For a chain with an open boundary described by the non-Hermitian Hamiltonian, the combination of the algebraic damping and the non-Hermitian skin effect leads to an edge burst. It is interesting to ask whether these features remain when the system is described by a master equation with periodic drives. In this paper, taking the Su-Schrieffer-Heeger model as an example, we explore the relaxation dynamics of the system with time periodically modulated intracell tunneling and single-particle dissipations. We find that for systems with periodic boundary conditions, the relaxation in the infinite-frequency driving limit can be algebraic or exponential depending on the intracell and intercell tunneling amplitudes. Finite-frequency driving can generally open the Liouvillian gap regardless of its strength and can turn the dynamics from algebraic damping to an exponential one. The Liouvillian gap increases with the increase of the driving period until a critical driving period is reached, and then it tends to a small, nonzero value for a sufficiently slow drive. For open boundary systems, we find that the dynamics of the localized excitation is still closely related to the Liouvillian gap before the excitation reaches the system boundary. The propagation speed of the localized excitation that controls the transition relaxation timescale can be slowed down by properly increasing the driving period and amplitude.

Starting from a microscopic system-baths description, we derive the general conditions for a time-local quantum master equation (QME) to satisfy the first and second laws of thermodynamics at the fluctuating level. Using counting statistics, we show that the fluctuating second law can be rephrased as a generalized quantum detailed balance condition (GQDB), i.e., a symmetry of the time-local generators which ensures the validity of the fluctuation theorem. When requiring in addition a strict system-bath energy conservation, the GQDB reduces to the usual notion of detailed balance which characterizes QMEs with Gibbsian steady states. However, if energy conservation is only required on average, QMEs with non-Gibbsian steady states can still maintain a certain level of thermodynamic consistency. Applying our theory to commonly used QMEs, we show that the Redfield equation breaks the GQDB, and that some recently derived approximation schemes based on the Redfield equation (which hold beyond the secular approximation and allow one to derive a QME of Lindblad form) satisfy the GQDB and the average first law. We find that performing the secular approximation is the only way to ensure the first and second laws at the fluctuating level.

Quantum complexity is emerging as a key property of many-body systems, including black holes, topological materials, and early quantum computers. A state's complexity quantifies the number of computational gates required to prepare the state from a simple tensor product. The greater a state's distance from maximal complexity, or “uncomplexity,” the more useful the state is as input to a quantum computation. Separately, resource theories—simple models for agents subject to constraints—are burgeoning in quantum information theory. We unite the two domains, confirming Brown and Susskind's conjecture that a resource theory of uncomplexity can be defined. The allowed operations, fuzzy operations, are slightly random implementations of two-qubit gates chosen by an agent. We formalize two operational tasks, uncomplexity extraction and expenditure. Their optimal efficiencies depend on an entropy that we engineer to reflect complexity. We also present two monotones, uncomplexity measures that decline monotonically under fuzzy operations, in certain regimes. This work unleashes on many-body complexity the resource-theory toolkit from quantum information theory.

One of the most challenging problems for the realization of a scalable quantum computer is to design a physical device that keeps the error rate for each quantum processing operation low. These errors can originate from the accuracy of quantum manipulation, such as the sweeping of a gate voltage in solid state qubits or the duration of a laser pulse in optical schemes. Errors also result from decoherence, which is often regarded as more crucial in the sense that it is inherent to the quantum system, being fundamentally a consequence of the coupling to the external environment. Grouping small collections of qubits into clusters with symmetries may serve to protect parts of the calculation from decoherence. In this work, we use four-level cores with a straightforward generalization of discrete rotational symmetry, called $\omega$-rotation invariance, to encode pairs of coupled qubits and universal two-qubit logical gates. We include quantum errors as a main source of decoherence, and show that symmetry makes logical operations particularly resilient to untimely anisotropic qubit rotations. We propose a scalable scheme for universal quantum computation where cores play the role of quantum-computational transistors, or quansistors for short. Initialization and readout are achieved by tunnel-coupling the quansistor to leads. The external leads are explicitly considered and are assumed to be the other main source of decoherence. We show that quansistors can be dynamically decoupled from the leads by tuning their internal parameters, giving them the versatility required to act as controllable quantum memory units. With this dynamical decoupling, logical operations within quansistors are also symmetry-protected from unbiased noise in their parameters. We identify technologies that could implement $\omega$-rotation invariance. Many of our results can be generalized to higher-level $\omega$-rotation-invariant systems, or adapted to clusters with other symmetries.

