Despite their importance, there is an ongoing challenge characterizing multipartite quantum correlations. The Seevinck-Svetlichny (SS) and Mermin-Klyshko (MK) inequalities present constraints on correlations in multipartite systems, a violation of which allows to classify the correlations by using the nonseparability property. In this work we present refined Tsirelson (quantum) bounds on these inequalities, derived from inequalities stemming from a fundamental constraint, tightly akin to quantum uncertainty. Unlike the original, known inequalities, our bounds do not consist of a single constant point but rather depend on correlations in specific subsystems (being local correlations for our bounds on the SS operators and bipartite correlations for our bounds on the MK operators). We analyze concrete examples in which our bounds are strictly tighter than the known bounds, i.e., lie beneath the previously found constants, thus better characterizing the set of allowed quantum correlations. We interpret the results as complementarity relations between multipartite and local correlations, as well as multipartite and bipartite correlations.

A significant hurdle for quantum information and processing using bosonic systems is stochastic phase errors which occur as the photons propagate through a channel. These errors will reduce the purity of states passing through the channel and therefore reduce the channels capacity. We present a scheme of passive linear optical unitary averaging for protecting unknown Gaussian states transmitted through an optical channel. The scheme reduces the effect of phase noise on purity, squeezing, and entanglement, thereby enhancing the channel via a probabilistic-error-correcting protocol. The scheme is robust to loss and typically succeeds with high probability. We provide both numerical simulations and analytical approximations tailored for relevant parameters with the improvement of practical and current technology. We also show the asymptotic nature of the protocol, highlighting both current and future relevance.

We report on the absolute energy of the $2s3s{}^1{S}_0$ state in neutral ${}^9\mathrm{Be}$. In doing so, we measure the transition frequencies of the $2s^2{}^1{S}_0\to$ $2s2p{}^1{P}_1^{\circ }$ and $2s2p{}^1{P}_1^{\circ }\to$ $2s3s{}^1{S}_0$ transitions, also determining the absolute energy of the $2s2p{}^1{P}_1^{\circ }$ state. The results are obtained from a home-built hollow cathode discharge lamp using saturated absorption spectroscopy with laser frequencies referenced to a frequency comb. The transition frequencies are determined to be $1276080080\left(50\right)$ and $363105650\left(130\right)$ MHz for the $2s^2{}^1{S}_0\to$ $2s2p{}^1{P}_1^{\circ }$ and $2s2p{}^1{P}_1^{\circ }\to$ $2s3s{}^1{S}_0$ transitions, respectively. The absolute energies are determined to be $42565.4498\left(17\right)$ and $54677.350\left(5\right)$ ${\mathrm{cm}}^{-1}$ for the $2s2p{}^1{P}_1^{\circ }$ and $2s3s{}^1{S}_0$ states, respectively. The results confirm recent theoretical calculations that included ${\alpha }^6$ (QED) corrections. All experimental results are limited by unresolved hyperfine structure.

The multichannel single photoionization of cold strontium atoms is investigated experimentally and theoretically with 800-nm femtosecond laser pulses of 35-fs duration at laser intensities of 3–6 $\mathrm{TW}/{\mathrm{cm}}^2$. The photoelectron energy spectra exhibit rich multipeak structures, which are identified as the multichannel ionization of the ground state $5s^2{}^1{S}_0$ and excited states $5s5p{}^1{P}_1$, $5s5p{}^3{P}_2$, and $5s4d{}^1{D}_2$. The photoelectron momentum distributions reveal distinct structures, and the corresponding photoelectron angular distributions provide valuable insights into the multiphoton ionization dynamics. As the laser intensity increases, the locations of the photoelectron energy peaks originating from the ionization of the excited states $5s5p{}^3{P}_2$ and $5s5p{}^1{D}_2$ remain unshifted, indicating the existence of Freeman resonance. The profiles from numerical solutions of the time-dependent Schrödinger equation (TDSE) are generally in agreement with the experimental results. However, some discrepancies, i.e., the ratio of partial waves contributing to the final states and tiny structures caused by resonance, may arise from the incomplete modeling of the valence-electron correlation effect in the present TDSE simulations based on the single-active-electron approximation.

We investigate sideband spectroscopy of a trapped ion using a probe laser phase modulated at the trap drive frequency. The enhanced sensitivity of our technique provides us 1000-fold improvement in our control of excess micromotion. It allows us to monitor the evolution of stray fields on a timescale of a few minutes and detect differential phases of $5\mu \mathrm{rad}$ between applied ac potentials. We also demonstrate the ability to suppress Doppler shifts from excess motion to well below the limit imposed by the intrinsic motion of the ion in its motional ground state. The technique we introduce can be readily implemented in any ion-trap system that utilizes sideband spectroscopy for micromotion compensation and can be seamlessly integrated into experiments in a fully automated way.

