We study the ground-state properties of the extended Bose-Hubbard model in a one-dimensional dimerized optical lattice. In the limit of strong on-site repulsion, i.e., hardcore bosons, and strong nearest-neighbor interaction, a stable density-wave (DW) phase is obtained at half-filling as a function of lattice dimerization. Interestingly, at quarter-filling we obtain the signatures of an insulating phase which has the character of both the bond order (BO) and the DW insulators, which we call a bond-order density-wave (BODW) phase. Moreover, we show that for a fixed hopping dimerization there occurs a BO-DW phase crossover as a function of the nearest-neighbor interaction and the BODW phase is more robust when the hopping dimerization is stronger. We further examine the stability of the BODW phase in the limit of finite on-site interactions.

The interplay of topology and non-Hermiticity opens a new avenue for engineering novel topological matter and generating various unique effects. Here, we demonstrate that the non-Hermiticity can induce rich topological phase transitions in a long-range Su-Schrieffer-Heeger model. We find that the non-Hermiticity is able to drive topological transitions between different winding numbers: $\nu =0\to 1$ and $2\to 1$. These topological phase transitions can be characterized by the bulk band gap, edge states, complex Zak phase, and hidden Chern number. Interestingly, by extending to more general long-range Su-Schrieffer-Heeger lattices, the non-Hermiticity can drive exotic transitions associated with the corresponding Hermitian topological phases. Finally, we demonstrate the experimental feasibility of our scheme in an electric circuit system. Our paper could be useful for the study of non-Hermitian topological states and their device applications.

In the vicinity of ground-state phase transitions quantum correlations can display nonanalytic behavior and critical scaling. This signature of emergent collective effects has been widely investigated within a broad range of equilibrium settings. However, under nonequilibrium conditions, as found in open quantum many-body systems, characterizing quantum correlations near phase transitions is challenging. Moreover, the impact of local and collective dissipative processes on quantum correlations is not broadly understood. Here we consider as a paradigmatic setting the superradiant phase transition of the open quantum Dicke model and characterize quantum and classical correlations across the phase diagram. We develop an approach to quantum fluctuations which allows us to show that local dissipation, which cannot be treated within the commonly employed Holstein-Primakoff approximation, rather unexpectedly leads to an enhancement of collective quantum correlations, and to the emergence of a nonequilibrium superradiant phase in which the bosonic and spin degrees of freedom of the Dicke model are entangled.

Bohr's complementarity principle is quantitatively formulated in terms of the distinguishability of various paths a quanton can take and the measure of the interference it produces. This phenomenon results from the interference of single-quanton amplitudes for various paths. The distinguishability of paths puts a bound on the sharpness of the interference the quanton can produce. However, there exist other kinds of quantum phenomena where interference of two-particle amplitudes results in a two-particle interference, if the particles are indistinguishable. The Hong-Ou-Mandel (HOM) effect and the Hanbury-Brown-Twiss (HBT) effect are two well-known examples. However, two-particle interference is not as easy to define as its single-particle counterpart, and the realization that it involves interference of two-particle amplitudes came much later. In this work, a duality relation, between the particle distinguishability and the visibility of two-particle interference, is derived. The distinguishability of the two particles, arising from some internal degree of freedom, puts a bound on the sharpness of the two-particle interference they can produce, in a HOM or HBT kind of experiment. It is argued that the existence of this kind of complementarity can be used to characterize two-particle interference, which in turn leads one to the conclusion that the HOM and the HBT effects are equivalent in essence and may be treated as a single two-particle-interference phenonmenon.

In the process of using quantum resources to improve the performance of quantum battery devices, device performance is highly restricted by decoherence due to the influence of the external environment. In particular, the fact that such devices are subject to complex environmental influences in real scenarios should be taken into account. Here, we investigate the performance (i.e., the internal energy, charging power, and ergotropy) of a quantum battery in a double-layer environment, where the first-layer environment is a single-mode cavity and the second-layer environment is a single reservoir. We are surprised to find that the memory effect of the reservoir does not affect the charging performance of a quantum battery; in sharp contrast, the reduction in the coupling between the quantum battery and the first-layer environment can improve the charging performance of a quantum battery. We then extend our discussion to the case where the second-layer environment consists of multiple reservoirs, each consisting of a single-mode cavity dissipated to the Markovian reservoir. It is shown that the charging performance of the quantum battery can be enhanced by increasing the number of cavities in the second-layer environment and the coupling strength between the first-layer environment and the second-layer environment. Our results may be helpful for realizing optimal charging performance of quantum batteries in a complex environment.

