By fully taking into account the virtual interband processes along with the intraband ones, here we propose an effective-band-mass theorem that is suitable for a wide class of single-particle Hamiltonians exhibiting multiple energy bands. Then, for the special case of two-band systems, we show that the interband contribution to the effective band mass of a particle at a given quantum state is directly controlled by the quantum metric of the corresponding state. As an illustration, we consider a spin-orbit-coupled spin-$\frac{1}{2}$ particle and calculate its effective band mass at the band minimum of the lower helicity band. Independent of the coupling strength, we find that the bare mass $m_0$ of the particle jumps to $2m_0$ for the Rashba coupling and to $3m_0$ for the Weyl coupling. This geometric mass enhancement is a nonperturbative effect, uncovering the mystery behind the effective mass of the two-body bound states in the noninteracting limit. As a further illustration, we show that a massless Dirac quasiparticle acquires a linearly dispersing band mass (equivalent to the effective cyclotron mass up to a prefactor) with its momentum through the same mechanism.

Correlation plenoptic imaging (CPI) is an imaging technique that exploits the correlations between the intensity fluctuations of light to perform the typical tasks of plenoptic imaging (namely, refocusing out-of-focus parts of the scene, extending the depth of field, and performing three-dimensional reconstruction) without entailing a loss of spatial resolution. Here, we consider two different CPI schemes based on chaotic light, both employing ghost imaging: the first one to image the object, the second one to image the focusing element. We characterize their noise properties in terms of the signal-to-noise ratio (SNR) and compare their performances. We find that the SNR can be significantly easier to control in the second CPI scheme involving standard imaging of the object; under adequate conditions, this scheme enables the number of frames for achieving the same SNR to be reduced by 1 order of magnitude.

We present a theoretical model of a system with time-delayed coherent feedback and show how this feedback can lead to the preservation of a substantial level of coherence for a two-level system embedded in a phononic cavity, even at a finite temperature. The exact nature of the solution obtained from the model enables transparent control of the polarization properties through appropriate choices of phase relations and initial conditions.

The Cramér-Rao bound and the quantum Fisher information have been tools used extensively for interferometric phase sensitivity. Most scenarios considering a Mach-Zehnder interferometer with two input sources focused on the phase-matched case, when the Fisher information is maximal. Under this constraint, the best sensitivity is achieved for a balanced (50/50) input beam splitter. In this paper, we take a different approach: we allow the beam splitter transmission coefficient as well as the input phase mismatch to be variable parameters. We then search for a pair of these parameters that maximizes the Fisher information. We find that for the double coherent input the maximum Fisher information can always be reached in the unbalanced case for a carefully chosen input phase mismatch. For the coherent plus squeezed vacuum case we find that under certain circumstances a threshold phase mismatch exists, beyond which the optimum Fisher information is found for the degenerate case. For the squeezed-coherent plus squeezed vacuum case we find that the optimum actually occurs when the squeezing angles of the two inputs are in antiphase.

When a quantum system is coupled to several heat baths at different temperatures, it eventually reaches a nonequilibrium steady state featuring stationary internal heat currents. These currents imply that entropy is continually being produced in the system at a constant rate. In this paper we apply phase-space techniques to the calculation of the Wigner entropy production on general linear networks of harmonic nodes. Working in the ubiquitous limit of weak internal coupling and weak dissipation, we obtain simple closed-form expressions for the entropic contribution of each individual quasiprobability current. Our analysis highlights the essential role played by the internal unitary interactions (node-node couplings) in maintaining a nonequilibrium steady state and hence a finite entropy production rate. We also apply this formalism to the paradigmatic problem of energy transfer through a chain of oscillators subject to self-consistent internal baths that can be used to tune the transport from ballistic to diffusive. We find that the entropy production scales with different power law behaviors in the ballistic and diffusive regimes, hence allowing us to quantify what is the “entropic cost of diffusivity.”

