Thermodynamically consistent measurements can preserve either statistics (unbiased) or marginal states (noninvasive) but not both. Here we show the existence of metrological tasks which unequally favor each of the aforementioned measurement types. We consider two different metrology tasks, namely, the weak value amplification technique and repeated metrology without resetting. We observe that unbiased measurement is better than noninvasive measurement for the former and the converse is true for the latter. We provide finite-temperature simulations of transmon sensors which estimate how much cooling, a resource for realistic measurements, is required to perform these metrology tasks.

Quantum information science may lead to technological breakthroughs in computing, cryptography, and sensing. For the implementation of these tasks, however, complex devices with many components are needed and the quantum advantage may easily be spoiled by the failure of only a few parts. A paradigmatic example is quantum networks. There, not only do noise sources such as photon absorption or imperfect quantum memories lead to long waiting times and low fidelity, but also hardware components may break, leading to a dysfunctionality of the entire network. For the successful long-term deployment of quantum networks in the future, it is important to take such deterioration effects into consideration during the design phase. Using methods from reliability theory and the theory of aging, we develop an analytical approach for characterizing the functionality of networks under aging and repair mechanisms, also for nontrivial topologies. Combined with numerical simulations, our results allow us to optimize long-distance entanglement distribution under aging effects.

This study examines the orientation-dependent high-order harmonic spectra of HCN, using time-dependent density-functional-theory calculations. In HCN, which lacks inversion symmetry, the emergence of even harmonics is a notable feature. At a parallel orientation to the laser polarization, these even harmonics manifest with intensities surpassing those of odd harmonics. However, their magnitudes diminish as the orientation shifts away from parallel. Conversely, at a perpendicular orientation to the laser polarization, the spectrum predominantly displays odd harmonics, although an exclusively even harmonic spectrum is generated with the harmonic polarization perpendicular to the laser. The intensities of these even harmonics wane as the molecular orientation deviates from the perpendicular. The behavior of odd harmonics is more intricate, generally showing heightened intensities at perpendicular orientations. The influence of laser parameters on the orientation dependence is minimal. Across various orientations and laser settings, a consistent minimum is observed near 22–23 eV, underscoring a universal feature of the high-order harmonic spectra of HCN. Regardless of the orientation, the highest occupied molecular orbitals (HOMOs) contribute little to the high-order even harmonics. As far as the high-order odd harmonics are concerned, the HOMOs only dominate when the orientation angle is greater than ${30}^{\circ }$.

The development of quantum gas microscopy for two-dimensional optical lattices has provided an unparalleled tool to study the Fermi-Hubbard model (FHM) with ultracold atoms. Spin-resolved projective measurements, or snapshots, have played a significant role in quantifying correlation functions, theory verification, and thus the uncovering of underlying physical phenomena such as antiferromagnetism at commensurate filling on bipartite lattices and other charge and spin correlations, as well as dynamical properties at various densities. Here we employ a recent concept, the multiscale structural complexity, and show that when computed for the snapshots (of single spin species, local moments, or total density) it can provide a theory-free property, immediately accessible to experiments. Specifically, after benchmarking results for Ising and $XY$ models, we study the structural complexity of snapshots of the repulsive FHM in the two-dimensional square lattice as a function of doping and temperature. We generate projective measurements using determinant quantum Monte Carlo and compare their complexities against those from the experiment. We demonstrate that these complexities are linked to relevant physical observables such as the entropy and double occupancy. Their behaviors capture the development of correlations and relevant length scales in the system. We provide an open-source code in python which can be implemented into data analysis routines in experimental settings for the square lattice.

Recently, synthetic spin-orbit coupling has been introduced into cold-atom systems for more flexible control of the Hamiltonian, which was further made time varying through two-photon detuning to achieve dynamic control of the cold-atom state. While an intraband transition can be adiabatically obtained, a complete interband transition, rather than a superposition of different bands, obtained through fast sweeping is usually guaranteed by having the positions of the initial and final states be far away from any band gap in the quasimomentum space. Here, by introducing an additional non-Hermitian parameter through an atom-loss contrast together with two-photon detuning as two controllable external parameters, both intraband and complete interband transitions can be achieved independent of the positions of the initial and final states. In addition, a “point-source diagram” approach in the two-dimensional external parameter space is developed to visualize and predict the locations of any nonadiabatic transitions. This control protocol can have potential applications in quantum state control and quantum simulations using cold-atom systems.

