In this paper, a one-step protocol is proposed for the parity measurement of $N$ cat-state qubits which are encoded on the cat states of the modes in superconducting Kerr-nonlinear cavities. The parity measurement is performed with the help of an auxiliary qutrit. Especially, the auxiliary qutrit can (cannot) be excited to the higher-energy levels when the cat-state qubits are in the even- (odd-) parity state. By designing the Rabi frequency of the classical fields via reverse engineering and optimal control, the qutrit is driven to an excited dressed state in the even-parity case, which is robust to the systematic errors of the qutrit-cavity coupling strengths. Accordingly, the parity of the cat-state qubits can be distinguished with high accuracy by measuring the final population of the ground state of the auxiliary qutrit. Numerical simulations also show that the protocol is insensitive to the systematic errors of the classical fields, the inhomogeneity of the coupling strengths, intercavity cross talk, unwanted qutrit transitions, and decoherence. Therefore, the protocol may provide an effective approach for parity measurement of multiple cat-state qubits.

The extension from point-to-point quantum key distribution (QKD) to multipoint communication is an inevitable trend toward the large-scale development of QKD networks. At present, it is a relatively simple and cost-effective implementation to integrate quantum access networks (QANs) into existing Ethernet passive optical access networks (EPONs). This work proposes a quantum secured 10 Gbit/s EPON (10G-EPON), in which a plug-and-play twin-field QKD (PnP-TF-QKD) architecture is developed, requiring a single untrusted laser and a pair of shared detectors. The PnP-TF-QKD implementation is compatible with standard 10G-EPON and can support up to 64 users. In addition, prior to the deployment of QKD on EPON, it is crucial to predict in advance whether the attainable QAN indicators, such as the secure key rate, the maximum feeder fiber length, etc., meet the requirements of a certain circumstance. However, this task is often complex and time consuming. Therefore, we present a machine-learning-based user-demand-oriented prediction model to facilitate the evaluation of QAN parameters. The plug-and-play twin-field quantum access network (PnP-TF-QAN) and its machine-learning-assisted implementation method provide guidance for further experiments and actual deployments of large-scale QANs.

In the last years, we have been witnessing a tremendous push to demonstrate that quantum computers can solve classically intractable problems. This effort, initially focused on the hardware, progressively included the simplification of the models to be simulated. We consider Hamiltonians written in terms of Pauli operators and systematically cut all qubits that are not essential to simulate the system. Our approach is universally applicable and lowers the complexity by first ensuring that the largest possible portion of the Hilbert space becomes irrelevant, and then by finding and exploiting all conserved charges of the system, i.e., symmetries that can be expressed as Pauli operators. Remarkably, both processes are classically efficient and optimal. To showcase our algorithm, we simplify chemical molecules, lattice gauge theories, and the Hubbard and Kitaev models.

Doppler-broadening gas thermometry is extended to the deep-ultraviolet domain using a comb-locked laser frequency chain, expressly developed for precision spectroscopy of mercury vapors at the wavelength of 253.7 nm, in coincidence with the $\left(6s^2\right){}^{1}S_0\to \left(6s6p\right){}^{3}P_1$ intercombination line. The system is based on a double-stage second-harmonic generation process of an external-cavity diode laser at 1014.8 nm, which is frequency locked to a self-referenced optical-frequency-comb synthesizer by means of an efficient nonlinear frequency-mixing scheme. An absolute frequency axis in the UV region is produced by scanning the comb repetition rate, with the entire frequency chain following the reference comb tooth. The complete characterization of the phase noise of the comb-locked near-infrared laser demonstrates line-emission narrowing down to the limit imposed by the coherence of the comb teeth. After a proper processing of the phase noise, the Gaussian and Lorentzian components of the width of the UV radiation are determined. This is useful to estimate the instrumental perturbation to the probed mercury line for the aims of thermodynamic temperature metrology. In this regard, the preliminary spectroscopic results are quite promising, despite the difficulties arising from the short wavelength of operation.

We describe a theoretical scheme for generating scalable spin squeezing with nearest-neighbor interactions between spin-1/2 particles in a three-dimensional (3D) lattice, which are naturally present in state-of-the-art 3D optical lattice clocks. We propose to use strong isotropic Heisenberg interactions within individual planes of the lattice, forcing the constituent spin-1/2s to behave as large collective spins. These large spins are then coupled with $XXZ$ anisotropic interactions along a third direction of the lattice. This system can be realized via superexchange interactions in a 3D optical lattice subject to an external linear potential, such as gravity, and in the presence of spin-orbit coupling to generate spin anisotropic interactions. We show there is a wide range of parameters in this setting where the spin squeezing improves with increasing system size even in the presence of holes.