The sequential quadruple Auger decay of the Xe $4s$ core-hole state has been investigated by multielectron-ion coincidence spectroscopy using a magnetic bottle electron spectrometer. The high collection efficiency for electrons and ions enables sixfold coincidence measurement of all the emitted electrons and the product ${\mathrm{Xe}}^{5+}$ ion. While the Auger electrons emitted in the different Auger steps greatly overlap each other in energy, step-by-step analysis of the energy correlation among Auger electrons in coincidence discloses two important quadruple Auger decay pathways.

We study the three-body recombination of identical fermionic atoms. Using a zero-range model for the $p$-wave interaction, we show that the rate constant of three-body recombination into weakly bound $p$-wave dimers can be written as ${\alpha }_{\mathrm{rec}}\propto v^{5/2}{R}^{1/2}{k}_T^4(1+C{k}_T^2{l}_d^2)$ for large and positive scattering volume $v$. Here $R$ is the $p$-wave effective range, $k_T^2$ gives the average thermal kinetic energy of the colliding atoms, and $l_d$ is the size of the $p$-wave dimer. The leading term is different from the usually stated $v^{8/3}$-scaling law, but is consistent with an earlier two-channel calculation. For the subleading term, we compute the constant $C$ by solving the relevant three-body problem perturbatively when the parameter $\gamma \equiv R/v^{1/3}$ is small. The additional $C{k}_T^2{l}_d^2$ term provides important corrections for the temperature and interaction dependence of ${\alpha }_{\mathrm{rec}}$, especially close to resonance when $k_T{l}_d$ is relatively large.

The transverse momentum correlation of photon pairs generated by noncollinear spontaneous parametric down-conversion with type-I phase matching is transformed into polarization entanglement. A robust wavefront-splitting interferometer scheme is used, due to which the phase of the entangled state does not require active stabilization. Frequency-degenerate signal and idler photons are separated and coupled into single-mode optical fibers. The concept is demonstrated in a compact and modular setup. A straightforward alignment process for producing any of the Bell states is presented. The entangled source has an output photon flux of $8.33\pm 1.27$ kcps/mW normalized to pump power (150 mW), and $17\pm 3\%$ heralding ratio. The measured state fidelity with respect to the negative anticorrelated Bell state is $0.951\pm 0.004$.

Spontaneous parametric down-conversion (SPDC) is a widely used source for photonic entanglement. Years of focused research have led to a solid understanding of the process, but a cohesive analytical description of the paraxial biphoton state has yet to be achieved. We derive a general expression for the spatio-temporal biphoton state that applies universally across common experimental settings and correctly describes the nonseparability of spatial and spectral modes. We formulate a criterion on how to decrease the coupling of the spatial from the spectral degree of freedom by taking into account the Gouy phase of interacting beams. This work provides new insights into the role of the Gouy phase in SPDC, and also into the preparation of engineered entangled states for multidimensional quantum information processing.

We extend Pearl's definition of causal influence to the quantum domain, where two quantum systems $A, B$ with finite-dimensional Hilbert space are embedded in a common environment $C$ and propagated with a joint unitary $U$. For a finite-dimensional Hilbert space $C$, we find the necessary and sufficient condition on $U$ for a causal influence of $A$ on $B$ and vice versa. We introduce an easily computable measure of the causal influence and use it to study the causal influence of different quantum gates, its mutuality, and quantum superpositions of different causal orders. For two two-level atoms dipole-interacting with a thermal bath of electromagnetic waves, the space-time dependence of causal influence almost perfectly reproduces the one of reservoir-induced entanglement.

