Quantum signal processing provides an optimal procedure for simulating Hamiltonian evolution on a quantum computer using calls to a block encoding of the Hamiltonian. In many situations it is possible to control between forward and reverse steps with almost identical cost to a simple controlled operation. We show that it is then possible to reduce the cost of Hamiltonian simulation by a factor of 2 using the recent results of generalized quantum signal processing.

A method for calculating the field shift contribution to isotope shifts (IS) in many-electron atoms, incorporating quantum electrodynamics (QED) effects, is introduced. We also implement the model QED approach to incorporate QED contribution to the nuclear recoil effect at the high-order correlation effects treatment level. The proposed computational scheme is used to revise the value of the root-mean-square (rms) nuclear charge radius of the isomer of aluminum-26, ${}^{26m}\mathrm{Al}$. This radius is important for the global analysis of the $V_{ud}$ element of the Cabibbo-Kobayashi-Maskawa matrix. The difference in mean-square nuclear charge radii of ${}^{27}\mathrm{Al}$ and ${}^{26m}\mathrm{Al}$, obtained by combining the calculated atomic factors with recently measured isotope shift of of the $3s^{2}3p{}^{2}P_{3/2}\to 3s^{2}4s{}^{2}S_{1/2}$ transition in Al, is $0.443\left(44\right)\left(19\right){\mathrm{fm}}^2$, where the first and second uncertainties are experimental and theoretical ones, respectively. The latter is reduced by a factor of 4 with respect to the previous study. Using this value and the known value of the rms charge radius of ${}^{27}\mathrm{Al}$, the resultant value $R_c{(}^{26m}\mathrm{Al})=3.132(10)$ fm is obtained. With the improved accuracy of the calculated IS factors, the error in $R_c{(}^{26m}\mathrm{Al}$) is now dominated by the experimental uncertainty. Similar revision of rms charge radii is made for the ${}^{28}\mathrm{Al}$, ${}^{29}\mathrm{Al}$, ${}^{30}\mathrm{Al}$, ${}^{31}\mathrm{Al}$, and ${}^{32}\mathrm{Al}$ isotopes using existing IS measurements. Additionally, atomic factors are computed for the $3s^{2}3p{}^{2}P_{3/2}\to 3s^{2}4s{}^{2}S_{1/2}$, $3s^{2}3p{}^{2}P_{1/2}\to 3s^{2}5s{}^{2}S_{1/2}$, and $3s^{2}3p{}^{2}P_{3/2}\to 3s^{2}5s{}^{2}S_{1/2}$ transitions in Al, which can be used in future experimental studies.

A picosecond laser is used to realize atomic beam splitters based on stimulated Raman transitions. With this approach, the interaction between the laser and the atom is localized in the overlap zone of pairs of counterpropagating picosecond pulses. This imposes constraints for implementing interferometers on free-falling atoms. We have developed a robust technique to control the pulses' overlap and ensure that it follows the atom's trajectory while minimizing the induced laser phase noise. We also demonstrate an atom interferometer where the atomic beam splitters are applied to one arm of the interferometer without interacting with the atomic wave packet propagating along the other arm.

An easy-plane ferromagnetic spin-1 Bose gas undergoes two Berezinskii-Kosterlitz-Thouless transitions, associated with mass and spin superfluidity, respectively. We study the effect of axial magnetization on the superfluid properties of this system. We find that nonzero axial magnetization couples mass and spin superflow, via a mechanism analogous to the Andreev-Bashkin effect present in two-component superfluids. With sufficiently large axial magnetization mass and spin superfluidity arise simultaneously. The crossover to this phase provides a finite-temperature generalization of the zero-temperature broken-axisymmetric to easy-axis transition. We present analytic relations connecting mass and spin superfluidity with experimentally observable coherence of the three spinor components and local magnetization.

Recently, the statistical effect of fermionic superradiance has been verified by a series of experiments both in free space and in a cavity. The Pauli blocking effect can be visualized by a 1/2 scaling of the Dicke transition critical pumping strength against the particle number $N_{\mathrm{at}}$ for fermions in a trap. However, the evidence for the Fermi surface (FS) nesting effect, which manifests the enhancement of superradiance by Fermi statistics, is still missing. Here we report a scheme for detecting FS nesting with the help of the trap-induced localization on the trap edge. We find two scalings of the critical pumping strength as $N_{\mathrm{at}}^{-1.33}$ and $N_{\mathrm{at}}^{-0.67}$ for a moderate particle number when localization enters and the Pauli blocking scaling $1/3$ (two-dimensional case) in the $N_{\mathrm{at}}\to \infty$ limit is unaffected. Further, we find the scaling of the critical pumping strength against the particle number increases with the ratio between the recoil energy and the trap frequency in the direction orthogonal to the pumping direction $E_R/{\omega }_z$. The scaling larger than 1 can be identified as a result of the Fermi surface nesting effect. Thus, we find a practical experimental scheme for visualizing the long-desired Fermi surface nesting effect with the help of trap-induced localization in a two-dimensional Fermi gas in a cavity.

