The challenge of developing high-precision temperature sensors is an important issue that has recently received a lot of attention. In this work, we introduce an estimation technique to precisely measure the temperature of a quantum reservoir using a Kerr-nonlinear resonator with drive. Thermalization in our suggested protocol is assessed using Uhlmann-Jozsa fidelity, then we utilize quantum Fisher information to evaluate the metrological potential of our thermometery scheme. We observe that increasing the Kerr-nonlinearity coefficient and driving amplitude significantly enhances precision in the temperature estimation. Furthermore, we also explore the underlying physical mechanisms by analyzing probe purity in the steady state and evaluating the performance of homodyne versus heterodyne detection methods. Our results demonstrate that neither of these Gaussian measurements is optimal; instead, optimal homodyne detection always surpasses heterodyne detection.

We show that genuine multipartite entanglement of an arbitrary number of qubits can be witnessed sequentially for an arbitrary number of times. In the simplest scenario where only one qubit is subjected to sequential measurements, we show that genuine multiparty entanglement can be detected arbitrarily many times starting from generalized Greenberger-Horne-Zeilinger, cluster, graph, and a class of mixed states. For the case where multiple qubits are subjected to sequential measurements, we again show that detection of genuine multiparty entanglement is possible for an arbitrary number of times for multipartite Greenberger-Horne-Zeilinger and a class of mixed states.

Photonics integrated circuits provide a stable, compact and robust platform for the implementation of quantum key distribution (QKD), which can generate information-theoretic secure keys between distant parties. Here, we present a high-speed QKD tests with silica planar lightwave circuits based asymmetric Mach-Zehnder interferometer modules. To further enhance system performance and reduce system complexity, we employ the advantage distillation (AD) algorithm alongside the one-decoy method. The AD algorithm can effectively extract strongly correlated bit pairs from weakly correlated bit pairs, thereby significantly improving the key rate over long distances, where traditional post-processing methods fail to generate keys. With a clock rate of 625 MHz, our system achieves composable secret key rates of 3.32 Mbps at 2 dB loss and 1.85 kbps at 18 dB loss, respectively. These results represent the first experimental validation of AD algorithm, and pave the way for the development of QKD networks with photonic integration.

The polarizability of the hydrogen molecule can be precisely calculated based on the quantum electrodynamics (QED) theory and a few fundamental physical constants. Therefore, experimental measurements with comparable accuracy can test the theory and establish gas metrology with hydrogen as the working gas. An optical refractometer was built and its performance was tested by measuring the electric dipole polarizabilities of argon, neon, and nitrogen. The polarizability of the ${\mathrm{H}}_2$ molecule was determined to be 2.031 027(71) ${\mathrm{cm}}^3{\mathrm{mol}}^{-1}$. The result agrees with the current calculated value, indicating the potential of a primary standard for pressure or temperature through measuring the polarizability of ${\mathrm{H}}_2$.

Numerical calculations of the electron self-energy without any expansion in the binding nuclear field are required in order to match the rapidly advancing precision of experimental spectroscopy. For the lightest elements, particularly hydrogen, these computations are complicated by large numerical cancelations and the slow convergence of the partial-wave expansion. Methods with accelerated convergence of the partial-wave expansion have been recently put forward [V. A. Yerokhin, K. Pachucki, and V. M. Shabaev, Phys. Rev. A 72, 042502 (2005); J. Sapirstein and K. T. Cheng, Phys. Rev. A 108, 042804 (2023)]. In our work we extend the accelerated-convergence methods to the previously hardly accessible region of nuclear charges $Z<5$ and higher excited states.

Enhancing nonlinear optical effects is critical to improving the performance of many functional devices for nonlinear and quantum optical applications. Here, we study the possibility of bending a waveguide to enhance the photon pair generation rate in a spontaneous four-wave mixing process. Whereas intuition might suggest that bending makes the process less efficient because the waveguides are more lossy when bent, we show that the increase in the effective nonlinearity outperforms the loss disadvantage for moderate bending radii. That can be explained by a better localization of the guided eigenmodes, leading to a reduced mode area. By studying selected waveguide devices with a fixed length, we demonstrate an improvement in the photon pair generation rate between 5% and 210% for an optimal bending radius. These findings have implications for the future design of integrated photonic devices for quantum optical applications, especially in cases where the chip estate tends to be a limiting factor.

