Photon-number-resolving detectors with hundreds of pixels are now readily available, while the characterization of these detectors using detector tomography is computationally intensive. Here, we present a modified detector tomography model that reduces the number of variables that need optimization. To evaluate the effectiveness and accuracy of our model, we reconstruct the photon number distribution of optical coherent and thermal states using the expectation-maximization-entropy algorithm. Our results indicate that the fidelity of the reconstructed states remains above 99%, and the second- and third-order correlations agree well with the theoretical values for a mean number of photons up to 100. We also investigate the computational resources required for detector tomography and find out that our approach reduces the solving time by around half compared to the standard detector tomography approach, and the required memory resources are the main obstacle for detector tomography of a large number of pixels. Our results suggest that detector tomography is viable on a supercomputer with 1 TB RAM for detectors with up to 340 pixels.

Quantum communications introduce a paradigm change in internet security by using quantum resources to establish secure keys between parties. Present-day quantum communication networks are mainly point to point and use trusted nodes and key management systems to relay the keys. Future quantum networks, including the quantum internet, will have complex topologies in which groups of users are connected and communicate with each other. Here we investigate several architectures for quantum communication networks. We show that photonic orbital angular momentum (OAM) can be used to route quantum information between different nodes. Starting from a simple point-to-point network, we will gradually develop more complex architectures: point-to-multipoint, fully connected, and entanglement-distribution networks. As a particularly important result, we show that an $n$-node fully connected network can be constructed with a single OAM sorter and $n-1$ OAM values. Our results pave the way to construct complex quantum communication networks with minimal resources.

With the advent of quantum processors exceeding 100 qubits and the high engineering complexities involved, there is a need for holistically benchmarking the processor to have quality assurance. Linear cross-entropy benchmarking (XEB) has been used extensively for systems with 50 or more qubits but is fundamentally limited in scale due to the exponentially large computational resources required for classical simulation. In this work we propose conducting linear XEB with random Clifford circuits of constant to logarithmic depth, a scheme we call Clifford XEB. Since Clifford circuits can be simulated in polynomial time, Clifford XEB can be scaled to much larger systems. To validate this claim, we run numerical simulations for the classes of Clifford circuits we propose with noise and observe exponential decays. When noise levels are low, the decay rates are well correlated with the noise of each cycle assuming a multiplicative error accumulation, i.e., where the fidelities of individual gates multiply. We perform simulations of systems up to 1225 qubits, where the classical processing task can be easily dealt with by a workstation. Furthermore, using the theoretical guarantees in Chen et al. [PRX Quantum 3, 030320 (2022)], we prove that Clifford XEB with our proposed Clifford circuits must yield exponential decays under a general error model for sufficiently low errors. Our theoretical results explain some of the phenomena observed in the simulations and shed light on the behavior of general linear XEB experiments.

Two branches of bistable temporal cavity solitons are found in models of quantum interference in microresonators with a $\mathrm{\Lambda }$ three-level medium in the anomalous dispersion regime. The cavity solitons are due to the locking of moving domain walls. We identify two distinct Maxwell points on opposite sides of the cavity resonance where domain walls are stationary and two distinct temporal cavity solitons, one narrow and with a high peak intensity, the other broader and with a lower peak intensity, coexist over wide parameter ranges and without the need of secondary cavity resonances. Localized structures combining the two soliton branches oscillate on timescales of tens of cavity round trips. Frequency combs generated by combinations of different types of multistable temporal cavity solitons lead to enhanced bandwidths and their control.

Cavity-enhanced spontaneous parametric down-conversion (SPDC) can be used to implement heralded single-photon sources with narrow bandwidth down to tens of megahertz, thus compatible with atom-based quantum memories. In this work, we propose and study the cavities that are either singly resonant at the frequency of only one of the generated photons or doubly resonant at the frequency of one generated photon and for the pump field. We derive analytical expressions for the generation rate and the spectral brightness as a function of the main structure parameters. Our analysis shows that by exploiting counterpropagating SPDC inside a cavity with properly designed distributed Bragg reflectors, pure narrow-band heralded single photons can be generated. In particular, when a pulsed pump illuminates a 1-mm-long singly resonant cavity, the bandwidth of the resonant and the nonresonant photons will be 120 MHz and 5.7 GHz, respectively, with heralded single-photon purity as high as 0.993. Due to its small absorption probability inside the cavity, the heralding efficiency can approach unity if the nonresonant partner is used as a heralded single photon. Finally, we also show that continuous-wave excitation of a 1-cm-long cavity resonant for the pump and copropagating fields can provide photon pairs in a narrow bandwidth of 10 MHz with spectral brightness as high as $2\times {10}^6$ $({\text{s}\text{mW}\text{MHz})}^{-1}$. This source can efficiently couple with quantum memories and can find applications in quantum information processing and communication.