The unavoidable interaction of a quantum system with its surrounding environment (its bath) is not always detrimental for quantum properties. For instance, under some specific conditions (that we identify as indistinguishability), a many-body system can gain internal coherences thanks to the interaction with its bath. The most famous consequence of this phenomenon is super-radiance. Beyond that, the thermodynamic effects on the system of these bath-induced coherences have been mostly unexplored. We show here, for a simple and common system (a pair of two-level systems), that the energetic and entropic impacts can indeed be dramatic and diverse, including amplification of the action of the bath but also its mitigation. Our results can be tested experimentally. They suggest that bath-induced coherences can be harnessed to enhance thermodynamic tasks, opening up interesting perspectives for thermal machines, quantum battery charging, natural or artificial energy harvesting systems, and state preparation and protection.

In this work we study how the non-Markovian character of the dynamics can affect the thermodynamic performance of a quantum thermal engine, by analyzing the maximum power output of Carnot and Otto cycles departing from the quasistatic and infinite-time-thermalization regime respectively, introducing techniques for their control optimization in general dynamical models. In our model, non-Markovianity is introduced by allowing some degrees of freedom of the reservoirs to be taken into account explicitly and share correlations with the engine by Hamiltonian coupling. It is found that the non-Markovian effects can fasten the control and improve the power output.

Quantum random numbers generators (QRNGs) rely on quantum systems to produce sequences of random numbers with an overall lower level of predictability than classical algorithmic systems. Over the past two decades, phase randomizations of coherent sources from quantum spontaneous emission effects have gained a lot of interest due to their operational simplicity, cost-contained components, and ability to generate random numbers at high rates. However, many QRNGs require optimal calibration and alignment to ensure efficient and effective random-number generation. This work demonstrates a detailed analysis of a heterodyne measurement based QRNG, which implements phase randomization from two independent laser sources. The analysis also quantifies the effects of setup misalignments using the Kullback-Leibler divergence as a benchmark to assess the limiting conditions of secure random-number generation.

Quantum mechanics is inherently probabilistic in light of Born's rule. Using quantum circuits as probabilistic generative models for classical data exploits their superior expressibility and efficient direct sampling ability. However, training of quantum circuits can be more challenging compared to classical neural networks due to the lack of an efficient differentiable learning algorithm. We devise an adversarial quantum-classical hybrid training scheme via coupling a quantum circuit generator and a classical neural network discriminator together. After training, the quantum circuit generative model can infer missing data with quadratic speed-up via amplitude amplification. We numerically simulate the learning and inference of generative adversarial quantum circuits using the prototypical bars-and-stripes dataset. Generative adversarial quantum circuits are a fresh approach to machine learning which may enjoy the practically useful quantum advantage of near-term quantum devices.

Bravyi and Gosset [S. Bravyi and D. Gosset, Phys. Rev. Lett. 116, 250501 (2016)] recently gave classical simulation algorithms for quantum circuits dominated by Clifford operations. These algorithms scale exponentially with the number of $T$ gates in the circuit, but polynomially in the number of qubits and Clifford operations. Here we extend their algorithm to qudits of odd prime dimension. We generalize their approximate stabilizer rank method for weak simulation to qudits and obtain the scaling of the approximate stabilizer rank with the number of single-qudit magic states. We also relate the canonical form of qudit stabilizer states to Gauss sum evaluations and give an $O\left(n^3\right)$ algorithm for calculating the inner product of two $n$-qudit stabilizer states.

Bremsstrahlung spectra generated by electrons with incident energies of 5–25 keV bombarding thick targets of $\mathrm{Al}(Z=13), \mathrm{Cu}(Z=29), \mathrm{Ag}(Z=47), \mathrm{Te}(Z=52)$, and $\mathrm{Au}(Z=79)$ are measured with high accuracy. Comparing the experimental spectra with the simulated spectra obtained from the Monte Carlo code penelope, it is found that they are in excellent agreement. The penelope code contains only the cross sections for ordinary bremsstrahlung, while disregarding polarization bremsstrahlung. The results quite definitely indicate that there is no polarization bremsstrahlung contribution to the x-ray radiation spectra produced by incident electrons with thick solid targets for the x-ray energies of interest in this paper ($>1\mathrm{keV}$).

