We propose a systematic method to construct robust optimal quantum control at high orders by inverse optimization and apply it for the generation of ultrarobust optimal quantum gates. We derive explicit integrals that characterize robustness up to seventh order against pulse amplitude for time or energy minimization. At fifth order, time optimization is achieved for a flat pulse and a detuning featuring a dual-frequency oscillation. We analyze its performance and compare it to composite pulse techniques: Ultrarobust inverse optimization is considerably faster (in the range 16%–40% depending on the considered gate) than the best composite pulses with similar performance.

Entering the era of post-quantum supremacy has given one the ability to precisely control noisy intermediate-scale quantum (NISQ) processors with multiqubits and extract valuable quantum many-body correlation resources for many distinct quantum applications. We here construct quantum many-body thermalized states on a 62-qubit superconducting quantum processor and use them to demonstrate the principle of Maxwell's demon. We further demonstrate the direct effect caused by Maxwell's demon on the charging process of a quantum battery (QB). We depicted the nonequilibrium transportation in our QB through measuring the dynamics of the Shannon entropy to explore its working conditions. Finally, we evaluate the information-to-work conversion by varying the readout fidelity to verify the validity of the Sagawa-Ueda equality within the NISQ processor environment and evaluate the qubit-environment interaction such as the measurement backaction. Our experiment suggests that the superconducting NISQ processor with appropriate error mitigation methods will be an ideal platform for studying quantum information thermodynamics through quantum many-body simulations.

Signals of entanglement and nonlocality are quantitatively evaluated at zero and finite temperature in an analog black hole realized in the flow of a quasi-one-dimensional Bose-Einstein condensate. The violation of Lorentz invariance inherent to this analog system opens the prospect to observe three-mode quantum correlations and we study the corresponding violation of bipartite and tripartite Bell inequalities. It is shown that the long-wavelength modes of the system are maximally entangled, in the sense that they realize a superposition of continuous variable versions of Greenberger-Horne-Zeilinger states the entanglement of which resists partial tracing.

In this paper, we introduce a two-dimensional all-dielectric photonic higher-order topological insulator that is driven by the bianisotropic responses of electromagnetic fields. The nontrivial topology originates from staggered bianisotropy effects, which result in modulated coupling between neighbor sites under a fixed lattice constant. This model is generalized from an analogous Su-Schrieffer-Heeger model in one dimension and we show that it realizes a second-order photonic topological insulator with robust symmetry-protected corner modes in two dimensions. We also demonstrate that the topological property of such a model can be characterized by a quantized quadrupolar moment. Our paper introduces a different platform for studying higher-order topological photonics via bianisotropy, as well as a distinct method to engineer all-dielectric topological photonic meta-atoms.

Excited-state quantum phase transitions (ESQPTs) strongly influence the spectral properties of collective many-body quantum systems, changing degeneracy patterns in different quantum phases. Level degeneracies in turn affect the system's dynamics. We analyze the degeneracy dependence on the size of two-level boson models with a $u(n+1)$ dynamical algebra, where $n$ is the number of collective degrees of freedom. Below the ESQPT critical energy of these models, the energy gap between neighboring levels that belong to different symmetry sectors gets close to zero as the system size increases. We report and explain why this gap goes to zero exponentially for systems with one collective degree of freedom but algebraically in models with more than one degree of freedom. As a consequence, we show that the infinite-time average of out-of-time-order correlators is an ESQPT order parameter in finite systems with $n=1$, but in systems with $n>1$, this average only works as an order parameter in the mean-field limit.

In this work, we study the geometric phases and quantum entanglement of a composite system consisting of two interacting spin-1/2 particles in the presence of an external magnetic field. The conditions for real and complex eigenvalues are derived and discussed. Based on the non-Hermitian Berry phase, we propose the geometric phase of projective quantum states, discuss their relationship, and analyze their geometric properties. We expound on how the geometric phase of projective quantum states is contingent upon the coupling strength and the nonreciprocity between two subsystems, illustrating the connection between the geometric phase of the composite system and the geometric phases of its subsystems. By studying the quantum entanglement of the subsystems, the role of the correlation between the subsystem and the nonreciprocity is discussed. The results show that the quantum entanglement is influenced and determined by the nonreciprocity of the coupling. These schemes can provide a way to manipulate the nonreciprocal correlation between the subsystems to control the non-Hermiticity, geometric phase, and entanglement of the subsystems.

A conserved photon current is derived from the commutation relations satisfied by the electromagnetic four-potential and field tensor operators. The density is found to be a sum over positive and negative frequency terms, both of which contribute a positive number density and propagate in a common direction. Discrete positive and negative frequency excitations are both identified as photons. The photon number, equal to the spatial integral of the photon density, is conserved in the absence of sources and sinks.

Quantum coherence is an intrinsic property within a single quantum system. Many different coherent resource theories have been proposed to characterize this property within the framework of resource theory. This difference is based on the difference in the identification of “free” operations. In this article, generalized physically incoherent operations, which are strictly contained in the intersection of incoherent operations and dephasing-covariant operations (dephasing-covariant incoherent incoherent operations), are introduced. We characterize the Kraus operators of dephasing-covariant incoherent incoherent operations and generalized physically incoherent operations, and further study the coherent state transformations under generalized physically incoherent operations. Specifically, we investigate the constraints imposed by the rank of the initial state on the extraction of a pure coherent state. Additionally, we present a sufficient condition for the transformation from a mixed state to a pure state ensemble.

