We study the long-time average of the reduced density matrix (RDM) of a central system that is locally coupled to a large environment, under a Schrödinger evolution of the total system. We consider a class of interaction Hamiltonian, whose environmental part satisfies the so-called eigenstate thermalization hypothesis ansatz with a constant diagonal part in the energy region concerned. Relations among elements of the averaged RDM are derived. When steady states of the central system exist, these relations imply the existence of a preferred basis, which is given by the eigenbasis of a renormalized self-Hamiltonian that includes certain averaged impact of the system-environment interaction. Numerical simulations performed for a qubit coupled to a defect Ising chain confirm the analytical predictions.

We address the question of how to compute the probability distribution of the time at which a detector clicks in the situation of $n$ nonrelativistic quantum particles in a volume $\mathrm{\Omega }\subset {\mathbb{R}}^3$ in physical space and detectors placed along the boundary $\partial \mathrm{\Omega }$ of $\mathrm{\Omega }$. We previously [Tumulka, Ann. Phys. (NY) 442, 168910 (2022)] argued in favor of a rule for the one-particle case that involves a Schrödinger equation with an absorbing boundary condition on $\partial \mathrm{\Omega }$ introduced by Werner; we call this rule the “absorbing boundary rule.” Here, we describe the natural extension of the absorbing boundary rule to the $n$-particle case. A key element of this extension is that, upon a detection event, the wave function gets collapsed by inserting the detected position, at the time of detection, into the wave function, thus yielding a wave function of $n-1$ particles. We also describe an extension of the absorbing boundary rule to the case of moving detectors.

In the Schrödinger picture, the state of a quantum system evolves in time and the quantum speed limit describes how fast the state of a quantum system evolves from an initial state to a final state. However, in the Heisenberg picture the observable evolves in time instead of the state vector. Therefore, it is natural to ask how fast an observable evolves in time. This can impose a fundamental bound on the evolution time of the expectation value of quantum-mechanical observables. We obtain the quantum speed limit time bound for observable for closed systems, open quantum systems, and arbitrary dynamics. Furthermore, we discuss various applications of these bounds. Our results can have several applications ranging from setting the speed limit for operator growth, correlation growth, quantum thermal machines, quantum control, and many-body physics.

Is quantum communication possible over an optical fiber with transmissivity $\lambda \le 1/2$? The answer is well known to be negative if the environment with which the incoming signal interacts is initialized in a thermal state. However, Lami et al. [Phys. Rev. Lett. 125, 110504 (2020)] found the quantum capacity to be always bounded away from zero for all $\lambda >0$, a phenomenon dubbed “die-hard quantum communication” (D-HQCOM), provided that the initial environment state can be chosen appropriately, depending on $\lambda$. Here we show an even stronger version of D-HQCOM in the context of entanglement-assisted classical communication: entanglement assistance and control of the environment enable communication with performance at least equal to that of the ideal case of absence of noise, even if $\lambda >0$ is arbitrarily small. These two phenomena of D-HQCOM have technological potential provided that we are able to control the environment. How can we achieve this? Our second main result answers this question. Here we provide a fully consistent protocol to activate the phenomena of D-HQCOM without directly accessing the environment state. This is done by sending over the channel “trigger signals,” i.e., signals which do not encode information, prior to the actual communication, with the goal of modifying the environment in an advantageous way. This is possible due to the memory effects which arise when the sender feeds signals separated by a sufficiently short temporal interval. Our results may offer a concrete scheme to communicate across arbitrarily long optical fibers, without using quantum repeaters. As a by-product of our analysis, we derive a simple Kraus representation of the thermal attenuator exploiting the associated Lindblad master equation.

We study the role of intermediate excitations of Rydberg states as an example of Kuchiev's “atomic antenna” in above-threshold ionization of xenon, in particular their effect on the coherence between the spin-orbit-split states of the ion. We focus on the case of a laser frequency close to resonant with the spin-orbit splitting, where a symmetry (parity) argument would preclude any coherence being directly generated by strong-field ionization. Using ab initio simulations of coupled multielectron spin-orbit dynamics in strong laser fields, we show how field-driven rescattering of the trapped Rydberg electrons introduces efficient coupling between the spin-orbit-split channels, leading to substantial coherences, exceeding 10% for some photon energies.

We revisit high-order-harmonic generation (HHG) in corotating bicircular fields, which possess high optical chirality, and extend the study to a frequency ratio higher than 1:2. By analyzing the classical trajectories in the corotating bicircular fields in the rotating frame, we show that the Coriolis-force effect that hinders HHG decreases with the frequency ratio. Given these features, we propose a method for producing highly elliptically polarized attosecond pulses (with ellipticity up to $\epsilon =0.88$) using the corotating bicircular field with a frequency ratio of 1:3. A comparison is also made with the extensively studied counter-rotating configurations with a frequency ratio of 1:2 considering various laser parameters. The results affirm the validity and superiority of the corotating configurations, from which both the ellipticity and yield of generated attosecond pulses are higher than those from the counter-rotating configurations.

Optical wave propagation in an assembly of two-level atoms is considered. An asymptotic model of nonlinear Schrödinger (NLS) type is derived in a rigorous way by means of the perturbative expansion method. The results of the obtained model are then compared with both a numerical solution of the initial set of equations of the two-level model and an exact analytical cnoidal wave solution. It is seen that the evolution of the population difference is a key factor for the accurate determination of the nonlinear index, and that the NLS approximation allows one to determine it theoretically. The knowledge of the population difference allows one to correct the cnoidal wave solution, which then matches the numerical solution up to the limits of our numerical accuracy.