The radiative electron capture (REC) into the $K$ shell of bare Xe ions colliding with a hydrogen gas target has been investigated. In this study, the degree of linear polarization of the K-REC radiation was measured and compared with rigorous relativistic calculations as well as with the previous results recorded for ${\mathrm{U}}^{92+}$. Owing to the improved detector technology, a significant gain in precision of the present polarization measurement is achieved compared to the previously published results. The obtained data confirms that for medium-Z ions such as Xe, the REC process is a source of highly polarized x rays which can easily be tuned with respect to the degree of linear polarization and the photon energy. We argue, in particular, that for relatively low energies the photons emitted under large angles are almost fully linear polarized.

We investigate the role of the Coulomb interaction in strong field processes. We find that the Coulomb field of the ion makes its presence known even in highly intense laser fields, in contrast to the assumptions of the strong field approximation. The dynamics of the electron after ionization is analyzed with four models for an arbitrary laser polarization: the Hamiltonian model in the dipole approximation, the strong field approximation, the Coulomb-corrected strong field approximation, and the guiding center. These models illustrate clearly the Coulomb effects, in particular Coulomb focusing and Coulomb asymmetry. We show that the Coulomb-corrected strong field approximation and the guiding center are complementary, in the sense that the Coulomb-corrected strong field approximation describes well short timescale phenomena (shorter than a laser cycle) for which the Coulomb interaction is significant on short timescales, such as in subcycle recollisions, while the guiding center is well suited for describing long timescale phenomena (longer than a laser cycle) for which the Coulomb interaction is significant on long timescales, such as in Coulomb-driven recollisions and Rydberg state creation.

We study the photoelectron energy and momentum distributions of strong-field tunneling ionization in the relativistic regime. Using the relativistic strong-field approximation, we have derived the position of the tunnel exit, the initial momentum distributions at the tunnel exit, and the instantaneous ionization rate when the electron tunnels through the Coulomb barrier in the relativistic regime. Since the tunneling electron energy is of the order of 1% of the rest of its mass, those nonadiabatic tunneling coordinates are quite different from the nonrelativistic case. We further incorporate the nonadiabatic relativistic tunneling coordinates into the classical trajectory Monte Carlo model and have calculated photoelectron energy and momentum distributions by accurately considering the field distribution in the focus of relativistic femtosecond laser pulses. We show that the nonadiabatic tunneling effect and the focal field distribution are very crucial to modeling strong-field ionization in the relativistic regime because the quiver radius of electrons can be as large as the beam waist.

We theoretically explore a variant of RABBITT spectroscopy in which the attosecond-pulse train comprises isolated pairs of consecutive harmonics of the fundamental infrared probe frequency. In this scheme, one-photon and two-photon amplitudes interfere, resulting in an asymmetric photoelectron emission. This interferometric principle has the potential of giving access to the time-resolved ionization of systems that exhibit autoionizing states, since it imprints the group delay of both one-photon and two-photon resonant transitions in the energy-resolved photoelectron anisotropy as a function of the pump-probe time delay. To bring to the fore the connection between the pump-probe ionization process and its perturbative analysis on the one side and the underlying field-free scattering observables as well as the radiative couplings in the target system on the other side, we test this scheme with an exactly solvable analytical one-dimensional model that supports both bound states and shape resonances. The asymmetric photoelectron emission near a resonance is computed using perturbation theory as well as solving the time-dependent Schödinger equation; the results are in excellent agreement with the field-free resonant scattering properties of the model.