Thermal-energy atom scattering at a surface under grazing incidence conditions is an innovative method for investigating dispersive atom-surface interactions with potential application in quantum sensing interferometry. The complete establishment of this technique requires a detailed peak analysis, which is yet to be achieved. We examine peak-width fluctuations in atomic and molecular beams diffracted by a grating under grazing incidence conditions. Careful measurements and analyses of the diffraction patterns of He atoms and ${\mathrm{D}}_2$ molecules from three square-wave gratings with different periods and radii of curvature enable the identification of factors influencing the peak-width variations as a function of incidence angle. The effects of macroscopic surface curvature, grating magnification, and beam emergence are substantial under these conditions but negligible for incidence angles close to the normal. Our results shed light on the phenomena occurring in grazing incidence thermal-energy atom scattering.

Orbital angular momentum (OAM), as a physical dimension of light, has been demonstrated to enhance the channel capacity and turbulence resistance of free-space optical (FSO) communication. However, the channel crosstalk in OAM-based FSO communication inevitably increases with transmission distance and turbulence intensity. Here, we propose a deep-learning-based recognition of a composite vortex beam to extend the regime of moderate-to-strong turbulence and long-distance FSO links. The composite vortex beam is generated by a coherent combination of two subbeams carrying different helical charges and phase delays, providing its helical charges and phase delay as new multiplexing dimensions and exhibiting better turbulence resistance compared to a single subbeam. We also developed a modified regular network to achieve the high-accuracy recognition of a composite vortex beam over a long distance at moderate-to-strong atmospheric turbulence. We believe that our approach has potential in deep-learning-based OAM high-capacity communication systems.

Bell correlations are among the most exotic of the phenomena through which quantum mechanics manifests itself. Their presence signals that the system may violate the postulates of local realism—conjectures once thought to be to be a non-negotiable property of the physical world. The importance of Bell correlations from this fundamental point of view is even further clarified by their applications, ranging from quantum cryptography to quantum metrology and quantum computing. It is therefore of growing interest to characterize the “Bell content” of complex, scalable many-body systems. Here, we perform a detailed analysis of the character and strength of many-body Bell correlations in interacting multiqubit systems with particle exchange symmetry. Such a configuration can be described by an effective Schrödinger-like equation, which allows precise analytical predictions. We show that in the vicinity of the quantum critical point, these correlations quickly become so strong that only a fraction of the qubits remain uncorrelated. We also identify the threshold temperature above which thermal fluctuations destroy Bell correlations. We hope that the approach presented here, due to its universality, will be useful for upcoming research on genuinely nonclassical Bell-correlated complex systems.

We investigate the temperature uncertainty relation in the nonequilibrium probe-based temperature estimation process. We demonstrate that it is the fluctuation of heat that fundamentally determines temperature precision through the temperature-heat uncertainty relation. Specifically, we find that heat is divided into trajectory heat and correlation heat, which are associated with the heat exchange along the thermometer's evolution and the correlation between the thermometer and the sample, respectively. Based on two types of thermometers, we show that both of these heat terms are resources for enhancing temperature precision. By clearly distinguishing the resources for enhancing estimation precision, our findings not only explain why various quantum features are crucial for accurate temperature sensing but also provide valuable insights for designing ultrahigh-sensitive quantum thermometers. Additionally, we demonstrate that the temperature-heat uncertainty relation is consistent with the well-known temperature-energy uncertainty relation in thermodynamics. It establishes a connection between the information theory and the thermodynamics.

Quantum digital signatures (QDSs), which distribute and measure quantum states by key generation protocols and then sign messages via classical data processing, are a key area of interest in quantum cryptography. However, the practical implementation of a QDS network has many challenges, including complex interference technical requirements, linear channel loss of quantum state transmission, and potential side-channel attacks on detectors. Here, we propose an asynchronous measurement-device-independent (MDI) QDS protocol with asynchronous two-photon interference strategy and one-time universal hashing method. The two-photon interference approach protects our protocol against all detector side-channel attacks and relaxes the difficulty of experiment implementation, while the asynchronous strategy effectively reduces the equivalent channel loss to its square root. Compared to previous MDI-QDS schemes, our protocol shows several orders of magnitude performance improvements and doubling of transmission distance when processing multibit messages. Our findings present an efficient and practical MDI-QDS scheme, paving the way for large-scale data processing with nonrepudiation in quantum networks.