Extensive studies have been conducted on the lateral optical force, which is perpendicular to the direction of light propagation. Current research is primarily focused on the lateral force experienced by a single sphere. In this study, we investigate the lateral optical force exerted on a double-sphere system aligned along the direction of evanescent-wave propagation, utilizing full-wave simulations and the multipole-expansion method. Despite no apparent lateral asymmetry, we find that each sphere can experience either positive or negative lateral optical force by tuning the incident wave's frequency and polarization. Unlike the lateral force experienced by a single sphere in an evanescent wave, arising solely from recoil force due to Belinfante's spin momentum in a circularly polarized evanescent wave or spin angular momentum in a linearly diagonally polarized evanescent wave, the lateral force exerted on each sphere in the double-sphere system can also stem from an interception force. This interception force occurs due to the field gradient caused by the spheres' asymmetric scattering. This asymmetry is rooted in the real or imaginary part of the complex Poynting vector of the incident wave. It is the interception force that enables continuous tuning of the lateral optical force of each sphere and their sign flip. This research advances the understanding of lateral optical force and offers more flexibility for optical manipulation in multiparticle systems, opening up new opportunities for lateral-force-based applications.

The dynamic performance of an interband cascade laser (ICL) with optical injection is studied within the framework of rate equations. The effect of stage number $m$ and active area $A$ on the ICL optical injection-locking diagram is numerically simulated. ICLs with a small stage number and large active area have a relatively large phase-locked region. When the stage number is small, the system exhibits a large number of extreme pulses in the frequency-unlocked region. We show that step-down perturbations of the laser pump current can trigger and suppress extreme pulses by changing the parameters of amplitude, duration, and modulation time points. The amplitude and duration of current perturbation have a significant impact on the number of extreme pulses. This work provides different insight into the generation and control of extreme pulses based on ICLs.

The concept of superbandwidth refers to the fact that a band-limited signal can exhibit, locally, an increase of its bandwidth, i.e., an effective bandwidth greater than that predicted by its Fourier transform. In this work, we study the propagation of superbandwidth laser pulses in a dispersive medium, characterized by the group velocity dispersion. In particular, two important results arise from the analysis of the instantaneous frequency of the pulse obtained through the Wigner function distribution. First, local oscillations of the electric field which are beyond the Fourier spectrum of the incoming pulse can be observed. Second, for a range of values of the pulse synthesis parameters, surprisingly, the dynamics of the instantaneous frequency within certain temporal regions, corresponds to that of a pulse propagating in a medium with a group velocity dispersion of opposite sign. This phenomenon is intrinsic to the special characteristics of the pulse and not to the dispersive properties of the medium.

We combine lossless, phase-only transformations with fully transmitting linear-optical scatterers to define the principle of linear-optical phase amplification. This enables a physical phase shift $\varphi$ to be nonlinearly mapped to a new space $\gamma \left(\varphi \right)$ using linear optics, resulting in a completely general and enhanced phase shifter that can replace any standard one. A particular phase amplifier is experimentally realized, allowing the phase enhancement parameter $d\gamma /d\varphi$ to be continuously tuned. Placing this enhanced phase shifter in one arm of a Mach-Zehnder interferometer led to an intensity-phase slope more than 20 times steeper than what can be obtained with its unamplified counterpart.

The sensitivity of electric field sensors based on two-photon electromagnetically induced transparency (EIT) involving highly excited Rydberg states in thermal atoms is often limited by the residual Doppler effect and optical power broadening. Here, we propose a method to reduce the EIT spectral linewidth using a Ramsey interrogation approach, allowing multiple interrogations of atomic coherence, using either temporally or spatially separated laser beams. Our theoretical calculations predict that the linewidth of such Raman-Ramsey spectral features can be substantially reduced compare to a standard Doppler-broadened EIT, opening a possibility to improve sensitivity of Rydberg atomic vapor-based sensors.