We consider the dynamical model of a binary bosonic gas trapped in a symmetric dual-core cigar-shaped potential. The setting is modeled by a system of linearly coupled one-dimensional Gross-Pitaevskii equations with cubic self-repulsive terms and quadratic attractive ones, which represent the Lee-Huang-Yang corrections [T. D. Lee, K. S. Huang, and C. N. Yang, Phys. Rev. 106, 1135 (1957).] to the mean-field theory in this geometry. The main subject is spontaneous symmetry breaking (SSB) of quantum droplets (QDs), followed by restoration of the symmetry, with respect to the identical parallel-coupled trapping cores, following the increase of the QDs' total norm. The SSB transition and inverse symmetry-restoring transition form a bifurcation loop, whose shape is concave at small values of the intercore coupling constant $\kappa$ and convex at larger $\kappa$. The loop does not exist above a critical value of $\kappa$. At very large values of the norm, QDs do not break their symmetry, featuring a flat-top shape. Some results are obtained in an analytical form, including an exact front solution connecting asymptotically constant zero and finite values of the wave function. Collisions between moving QDs are considered too, demonstrating a trend to merge into breathers.

We use a non-Lindbladian master equation of the Scully-Lamb laser model for the analysis of light propagation in a parity-time symmetric photonic system composed of coupled active and passive whispering-gallery microresonators. Performing the semiclassical approximation, we obtain a set of two nonlinear coupled differential equations describing the time evolution of intracavity fields. These coupled equations are able to explain the experimentally observed light nonreciprocity [Peng et al., Nat. Phys. 10, 394 (2014); Chang et al., Nat. Photon. 8, 524 (2014)]. We show that this effect arises from the interplay between gain saturation in the active microcavity, intercavity coupling, and losses in the cavities. Additionally, using this approach, we study the effect of the gain saturation on exceptional points, i.e., exotic degeneracies in non-Hermitian systems. Namely, we demonstrate that the inclusion of gain saturation leads to a modification of the exceptional points in the presence of intense intracavity fields. The Scully-Lamb master equation for systems of coupled optical structures, as proposed and applied here, constitutes a promising tool for the study of quantum optical effects in coupled systems with losses, gain, and gain saturation.

We investigate the temporal accelerating self-imaging effect for a train of Airy pulses propagating in optical fibers. The group velocity of the Airy pulses is varying during propagation, resulting in a parabolic time-space trajectory of self-imaging. The acceleration is determined by the main-lobe width of the Airy pulses. Meanwhile, the self-imaging distance depends both on the main-lobe width and the time interval of the pulses. In addition, the trajectory of self-imaging can be modified by imposing a linearly varying phase on the input pulse train. By applying third-order dispersion, we also realize the temporally magnified self-imaging of the Airy pulse trains. As the Airy pulses possess infinite energy, the self-imaging can be observed for infinite times. If the pulses are truncated to have finite-energy, the self-imaging maintains only in a limited distance. The study provides a promising way to control the self-imaging of optical pulses and may find applications in optical communication and signal processing systems.

A protocol for explicitly constructing the exact time-evolution operators generated by $2\times 2$ time-dependent $PT$-symmetry Hamiltonians is reported. Its mathematical applicability is illustrated with the help of appropriate examples. The physical relevance of the proposed approach within gain-loss system scenarios, like two coupled waveguides, is discussed in detail.

Measurement-device-independent quantum key distribution (MDI-QKD) is proposed to close the loopholes of the detection part in QKD systems, while it still requires the light source to be trusted. This assumption is not guaranteed in practice, leading to practical secure issues. In this paper, we propose a light source monitoring scheme with single-photon detectors for MDI-QKD, which can guarantee the security of the system under the untrusted source condition with both common decoy state method and the newly proposed five-intensity decoy state method. Moreover, our monitoring scheme performs almost the same as the ideal MDI protocol even considering the imperfect factors in practical systems. Compared to the previous scheme, our work improves the secure key rate and the transmission distance of practical MDI-QKD systems significantly.