Manipulating the global $\mathcal{PT}$ symmetry of a non-Hermitian composite system is a rather significative and challenging task. Here, we investigate Floquet control of global $\mathcal{PT}$ symmetry in quadrimer waveguide arrays with transverse periodic structure along the $x$ axis and longitudinal periodic modulation along the $z$ axis. For the unmodulated case with inhomogeneous inter- and intraquadrimer coupling strength ${\kappa }_1\ne \kappa$, in addition to the conventional global $\mathcal{PT}$ symmetric phase and the $\mathcal{PT}$ symmetry breaking phase, we find that there is an exotic phase in which global $\mathcal{PT}$ symmetry is broken under open boundary condition, whereas it still is unbroken under periodical boundary condition. Especially, the domain of the exotic phase induced by boundary effect can shrink and even disappear by tuning modulation parameter. More interestingly, whether or not the array has initial global $\mathcal{PT}$ symmetry, periodic modulation can not only restore the broken global $\mathcal{PT}$ symmetry, but also control it by tuning modulation amplitude. Therefore, the global property of transverse periodic structure of such an array can be manipulated by only tuning modulation amplitude of longitudinal periodic modulation.

There is renewed interest in using the coherence between beams generated in separate down-converter sources for new applications in imaging, spectroscopy, microscopy, and optical coherence tomography (OCT). These schemes make use of continuous-wave (cw) pumping in the low parametric gain regime, which produces frequency correlations and frequency entanglement between signal-idler pairs generated in each single source. But can induced coherence still be observed if there is no frequency correlation, so the biphoton wave function is factorable? We will show that this is the case and might be an advantage for OCT applications. High axial resolution requires a large bandwidth. For cw pumping, this requires the use of short nonlinear crystals. This is detrimental since short crystals generate small photon fluxes. We show that the use of ultrashort pump pulses allows one to improve the axial resolution even for a long crystal that produces higher photon fluxes.

Bipartite temporal Bell inequalities are similar to the usual Bell inequalities except that, instead of changing the direction of the polarizer at each measurement, one changes the time at which the measurement is being performed. By doing so, one is able to test for realism and locality, but relying on position measurements only. This is particularly useful in experimental setups where the momentum direction cannot be probed (such as in cosmology for instance). We study these bipartite temporal Bell inequalities for continuous systems placed in two-mode squeezed states and find some regions in parameter space where they are indeed violated. We highlight the role played by the rotation angle, which is one of the three parameters characterizing a two-mode squeezed state (the other two being the squeezing amplitude and the squeezing angle). In single-time measurements, it only determines the overall phase of the wave function and can therefore be discarded, but in multiple-time measurements, its time dynamics becomes relevant and crucially determines when bipartite temporal Bell inequalities can be violated. Our study provides an opportunity for experimental designs for the observation of Bell inequality violations.

Einstein-Podolsky-Rosen (EPR) steering has potential applications in quantum information processing. Here the genuine tripartite EPR steering is investigated in cascaded nonlinear process of spontaneous parametric down-conversion cascaded with a sum-frequency generation in an optical cavity for a wide range of the nonlinear parameters. The threshold properties of the cascaded nonlinear precess are also analyzed both below and without threshold regime, respectively. The genuine tripartite EPR steering is demonstrated based on the criteria for genuine multipartite EPR steering. Our scheme of the generation of genuine tripartite EPR steering can be used as a suggestion for the potential experiments and the applications in quantum communication and computation.