We developed a full quantum time-dependent wave-packet evolution method to simulate the multibody breakups of polyatomic molecular ions. The interaction picture theory was used to propagate the wave packet, where the effects of strong and long-range interactions are considered by basis functions and the weak and short-range interactions are included in the perturbation term. In this method, the Coulomb interactions, polycentric interactions, vibration-rotation coupling of molecular fragments, and the rotation-rotation coupling between molecular fragment and parent molecule are considered accurately. Using this method, we simulate the two-body breakups of ${\mathrm{H}}_2{\mathrm{Ar}}^{2+}$ and ${\mathrm{N}}_2{\mathrm{Ar}}^{2+}$. The simulated results show that the ultrafast rotation of the molecular fragment generally exists in the breakup of the polyatomic molecular ion. For ${\mathrm{H}}_2{\mathrm{Ar}}^{2+}, {\mathrm{H}}_2^{+}$ can rotate from an initial broad distribution around ${35}^{\circ }$ and ${145}^{\circ }$ to a narrow distribution around ${75}^{\circ }$ and ${105}^{\circ }$ within 30 fs. For ${\mathrm{N}}_2{\mathrm{Ar}}^{2+}$, the initial sine-shaped angular distribution ($j$ = 0) shrinks to a narrow range (${90}^{\circ }\pm {20}^{\circ }$) within 120 fs and then spreads to an approximately random distribution within 300 fs. Such ultrafast rotations are dominated by the polycentric interaction between fragments and impacted by the vibration-rotation and rotation-rotation couplings.

Spectroscopy of alkali-metal–buffer-gas mixtures at high pressures from single-digit to several hundred bars in the regime of substantial collisional broadening is relevant in a wide range of fields, ranging from collisional redistribution cooling to laboratory astrophysics. Here we report on spectroscopic measurements of dense rubidium–noble-gas mixtures recorded in a pressure cell equipped with soldered sapphire optical viewports, which allows for the controlled realization of extreme conditions of high temperature and high pressure in a table top laboratory experiment. In the gas cell, we have recorded absorption and emission spectra of rubidium subject to 188 bars of helium buffer-gas pressure at 548-K temperature. The spectra to good accuracy follow a Boltzmann-type Kennard-Stepanov frequency scaling of the ratio of absorption and emission spectral profiles. Further, the long optical path length in the cell allowed us to both record spectra of rubidium-argon mixtures at moderate temperatures and high pressures and observe redistributional laser cooling in this system.

Motivated by the possibility of multiphoton-driven pump-probe experiments, such as time-resolved photoelectron spectroscopy, we carry out essential states's calculations of strong-field molecular excitation by solving the time-dependent Schrödinger equation for a molecule in a high-intensity laser field. Usually such calculations rely on adiabatic elimination, but here we make direct use of a large number of energies and transition dipole moments obtained from electronic structure calculations. In this way, we capture a range of multiphoton absorption orders, from 2 to 5, and include dynamic Stark shifts naturally. We consider a range of laser frequencies and intensities to characterize several multiphoton resonances and dynamic Stark shifts. The calculations also include averaging over molecular orientation and geometry, but are carried out for frozen nuclei, which is relevant in the limit of very short laser pulses ($<10$ fs). Here we focus on the molecule thiophene; however, these calculations can easily be implemented for other molecules with appropriate electronic structure input.

The coherent population trapping (CPT) effect is used for making compact atomic clocks. There are two types of CPT clocks: the one in which the Raman beams are applied continuously and the one in which two CPT pulses separated by a dark period are applied (Ramsey scheme). It is obvious that the technique of spin squeezing can only be applied to the Ramsey CPT clock to enhance the sensitivity. However, it is not apparent how to adapt to the CPT clock the protocols for the microwave clock using one-axis-twist squeezing (OATS), since the Ramsey CPT clock is not trivially equivalent to the Ramsey microwave clock. In this paper, we show explicitly how to adapt two protocols using OATS, namely, the Schrödinger cat state protocol (SCSP) and the generalization thereof, and the echo squeezing protocol (ESP), to the CPT clock. The ESP magnifies the phase shift by a factor of $\sqrt{N/e}$, while the SCSP magnifies the phase shift by a factor of $N/2$, making it able to achieve a higher sensitivity in the presence of excess noise.

Kerr frequency combs (KFCs) generated from continuous-wave pumped microresonators have been vastly exploited for a plethora of applications. Along with an appreciable bandwidth, most of the applications demand a stable and coherent frequency comb, which is a challenging quest. Several complex experimental approaches were reported to attain stable frequency combs. In this paper, we report an innovative and simple approach to achieve stabilized KFCs in a ${\text{Si}}_3{\text{N}}_4$ racetrack microring resonator. Intensive numerical simulations reveal an enhancement of the comb bandwidth when the temperature is reduced slightly lower than the room temperature. The maximum temperature rise due to the propagating dissipative Kerr soliton (DKS) has also been studied through finite element simulations. Through homogeneous steady-state analysis we validate that the stability of a single DKS state is enhanced at the temperatures reported in this paper. We believe that the proposed thermal route may help in reducing the complex experimental procedures for stabilization of KFCs.

