We study non-Markovianity and the Landauer principle in a composite thermal environment consisting of a memory dissipated to a Markovian reservoir. We consider non-Markovianity with respect to four different combinations of two parts of the environment regarding their bosonic and fermionic natures represented by a quantum harmonic oscillator and a qubit, respectively. It is found that non-Markovianity can be suppressed, kept invariant, or enhanced by increasing environmental temperature conditioned on different combinations. Focusing on our model, we establish a modified Landauer principle which holds in the non-Markovian process. We then generalize the model to the case with multiple environments to explore scaling effects of the environments on non-Markovianity. We find that non-Markovianity can be increased by enlarging the size of the environments. We also derive a Landauer bound in these multiple non-Markovian thermal environments. Our results provide strategies for acquiring and enhancing non-Markoviaity in the thermal environments and show the possibility to generalize the conventional Landauer principle to the non-Markovian process.

The Dirac-Frenkel variational method with Davydov ${\text{D}}_2$ trial wavefunction is extended by introducing a thermalization algorithm and applied to simulate dynamics of a general open quantum system. The algorithm allows to control temperature variations of a harmonic finite-size bath when in contact with the quantum system. Thermalization of the bath vibrational modes is realized via stochastic scatterings, implemented as a discrete-time Bernoulli process with Poisson statistics. It controls bath temperature by steering vibrational modes' evolution towards their canonical thermal equilibrium. Numerical analysis of the exciton relaxation dynamics in a small molecular cluster reveals that thermalization additionally provides significant calculation speedup due to the reduced number of vibrational modes needed to obtain the convergence.

Complex numbers are widely used in both classical and quantum physics and are indispensable components for describing quantum systems and their dynamical behavior. Recently, the resource theory of imaginarity has been introduced, allowing for a systematic study of complex numbers in quantum mechanics and quantum information theory. In this work we develop theoretical methods for the resource theory of imaginarity, motivated by recent progress within theories of entanglement and coherence. We investigate imaginarity quantification, focusing on the geometric imaginarity and the robustness of imaginarity, and apply these tools to the state conversion problem in imaginarity theory. Moreover, we analyze the complexity of real and general operations in optical experiments, focusing on the number of unfixed wave plates for their implementation. We also discuss the role of imaginarity for local state discrimination, proving that any pair of real orthogonal pure states can be discriminated via local real operations and classical communication. Our study reveals the significance of complex numbers in quantum physics and proves that imaginarity is a resource in optical experiments.

We propose a superconducting qutrit-resonator chain model, and analytically work out forms of its topological edge states. The existence of the zero-energy mode enables one to generate a state transfer between two ends of the chain, accompanied with state flips of all intermediate qutrits, based on which $N$-body Greenberger-Horne-Zeilinger (GHZ) states can be generated with great robustness against disorders of coupling strengths. Three schemes of generating large-scale GHZ states are designed, each of which possesses the robustness against loss of qutrits or of resonators, meeting a certain performance requirement of different experimental devices. With experimentally feasible qutrit-resonator coupling strengths and available coherence times of qutrits and resonators, it has a potential to generate large-scale GHZ states among dozens of qutrits with a high fidelity. Further, we show the experimental consideration of generating GHZ states based on the circuit QED system, and discuss the prospect of realizing fast GHZ states.

We study the electronic resonant states of ${\mathrm{H}}_3$ with energies above the potential energy surface of the ${\mathrm{H}}_3^{+}$ ground state. These resonant states are important for the dissociative recombination of ${\mathrm{H}}_3^{+}$ at higher collision energies, and previous studies have indicated that these resonant states exhibit a triple intersection. We introduce a complex generalization of the pseudo–Jahn-Teller model to describe these resonant states. The potential energies and the autoionization widths of the resonant states are computed with electron scattering calculations using the complex Kohn variational method, and the complex model parameters are extracted by a least-square fit to the results. This treatment results in a non-Hermitian pseudo–Jahn-Teller Hamiltonian describing the system. The nonadiabatic coupling and geometric phase are further calculated and used to characterize the enriched topology of the complex adiabatic potential energy surfaces.

We use the adiabatic-nuclei molecular convergent close-coupling method to perform calculations of 0.01–1000 eV electrons scattering on the $c{}^3\mathrm{\Pi }_u$, $a{}^3\mathrm{\Sigma }_g^{+}$, $B{}^1\mathrm{\Sigma }_u^{+}$, $C{}^1\mathrm{\Pi }_u$, and $EF{}^1\mathrm{\Sigma }_g^{+}$ states of ${\mathrm{H}}_2$ in the $v=0$ vibrational level. Elastic, superelastic, ionization, and grand-total cross sections are presented, as well as cross sections for excitation of the $n=2$–3 singlet and triplet states of ${\mathrm{H}}_2$ (where $n$ is the atomic-limit principle quantum number). Comparison with available theoretical results is made. Good agreement is found with the recent $R$-matrix results [J. Phys. B 53, 245203 (2020)] for most of the exchange and dipole-forbidden transitions, but not for the dipole-allowed transitions. The sources of disagreement were found to be an unconverged partial-wave expansion and the utilization of the fixed-nuclei approximation (as opposed to adiabatic-nuclei) in the $R$-matrix calculations.

