Decoherence affects all quantum systems, natural or artificial, and is the primary obstacle impeding quantum technologies. We show theoretically that for a Rydberg qubit in a Bose condensed environment, experiments can image the system-environment interface that is central for decoherence. High-precision absorption images of the condensed environment will be able to capture transient signals that show the real-time buildup of a mesoscopic entangled state in the environment. This is possible before decoherence sources other than the condensate itself can kick in, since qubit decoherence timescales can be tuned from the order of nanoseconds to microseconds by a choice of the excited Rydberg principal quantum number $\nu$. Imaging the interface will allow detailed explorations of open quantum system concepts and may offer guidance for coherence protection in challenging scenarios with non-Markovian environments.

The von Neumann entropy rate for open quantum systems is, in general, written in terms of entropy production and entropy flow rates, encompassing the second law of thermodynamics. For memoryless open quantum systems, whose evolutions correspond to one-parameter quantum dynamical semigroups, we find a decomposition of the infinitesimal generator of the dynamics that unveils the fluctuation-dissipation structure of these evolutions. Thus, the fluctuation part of this decomposition allows to relate the von Neumann entropy rate with the divergence-based quantum Fisher information, at any time. Applied to continuous-in-time quantum Gaussian channels, our decomposition leads to the quantum analog of the generalized classical de Bruijn identity, thus expressing the quantum fluctuation-dissipation relation in that kind of channels. Finally, from this perspective, we analyze how stationarity arises.

The problem of fermion dynamics is studied using the $Q$ function for fermions. This is a probabilistic phase-space representation, which we express using Majorana operators and a phase-space variable of real antisymmetric matrices. We consider a general Hamiltonian with quadratic and quartic Majorana interactions and arbitrary couplings. Our model includes the Majorana-Hubbard and Fermi-Hubbard Hamiltonians, as well as general quantum field theories of interacting fermions. The relevant coefficients are calculated for a number of both local and nonlocal Hamiltonian models. We also give a detailed example of a four-Majorana interaction. Using the Majorana $Q$ function we derive a generalized Fokker-Planck equation, with results for the drift and diffusion terms. The diffusion term is shown to be traceless, which has a dynamical interpretation as a forward-backward stochastic process. We prove that an initial pure state has trajectories that remain on a surface of pure states. This approach leads to a model of quantum measurement in terms of an ontology with real vacuum fluctuations.

The appearance of steady-state coherence (SSC) from system-bath interactions proves that quantum effects can appear without an external drive. Such SSC could become a resource to demonstrate a quantum advantage in the applications. We predict the generation of SSC if the target system repeatedly interacts with independent and noncorrelated bath elements. To describe their behavior, we use the collision model approach of system-bath interactions, where the system interacts with one bath element (initially in an incoherent state) at a time, asymptotically (in the fast-collision regime) mimicking a macroscopic Markovian bath coupled to the target system. Therefore, the SSC qualitatively appears to be the same as if the continuous Markovian bath were used. We confirm that the presence of composite system-bath interactions under the rotating-wave approximation is the necessary condition for the generation of SSC using thermal resources in collision models. Remarkably, we show that SSC substantially increases if the target system interacts collectively with more than one bath element at a time. A few bath elements collectively interacting with the target system is sufficient to increase SSC at nonzero temperatures at the cost of a tolerable lowering of the final state purity. From the thermodynamic perspective, the SSC generation in our collision models is inevitably linked to a nonzero power input (and thus heat dissipated to the bath) necessary to reach the steady state, although such energetic cost can be lower compared to cases relying on SSC nongenerating interactions.

Recent advances in the study of thermodynamics of microscopic processes have driven the search for new developments in energy converters utilizing quantum effects. Here we propose a modified Otto cycle to design an engine fueled by quantum projective measurements. Standard quantum thermal machines operating in a finite-time regime with a driven Hamiltonian that does not commute at different times have their performance decreased by the presence of coherence, which is associated with a larger entropy production and irreversibility degree. However, we show that replacing the standard hot thermal reservoir by a projective measurement operation with a general basis in the Bloch sphere and controlling the basis angles suitably could improve the performance of the quantum engine as well as decrease the entropy change during the measurement process. Our results follow a generalization of quantum thermal machine models where the fuel comes from general sources beyond the standard thermal reservoir.