The adiabatic approximation exhibits wide applicability in quantum mechanics, providing a simple approach for nontransitional dynamics in quantum systems governed by slowly varying time-dependent Hamiltonians. However, the standard adiabatic theorem is specifically derived for closed quantum systems. In a realistic open system scenario, the inevitable system-reservoir interaction must be taken into account, which strongly impacts the generalization of the adiabatic behavior. In this paper, we introduce sufficient conditions for the adiabatic approximation in open quantum systems. These conditions are simple yet general, providing a suitable instrument to investigate adiabaticity for arbitrary initial mixed states evolving under time local master equations. We first illustrate our results by showing that the adiabatic approximation for open systems is compatible with the description of quantum thermodynamics at thermal equilibrium, where irreversible entropy production is vanishing. We also apply our sufficient conditions as a tool in quantum control, evaluating the adiabatic behavior for the Hamiltonians of both the Deutsch algorithm and the Landau-Zener model under decoherence.

We study quantum coarse-grained entropy and demonstrate that the gap in entropy between local and global coarse-grainings is a natural generalization of entanglement entropy to mixed states and multipartite systems. This “quantum correlation entropy” $S^{q}c$ is additive over independent systems, is invariant under local unitary operations, measures total nonclassical correlations (vanishing on states with strictly classical correlation), and reduces to the entanglement entropy for bipartite pure states. It quantifies how well a quantum system can be understood via local measurements and ties directly to nonequilibrium thermodynamics, including representing a lower bound on the quantum part of thermodynamic entropy production. We discuss two other measures of nonclassical correlation to which this entropy is equivalent and argue that together they provide a unique thermodynamically distinguished measure.

We study the code obtained by concatenating the standard single-mode Gottesman-Kitaev-Preskill (GKP) code with the surface code. We show that the noise tolerance of this surface–GKP code with respect to (Gaussian) displacement errors improves when a single-mode squeezing unitary is applied to each mode, assuming that the identification of quadratures with logical Pauli operators is suitably modified. We observe noise-tolerance thresholds of up to $\sigma \approx 0.58$ shift-error standard deviation when the surface code is decoded without using GKP syndrome information. In contrast, prior results by K. Fukui, A. Tomita, A. Okamoto, and K. Fujii, High-Threshold Fault-Tolerant Quantum Computation with Analog Quantum Error Correction, Phys. Rev. X 8, 021054 (2018) and C. Vuillot, H. Asasi, Y. Wang, L. P. Pryadko, and B. M. Terhal, Quantum error correction with the toric Gottesman-Kitaev-Preskill code, Phys. Rev. A 99, 032344 (2019) report a threshold between $\sigma \approx 0.54$ and $\sigma \approx 0.55$ for the standard (toric, respectively) surface–GKP code. The modified surface–GKP code effectively renders the mode-level physical noise asymmetric, biasing the logical-level noise on the GKP qubits. The code can thus benefit from the resilience of the surface code against biased noise. We use the approximate maximum likelihood decoding algorithm of S. Bravyi, M. Suchara, and A. Vargo, Efficient algorithms for maximum likelihood decoding in the surface code, Phys. Rev. A 90, 032326 (2014) to obtain our threshold estimates. Throughout, we consider an idealized scenario where measurements are noiseless and GKP states are ideal. Our paper demonstrates that Gaussian encodings of individual modes can enhance concatenated codes.

Flag verification techniques are useful in quantum error correction for detecting critical faults. Here we present an application of flag-verification techniques for improving postselected performance of near-term algorithms. We extend the definition of what constitutes a flag by creating error-detection gadgets based on known transformations of unitary operators. In the case of Clifford or near-Clifford circuits, these unitary operators can be chosen to be controlled Pauli gates, leading to gadgets which require only a small number of additional Clifford gates. We show that such flags can improve circuit fidelities by up to a factor of 2 after postselection, and demonstrate their effectiveness over error models featuring single-qubit depolarizing noise, crosstalk, and two-qubit coherent overrotation.