We examine the problem of exhibiting Bell nonlocality for a two-qudit entangled pure state using a randomly chosen set of mutually unbiased bases (MUBs). Interestingly, even if we employ only two-setting Bell inequalities, we find a significant chance of obtaining a Bell violation if the two parties are individually allowed to measure a sufficient number of MUBs. In particular, for the case of maximally entangled qutrits and ququarts, our numerical estimates indicate that we can obtain near-guaranteed Bell violation by considering only such Bell inequalities. The case of maximally entangled ququints is similar, albeit the chance of ending up with a successful trial decreases somewhat to approximately $99.84\%$. Upon a closer inspection, we find that even all these no-violation instances violate some more-setting Bell inequalities. These results suggest that the experimental tests of Bell nonlocality for these higher-dimensional entangled states remain viable even if the two parties do not share a common reference frame.

Quantum systems can support irregular eigenstates at their threshold, which can be bound, loosely bound, or half bound. Such states are physically significant, and for instance half-bound states are known to lead to anomalous quantum scattering, where the reflection coefficient vanishes at the threshold rather than approach unity. Here, we present irregular threshold states which are generalizations of the above cases. The asymptotic behavior of these states can be tuned arbitrarily by precise control of the potential; hence, they are denoted “asymptotically tunable.” We provide exact analytical prescriptions on how to generate and control these systems. We explore several examples in 1D, including states that exhibit a power-law–like asymptotic scaling, and hybrid states that exhibit asymmetric boundary conditions (e.g., are fully bounded for $x\to \infty$ but unbounded for $x\to -\infty$, etc.). We numerically explore the scattering properties of these systems and find a close connection between the asymptotic behavior of the threshold states, and the appearance of anomalous scattering. We show that the threshold reflection coefficient can exhibit both discontinuities and derivative discontinuities as the system transitions from regular to irregular, which persist even under perturbations, and thus seem to not be quantum critical. These states could be useful for quantum system engineering, and for potential applications in optical systems for manipulating light.

We develop a classically verifiable quantum scheme for blindly factorizing the semiprime 21 for a classical client who does not trust the remote quantum servers. Our scheme advances state of the art, which achieves blind quantum factorization of 15, by increasing the problem to factorizing the next semiprime, choosing a harder base, executing a non-Clifford gate, and showing that the security check for 15 also works for 21. Our algorithmic approach to incorporating non-Clifford operations sets the stage for scaling blind quantum factorization, whereas our five-EPR-pair (Einstein-Podolsky-Rosen pair) scheme motivates a photonic experiment that supplants current demonstrations of blind factorization.

We implement nonlinear anharmonic interaction in the coupled van der Pol oscillators to investigate the quantum synchronization behavior of the systems. We study the quantum synchronization in two oscillator models, coupled quantum van der Pol oscillators and anharmonic self-oscillators. We demonstrate that the considered systems exhibit a high-order synchronization through coupling in both classical and quantum domains. We show that, due to the anharmonicity of the nonlinear interaction between the oscillators, the system exhibits phonon blockade in the phase-locking regime, which is a pure nonclassical effect and has not been observed in the classical domain. We also demonstrate that, for coupled anharmonic oscillators, the system shows a multiple resonance phase-locking behavior due to nonlinear interaction. We point out that the synchronization blockade arises due to strong anticorrelation between the oscillators, which leads to phonon antibunching in the same parametric regime. In the anharmonic oscillator case we illustrate the simultaneous occurrence of bunching and antibunching effects as a consequence of simultaneous negative and positive correlation between the anharmonic oscillators. We examine the aforementioned characteristic features in the frequency entrainment of the oscillators using a power spectrum where one can observe normal mode splitting and the Mollow triplet in the strong coupling regime. Finally, we propose a possible experimental realization for the considered system in the trapped ion and optomechanical setting.

The readout error on near-term quantum devices is one of the dominant noise factors, which can be mitigated by classical postprocessing called quantum readout error mitigation (QREM). The standard QREM applies the inverse of noise calibration matrix to the outcome probability distribution using exponential computational resources to the number of measured qubits. This becomes infeasible for the current quantum devices with tens of qubits or more. Here we propose two efficient QREM methods finishing in $O\left(n{s}^2\right)$ time for probability distributions of $n$ qubits and $s$ shots, which mainly aim at mitigating sparse probability distributions such that only a few states are dominant. We compare the proposed methods with several recent QREM methods in the following three cases: expectation values of the GHZ state, its fidelities, and the estimation error of maximum likelihood amplitude estimation (MLAE) algorithm with a modified Grover iterator. The two cases of the GHZ state are on real IBM quantum devices, while the third is with numerical simulation. Using the proposed method, the mitigation of the 65-qubit GHZ state takes only a few seconds, and we witness the fidelity of the 29-qubit GHZ state exceeding 0.5. The proposed methods also succeed in reducing the estimation error in the MLAE algorithm, outperforming the results by other QREM methods in general.