The lackadaisical quantum walk is a lazy version of a discrete-time, coined quantum walk, where each vertex has a weighted self-loop that permits the walker to stay put. They have been used to speed up spatial search on a variety of graphs, including periodic lattices, strongly regular graphs, Johnson graphs, and the hypercube. In these prior works, the weights of the self-loops preserved the symmetries of the graphs. In this paper, we show that the self-loops can break all the symmetries of vertex-transitive graphs while providing the same computational speedups. Only the weight of the self-loop at the marked vertex matters, and the remaining self-loop weights can be chosen randomly, as long as they are small compared to the degree of the graph.

We present a method to perform a variational analysis of the quantum master equation for driven-disspative bosonic fields with arbitrary large occupation numbers. Our approach combines the $P$ representation of the density matrix and the variational principle for open quantum system. We benchmark the method by comparing it to wave-function Monte Carlo simulations and the solution of the Maxwell-Bloch equation for the Jaynes-Cummings model. Furthermore, we study a model describing Rydberg polaritons in a cavity field and introduce an additional set of variational parameters to describe correlations between different modes.

We investigate coherent single-photon transport in a waveguide-QED structure containing two giant atoms. The unified analytical expressions of the single-photon scattering amplitudes applicable for different topological configurations are derived. The spectroscopic characteristics in different parameter regimes, especially the asymmetric Fano line shapes and the electromagnetically induced transparency (EIT)-like spectra, are analyzed in detail. Specifically, we find that the appearance of Fano line shapes is influenced by not only the phase delays between coupling points but also the topologies of system. We also summarize the general conditions for appearance of EIT-like spectra by analyzing the master equation and verify these conditions by checking the corresponding analytical expressions of the scattering spectra. These phenomena may provide powerful tools for controlling and manipulating photon transport in future quantum networks.

Starting from a general wave-function described on a set of spins or qubits, we propose several quantum algorithms to extract the components of this state on eigenstates of total spin ${\mathbf{S}}^2$ and its $z$ component $S_z$. The method plays the role of total spin projection and gives access to the amplitudes of the initial state on a total spin basis. The different algorithms have various degrees of sophistication depending on the requested tasks. They can either solely project onto the subspace with good total spin or completely uplift the degeneracy in this subspace. In the former case, when the projection on the total spin $j$ is made, we show that the number of operations for the projection can be reduced from a quadratic to a linear dependence in $j$. After each measurement, the state collapses to one of the spin eigenstates that could be used for postprocessing. For this reason, we call the method total quantum spin filtering.

We demonstrate a hierarchy of various classes of quantum correlations on experimentally prepared two-qubit Werner-like states with controllable white noise. Werner states, which are white-noise-affected Bell states, are prototypal examples for studying such a hierarchy as a function of the amount of white noise. We experimentally generate Werner states and their generalizations, i.e., partially entangled pure states affected by white noise. These states enable us to study the hierarchy of the following classes of correlations: separability, entanglement, steering in three- and two-measurement scenarios, and Bell nonlocality. We show that the generalized Werner states (GWSs) reveal fundamentally different aspects of the hierarchy compared to the Werner states. In particular, we find five different parameter regimes of the GWSs, including those steerable in a two-measurement scenario but not violating Bell inequalities. This regime cannot be observed for the usual Werner states. Moreover, we find threshold curves separating different regimes of the quantum correlations and find the optimal states which allow for the largest amount of white noise which does not destroy their specific quantum correlations (e.g., unsteerable entanglement). Thus, we could identify the optimal Bell-nondiagonal GWSs which are, for this specific meaning, more robust against the white noise compared to the Bell-diagonal GWSs (i.e., Werner states).