Quantum device characterization via state tomography plays an important role in both validating quantum hardware and processing quantum information, but it needs the exponential number of the measurements. For the systems with XX$+$YY-type couplings and $Z$ readouts, such as superconducting quantum computing (SQC) systems, traditional quantum state tomography (QST) using single-qubit readout operations at least requires $3^n$ measurement settings in reconstructing an $n$-qubit state. In this paper, I propose an improved QST by adding two-qubit evolutions as the readout operations and obtain an optimal tomographic scheme using the integer programming optimization. I, respectively, apply the alternative scheme on SQC systems with the nearest-neighbor, two-dimensional, and all-to-all connectivities on qubits. It shows that this method can reduce the number of measurements by over 60% compared with the traditional QST. In addition, comparison with the traditional scheme in the experimental feasibility and robustness against errors was made by numerical simulation. It is found that the alternative scheme has good implementability and can achieve comparable or even better accuracy than the traditional scheme. It is expected that the experimentalist from related fields can directly utilize the ready-made results for reconstructing quantum states involved in their research.

I propose a quantum sensor based on a driven-dissipative quantum system for the joint estimation of two conjugated variables characterizing the phase-space displacement. The quantum probe consists of a lattice system with two-level atoms and bosonic modes which interact via dipolar coupling. Interplay between the coherent dynamics and dissipative processes of losses of bosonic excitations leads to a steady state which exhibits a nonanalytical behavior. I show that close to the dissipative phase transition, the sensitivity of one of the conjugated parameters, i.e., either the magnitude or the phase of the displacement, can be significantly enhanced. Moreover, I show that the sum of the measurement uncertainties of the two parameters can overcome the standard quantum limit.

The spontaneous two-photon decay rates from the $1s2s{}^{1}S_0$ level to the ground state $1s^2{}^{1}S_0$ in helium and its isoelectronic ions through neon ($Z=10$) are calculated, including the effects of finite nuclear mass. In all cases the length and velocity results agree to eight or more figures, demonstrating that the theoretical formulation correctly takes into account the effects of mass scaling, mass polarization, and motion of the nucleus in the center-of-mass frame. Algebraic relationships are derived and tested relating the expansion coefficients in powers of $\mu /M$ for mass polarization, where $\mu$ is the electron reduced mass and $M$ is the nuclear mass. Muonic, pionic, and antiprotonic helium are included as extreme test cases where mass polarization is large. The results are compared with experiment and previous calculations.

The strong-field approximation (SFA) has been widely applied to model ionization processes in short and intense laser pulses. Several approaches have been suggested in order to overcome certain limitations of the original SFA formulation with regard to the representation of the initial bound and final continuum states of the emitted electron as well as a suitable description of the driving laser pulse. We here present a reformulation of the SFA in terms of partial waves and spherical tensor operators that supports a quite simple implementation and the comparison of different treatments of the active (photo)electron and the laser pulses. In particular, this reformulation helps to adapt the SFA to experimental setups, and it paves the way to extend the strong-field theory toward the study of nondipole contributions in light-atom interactions as well as of many-particle correlations in strong-field ionization processes. A series of detailed computations have been carried out in order to confirm the validity of the reformulation and to show how the representation of the bound and continuum states affects the predicted above-threshold ionization spectra and related observables.

We developed a frequency-domain model describing optical frequency-comb generation in optical resonators with second- and third-order nonlinearities. Compared with time-domain models, our model in principle allows one to express the cavity dispersion accurately, avoiding the dispersion being truncated beyond a certain order. Moreover, the frequency-domain model can readily include frequency dependence of system parameters, such as the linear absorption and the cavity coupling ratio. To demonstrate the validity of our model, we numerically simulated quadratic combs in a singly resonant second-harmonic-generation cavity and Kerr combs in a microresonator as two examples. The simulated results obtained from the frequency-domain model agree well with those given by previous time-domain models. A system containing both second- and third-order nonlinearities can give rise to many novel physical dynamics. The developed frequency-domain model will contribute to understanding optical frequency-comb generation assisted by multiorder nonlinear processes in various optical resonators.