We investigate the homogeneous superfluid mixtures of a Bardeen-Cooper-Schrieffer (BCS) superfluid originating from pairing two-species fermionic atoms and superfluidity stemming from condensation of bosonic atoms. By integrating out the freedoms associated with the BCS superfluid, we derive the fermion-mediated interactions between bosons, which is attractive and can be tuned from long range in the BCS region to short range in the region of Bose-Einstein condensation of molecular dimers. By analyzing the Bogoliubov spectrum and the damping rate of bosonic superfluid, we map out the phase diagram spanned by the boson-fermion mass ratio and the boson-fermion coupling strength, which consists of a phase separation region and two phase-mixing regions with and without Landau damping. The three different phases can coexist at a tricritical point, which moves toward a low boson-fermion mass ratio and a high boson-fermion scattering length as the fermion-fermion interaction strength is tuned up on the BCS side.

We provide a theoretical framework based on classical electromagnetism to describe optical properties of Fabry-Pérot cavities filled with multilayered and linear chiral materials. We find a formal link between transfer-matrix, scattering-matrix, and Green-function approaches to compute the polarization-dependent optical transmission, and cavity-modified circular dichroism signals. We show how general symmetries like Lorentz's reciprocity and time-reversal symmetry constrain the modeling of such cavities. We apply this approach to investigate numerically and analytically the properties of various Fabry-Pérot cavities, made of either metallic or helicity-preserving dielectric photonic crystal mirrors. In the latter case, we analyze the onset of chiral cavity polaritons in terms of partial helicity preservation of electromagnetic waves reflected at the mirror interfaces. Our approach is relevant for designing innovative Fabry-Pérot cavities for chiral sensing and for probing cavity-modified stereochemistry.

We investigate theoretically the chaotic dynamics in an optomechanical system composed of coupled optical resonators. We find that introducing additional loss through a nanotip can induce chaotic motion. The underlying reason for this unconventional phenomenon lies in steering the system parameters via an additional loss that can bring the system to the vicinity of a chaotic regime, which dynamically enhances the optomechanical nonlinearity and suppresses the negative influence of loss, giving rise to the emergence of chaotic motion. Our work may open a different avenue for designing and developing chaotic systems in optomechanics and provide theoretical guidance for chaotic secure communication.

We investigate the scattering processes of two photons in a one-dimensional waveguide coupled to two giant atoms. By adjusting the accumulated phase shifts between the coupling points, we are able to effectively manipulate the characteristics of these scattering photons. Utilizing the Lippmann-Schwinger formalism, we derive analytical expressions for the wave functions describing two-photon interaction in separate, braided, and nested configurations. Based on these wave functions, we also obtain analytical expressions for the incoherent power spectra and second-order correlation functions. In contrast to small atoms, the incoherent spectrum, which is defined by the correlation of the bound state, can exhibit more tunability due to the phase shifts. Additionally, the second-order correlation functions in the transmission and reflection fields could be tuned to exhibit either bunching or antibunching upon resonant driving. These unique features offered by the giant atoms in waveguide QED could benefit the generation of nonclassical itinerant photons in quantum networks.

To prepare quantum states and extract information, it is often assumed that one can perform a perfectly projective measurement. Such measurements can achieve an uncorrelated system and environment state. However, perfectly projective measurements can be difficult or impossible to perform in practice. We investigate the limitations of repeated nonprojective measurements in preparing a quantum system. For an $n$-qubit system initially entangled with its environment and subsequently prepared with measurements, using a sequence of weak measurements, we show that some entanglement remains unless one of the measurement operators becomes perfectly projective through an extreme limiting process. Removing initial (unentangled) correlations between a system and its environment and the scenario where measurement outcomes are not tracked are also discussed. We present results for $n$-qubit and $n$-dimensional input states.

While the Landau-Lifshitz equation, which describes classical radiation reaction, can be solved exactly and analytically for a charged particle accelerated by a plane electromagnetic wave, no such solutions are available for quantum radiation reaction (the recoil arising from the successive, incoherent emission of hard photons). Yet upcoming experiments with ultrarelativistic electron beams and high-intensity lasers will explore the regime where both radiation-reaction and quantum effects are important. Here we present analytical solutions for the mean and variance of the energy distribution of an electron beam that collides with a pulsed plane electromagnetic wave, which are obtained by means of a perturbative expansion in the quantum parameter ${\chi }_0$. These solutions capture both the quantum reduction in the radiated power and the stochastic broadening, and are shown to be accurate across the range of experimentally relevant collision parameters, i.e., GeV-class electron beams and laser amplitudes $a_0\lesssim 200$.