A laser-assisted ($e, 2e$) [LA($e, 2e$)] process of Ar in an ultrashort (1.2 ps) near-infrared intense laser field, $\mathrm{Ar}+e^{-}+h\nu \to {\mathrm{Ar}}^{+}+e^{-}+e^{-}$, was observed by coincidence measurements of the fast electron scattered forward within the small-angle range and the slow electron emitted to the wide-angle range. The spectrum of the sum of the kinetic energies of fast and slow electrons exhibited a shoulder structure at the one-photon energy shift from the ($e, 2e$) peak for the Ar $3p$ orbital, which was unambiguously assigned to the LA($e, 2e$) process with the one-photon energy gain. The determined triply differential cross section of the LA($e, 2e$) process with one-photon gain from the laser field was approximately twice as large as a theoretical estimate and this enhancement was regarded as an evidence of the formation of light-dressed states of Ar.

In previous work devoted to ab initio calculations of hyperfine-structure constants in nitrogen and fluorine atoms, we observed sizable relativistic effects, a priori unexpected for such light systems, that can even largely dominate over electron correlation. We observed that the atomic wave functions calculated in the Breit-Pauli approximation describe adequately the relevant atomic levels and hyperfine structures, even in cases for which a small relativistic $LS$-term mixing becomes crucial. In the present work we identify levels belonging to the spectroscopic terms $2p^4{(}^{3}P)3d{}^{2,4}(P,D,F)$ of the fluorine atom, for which correlation effects on the hyperfine structures are small, but relativistic $LS$-term admixtures are decisive to correctly reproduce the experimental values. The Breit-Pauli analysis of the hyperfine matrix elements nails cases with large cancellation, either between $LS$ pairs for individual hyperfine operators or between the orbital and the spin dipole contributions. Multiconfiguration Dirac-Hartree-Fock calculations are performed to support the Breit-Pauli analysis.

An adiabatic cycle around a degeneracy point in the parameter space of a one-dimensional band insulator is known to result in an integer-valued noiseless particle transport in the thermodynamic limit. Recently, it was shown that in the case of an infinite bipartite lattice the adiabatic Thouless protocol can be continuously deformed into a fine-tuned finite-frequency cycle preserving the properties of noiseless quantized transport. In this paper, we numerically investigate the implementation of such an ideal rapid-cycle Thouless pumping protocol in a one-dimensional optical lattice. It is shown that the rapidity will cause first-order corrections due to next-to-nearest-neighbor hopping and second-order corrections due to the addition of a harmonic potential. Lastly, the quantization of the change in center of mass of the particle distribution is investigated and shown to have corrections in the first order of the potential curvature.

We propose the concept of one-sided destructive quantum interference based on lossy metasurfaces. It corresponds to one-sided beam splitting with 50:25:0 in percentage ratio among the transmission and the two reflection efficiencies, which can be realized by a metasurface with unidirectional zero reflection (UZR) from a non-Hermitian exceptional point. When two identical photons enter both sides of the lossy metasurface, quantum interference only occurs for the single-photon output state towards one side but not the other due to UZR. Such one-sided quantum interference can be further made totally destructive. This effect along with UZR provides more degrees of freedom in quantum interference control and entangled states construction.

Vortex solitons (dark solitons) are described in self-defocusing Kerr media whose phase line singularity is not parallel to the propagation direction, but is perpendicular or tilted almost arbitrary angles, depending on the medium linear dispersion. These solitons broaden the so-called spatiotemporal optical vortices recently observed in experiments to the nonlinear realm, opening alternative perspectives in practical applications of optical vortices.