We discuss the Heisenberg limit in the multiparameter metrology within two different paradigms: one where the measurement is repeated many times (so the Cramér-Rao bound is guaranteed to be asymptotically saturable). and the second one where all the resources are allocated into one experimental realization (analyzed with the mimimax approach). We investigate the potential advantage of measuring all the parameters simultaneously compared to estimating them individually, while spending the same total amount of resources. We show that in general the existence of such an advantage, its magnitude, and conditions under which it occurs depends on which of the two paradigms has been chosen. In particular, for the problem of magnetic field sensing using $N$ entangled spin $\frac{1}{2}$, we show that the predictions based purely on the Cramér-Rao formalism may be overly pessimistic in this matter: the minimax approach reveals the superiority of measuring all the parameters jointly whereas the Cramér-Rao approach indicates lack of such an advantage.

We implement in an IBM quantum computer a quantum algorithm for multivariate polynomial factoring and propose a noise-avoiding protocol to distill the experimental results in the presence of noise. This algorithm uses single-qubit quantum state tomography (QST) processes to factor a specific type of multivariate polynomials. In one-to-one correspondence, it encodes each multivariate polynomial to one quantum state. While the validity of the algorithm is experimentally verified, the quality of the final results is subjected to the decoherence levels of the preparation of the quantum states. In this paper we propose a protocol to ensure the validity of factors found by our algorithm in the presence of such decoherence and noise. This method might be, in fact, part of a larger class of methods based on that same premise and useful outside the implementation of this specific algorithm. In combination with the noise robustness of the single-qubit QST, our factorization algorithm performs perfectly, even reversing the effects of weak noise, for the second- to fifth-order polynomial cases for which it has been implemented.

We have studied the mutual neutralization reaction of atomic iodine ions (i.e., ${\mathrm{I}}^{+}+{\mathrm{I}}^{-}\to \text{I}+\text{I}$) in a cryogenic double electrostatic ion-beam storage-ring apparatus. Our results show that the reaction forms iodine atoms either in the ground-state configuration ($\mathrm{I}\left(5p^5{}^2{P}^{\circ }\right)$, ∼40%) or with one atom in an electronically excited state ($\mathrm{I}(6s{}^2\left[2\right]$), $\sim 60\%$), with no significant variation over the branching ratios in the studied collision-energy range (0.1–0.8 eV). We estimate the total charge-transfer cross section to be of the order of ${10}^{-13}{\mathrm{cm}}^2$ at 0.1 eV collision energy. Ab initio relativistic electronic structure calculations of the potential-energy curves of ${\mathrm{I}}_2$ suggest that the reaction takes place at short internuclear distances. The results are discussed in view of their importance for applications in electric thrusters.

We present a theoretical approach for ab initio calculations of the one-loop QED corrections to energy levels of heavy diatomic quasimolecules. This approach is based on the partial-wave expansion of the molecular wave and Green's functions in the basis of monopole solutions, written in spherical coordinates. By using so-generated molecular functions, we employ the existing atomic-physics techniques to evaluate the self-energy and vacuum-polarization corrections. In order to illustrate the application of our method, we perform detailed calculations of the Dirac energy and QED corrections for the 1${\sigma }_g$ ground state of homonuclear ${\mathrm{U}}_2{}^{183+}$ and heteronuclear U-${\mathrm{Pb}}^{173+}$ and Bi-${\mathrm{Au}}^{161+}$ quasimolecules.

High harmonic generation (HHG) promises to provide insight into ultrafast dynamics and has been at the forefront of attosecond physics since its discovery. One class of materials that demonstrates HHG are Mott insulators whose electronic properties are of great interest given their strongly correlated nature. Here, we use the paradigmatic representation of Mott insulators, the half-filled Fermi-Hubbard model, to investigate the potential of using the HHG response to distinguish these materials. We develop a heuristic argument based on the Magnus expansion which predicts that distinguishability of Mott insulators will in most cases diminish as the dimensionless parameter $g\equiv a{F}_0/{\omega }_0$ becomes large, a result confirmed by numerical simulation. The notable exception is when comparing to a conducting system, which becomes more distinguishable from in the frequency domain at high $g$.