The quantum routing of single photons takes the central role in an optical quantum network. The matching between the routing probability and the selected frequency of single photons limits the robustness and universality of the quantum router. Here, we investigate how to implement the quantum routing of single photons within bandwidth frequencies by two cavities embedding atoms acting as quantum nodes. We show that the routing capabilities of the single photons and the range of the frequencies can be manipulated by properly designing the chiral coupling strengths and the channel boundaries. Also, we demonstrate that the incident single photons, within bandwidth frequencies, can be completely routed to the targeted output port. This is particularly important to construct optical quantum networks within bandwidth frequencies in the future.

We investigate quantum objectivity in the boson-spin model, where a central harmonic oscillator interacts with a thermal bath of spin-$\frac{1}{2}$ systems. We analyze how information about a continuous position variable can be encoded into discrete, finite-dimensional environments. More precisely, we study conditions under which the so-called spectrum broadcast structures can be formed in the model. These are multipartite quantum-state structures, representing a mode-refined form of decoherence. Working in the recoil-less limit, we use the Floquet theory to show that despite its apparent simplicity, the model has a rich structure with different regimes, depending on the motion of the central system. In one of them, the faithful encoding of the position and hence objectivity are impossible irrespectively of the resources used. In another, large enough collections of spins will faithfully encode the position information. We derive the characteristic length scales, corresponding to decoherence and precision of the encoding.

Despite recent advances in quantum sciences, a quantum master equation that accurately and simply characterizes open quantum dynamics across extremely long timescales and in dispersive environments is still needed. In this study, we optimize the computation of the fourth-order time-convolutionless master equation to meet this need. Early versions of this master equation required computing a multidimensional integral, limiting its use. Our master equation accounts for simultaneous relaxation and dephasing, resulting in coefficients proportional to the system's spectral density over frequency derivative. In sub-Ohmic environments, this derivative induces infrared divergence in the master equation, invalidating the second-order Bloch-Redfield master equation findings. We analyze the approach to a ground state in a generic open quantum system and demonstrate that it is not reliably computed by the Bloch-Redfield equation alone. The optimized fourth-order equation shows that the ground-state approach is accurate to second order in bath coupling regardless of the dispersion, even though it can diverge in the fourth order.

R-matrix with pseudostates (RMPS) calculations have been carried out for photoionization of atomic sodium near threshold. The large RMPS atomic orbital and configuration basis allows for very accurate computations of low-energy photoionization cross sections up to $\approx 30$ eV, the energy range for which the RMPS calculations were optimized. Consistency checks for accuracy include, first, the excellent agreement found between length- and velocity-gauge theoretical results, a necessary but not sufficient requirement for having a converged wave function. A second accuracy quantification is the excellent prediction of the position of the Cooper minimum compared to experimental results. Particular attention is paid to the Cooper minimum occurring just above threshold, and the spin-orbit splitting of minima, resulting in a nonzero total cross section. Our RMPS results away from the minimum are found to be lower than the experimental data, and we make the case that the experimental magnitudes are an overestimate. A third important affirmation of the present accuracy is the continuity found between the $3s\to np$ bound-bound discrete oscillator strength density below threshold—see Wiese et al. [W. L. Wiese, M. W. Smith, and B. M. Miles, Atomic Transition Probabilities, Vol. 2: Sodium Through Calcium; A Critical Data Compilation (US Government Printing Office, Washington, DC, 1969)]—and the $3s\to \epsilon p$ bound-continuum RMPS oscillator strength density above threshold. These three somewhat independent tests of the accuracy of the computed cross sections add confidence to our recommending the present RMPS results as the most reliable extant data for low-energy Na photoionization (and the earlier Wiese et al. results for the discrete states).