We propose two semi-device-independent approaches that are able to quantify unknown multipartite quantum entanglement experimentally, where the only information that has to be known beforehand is quantum dimension, and the concept that plays a key role is nondegenerate Bell inequalities. Specifically, using the nondegeneracy of multipartite Bell inequalities, we obtain useful information on the purity of target quantum state. Combined with an estimate of the maximal overlap between the target state and pure product states, and a continuous property of the geometric measure of entanglement, we shall prove the information on purity allows us to give a lower bound for this entanglement measure. In addition, we show that a different combination of the above results also converts to a lower bound for the relative entropy of entanglement. As a demonstration, we apply our approach on 5-partite qubit systems with the Mermin-Ardehali-Belinskii-Klyshko (MABK) inequality, and show that useful lower bounds for the geometric measure of entanglement can be obtained if the Bell expression value is larger than 3.60, and those for the relative entropy of entanglement can be given if the Bell expression value is larger than 3.80, where the Tsirelson bound is 4.

The Gottesman-Kitaev-Preskill (GKP) code is an important type of bosonic quantum error-correcting code. Since the GKP code only protects against small shift errors in $\widehat{p}$ and $\widehat{q}$ quadratures, it is necessary to concatenate the GKP code with a stabilizer code for the larger error correction. In this paper, we consider the concatenation of the single-mode GKP code with the two-dimensional (2D) color code (color-GKP code) on the square-octagon lattice. We use the Steane-type scheme with a maximum-likelihood estimation for GKP error correction and show its advantage for the concatenation. In our main paper, the minimum-weight perfect matching algorithm is applied to decode the color-GKP code. Complemented with the continuous-variable information from the GKP code, the threshold of the 2D color code is improved. If only data GKP qubits are noisy, the threshold reaches $\sigma \approx 0.59(\overline{p}\approx 13.3\%)$ compared with $\overline{p}=10.2\%$ of the normal 2D color code. If measurements are also noisy, we introduce the generalized restriction decoder on the three-dimensional space-time graph for decoding. The threshold reaches $\sigma \approx 0.46$ when measurements in the GKP error correction are noiseless, and $\sigma \approx 0.24$ when all measurements are noisy. Lastly, the good performance of the generalized restriction decoder is also shown on the normal 2D color code giving the threshold at $3.1\%$ under the phenomenological error model.

We present a model describing ionization and excitation of nitrogen molecules by a strong and short laser pulse. Different from previous publications, both processes are considered within the same formalism of the density matrix. We account for the dependence of the dipole moment on the vibrational quantum number and for a large number of excited levels. Populations of the excited levels depend significantly on the laser intensity, wavelength, polarization, and pulse duration.

An important tool for understanding the effects of interactions in harmonically trapped atomic gases is the examination of their collective modes. One such mode is the breathing or monopole mode, which is special as it is constrained to occur at twice the harmonic trapping frequency when the interactions are scale invariant. When the interactions are not scale invariant, the frequency of the breathing mode will deviate from twice the trap frequency. The deviation itself depends on the thermodynamic contacts, which describe how the energy changes with the interactions. In this work I examine how the thermodynamic contacts and the breathing-mode frequency of a spin-polarized one-dimensional (1D) $p$-wave Fermi gas depend on the 1D scattering volume $\ell$ and the effective range $r$ in the high-temperature limit. Such dynamics can be studied in experiments and provide a tool for understanding how the dynamics depends on interactions with a finite effective range.

With the growing popularity of cryogenic correlative light and electron microscopy, it is becoming increasingly important to bridge the resolution gap between these two modalities. At cryogenic temperatures, the photon yield of fluorophores is a few orders of magnitude higher than at room temperature, enabling localization precisions on the Ångström scale. The current challenge is to induce sparsity at cryogenic temperatures such that individual fluorescent molecules can be localized. In this paper, we demonstrate the progress of using polarized stimulated-emission depletion (STED) to induce sparsity at cryogenic temperatures and in vacuum. We generate linear polarization of arbitrary in-plane orientations to achieve polarized STED with a sparsity of 3.3:1. Furthermore, we have probed the dark-state lifetime of ATTO 647N at cryogenic temperatures and in vacuum at room temperature. This dark state in vacuum is long-lived ($\tau =38$ ms) and could be the cause for reduced photostability of fluorophores under STED illumination in vacuum. The experiments were done on an in-house designed and built liquid nitrogen cryostat, enabling 30 hours of stable cryogenic fluorescence microscopy.