Nonlinear Compton scattering of ultrarelativistic electrons traversing high-intensity laser pulses generates hard photons. These photon high-energy tails are considered to be parameters within reach in the forthcoming LUXE and E-320 experiments. We consider the invariant differential cross sections $d\sigma /du$ between the IR and UV regions and analyze the impact of the laser polarization, and find $q$-deformed exponential shapes. (The variable $u$ is the light-cone momentum transfer from the initial electron to the final photon.) Optical laser pulses of various durations are compared with the monochromatic laser-beam model, which uncovers the laser-intensity parameter in the range $\xi =1,\cdots ,10$. Some supplemental information is provided for the azimuthal final-electron and final-photon distributions and the photon-energy differential cross sections.

We present a state-of-the-art theory of tunneling ionization of the hydrogen molecule in a static electric field. The theory is based on the leading-order many-electron weak-field asymptotic theory and successively incorporates core polarization, spectator nucleus, and internuclear motion effects. The predictions of the theory are compared with ab initio calculations of the ionization rate of ${\mathrm{H}}_2$ with frozen nuclei and experimental results on the anisotropy of strong-field ionization of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$. The comparison reveals the relative role of the different effects. We show that all the effects should be taken into account in order to achieve good agreement with the ab initio and experimental results.

This paper analyzes the existence of surface electromagnetic waves (SEWs) at the interface of bianisotropic bicrystals which are formed of half-infinite periodic superlattices or of homogeneous bianisotropic media. In either case we assume no absorption. Properties of the impedance matrices characterizing such bianisotropic media are established. On this basis, a series of statements is proved on the maximum total number of SEWs in two bicrystals composed of two superlattices or homogeneous media in such a way that the upper (lower) half of one bicrystal complements the lower (upper) part of the other to an infinite periodic superlattice or homogeneous media, respectively. It is shown that in superlattices the maximum number of SEWs at a fixed tangential wave number equals two in the lowest forbidden band (this band begins from zero frequency) and four in any upper forbidden band. It is also proved that at most two SEWs emerge in homogeneous bianisotropic media.

Nonlinear spectroscopy and microscopy techniques are ubiquitous in a wide range of applications across physics and biology. However, these usually rely on high-powered pulsed laser systems. A promising alternative is to exploit entangled two-photon absorption (ETPA), which can lead to tens of orders of magnitude lower incident fluxes than in conventional two-photon absorption schemes. However, the role of different entangled degrees of freedom in ETPA was unclear following recent experimental studies, when compared to earlier theoretical works. Here, we first demonstrate a linear dependence of the ETPA rate with the photon-pair rate, a clear signature of ETPA, and estimate the values for the concentration-dependent ETPA cross section for Rhodamine 6G. We then investigate the signature of energy-time entanglement and polarization dependence in the ETPA fluorescence rate and demonstrate a strong dependence of the signal on the interphoton delay that reflects the coherence time of the entangled two-photon wave packet.

We analyze recently published results about the Aharonov-Bohm effect for bound states from the interaction of the magnetic quadrupole moment of a neutral particle with axial fields. We show that the eigenvalues obtained by the authors from an arbitrary truncation of the Frobenius power series correspond to more than one model and, consequently, are unsuitable for drawing any sound physical conclusion. In addition, the prediction of allowed oscillator frequencies is a mere consequence of the truncation condition just mentioned.

Wavelength division multiplexing (WDM) of quantum key distribution (QKD) with classical optical channels has been proven to be feasible experimentally. In this paper, we will analyze the practical security of WDM-QKD theoretically. Specifically, we first establish the noise model of classical signals. Then, the influence of classical noises on the performance of decoy-state WDM-QKD is studied with a finite-key security analysis. For numerical simulations, the effect of classical noise-reduction methods for different experimental settings is given. The secret key rate and maximum transmission distance of WDM-QKD is simulated with various sent number of quantum pulses or transmitted power of classical signals. Furthermore, the performance of two kinds of decoy-state WDM-QKD is compared, which shows that the optimal decoy-state protocol for WDM-QKD is related to the sent number of quantum pulses. Since WDM-QKD is desired to reduce the cost of QKD networks and since experiments have already been carried out, our work can not only close the gap between theory and practice, but also be used to optimize the experimental parameters and improve its performance.