Non-Hermitian systems offer novel methods for manipulating electromagnetic waves, including applications in sensing and mode conversion, attracting significant research interest. In this work, we propose a hybrid photonic crystal comprised of two topologically distinct heterostructures with angle-insensitive band gaps. This hybrid photonic crystal can sustain two topological interface states (TISs), and is considered as an optical non-Hermitian system. By precisely tuning of the coupling between these TISs, we achieve perfect wave transmission, revealing a parity-time (PT) phase transition, as evidenced by the evolution of the transmission spectra. Remarkably, the perfect transmission of waves remains resilient against fluctuating incident angles, ensured by the angle-tolerant photonic band gaps of the heterostructures. Additionally, we explore another hybrid photonic crystal heterostructure featuring two interface channels supporting the TIS and the Tamm plasma polariton. As the coupling between these two modes strengthens, two distinct resonance peaks emerge. Gradually reducing this coupling results in the merging of these peaks into a single peak. As the PT phase transition case, perfect wave absorption can be realized. Notably, the frequency shifts of the transmission and absorption peaks remain minimal, demonstrating a remarkable stability with an angle variation from 0 ° to 90 ° ($\mathrm{\Delta }f=1.4\mathrm{THz}$). These results further expand and complement the ongoing exploration of non-Hermitian topological physics.

Besides exploring novel transition patterns, acquiring a full understanding of the transition nature is an ultimate pursuit in studies of phase transitions. The fundamental models of light-matter interactions manifest single-qubit topological phase transitions (TPTs), which call for an analytical demonstration apart from numerical studies. We present a rigorous study of TPTs in the Jaynes-Cummings model generally with Stark nonlinear coupling. In terms of the properties of Hermite polynomials, we show that the topological structure of the eigenfunction has an exact correspondence to the spin winding by nodes, which yields a full spin winding without antiwinding nodes. We find it is the superposition between neighboring Fock states that leads to such a nontrivial spin winding. The spurious fractional contribution to the winding number of the winding angle at infinity is found to be actually an integer. Thus, the phase transitions in the model have the nature of TPTs and the excitation number is endowed as a topological quantum number. The principal transition establishes a paradigmatic case in which a transition is of both the symmetry-breaking Landau class of transition and the symmetry-protected topological class of transition, while conventionally these two classes of transitions are incompatible due to the contrary symmetry requirements. Such a transition-class reconciliation is realized by a preserved higher symmetry (here the parity) which protects the TPTs, while the symmetry breaking involves the subsymmetries. We also explain the origin of unconventional TPTs in the presence of counterrotating terms. Our results may provide deeper insight into the few-body phase transitions in light-matter interactions.

The certification of quantum nonlocality, which has immense significance in designing device-independent technologies, confronts severe experimental challenges. Detection loopholes, originating from the unavailability of perfect detectors, are one of the major issues among them. In the present study we focus on the minimum detection efficiency (MDE) required to detect various forms of genuine nonlocality, originating from the type of causal constraints imposed on the involved parties. In this context, we demonstrate that the MDE needed to manifest the recently suggested $T_2$-type nonlocality deviates significantly from perfection. Additionally, we have computed the MDE necessary to manifest Svetlichny's nonlocality, with the state-independent approach markedly reducing the previously established bound. Finally, considering the inevitable existence of noise we demonstrate the robustness of the imperfect detectors to certify $T_2$-type nonlocality.

It is well known that the dynamics of a hybrid system, which consists of a quantum subsystem and a classical subsystem, can be studied by transforming the quantum subsystem into a classical one. Based on our previous work [Phys. Rev. A 102, 032213 (2020)], we present the formalism of the quantum-classical hybrid system containing a quantum subsystem with energy degeneracy. By simulating the non-Abelian dynamics and geometric phases for the quantum subsystem with classical non-Abelian dynamics and geometric angles, the effect of the non-Abelian gauge field and the geometric phase of the quantum (sub)system on the dynamics of the classical subsystem is revealed by a simple canonical transformation. Remarkably, the non-Abelian angle geometric vector potential is found not only to induce the Wilczek-Zee phase in the dynamics of the quantum subsystem but also to give rise to a Lorentz-like non-Abelian geometric force. To illustrate the dynamics of the hybrid system, we examine the interaction between a spin-half particle and a magnetic field coupled with a magnetic particle. The explicit expression of the non-Abelian magneticlike gauge field is presented, introducing a different form of non-Abelian geometric force in the realm of quantum-classical dynamics.