The neutralization probability in the scattering of $\mathrm{N}{\mathrm{e}}^{+}$ ions by a molybdenum disulfide semiconductor $\left(\mathrm{Mo}{\mathrm{S}}_2\right)$ surface in a backscattering geometric configuration is experimentally and theoretically analyzed. The low-energy ion scattering technique was used to experimentally determine the neutral fractions of Ne projectiles backscattered by the Mo atoms of the $\mathrm{Mo}{\mathrm{S}}_2$ target surface after a single collision. A high neutralization rate in the backscattered projectiles (of the order of 95%) was obtained, practically independent of the projectile incoming energy and the collision geometry. Additionally, the remaining dispersed ions were measured to be all positive, within the experimental error. An identical experiment was conducted on a polycrystalline metallic molybdenum surface to assess for differences between the two target surfaces, with completely different electronic properties. A quantum-mechanical formalism was employed to theoretically describe the projectile-surface resonant charge transfer during the binary collision of Ne projectiles with both surfaces. Our calculations show that the inner states of Mo atoms play a central role in the resonant charge transfer in the $\mathrm{Ne}\text{/}\mathrm{Mo}{\mathrm{S}}_2$ system. From the comparison with the experimental results, we can conclude that the resonant charge transfer to the projectile ground state is not sufficient to explain the large neutralization of the Ne projectiles observed for both surfaces. An Auger neutralization mechanism and resonant charge transfer to the excited states of the projectile are expected to be particularly relevant for polycrystalline Mo and $\mathrm{Mo}{\mathrm{S}}_2$ target surfaces, respectively.

The nature of electronic motion and structural information of atoms and molecules is encoded into strong-field-induced radiation ranging from terahertz (THz) to extreme ultraviolet wavelength. The dependence of THz yields in bichromatic laser fields on ellipticity and interpulse phase delay was experimentally measured, and the trajectory calculations establish the link between the THz emission and the motion of the photoelectron wave packet. The interaction between the photoelectron and the parent core transforms from a soft collision to recollision as the laser field is tuned from elliptical to linear polarization, which can be reflected in THz emission. The soft collision is found to be more effective in reconstructing electron dynamics through THz polarization, which enables us to construct the effective core potential of the generating medium with the Coulomb-reshaped THz radiation in an elliptically polarized laser field. Our work enables us to design innovative all-optical THz measurements of electronic and structural dynamics.

Quantum electrodynamics (QED) predicts the phenomenon of electron-positron pair production via the decay of a high-energy photon in the presence of strong external fields. Such photons can be generated due to nonlinear Compton scattering involving electrons propagating in an intense external background. Here we investigate the possibility of extracting superhigh laser intensities by measuring the positron yield. Our numerical simulations based on the QED rates of the two aforementioned processes provide the total number of positrons as a function of the laser intensity for two different setups: a single focused laser pulse and a combination of two counterpropagating pulses. As seed particles, we consider a free-electron gas and a gas of neutral Xe atoms. In this study, special focus is placed on taking into account the cascade process of pair production, which can occur in the case of superhigh laser intensities. It is demonstrated that the contribution of the QED cascade is not important in the vicinity of the positron generation threshold; i.e., an extended intensity domain where the laser field produces a substantial number of positrons but does not yet launch the cascade exists. Within this range, the intensity diagnostics is particularly accurate. By adjusting the geometry of the experimental setup, one can change the corresponding threshold value of the laser intensity in order to maximize the accuracy of the diagnostic scheme.

The recent observations of a stable molecular condensate emerging from a condensate of bosonic atoms and related “super-chemical” dynamics have raised an intriguing set of questions. Here we provide a microscopic understanding of this unexpected stability and dynamics in atom-molecule superfluids; we show one essential element behind these phenomena is an extremely narrow Feshbach resonance in ${}^{133}\mathrm{Cs}$ at 19.849G. Comparing theory and experiment, we demonstrate how this narrow resonance enables the dynamical creation of a large closed-channel molecular fraction superfluid, appearing in the vicinity of unitarity. Theoretically, the observed superchemistry (i.e., Bose-enhanced reactions of atoms and molecules) is found to be assisted by the formation of Cooper-like pairs of bosonic atoms that have opposite momenta. Importantly, this narrow resonance opens the possibility to explore the quantum critical point of a molecular Bose superfluid and related phenomena which would not be possible near a more typically broad Feshbach resonance.