We present a scheme for achieving broadband complete reflection by constructing photonic band gap via collective atom-atom interaction in a one-dimensional (1D) waveguide quantum electrodynamics (QED) system. Moreover, we propose several strategies to further expand the ultrahigh-reflection windows, including increasing the number of atoms with separations near the Bragg distance and inducing gradient frequency modulation among the atoms. The center frequency and bandwidth of the ultrahigh-reflection window are dynamically adjustable by applying external electromagnetic field. The results here can enrich the many-body physics of the waveguide-QED system and offer a pathway for achieving broadened ultrahigh reflection in a controllable way, which can find important applications in the realms of chip-integrated band filter, quantum storage, optical switching, and wavelength-selective devices.

Quantum optics with giant atoms provides a new approach for implementing optical memory devices at the atomic scale. Here, we theoretically investigate the relaxation dynamics of a single driven three-level atom interacting with a one-dimensional waveguide via two coupling points. Under certain conditions, after long-time dynamics, we find that the population of the giant atom can either remain stable or exhibit regular periodic oscillations, while photons are trapped within the region of the giant atom. This phenomenon cannot be realized using a two-level atom configured with two coupling points. It is noteworthy that the atomic excitation probability of a stable bound state is a constant value, which is determined by the size of the atom. Importantly, the size of the atom, defined as the distance between the two coupling points, is significantly larger than the wavelength of the light field, which is a necessary condition for the existence of oscillating bound states.

A stable and fast path linking two arbitrary states of a quantum system is generally required for state-engineering protocols, such as stimulated Raman adiabatic passage, shortcuts to adiabaticity, and holonomic transformation. Such a path is also fundamental to the exact solution of the time-dependent Schrödinger equation. We construct a universal control framework using an ancillary picture, in which the time-dependent Hamiltonian can be diagonalized. Multiple desired paths can be derived by the von Neumann equation for parametric ancillary projection operators. No transition exists among the ancillary basis states during the time evolution. Under various conditions, our control framework reduces to the nonadiabatic holonomic transformation, the Lewis-Riesenfeld theory for invariants, and counterdiabatic driving methods. In addition, it is applicable to the cyclic transfer of system populations that could be a hard problem for existing methods. Our work can provide a full-rank time-evolution operator for a time-dependent quantum system with a finite number of dimensions.

Adopting a statistical approach we study the degradation of entanglement of a quantum system under the action of an ensemble of randomly distributed Markovian noise. This enables us to address scenarios where only limited information is available on the mechanisms that rule the noisy evolution of the model. As an application, we characterize the statistic of entanglement deterioration for a quantum memory formed by $n$ qudits that undergo randomly distributed local, uniform, Markovian noise evolution.

In the geometry of quantum-mechanical processes, the time-varying curvature coefficient of a quantum evolution is specified by the magnitude squared of the covariant derivative of the tangent vector to the state vector. In particular, the curvature coefficient measures the bending of the quantum curve traced out by a parallel-transported pure quantum state that evolves in a unitary fashion under a nonstationary Hamiltonian that specifies the Schrödinger evolution equation. In this paper, we present an exact analytical expression of the curvature of a quantum evolution for a two-level quantum system immersed in a time-dependent magnetic field. Specifically, we study the dynamics generated by a two-parameter nonstationary Hermitian Hamiltonian with unit speed efficiency. The two parameters specify the constant temporal rates of change of the polar and azimuthal angles used in the Bloch sphere representation of the evolving pure state. To better grasp the physical significance of the curvature coefficient, showing that the quantum curve is nongeodesic since the geodesic efficiency of the quantum evolution is strictly less than 1 and tuning the two Hamiltonian parameters, we compare the temporal behavior of the curvature coefficient with that of the speed and the acceleration of the evolution of the system in projective Hilbert space. Furthermore, we compare the temporal profile of the curvature coefficient with that of the square of the ratio between the parallel and transverse magnetic field intensities. Finally, we discuss the challenges in finding exact analytical solutions when extending our geometric approach to higher-dimensional quantum systems that evolve unitarily under an arbitrary time-dependent Hermitian Hamiltonian.

We study the entanglement purification via entanglement swapping in terms of optimal output state concurrence. A strict formula that governs the process is first given analytically for the case where both input entangled pairs are in arbitrary pure states. Then, it is generalized, with strong numerical evidence, to the case with one of the entangled pairs being in an arbitrary mixed state. The formula subsequently gives a formal upper bound of entanglement purification via entanglement swapping, as well as how the maximum entanglement purification can be attained by engineering the pure state entangled pair. Moreover, the formalism of the upper bound in turn suggests that entanglement swapping is most suited for reducing the influence of pure dissipative noise, which can be applied to extend the lifespan of stationary qubits in quantum communications.

