer more complex quantum synchronizing systems, it is practically relevant to study composite oscillators built from basic quantum units that are commonly available and offer high controllability. Here we consider a minimal model for a composite oscillator consisting of two interacting qubits coupled to separate baths and show that this system exhibits a wide variety of synchronizing behaviors. We study the phase response of the constituent qubits as well as the system as a whole, when one of the qubits is weakly driven. We consider the thermal baths to have positive as well as effective negative temperatures and discover effects that occur only when the temperatures of the baths for the two qubits are of opposite signs. We propose and analyze a circuit quantum electrodynamics implementation of this model, which exploits recent advances in dissipation engineering to realize effective negative temperature baths. Our work demonstrates the potential for assembling complex quantum synchronizing systems from basic building units, which is of pragmatic importance for advancing the field of quantum synchronization.

Quantum parameter estimation holds the promise of quantum technologies, in which physical parameters can be measured with much greater precision than what is achieved with classical technologies. However, how to obtain the best precision when the optimal measurement is not accessible is still an open problem. In this paper, we present a theoretical framework to explore the parameter estimation with limited access of measurements by analyzing the effect of nonoptimal measurement on the estimation precision. We define a quantity $\mathrm{\Lambda }$ to characterize the effect and illustrate how to optimize observables to attain a bound with limited accessibility of observables. In addition, we introduce the minimum Euclidean distance to quantify the difference between an observable and the optimal ones in terms of the Frobenius norm and find that the measurement with a shorter distance to the optimal ones benefits the estimation. Two examples are presented to show our theory. In the first, we analyze the effect of nonoptimal measurement on the estimation precision of the transition frequency for a driven qubit. In the second example, we consider a bipartite system, in which one of them is measurement inaccessible. To be specific, we take a toy model, the nitrogen-vacancy (NV) center in diamond, as the bipartite system, where the NV-center electronic spin interacts with a single nucleus via the dipole-dipole interaction. We achieve a precise estimation for the nuclear Larmor frequency by optimizing only the observables of the electronic spin. In these two examples, the minimum Euclidean distance between an observable and the optimal ones is analyzed and the results show that the closer the observable to the optimal ones the better the estimation precision. This paper establishes a relation between the estimation precision and the distance of the nonoptimal observable to the optimal ones, which could be helpful for experiments seeking the best observable in parameter estimation.

We perform entanglement of spin and motional degrees of freedom of a single, ground-state trapped ion through the application of a $16\mathrm{ps}$ laser pulse. The duration of the interaction is significantly shorter than both the motional timescale ($30\mu \text{s}$) and spin precession timescale ($1\text{ns}$), demonstrating that neither sets a fundamental speed limit on this operation for quantum information processing. Entanglement is demonstrated through the collapse and revival of spin coherence as the spin components of the wave function separate and recombine in phase space. We infer the fidelity of these single-qubit operations to be $\left({97}_{-4}^{+3}\right)\%$.

The ionization of He by 10–45 keV proton impact was investigated employing recoil-ion momentum spectroscopy. For projectile velocities up to 1 a.u, the electron emission pattern in velocity space shows two intense “banana”-shaped areas similar to the emission pattern previously observed for transfer ionization in collision of $\mathrm{H}{\mathrm{e}}^{2+}$ with He. These structures extend from the target to the projectile velocities and exhibit a similar focusing towards either of the two nuclei. With increasing impact velocity, an accumulation of the electron velocity distribution at the nuclei becomes more pronounced. At 45 keV proton impact a target-cusp-like peak is observed. In addition to this peak the emission pattern shows signatures of the quasimolecular 2pπ promotion, which is characteristic for the process of saddle-point ionization.

We expand the theoretical toolbox for controllable quantum reflection by departing from a simple planar reflector. We introduce a circular hole (a micropore) of variable size, for which the electrostatic image potential can be exactly calculated. We combine this with two-dimensional simulations of wave-packet propagation at arbitrary angles of incidence to show that the quantum reflection probability can be tuned over a wide range of values.

We propose a theoretical model used for dynamically manipulating an ultracold weakly bound heteronuclear alkali-metal diatomic molecule by a nonresonant single-cycle THz laser pulse. Taking the ${}^6\mathrm{Li}{}^{23}\mathrm{Na}$ molecule as an example, we first calculate the variation of the weakly bound state and quasibound state as a function of external electric field, and then reveal abundant interference patterns during and after the laser pulse due to the superpositions between states involving weakly bound state, scattering state, and quasibound state. Moreover, one can map out the nodes of the probability density of the weakly bound molecules by measuring the wavepacket dynamics.