Spatial coherence is a fundamental property of light with many applications. However, if a light source is spatially incoherent and structured, its near-field coherence is difficult to evaluate directly because of the complexity of the emitted random light. In this work, we propose a general approach to calculate the near-field mutual intensity and transverse coherence length of propagating radiation from a periodically structured incoherent source using optical reciprocity. If the structure period is subwavelength and much smaller than the source, the mutual intensity reduces to a Fourier transform of the angular intensity spectrum of the radiation. Tailoring the near-field spatial coherence of incoherent sources by structuring can be used, e.g., for applications in optical sensing and imaging.

The generalized Rabi model (gRM) with both one- and two-photon coupling terms has been successfully implemented in circuit quantum electrodynamics systems. In this paper, we examine theoretically multiphoton resonances in the gRM and derive their effective Hamiltonians. We show that all three- to six-photon resonances can be achieved by involving two intermediate states with different detunings in the system. Furthermore, we study the interplay between multiphoton resonance and chiral transport of photon Fock states in a resonator junction with broken time-reversal symmetry. Depending on the qubit-photon interaction and photon-hopping amplitude, we find that the ultrastrong-coupling system can demonstrate different short-time dynamics.

We study multiphoton blockade and photon-induced tunneling effects in the two-photon Jaynes-Cummings model, where a single-mode cavity field and a two-level atom are coupled via a two-photon interaction. We consider both the cavity-field-driving and the atom-driving cases and find that single-photon blockade and photon-induced tunneling effects can be observed when the cavity mode is driven, while the two-photon blockade effect appears when the atom is driven. For the atom-driving case (the two-photon transition process), we present a criterion of the correlation functions for the multiphoton blockade effect. Specifically, we show that quantum interference can enhance the photon blockade effect in the single-photon cavity-field-driving case. Our results are confirmed by analytically and numerically calculating the correlation function of the cavity-field mode. Our work has potential applications in quantum information processing and paves the way for the study of multiphoton quantum coherent devices.

We propose and demonstrate real-time subwavelength cavity QED measurements of the spatial distribution of atoms in an optical lattice. Atoms initially confined in one “trap” standing wave of an optical cavity mode are probed with a second “probe” standing wave. With frequencies offset by one free spectral range, the nodes of the trap fall on the antinodes of the probe in the $\approx {10}^4$ lattice sites around the center of the cavity. This lattice site independent atom-cavity coupling enables high sensitivity detection of atom dynamics even with atoms spread over many lattice sites. To demonstrate, we measure the temperature of $20–70\mu \mathrm{K}$ atom ensembles in $<10\mu \mathrm{s}$ by monitoring their expansion by $\approx 100$ nm after sudden release from the trap lattice. Atom-cavity coupling imprints the atom dynamics on the probe transmission. The technique will enable improved nondestructive detection of Bloch oscillations and other atom dynamics in optical lattices.

We propose a protocol to examine many-polaron properties in a cold atom experiment. Initially, polaronic clouds are prepared around the opposite edges of a majority gas cloud. After time evolution, the collision of two clouds exhibits various polaronic effects. To see how collective properties of many polarons with mediated interactions appear in the case in which the impurity and majority gases are composed of mass-balanced fermions with different spin components, we perform a nonlinear hydrodynamic simulation for collisional dynamics of two Fermi polaronic clouds. We found that the dynamics is governed by the impurity Fermi pressure, polaron energy, and multipolaron correlations. In particular, shock waves occur in such a way as to reflect the many-body properties of polarons through the first sound of minority clouds. Our idea is applicable to other systems such as Bose polarons as well as mass-imbalanced mixtures.

The existence of nonvanishing Bohm potentials, in the Madelung-Bohm version of the Schrödinger equation, allows for the construction of particular solutions for states of quantum particles interacting with nontrivial external potentials that propagate as free classical particles. Such solutions are constructed with phases which satisfy the classical Hamilton-Jacobi for free particles and whose probability densities propagate with constant velocity, as free classical particles do.