We study the phenomenon of quantum synchronization from the viewpoint of quantum metrology. By interpreting quantum self-sustained oscillators as dissipative quantum sensors, we develop a framework to characterize several aspects of quantum synchronization. We show that the quantum Fisher information (QFI) serves as a system-agnostic measure of quantum synchronization that also carries a clear operational meaning, viz., it quantifies the precision with which the amplitude of a weak synchronizing drive can be measured. We extend our analysis to study many-body oscillators subjected to multiple drives. We show how the QFI matrix can be used to determine the optimal drive that maximizes quantum synchronization, and also to quantitatively differentiate the synchronization responses induced by different drives. Our work highlights multiple connections between quantum synchronization and quantum metrology, paving a route toward finding quantum technological applications of quantum synchronization.

To achieve an ultrafast long-distance quantum communication, the one-way quantum repeater scheme based on linear optics is a promising framework. However, photon loss becomes the dominant error in communication as the distance increases. To resolve this problem, a quantum parity code has been developed, which, however, suffers from a significant fluctuation in the success transmission probability as the code length increases. Here, to utilize all the available qubits to boost the coding gain, we propose a generalized version of quantum parity code by using the concatenated structure in a flexible way. Compared to quantum parity code, the generalized one does not increase the hardware overhead but offers superior performance with the continuous increase of code length. The gain comes from optimizing the length distribution of the inner codes. Given the current limited ability to realize quantum codes, generalized quantum parity code assisted by one-way quantum repeaters offers a promising way to achieve ultrafast long-distance quantum communication.

Enhanced quantum metrology is of vital importance for revealing the distinct features of quantum phenomena. Here we propose an effective scheme to significantly improve the quantum estimation of optomechanical coupling strength (OCS) with the assistance of two-level atoms, coherent feedback loop, and squeezed vacuum injection. Our results indicate that, by selecting appropriate atomic, quantum feedback and squeezing parameters, the estimation error of the OCS parameter can be greatly reduced. We explain the physical mechanisms behind these factors enhancing estimation precision of OCS parameter and explore the ideal parameter regions for estimating it. More importantly, the estimation precision obtained by our scheme exhibits stronger robustness against mechanical damping and ambient temperature in comparison with the unassisted case. In addition, we evaluate the performance of heterodyne detection, indicating that the measurement precision obtained through this method can approach the estimation precision obtained by our scheme. Further, the experimental feasibility of our scheme is discussed. The proposed scheme for enhancing quantum sensing is quite general and can also be applied to high-precision estimation of other physical parameters.

The study of magnetic-resonance linewidth is crucial in magnetic-resonance physics and its applications. Previous studies focused on the linewidth of alkali metal atoms within the spin-exchange relaxation-free regime near zero magnetic field and in strong magnetic fields where Zeeman resonances are well resolved due to the quadratic Zeeman effect. However, the linewidth in the unresolved-Zeeman-resonance regime, which is prevalent in various magnetometer and comagnetometer applications, is not well understood. To address this, we developed a theoretical framework based on the master equation for alkali metal atoms and solved it under the rotating-wave approximation and weak-driving conditions. Our numerical calculations and analytical expressions reveal that the light-narrowing effect occurs only when the ratio of the spin-exchange rate to the spin destruction rate exceeds a critical value. Additionally, we show that the linewidth in the unresolved Zeeman-resonance regime is significantly influenced by the mutual coupling of quantum coherence between different Zeeman sublevels. These findings provide a theoretical tool for understanding spin relaxation in alkali-metal atoms and optimizing the performance of atomic magnetometers and comagnetometers operating in this regime.

We investigate the dynamics of a Bose-Einstein condensate in a one-dimensional ring-shaped lattice with the Peierls phase and site-dependent modulations, where the condensate is confined in a single deep trap and the interparticle interaction strength is modulated by a time-periodic driving field. The system has a finite spectrum, which limits the excitation regimes, and the Peierls phase typically induces imbalanced complex hopping amplitudes in each direction, leading to nonzero net particle currents along the lattice chain, which can hold nearly persistent even when the driving field is turned off after half of the period. The configuration provides a specific way for the coherent control of particle currents in many-body quantum systems with the help of an external driving field and promotes the possible applications in future closed-loop atom circuits.

We consider a quantum droplet which is confined in a ring potential. We investigate the so-called yrast state, i.e., the lowest-energy state of the droplet assuming that it has some fixed expectation value of the angular momentum. Two of the most interesting aspects of this problem are the nonlinear term—which is partly attractive and partly repulsive—and the periodic boundary conditions. For some range of the parameters, the attractive or the repulsive part of the nonlinear term dominates and one gets the expected behavior. In some intermediate regime, the two nonlinear terms are of comparable size. In this case, both the solution and the corresponding dispersion relation show an interesting behavior. Finally, we make contact with the problem of solitary-wave excitation since the derived solutions are traveling-wave, i.e., solitary-wave, solutions.