Learning probability distribution is an essential framework in classical learning theory. As a counterpart, quantum state learning has spurred the exploration of quantum machine learning theory. However, as dimensionality increases, learning a high-dimensional unknown quantum state via conventional quantum neural network approaches remains challenging due to trainability issues. In this work we devise the quantum sequential scattering model, inspired by the classical diffusion model, to overcome this scalability issue. Training of our model could effectively circumvent the vanishing gradient problem for a large class of high-dimensional target states possessing polynomial-scaled Schmidt ranks. Theoretical analysis and numerical experiments provide evidence for our model's effectiveness in learning both physically and algorithmically meaningful quantum states and outperform the conventional approaches in training speed and learning accuracy. Our work indicates that an increasing entanglement (a property of quantum states) in the target states necessitates a larger-scaled model, which could reduce our model's learning performance and efficiency.

Magic-state distillation is a protocol to purify noisy quantum states using quantum stabilizer codes. In this paper, we provide conditions for identifying magic states along with analytical expressions for computing the noise threshold. Through quantum entropic results, we prove the convergence of the magic-state distillation procedure. We derive analytical results to show how the noise threshold scales as a function of the minimum distance and the number of physical qudits of the underlying quantum stabilizer code. We distill $|T\rangle$ and $|S\rangle$ states with thresholds 0.498 (theoretical limit is 0.5) and 0.71 (theoretical limit is 0.75) over the depolarizing channel using ${\left[[31,1,16]\right]}_2$ quantum Bose-Chaudhuri-Hocquenghem code and ${\left[[29,1,15]\right]}_3$ quantum qutrit code, respectively. Finally, encoding algorithms based on quantum Calderbank-Shor-Steane (CSS) and non-CSS qudit stabilizer codes along with circuits are proposed for magic-state distillation.

Quantum sensing utilizing unique quantum properties of non-Hermitian systems to realize ultraprecision measurements has been attracting increasing attention. However, the debate on whether non-Hermitian systems are superior to Hermitian counterparts in sensing remains an open question. Here, we investigate the quantum information in $\mathcal{PT}$-symmetric quantum sensing utilizing two experimental schemes based on the trapped-ion platform and explore the relationship between $\mathcal{PT}$-symmetric quantum sensors and quantum resources. It turns out that the existence of advantages of non-Hermitian quantum sensing heavily depends on additional information resources carried by the extra degrees of freedom introduced to construct $\mathcal{PT}$-symmetric quantum sensors. Moreover, the practical application of non-Hermitian quantum sensing with superior performance is primarily restricted by the extra resource consumption accompanied by the postselection, while the superiority of non-Hermitian quantum sensing can be efficiently achieved in the presence of reasonable quantum resources. Our study provides theoretical references for the efficient construction of non-Hermitian quantum sensors with superior performance based on quantum resources and has potential applications in the research field of quantum precision measurement.

Earlier research highlighted the absence of certain bright peaks in Autler-Townes laser excitation spectra of alkali-metal atoms and attributed this phenomenon to a specific architecture of adiabatic (or “laser-dressed”) states in hyperfine (HF) components. Here we have studied the dressed states' properties in several two-photon excitation schemes where constructive and destructive interferences between HF excitation pathways limit the available two-photon transitions. The analysis reveals a mechanism by which this interference reduces the conventional two-photon selection rule for the total angular momentum $F$, from $\mathrm{\Delta }F=0,\pm 1$ to $\mathrm{\Delta }F\equiv 0$. Our discovery presents a practical method for selectively controlling the populations of unresolvable HF $F$ components of $n{s}_{1/2}$ Rydberg states in alkali-metal atoms. Using numerical simulations with sodium and rubidium atoms, we demonstrate that by blocking the effects of HF interaction with a specially tuned auxiliary control-laser field, the deviations from the ideal selectivity of the HF components population can be lower than $0.01\%$ for Na and $0.001\%$ for Rb atoms.

Over the past couple of decades, quantum simulators have been probing quantum many-body physics with unprecedented levels of control. So far, the main focus has been on the access to novel observables and dynamical conditions related to condensed-matter models. However, the potential of quantum simulators goes beyond the traditional scope of condensed-matter physics: Being based on driven-dissipative quantum optical platforms, quantum simulators allow for processes that typically are not considered in condensed-matter physics. These processes can enrich in unexplored ways the phase diagram of well-established models. Taking the extended Bose-Hubbard model as the guiding example, in this work we examine the impact of coherent pair injection, a process readily available in, for example, superconducting circuit arrays. The interest behind this process is that, in contrast to the standard injection of single excitations, it can be configured to preserve the $\text{U}\left(1\right)$ symmetry underlying the model. We prove that this process favors both superfluid and density-wave order, as opposed to insulation or homogeneous states, thereby providing a route towards the access of lattice supersolidity.