In the spirit of device-independent cryptography, we present a two-party quantum authorization primitive with nonlocality as its fueling resource. Therein, users are attributed authorization levels granting them access to a private database accordingly. The authorization levels are encoded in the nonlocal resources distributed to the users and subsequently confirmed by their ability to win Clauser-Horne-Shimony-Holt games using such resources. We formalize the protocol, prove its security, and frame it in a device-independent setting employing the notion of Clauser-Horne-Shimony-Holt self-testing via simulation. Finally, we provide a proof-of-concept implementation using the Qiskit open-source framework.

Quantum low-density-generator-matrix (QLDGM) codes are known to exhibit great error correction capabilities, surpassing existing quantum low-density-parity-check (QLDPC) codes and other sparse-graph schemes over the depolarizing channel. Most of the research on QLDPC codes and quantum error correction (QEC) is conducted for the symmetric instance of the generic Pauli channel, which incurs bit flips, phase flips, or a combination of both with the same probability. However, due to the behavior of the materials they are built from, some quantum devices must be modelled using a different channel model capable of accurately representing asymmetric scenarios in which the likelihood of a phase flip is higher than that of a bit flip. In this work, we study the design of QLDGM CSS codes for such Pauli channels. We show how codes tailored to the depolarizing channel are not well suited to these asymmetric environments and we derive methods to aptly design QLDGM CSS codes for this paradigm.

Rare-earth ion doped crystals have proven to be solid platforms for implementing quantum memories. Their potential use for integrated photonics with large multiplexing capability and unprecedented coherence times is at the core of their attractiveness. The best performances of these ions are, however, usually obtained when subjected to a DC magnetic field, but consequences of such fields on the quantum memory protocols have only received little attention. In this paper, we focus on the effect of a DC bias magnetic field on the population manipulation of non-Kramers ions with nuclear quadrupole states, both in the spin and optical domains, by developing a simple theoretical model. We apply this model to explain experimental observations in a ${}^{151}\mathrm{Eu}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal, and highlight specific consequences on the atomic frequency comb spin-wave protocol. The developed analysis should allows predicting optimal magnetic field configurations for various protocols.

The annealing schedule is optimized for a parameter in the Lechner-Hauke-Zoller (LHZ) scheme for quantum annealing designed for the all-to-all-interacting Ising model representing generic combinatorial optimization problems. We adapt the variational approach proposed by Matsuura et al. (arXiv:2003.09913) to the annealing schedule of a term representing a constraint for variables intrinsic to the LHZ scheme with the annealing schedule of other terms kept intact. Numerical results for a simple ferromagnetic model and the spin-glass problem show that nonmonotonic annealing schedules optimize the performance measured by the residual energy and the final ground-state fidelity. This improvement does not accompany a notable increase in the instantaneous energy gap, which suggests the importance of a dynamical viewpoint in addition to static analyses in the study of practically relevant diabatic processes in quantum annealing.

Bose-Einstein condensation, an effect long known for material particles as cold atomic gases, has in recent years also been observed for photons in microscopic optical cavities. Here, we report absorption and emission spectroscopic measurements on the lowest electronic transition ($5p^6\to 5p^{5}6s$) of xenon, motivated by the search for a thermalization medium for photon Bose-Einstein condensation in the vacuum-ultraviolet spectral regime. We have recorded pressure-broadened xenon spectra in the 135- to 190-nm wavelength regime at conditions near the critical point. The explored pressure and temperature ranges include high-pressure gaseous xenon below the critical pressure and supercritical xenon at room temperature as well as liquid xenon close to the boiling point near the critical pressure.

We report angle-resolved measurements on photoelectrons emitted upon near-threshold two-photon double ionization (TPDI) of Kr irradiated by free-electron laser (FEL) pulses. These photoelectron angular distributions (PADs) are compared with the results of semirelativistic $R$-matrix calculations. As reported by Augustin et al. [Phys. Rev. A 98, 033408 (2018)], it is found that the presence of autoionizing resonances within the bandwidth of the exciting FEL pulse strongly influences the PADs. In contrast to measurements on lower-$Z$ targets such as Ne and Ar, the larger spin-orbit interaction, inherent in $4p$-subshell hole states of Kr, permits us to resolve and study PADs associated with some of the fine-structure components of the ${\mathrm{Kr}}^{+}$ and ${\mathrm{Kr}}^{2+}$ ions.

In the present paper, we consider nonresonant corrections to $2s\text{−}ns/nd$ transition frequencies in hydrogen for the experiments based on two-photon spectroscopy. A detailed study of angular correlations of quantum interference effects within the framework of rigorous quantum electrodynamics is given. Closed expressions for the resonant two-photon-scattering cross sections on an atomic level with dependence on all atomic quantum numbers including fine and hyperfine structure are derived. These expressions are applied for the description of experiments based on two-photon spectroscopy with fixing of incident (outgoing) photon polarizations. We demonstrate that, for certain experimental geometries, nonresonant corrections could be significant for the determination of Rydberg constant and proton charge radius.