Quantum batteries are quantum systems used to store energy to be later extracted by an external agent in the form of work to perform some task. Here we study the charging of a hybrid quantum battery via a collisional model mediated by an anti-Jaynes-Cummings interaction obtained from an off-resonant Raman configuration. The battery is made of two distinct components: a stationary infinite-dimensional single quantum system (e.g., a harmonic oscillator) and a stream of small-dimensional ones (e.g., qutrits). The charging protocol consists of sequentially interacting the harmonic oscillator with each element of the stream, one at a time, under the action of an external energy source and the goal is to analyze how the charging of both the harmonic oscillator and the qutrits is affected by the correlation properties of the stream.

In pursuit of enhancing the prediction capabilities of neural networks, it has been a longstanding objective to create datasets encompassing a diverse array of samples. The purpose is to broaden the horizons of the neural network and continually strive for improved prediction accuracy during the training process, which serves as the ultimate evaluation metric. However, in this paper, we explore an intriguing avenue for enhancing algorithm effectiveness through exploiting the knowledge blindness of the neural network. Our approach centers around a machine learning algorithm utilized for preparing arbitrary quantum states in a semiconductor double quantum dot system, a system characterized by highly constrained control degrees of freedom. We artificially define the characteristics of local optima in the state preparation task. When the optimization falls into a local optimum, disturbances are introduced through stochastic prediction, which gives a chance to escape from the local optimum. Notably, unlike previous methodologies that employ reinforcement learning to identify pulse patterns, we adopt a training approach akin to supervised learning, using it to dynamically design the pulse sequence. This approach has broad applicability and improves the efficiency of the algorithm.

We show that the method of iterative Bayesian unfolding for mitigating readout errors in quantum computers can be derived from an information theoretic analysis. This inspires more flexible applications of this error-mitigation scheme. In particular, we distinguish between structural mitigation and unstructural mitigation. Structural mitigation addresses nearly deterministic quantum computation, where the computer is expected to output a single- or few-outcome bit strings. It is shown that the readout errors alone can be corrected by few repetitions of the computation. In contrast, unstructural mitigation is designed for quantum simulation, where the computer outputs bit strings broadly distributed. In this case, one is interested in mitigating certain observables of interest. As most observables of interest are dependent on few bits and not the whole bit string, it is sufficient to mitigate the marginal distributions over these dependent bits. As long as the cross talk of readout errors can be ignored, it is shown that the iterative Bayesian unfolding applied locally for these marginal distributions gives similar results as mitigation using least squared errors. We illustrate our analysis using the data of the preparation of the Greenberger-Horne-Zeilinger state in a 127-qubit quantum computer.

We present a strong quantum entanglement generation protocol based on two-mode photon-subtracted squeezed vacuum (TMPSSV) states. The generated entanglement shows higher entropy than those based on quantum catalysis, quantum scissors, photon-subtracted two-mode squeezed vacuum (PSTMSV) states, nonlocal PSTMSV states, and two-mode squeezed vacuum (TMSV) states in the ideal case. The impacts of experimental imperfections such as losses in the squeezed vacuum states and detection efficiency of single-photon detectors on the generated entanglement based on TMPSSV states are analyzed. When practical experimental imperfections are considered, higher logarithmic negativity than those of ideal PSTMSV states and ideal TMSV states are obtained for low initial squeezing level. Enhancement factor over 2 is obtained when the entanglement generated from imperfect non-Gaussian states passes through two lossy quantum channels in low initial squeezing level. Strong correlation is kept in lossy channels, which shows good robustness of the entanglement generation protocol based on TMPSSV states.

Collinear laser spectroscopy experiments were performed on singly charged ${}^{45}\mathrm{Sc}$ (${}^{45}\mathrm{Sc}$ II). Fourteen fine-structure transitions in the $3d4s\leftrightarrow 3d4p$ configuration were studied for three sets of triplet states, the ${}^3{D}_1, {}^3{D}_2$, and ${}^3{D}_3$ states in the $3d4s$ configuration and the ${}^3{F}_2^o, {}^3{F}_3^o, {}^3{F}_4^o, {}^3{D}_1^o, {}^3{D}_2^o$, and ${}^3{D}_3^o$ states in the $3d4p$ configuration. Furthermore, the hyperfine magnetic dipole ($A$) and the electric quadrupole ($B$) coupling constants were determined. Detailed studies of experimental systematic uncertainties lead to accurate determination of the coupling constants and facilitate comparison with theoretical models, especially for $B$ constants. The improved experimental uncertainty reveals deviations from available theoretical calculations and suggests the need for further theoretical studies.