We study parametric resonances in a “temporal photonic crystal” or “photonic time crystal” (PTC) slab whose permittivity is assumed to be modulated periodically in time in a stepwise manner (with abrupt transitions). These resonances in the light reflected and transmitted by the PTC slab occur when the modulation frequency assumes a series of eigenvalues that are inversely proportional to the slab thickness, in addition to having a value that is twice that of the frequency of incidence. We compare the reflection and transmission spectra with those for a harmonically modulated slab and find marked, qualitative differences. These depend crucially on the modulation strength and on the impedance contrast at the slab's interfaces. In the special case of weak modulation, the differences in behavior between the two forms of modulation can be traced to a simple scaling factor. In principle, these findings could be corroborated with infrared light incident on PTC slabs tens of micrometers thick.

Quantumness refers to the peculiar and counterintuitive characteristics exhibited by quantum systems. Tsirelson inequalities have emerged as a powerful tool in quantum theory to detect quantumness and entanglement of harmonic oscillators, spins undergoing uniform precession, and anharmonic systems. In this paper we harness the versatility of Tsirelson inequalities to address two distinct problems: detecting cheating in classic shell games and probing quantumness in spatially separated systems and harmonic oscillators. By adopting a black-box approach and a geometric characterization of the space of conditional probabilities, we demonstrate that Tsirelson inequalities can be used in both scenarios, enabling us to uncover quantum signatures and identify cheaters in a single unified framework. This connection provides an intuitive different perspective on quantumness of mechanical systems.

The chiral dynamics of three-mode non-Hermitian systems with a periodical driving are studied. An exact chiral behavior of the relative phases of the final states after encircling with opposite directions in parameter space is obtained analytically with the solutions of the dynamical equations. It is shown that the relative phases of the evolution states are opposite numbers for the clockwise and counterclockwise loops of the driving parameters. But the density distributions have no chirality during the evolution. We examine the dynamical behavior by a numerical simulation of the evolution equations, which agrees well with our analytical predictions for the adiabatic evolution. The final density distributions are found to be independent of initial states and always the same for the clockwise and counterclockwise encircling, while in most examples discussed in the literature the amplitudes show a nonsymmetric behavior when the parameter space circles are in opposite directions. It is also found that the dynamical behavior is steady for the long-time evolution. We attribute the results to the special symmetry of the model, which might encourage others to work on physical realizations of the model.

We consider an all-optical system consisting of two whispering-gallery-mode microring resonators and two adjacent optical waveguides. Pumping fields from different directions can simultaneously excite both clockwise and counterclockwise modes in the two resonators. When the propagation direction of the excited photons aligns with the propagation direction of the photons excited by two-photon driving, satisfying the phase-matching conditions, stable nonreciprocal quantum entanglement and one-way Einstein-Podolsky-Rosen (EPR) steering can be effectively generated. This work demonstrates that quantum correlations can only be prepared when the system is driven by a pumping field from the unique port, a condition induced by the nonlinearity of the system. Furthermore, we show that the maximum values of quantum entanglement and one-way EPR steering can be achieved when the frequency detunings of the two optical modes are opposite, and higher two-photon driving strength and coupling strength significantly enhance quantum entanglement and one-way EPR steering. Our study provides a highly manipulable platform, offering promise for one-way quantum computing and quantum communication based on macroscopic entangled states.

In this paper, we propose a one-step scheme for implementing the nonadiabatic holonomic swap gate with Rydberg atoms. By applying invariant-based reverse engineering to design the effective Hamiltonian of the system, a suitable evolution path for implementing nonadiabatic holonomic quantum computation is found. In addition, the systematic-error-sensitivity nullified optimal control method is considered in the parameter selections, so that the scheme is insensitive to the systematic error of pulses. We also estimate the effects of random noise, the random initial phase of the pulses, the Doppler shift, and decoherence on the scheme. The numerical results show that the scheme exhibits fairly good performance against these negative factors. Finally, we generalize the scheme to realize the non-Clifford swap gates. Therefore, this scheme can provide a feasible framework for implementing high-fidelity and robust swap gates and non-Clifford swap gates with Rydberg atoms.

We investigate the double-ionization process of helium in brief and intense laser fields of 10, 16, and 40 optical cycles, linearly polarized with an intensity of $5\times {10}^{12}\text{W}{\text{cm}}^{-2}$. We calculate the total two-photon double-ionization cross section for photon energies ranging from 39.5 to 54 eV and the triple-differential cross sections at a photon energy of 42 eV, using the correlated Coulomb-Volkov approach, which takes into account the interaction of the laser field with each electron (Volkov phase) and interactions of Coulombian nature between electrons and between electrons and the nucleus. The proposed wave function has the advantage of being much less time consuming due to the fact that it does not require a numerical propagation of the wave packet. The results of our calculations obtained for the nonsequential double photoionization are compared with previously reported data and good agreement is found with approaches using projections onto uncorrelated continuum states. This indicates that the disagreements that remain between some theoretical models for the process studied cannot simply be attributed to the electronic correlation in the continuum states.