Through considering a two-component dipolar Bose-Einstein condensate, we investigate the influence of the angle between the polarization orientations of the two species on the ground states and show that the miscibility between the two components can be adjusted not only by the inter-component contact interaction but also by the polarization angle. Particularly, in the presence of an external confinement, the two species exhibit a preference for splitting into multiple droplets separated from each other in the immiscible regime, featuring a zig-zag-like profile. Furthermore, the number of separated droplets depends solely on the polarization angle. This introduces a promising degree of freedom for exploring emergent states of matter in dipolar quantum gases.

Ultracold atomic and molecular systems in the past years have significantly advanced the exploration of topological phases of matter. Notably, the recent experimental progress achieved in ultracold molecules, particularly within optical-tweezer lattices and leveraging long-range dipolar interactions, has opened up new avenues for this prospect. Here, we propose the creation of an ultracold-molecule trimerized Kagome optical-tweezer lattice to examine the topological phases induced by long-range dipolar interactions. Through a comparison of topological features in scenarios with and without long-range couplings, we find that the long-range interactions can induce higher-order topological phases with diverse topological corner modes at each corner, a phenomenon that can even emerge in nonstandard trimerized Kagome lattices. Furthermore, we develop a dynamic method to spectrally and spatially detect these topological corner modes. Our research not only underscores that long-range interactions can engender much richer topological phases but also suggests exciting possibilities for designing reconfigurable topological lattices.

We investigate the existence and stability of vortexlike collective excitations in nonlinear (NL) multicore fibers (MCFs). We focus on realistic cored and coreless hexagonal MCFs where every core operates in the single-mode regime while displaying polarization degeneracy. We model the propagation dynamics of vortex fields in such MCF structures by NL discrete Schrödinger equations, finding families of homogeneouslike NL vortex fields with constant power distribution over peripheral cores. By analyzing the stability of such collective modes with distinct topological charges against perturbations, we find that they are all unstable. We further investigate the existence and stability of inhomogeneous NL vortex fields, finding only one stable nonspinning localized NL mode with vanishing topological charge in a narrow power range only for cored hexagonal MCFs. Our results are relevant for the manipulation and control of collective excitations in NL MCFs, indicating that self-organization into stable localized collective modes can be exploited in innovative MCF-based communication systems.

We study discrete time crystals (DTCs) in periodically driven quantum systems, in the presence of non-Markovian dissipation. In contrast to DTCs observed in earlier works in the presence of Markovian dynamics, using the open Dicke model in presence of Jaynes-Cummings-like dissipation, we show that non-Markovian regime can be highly beneficial for stabilizing DTCs over a wide range of parameter values. This may be attributed to periodically varying dissipation rates even at long times in the case of non-Markovian dynamics. Further the Markovian and non-Markovian regimes show sharp distinctions for intermediate strengths of the dissipator coefficient, with a time-independent steady state in the Markovian regime being replaced by varied dynamical phases, including DTC order, in the non-Markovian regime. We also verify the robustness of the DTC phase in the non-Markovian regime by introducing errors both in the Hamiltonian as well as in the dissipation. Our study shows the possibility of using DTC as a probe for non-Markovian dynamics in periodically modulated open quantum systems, at long times.

Non-Hermitian periodic systems possess unique properties not found in their Hermitian counterparts, including non-Hermitian skin effects in the open-boundary-condition spectrum and nontrivial braiding of the bulk band structure. Here, by viewing one-dimensional non-Hermitian band structures as Riemann surfaces, we show that the monodromy representation, a group homomorphism from the fundamental group of a punctured complex plane to the permutation group operating on the ordering of the Riemann sheets, serves as a topological invariant of the system. The connection between monodromy representations and the experimental observable effects is established through the branch cuts induced by the underlying multivalued functions. An open-boundary spectrum is interpreted as branch cuts connecting certain branch points, and its consistency with the monodromy representation severely limits its possible morphology. A braid word along a closed loop is controlled by the number and permutation labels of branch points within the loop, and its crossing number is given by the winding number of the discriminant. The importance of the monodromy representation as a topological invariant and the analysis of the Riemann surface geometry as defined by the band structure can be used to generate important insights about the physical behaviors of non-Hermitian systems.

Estimating the fidelity with a target state is important in quantum information tasks. Many fidelity-estimation techniques present a suitable measurement scheme to perform the estimation. In contrast, we present techniques that allow the experimentalist to choose a convenient measurement setting. Our primary focus lies on a method that constructs an estimator with nearly minimax optimal confidence intervals for any specified measurement setting. We demonstrate, through a combination of theoretical and numerical results, various desirable properties of the method: robustness against experimental imperfections, competitive sample complexity, and accurate estimates in practice. We compare this method with maximum likelihood estimation and the associated profile likelihood method, a semi-definite programming based approach, direct fidelity estimation, quantum state verification, and classical shadows. Our method can also be used for estimating the expectation value of any observable with the same guarantees.