Variational quantum algorithms are promising tools whose efficacy depends on their optimization method. For noise-free unitary circuits, the quantum generalization of natural gradient descent has been introduced and shown to be equivalent to imaginary time evolution: the approach is effective due to a metric tensor reconciling the classical parameter space to the device's Hilbert space. Here we generalize quantum natural gradient to consider arbitrary quantum states (both mixed and pure) via completely positive maps; thus our circuits can incorporate both imperfect unitary gates and fundamentally nonunitary operations such as measurements. We employ the quantum Fisher information (QFI) as the core metric in the space of density operators. A modification of the error suppression by derangements (ESD) and virtual distillation (VD) techniques enables an accurate and experimentally efficient approximation of the QFI via the Hilbert-Schmidt metric tensor using prior results on the dominant eigenvector of noisy quantum states. Our rigorous proof also establishes the fundamental observation that the geometry of typical noisy quantum states is (approximately) identical in either the Hilbert-Schmidt metric or as characterized by the QFI. In numerical simulations of noisy quantum circuits we demonstrate the practicality of our approach and confirm it can significantly outperform other variational techniques.

We have studied soft-x-ray absorption in coronene and in singly hydrogenated coronene cations. Hydrogenation on the one hand closes the singly occupied molecular orbital of the coronene radical cation and thus decreases the reactivity of the molecule. On the other hand, the additional H atom breaks the coronene $D_{6h}$ symmetry and exhibits a comparably small binding energy. The effect of single hydrogenation on soft-x-ray photoionization is therefore hard to predict. In our experiments, the mass selected polycyclic aromatic hydrocarbons' cations were stored in a cryogenic radio-frequency ion trap and exposed to monochromatic x rays with energies around the carbon $K$ edge. The partial photoion yields were recorded as a function of photon energy by means of time-of-flight mass spectrometry. The experimental data was compared to time-dependent density-functional theory calculations based on a short-range corrected functional. The combination of experiment and theory allowed for precise assignment of $1s$ excitations to the resonances in the spectra. We found that these resonant transitions as well as direct $1s$ ionization into the continuum could induce nondissociative ionization, H/${\mathrm{H}}_2$ loss, as well as fragmentation of the carbon backbone. For multiple ionization, neutral hydrogen loss was found to compete with fission reactions where ${\mathrm{H}}^{+}$ or ${\mathrm{H}}_2{}^{+}$ is released. H attachment was found to substantially decrease backbone fragmentation.

Quantitative numerical analyses of interacting dilute Bose-Einstein condensates are most often based on semiclassical approximations. Since the complex-valued field-theoretic action of the Bose gas does not offer itself to the direct application of standard Monte Carlo techniques, simulations beyond their scope by now almost exclusively rely on quantum-mechanical techniques. Here we explore an alternative approach based on a Langevin-type sampling in an extended state space, known as the complex Langevin (CL) algorithm. While the use of the CL technique has a long-standing history in high-energy physics, in particular in the simulation of QCD at finite baryon density, applications to ultracold atoms are still in their infancy. Here we examine the applicability of the CL approach for a one- and two-component, three-dimensional nonrelativistic Bose gas in thermal equilibrium, below and above the Bose-Einstein phase transition. By comparison with analytic descriptions at the Gaussian level, including Bogoliubov and Hartree-Fock theory, we find that the method allows computing reliably and efficiently observables in the regime of experimentally accessible parameters. Close to the transition, quantum corrections lead to a shift of the critical temperature which we reproduce within the numerical range known in the literature. With this work, we aim to provide a first test of CL as a potential out-of-the-box tool for the simulation of experimentally realistic situations, including trapping geometries and multicomponent (or multiple species) models.