We study how the successive scattering of spin-1/2 particles with a central spin-1/2 target particle can generate entanglement between the helicity degrees of freedom of all scattered particles, effectively producing a multipartite entangled state. We show that the bipartite entanglement between each pair of scattered particles, as quantified by the concurrence, is largest for reflected particles and decreases with the number of scatterings. We study the entanglement generation as a function of the scattered particles momenta, angular distribution, and mass ratios, and show that there is always a combination of optimal helicities and momentum, which generate the largest amount of bipartite entanglement.

The spectra of rogue waves of Manakov equations that exist in both focusing and defocusing regimes are derived in analytic form. These spectra are asymmetric during their whole expansion-contraction cycle. They have triangular shape at each side of the spectrum in the log scale. Such spectra are characterized by two quantities: the slopes of the spectra and the spectral jump at zero frequency. We confirm our analytical results using numerical simulations.

Non-Gaussian quantum states are crucial to fault-tolerant quantum computation with continuous-variable systems. Usually, generation of such states involves trade-offs between success probability and quality of the resultant state. For example, injecting squeezed light into a multimode interferometer and postselecting on certain patterns of photon-number outputs in all but one mode, a fundamentally probabilistic task, can herald the creation of cat states, Gottesman-Kitaev-Preskill (GKP) states, and more. We consider the addition of a non-Gaussian resource state, particularly single photons, to this configuration and show how it improves the qualities and generation probabilities of desired states. With only two modes, adding a single-photon source improves GKP-state fidelity from 0.68 to 0.95 and adding a second then increases the success probability eightfold; for cat states with a fixed target fidelity, the probability of success can be improved by factors of up to four by adding single-photon sources. These demonstrate the usefulness of additional commonplace non-Gaussian resources for generating desirable states of light.

We study the long-range hopping limit of a one-dimensional array of $N$ equal-distanced quantum emitters in free space, where the hopping amplitude of emitter excitation is proportional to the inverse of the distance and equals the lattice dimension. For two species of emitters in an alternating arrangement, the single excitation sector exhibits non-Hermitian spectral singularities known as exceptional points. We unveil an unconventional phase transition, dubbed exceptional-point phase transition, from the collective to individual spontaneous emission behaviors. At the transition point, the $N\times N$ Hamiltonian fragments into $N/2-1$ many two-dimensional nondiagonalizable blocks. The remaining diagonalizable block contains a dissipation-induced edge state with algebraically localized profiles, and we provide numerical evidence for its existence in the infinite-array limit. We demonstrate that the edge state can be eliminated via a continuous deformation, consistent with the ill-definedness of bulk topological invariant. We also propose a spatially resolved character to quantify the incoherent flow and loss in the nonunitary quantum walks of single atomic excitations.

A method is presented for creating single- and double-cavity-induced transparency based on a cavity QED framework. Using this method we develop a fully quantized model of a cavity-induced transparency (CIT) scheme previously proposed [P. R. Rice and R. J. Brecha, Opt. Commun. 126, 230 (1996)] adopting the quantum-jump approach. This allows us to analyze the interaction of $N$ cavity fields with an $(N+1)$-level quantum system where $N=2,3$ for the cases of single- and double-cavity-induced transparency, respectively. In the steady-state limit, a general analytic expression for the linear susceptibility of a weak cavity field is obtained and $N-1$ transparency windows are observed. In the model we consider, no external fields are required to induce the transparency and the CIT effect is fully generated by cavity QED photons. We also compare the numerical integration of density-matrix equations with the analytical solutions provided by the quantum-jump approach and show that both results are in excellent agreement.

In the present work, the results obtained by Strange [Phys. Rev. Lett. 104, 120403 (2010)] about the revivals of a relativistic fermion wave function on a torus are considerably expanded. In fact, all the possible quantum states exhibiting revivals are fully characterized. The revivals are exact, that is, are true revivals without taking any particular limit such as the nonrelativistic one. The present results are of interest since they generalize the Talbot effect and the revivals of the Schrödinger equation to a relativistic situation with nonzero mass. This makes the problem nontrivial, as the dispersion relation is modified and is not linear. The present results are obtained by the use of arithmetic tools, which are described in certain detail. In addition, several plots of the revivals are presented, which are useful for exemplifying the procedure proposed in the present paper.