We examine the potential-energy curves and polarization of the dipole moments of two static polar molecules under the influence of an external dc electric field and their anisotropic dipole-dipole interaction. We model the molecules as quantum rigid rotors to take their rotational degrees of freedom into account and consider a selection of linear, symmetric, and asymmetric top molecules. We provide a comprehensive examination of the energy curves and polarization of the dipoles for varying intermolecular separation and direction of the electric field and find that the properties of the molecules depend strongly on the field's direction at short separations, showing the importance of accounting for molecular rotation. The latter provides insight into the possible effects of accounting for rotational degrees of freedom in molecular dipolar gases.

We study static superfluid properties of alkali-earth-like Fermi atomic systems in the presence of orbital Feshbach resonance. Using a two-band description of the ground state and excited state and a mean-field approximation of the intraband atomic pairing, we investigate the phase transitions and crossover between BCS and Bose-Einstein-condensate (BEC) superfluidity. At zero temperature, we find exact BCS analytical solutions for the mean-field gap equations and number equations. Using these exact solutions, we calculate various static properties, such as superfluid order parameters, chemical potentials, density variations, density profiles, correlation and coherence lengths, ground-state energy, and Tan's contact density across the entire BCS-BEC crossover region. We anticipate that our closed-form analytical results can be used as a benchmark for future experimental and theoretical investigations and will have an impact on the current understanding of two-band superconductors such as ${\mathrm{MgB}}_2$.

We explore topological excitations in a spin-1 Bose-Einstein condensate subjected to an in-plane sinusoidally varying magnetic field and Rashba spin-orbit coupling (SOC). In the absence of SOC, the periodic magnetic field induces vortex-antivortex structures in the $|F=1,m_F=\pm 1\rangle$ condensates at saddle points such that the net topological charge remains zero. The introduction of Rashba SOC breaks the system's symmetry, leading to nonconservation of overall angular momentum in the spin-1 condensate. This anisotropy results in the emergence of certain skyrmion spin textures. We provide a comparative study for various in-plane magnetic-field configurations while keeping the SOC strength constant. Our numerical simulations within the mean-field framework reveal the potential to engineer diverse topological excitations controlled by the interplay between the spin-orbit coupling and in-plane magnetic field in a spinor condensate.

We present a method for producing a flat, large-area Fermi gas of ${}^6\mathrm{Li}$ with a uniform area density. The method uses a programmable optical potential within a feedback loop to flatten the in-plane trapping potential for atoms. The optical potential is generated using a laser beam, whose intensity profile is adjusted by a spatial light modulator and optimized through measurements of the density distribution of the sample. The resulting planar sample exhibits a uniform area density within a region of about 480 $\mu \mathrm{m}$ in diameter, and the standard deviation of the trap bottom potential is estimated to be $\approx k_B\times 6.1$ nK, which is less than 20% of the transverse confinement energy. We discuss a dimensional crossover toward the two-dimensional regime by reducing the number of atoms in the planar trap, including the effect of the spatial variation of the transverse trapping frequency in the large-area sample.

The essential role of synthetic spin-orbit coupling in discovering new topological matter phases with cold atoms is widely acknowledged. However, the engineering of spin-orbit coupling remains unclear for arbitrary-spin models due to the complexity of spin matrices. In this paper, we develop a more general but relatively straightforward method to achieve spin-orbit coupling for multispin models. Our approach hinges on controlling the coupling between distinct pseudospins through two intermediary states, resulting in tunneling with spin flips that have direction-dependent strength. The engineered spin-orbit coupling can facilitate topological phase transitions with Chern numbers over 1, a unique characteristic of multispin models compared to spin-1/2 models. By utilizing existing cold atom techniques, our proposed method provides an ideal platform for investigating topological properties related to large Chern numbers.

The quantum Cramér-Rao bound for the joint estimation of the centroid and the separation between two incoherent point sources cannot be saturated. As such, the optimal measurements for extracting maximal information about both at the same time are not known. In this paper, we ascertain these optimal measurements for an arbitrary point spread function, in the most relevant regime of a small separation between the sources. Our measurement can be adjusted within a set of tradeoffs, allowing more information to be extracted from the separation or the centroid while ensuring that the total information is the maximum possible.

In optical quantum information processing with continuous variables, optical non-Gaussian quantum states are essential for universal and fault-tolerant quantum computation. Experimentally, their most typical generation method is photon subtraction (PS), where single-photon detection by an on/off detector probabilistically heralds the generation of squeezed single-photon states. In PS, however, trying to avoid unwanted multiphoton detection inevitably limits the generation rate, hindering the application of squeezed single-photon states. Here, we theoretically show that generalized photon subtraction (GPS), a simple extension of PS, can improve the generation rate while maintaining the quality of the generated states. Furthermore, we experimentally demonstrate the generation rate improvement for 2-dB- and 4-dB-squeezed single-photon states compared to PS, by more than one order of magnitude, particularly for the case of 2 dB. Our results will accelerate the application of squeezed single-photon states to more advanced quantum information protocols.