Besides the direct Coulomb and multiple ionization, ionization caused by the electron capture is found to be a vital mechanism for the x-ray emissions in heavy-ion collisions. This mechanism associated with $M$-shell ionization is more complex than that for the $K$- and $L$-shell ionization and not understood yet. We have developed a theoretical method to understand the $M$ x-ray production cross sections. Here we have considered both the $M\text{−}K$ and $M\text{−}L$ capture processes of the target atoms by the changed charge state of the projectile ions to witness a good agreement between the present theory and past experiments. We have first verified the present theory with a few representative cases of the carbon, oxygen, and fluorine projectile ions colliding with the lead and bismuth targets at energies ranging from 0.8 to 6.0 MeV/u. Next, we have validated the worth of its predictions through neon ion-induced $M$ x-ray production of lead and bismuth targets. We believe the current knowledge of the $M$ x-ray production mechanisms will play a significant role in understanding the different aspects of the heavy-ion collision physics.

The self-consistent-field method proposed recently [Wang et al., Phys. Rev. Lett. 128, 013001 (2022)] is discussed in more detail. The method leads to self-consistent eigenvalue equations for the natural spin orbitals and Fermi-Dirac distribution for the orbital occupation numbers. The entropic functional contains two parameters ($\kappa$ and $b$), which can be fitted to various experimental or theoretical data such as the dissociation energy, entropy, and total energy at a given geometry, etc. Calculations are demonstrated on the square ${\mathrm{H}}_4$ and hydrogen chains of ${\mathrm{H}}_{50}$, which are representative cases of degenerate or nondynamically correlated systems.

A recently proposed experiment on the observation of the $\mathcal{P},\mathcal{T}$-odd ($\mathcal{P}$ is space parity and $\mathcal{T}$ is time reflection) Faraday effect (optical rotation in an external electric field) (PTFE) in intracavity absorption spectroscopy is discussed. Two versions of this experiment are considered: the propagation of a continuous laser beam through an optical cavity filled with molecular gas (ThO vapor) and propagation of a continuous laser beam through an optical cavity crossed by the molecular beam (ThO molecules). With the use of optical cavities of high fineness, the accuracy of the experiment may reach a level exceeding the recently attained one by several orders of magnitude. The statistics of both types of experiment are analyzed. Ways to avoid experimentally the dangerous systematic errors, which can mimic the PTFE, are discussed.

In the description of resonant inelastic x-ray scattering (RIXS) from inversion-symmetric molecules the small core-level splitting is typically neglected. However, the spacing $\mathrm{\Delta }$ between gerade and ungerade core levels in homonuclear diatomic molecules can be comparable with the lifetime broadening of the intermediate core-excited state $\mathrm{\Gamma }$. We show that when $\mathrm{\Delta }\sim \mathrm{\Gamma }$ the scattering becomes nonlocal in the sense that x-ray absorption at one atomic site is followed by emission at the other one. This is manifested in an unusual dependence of the RIXS cross section on the sum of the momenta of incoming and outgoing x-ray photons $\mathbf{k}+{\mathbf{k}}^{\prime }$, contrary to the normal $\mathbf{k}-{\mathbf{k}}^{\prime }$ dependence in the conventional local RIXS theory. The nonlocality of the scattering influences strongly the scattering angle and excitation energy dependence of the intensity ratio between parity forbidden and allowed RIXS channels. Numerical simulations for ${\mathrm{N}}_2$ show that this effect can readily be measured at present-day x-ray radiation facilities.

Vacuum ultraviolet (UV) light at a well-defined wavelength excites ${\mathrm{N}}_2$ to different vibronic levels of the singlet electronic states that strongly interact through nonadiabatic coupling. Each discrete vibronic state acts as an isolated resonance: it is weakly coupled to a dissociative continuum of triplet states via a weak spin-orbit coupling. Here, we seek to compare this decay to that of a coherent superposition of bound singlets pumped by a broad in energy — ultrafast pulse. Despite the strong intersinglets and intertriplets nonadiabatic couplings, the coherent set of states decays as a $mixture$ of the isolated vibronic states with essentially their individual lifetimes as determined separately in a sharp wavelength excitation. The vibrational quantum number of the vibronic states is a nearly good one when the spin-orbit coupling is as weak as is the case for ${\mathrm{N}}_2$. Numerically converged dynamical computations valid for longer times show that for nonrotating molecules the individual vibronic resonances overlap and interfere only upon an artificially order of magnitude increase of the strength of the spin-orbit coupling. The resonances strongly overlap only at an even stronger coupling to the dissociative continuum.