We consider thermo-optic hysteresis in a silicon structure supporting bound states in the continuum. Taking into account radiative heat transfer as a major cooling mechanism we constructed a nonlinear model describing the optical response. It is shown that the thermo-optic hysteresis can be obtained with low intensities of incident light $I_0\approx 1\mathrm{W}/{\mathrm{m}}^2$ at the red edge of the visible under the critical coupling condition.

We investigate the Landau-Zener tunneling (LZT) of a self-interacting two-level system in which the coupling between the levels is nonreciprocal. In such a non-Hermitian system, when the energy bias between two levels is adjusted very slowly, i.e., in the adiabatic limit, we find that a quantum state can still closely follow the eigenstate solution until it encounters the exceptional points (EPs) at which two eigenvalues and their corresponding eigenvectors coalesce. In the absence of the nonlinear self-interaction, we can obtain explicit expressions for the eigenvectors and eigenvalues and analytically derive the adiabatic LZT probability from invariants at EPs. In the presence of the nonlinear interaction, the dynamics of the adiabatic evolutions are explicitly demonstrated with the help of classical trajectories in the plane of the two canonical variables of the corresponding classical Josephson Hamiltonian. We show that the adiabatic tunneling probabilities can be precisely predicted by the classical action at EPs in the weak nonreciprocal regime. In a certain region of strong nonreciprocity, we find that interestingly, the nonlinear interaction effects can be completely suppressed so that the adiabatic tunneling probabilities are identical to their linear counterparts. We also obtain a phase diagram for large ranges of nonreciprocity and nonlinear interaction parameters to explicitly demonstrate where the adiabaticity can break down, i.e., the emergence of the nonzero tunneling probabilities even in adiabatic limit.

Stimulated emission tomography (SET) is a powerful and successful technique to both improve the resolution and experimentally simplify the task of determining the modal properties of biphotons. In the present paper we provide a theoretical description of SET valid for any quadratic coupling regime between a nonlinear medium and pump fields generating photons by pairs in the low-gain regime. We use our results to obtain not only information about the associated modal function modulus but also its phase, for any mode, and we discuss the specific case of time-frequency variables as well as the quantities and limitations involved in the measurement resolution.

We present the variational separability verifier (VSV), which is a variational quantum algorithm that determines the closest separable state (CSS) of an arbitrary quantum state with respect to the Hilbert-Schmidt distance (HSD). We first assess the performance of the VSV by investigating the convergence of the optimization procedure for Greenberger-Horne-Zeilinger states of up to seven qubits, using both state-vector and shot-based simulations. We also numerically determine the CSS of maximally entangled mixed $X$ states, and subsequently use the results of the algorithm to surmise the analytical form of the aforementioned CSS. Our results indicate that current noisy intermediate-scale quantum devices may be useful in addressing the NP-hard full separability problem using the VSV, due to the shallow quantum circuit imposed by employing the destructive swap test to evaluate the HSD. The VSV may also possibly lead to the characterization of multipartite quantum states, once the algorithm is adapted and improved to obtain the closest $k$-separable state of a multipartite entangled state.

The prospect of using quantum computers to solve combinatorial optimization problems via the quantum approximate optimization algorithm (QAOA) has attracted considerable interest in recent years. However, a key limitation associated with QAOA is the need to classically optimize over a set of quantum circuit parameters. This classical optimization can have significant associated costs and challenges. Here we provide an expanded description of Lyapunov control-inspired strategies for quantum optimization, as presented in [Magann et al., Phys. Rev. Lett. 129, 250502 (2022)], that do not require any classical optimization effort. Instead, these strategies utilize feedback from qubit measurements to assign values to the quantum circuit parameters in a deterministic manner, such that the combinatorial optimization problem solution improves monotonically with the quantum circuit depth. Numerical analyses are presented that investigate the utility of these strategies towards MaxCut on weighted and unweighted 3-regular graphs, both in ideal implementations and in the presence of measurement noise. We also discuss how how these strategies compare with QAOA and how they may be used to seed QAOA optimizations in order to improve performance for near-term applications, and we explore connections to quantum annealing.