Local measurements cannot create entanglement, but they can convert short-range entanglement to long-range entanglement, as in quantum teleportation. This phenomenon of measurement-induced entanglement (MIE) has been widely discussed in recent work on measurement-induced entanglement phase transitions and related phenomena. Here, we situate MIE in a broader context of the growth of long-range conditional mutual information (CMI) under decoherence. We upper-bound the rate at which decoherence can generate long-range CMI and derive a characterization of states that saturate this bound. We point out that the structure of states saturating the CMI upper bound can be very different under different decoherent dynamics and provide explicit examples. We additionally explore the dynamics of CMI in random quantum circuits subject to random local decoherence, as a function of circuit depth. We argue that the universality class of the finite-depth teleportation transition, and its lower critical dimension, are different for erasures than for measurements.

Self-testing is a powerful method for certifying quantum systems. Initially proposed in the device-independent (DI) setting, self-testing has since been relaxed to the semi-device-independent (semi-DI) setting. In this study, we focus on the self-testing of a specific type of nonprojective qubit measurements belonging to a one-parameter family, using the semi-DI prepare-and-measure (PM) scenario. Remarkably, we identify the simplest PM scenario discovered so far, involving only four preparations and four measurements, for self-testing the fourth measurement. This particular measurement is a four-outcome nonprojective positive operator-valued measure (POVM) and falls in the class of semisymmetric informationally complete POVMs introduced by Geng et al. [Phys. Rev. Lett. 126, 100401 (2021)]. To achieve this, we develop analytic techniques for semi-DI self-testing in the PM scenario. Our results pave the way toward self-testing any extremal qubit POVM within a potentially minimal PM scenario.

Non-Markovianity, the intricate dependence of an open quantum system on its temporal evolution history, holds tremendous implications across various scientific disciplines. However, accurately characterizing the complex non-Markovian effects has posed a formidable challenge for numerical simulations. While quantum computing technologies show promise, a universal theory enabling practical quantum algorithm implementation has been elusive. We address this gap by introducing the dissipaton-embedded quantum master equation in second quantization (DQME-SQ). This exact and compact theory offers two key advantages: representability by quantum circuits and universal applicability to any Gaussian environment. We demonstrate these capabilities through digital quantum simulations of non-Markovian dissipative dynamics in both bosonic and fermionic environments. The DQME-SQ framework opens a new horizon for the efficient exploration of complex open quantum systems by leveraging the rapidly advancing quantum computing technologies.

With high-order harmonic generation (HHG) from Su-Schrieffer-Heeger (SSH) chains considered as a paradigm example, the mechanism of HHG in the presence of edge states (ESs) is clarified from the perspective of the underlying three-step processes and their interference. This is done by analyzing the HHG contributions from the states of different types, with the energy of the ESs possibly being modified within the band gap. The ES energy shift introduces asymmetry in the transitions between the ESs and the two bands; in such a generic setting, it is found that the ES-induced HHG plateau can be either attributed to the contribution from the initially occupied ES alone, or alternatively understood as a consequence of interfering contributions from the ES and the valence band. On the basis of the three-step processes, it is shown that the ES-induced plateau can be described with semiclassical trajectories for the electron-hole dynamics in real space. This work also demonstrates the possibility of the ES-induced HHG being outweighed by the interband HHG for long chains, underlining the importance of the length scale of the system for observing the ES-induced HHG enhancement in practice.

Motivated by the recent realization of space-borne Bose-Einstein Condensates (BECs) under microgravity conditions, we extend the understanding of ultracold dipolar bosonic gases by exploring their behavior in a novel trapping configuration known as the “bubble trap” topology. Utilizing the three-dimensional numerical simulations within the extended Gross-Pitaevskii framework, we unveil diverse ground-state phases in this static curved topology. Subsequently, we investigate the influence of rotation on a dipolar BEC confined to the surface of a spherical bubble. Our findings reveal that the rotation of a bubble trap at certain rotation frequencies can modify the effective local dipole-dipole interaction strength, leading to the induction of supersolidity and the formation of quantum droplets. In addition, we demonstrate that a bubble trap can sustain high circulation, with the flow persisting for a longer time. Significantly, adjusting the rf detuning parameter allows the condensate to achieve hypersonic velocity. Finally, we also explore the impact of drastic change in the topological nature of the trap on the rotating dipolar BEC, transitioning from a filled shell trap to a bubble trap and vice versa. Based on the results of the topological transition, we propose a dynamic protocol to drive the interacting atomic gas into the quantum Hall regime.

We present a theoretical study of a polarization beam splitter (PBS), different from the conventional time-reversal symmetry one, where a specific linearly polarized light on two opposite sides can be totally reflected into the incident one and can be transparent to incident to its orthogonal linearly polarized light on only one side. We employ the mechanism via an exceptional point and finite periodicity of non-Hermitian $\mathcal{PT}$ symmetry. We design such a PBS made of a finite periodic structure in which each unit cell has a delicate balance of gain and loss in spatial distribution. In order to obtain total reflection for one of two orthogonal polarizations of the incident wave, the unit cell has to be operated at some $\mathcal{PT}$-symmetry phase of the reflection band. To have a single-sided transparency for its orthogonal polarized light, the unit cell should be designed at an exceptional point as well as have asymmetric reflectance. Interestingly, such a single-sided transparent phenomenon is independent of the total number of unit cells. We believe this unidirectional PBS may stimulate a new route to polarization control.