The analysis of nonlinear spectroscopy, widely used to study the dynamics and structures of condensed-phase matter, typically employs a perturbative approach noticing the weak interaction between the laser and the matter of interest. However, such a perturbative approach is no longer applicable once the interaction between the laser and the matter is strong. We adapt the method of quantum dynamical evolution into the calculation of signal and present the response formalism of the nonlinear spectroscopy in a nonperturbative approach. In this approach, we demonstrate that, in addition to the third-order term in the perturbative method, the higher-order terms have essential contributions to the nonlinear signal of the two-pulse and three-pulse photon echo (2PPE and 3PPE). The detailed calculations are demonstrated with the example of a three-level V-type system, which is widely used in the studies of quantum optics. We consider the effect of the environment via a pure dephasing mechanism with both the localized modes of each molecule and the shared modes between molecules.

We present a theory of magnon polaritons in quantum Ising materials, and develop a formalism describing the coupling between light and matter in an Ising system that is tuned through its quantum critical point. The theory is applied to Ising materials having multilevel single-site Hamiltonians, in which multiple magnon modes are present, such as the insulating Ising magnet ${\mathrm{LiHoF}}_4$. We find that the magnon-photon coupling strengths may be tuned by the applied transverse field, with the coupling between the soft mode present in the quantum Ising material and a photonic resonator mode diverging at the quantum critical point of the material. A fixed system of spins will not exhibit the diamagnetic response expected when light is coupled to mobile spins or atoms. Without the diamagnetic response, one expects a divergent magnon-photon coupling strength to lead to a superradiant quantum phase transition. However, this neglects the effects of damping and decoherence present in any real system. We show that damping and decoherence may block the superradiant quantum phase transition, and lead to weak coupling between the soft magnon mode and the resonator mode. The results of the theory are applied to experimental data on the model system ${\mathrm{LiHoF}}_4$ in a microwave resonator.

Non-Hermitian systems have recently attracted broad interest and exhibited intriguing physical phenomena, including the non-Hermitian skin effect, which have been widely studied in various fermionic and bosonic systems. Here we propose a non-Hermitian atom-waveguide system composed of a tilted one-dimensional atomic array coupled with two identical waveguides with opposite chiralities. Such system creates an effective lattice model including nonreciprocal long-range hoppings through the chiral-waveguide photon-mediated interactions. We find the excitations of the collective atomic states concentrate in the middle interface associated with subradiant modes, while, on the contrary, superradiant modes exhibit extended features. Such a unique feature in our proposed system is linked to the non-Hermitian skin effect. Simulation results present a subradiant funneling effect, with robustness against small atomic position disorders. Our work underpins the fundamental comprehension towards the non-Hermitian skin effect in open quantum systems and also provides prospective paths to study non-Hermitian systems in the area of quantum optics.

While quantum probes surely feel gravity, no source of gravity has been prepared in a delocalized quantum state yet. Two basic questions need to be addressed: how to delocalize a mass sufficiently large to generate detectable gravity and, once that state has been prepared, how to fight localization by decoherence. We propose to fight decoherence by freezing the source in the desired state through the Zeno effect. Successful implementation can be verified by scattering a probe in the effective potential generated by the source. Besides putting forward the idea, we provide an estimation of the values of the parameters required for the proposal to be feasible. Overall, the proposal seems as challenging as other existing ones, although the specific challenges are different (e.g., no entanglement needs to be preserved or detected, but the Zeno freezing must be implemented).

Decoherence of a quantum system is one of the main difficulties for quantum information processing. Continuous dynamical decoupling has achieved great success in the improvement of the coherence of a quantum state but is difficult to optimize. Here we exploit the black-box optimization process in a machine learning algorithm to optimize the dynamical decoupling scheme. By applying the discrete optimization process to continuous driving fields, we achieve a longer coherence time in comparison to representative schemes of dynamical decoupling. Our black-box-optimization-based machine learning algorithm provides a general routine to tackle the challenging task of improving the coherence of a quantum state for which the dephasing of the system is crucial for operation.

Quantum state tomography is a key technique for quantum information processing but is challenging due to the exponential growth of its complexity with the system size. In this work we propose an algorithm which iteratively finds the best non-negative matrix product state approximation based on a set of measurement outcomes whose size does not necessarily grow exponentially. Compared to the tomography method based on neural network states, our scheme utilizes a so-called tensor train representation that allows straightforward recovery of the unknown density matrix in the matrix product operator form. As applications, the effectiveness of our algorithm is numerically demonstrated to reconstruct the ground state of the XXZ spin chain under depolarizing noise.

Experimental near-edge x-ray-absorption fine-structure (NEXAFS) spectra of the nitrosonium ${\mathrm{NO}}^{+}$ ion are presented and theoretically analyzed. While neutral NO has an open shell, the cation is a closed-shell species, which for NEXAFS leads to the simplicity of a closed-shell spectrum. Compared to neutral NO, the electrons in the cation experience a stronger Coulomb potential, which introduces a shift of the ionization potential towards higher energies, a depletion of intensity in a large interval above the ${\pi }^{*}$ resonance, and a shift of the ${\sigma }^{*}$ resonance from the continuum to below the ionization threshold. NEXAFS features at the nitrogen and oxygen $K$ edges of ${\mathrm{NO}}^{+}$ are compared, as well as NEXAFS features at the nitrogen edges of the isoelectronic closed-shell species ${\mathrm{NO}}^{+}, {\mathrm{N}}_2$, and ${\mathrm{N}}_2{\mathrm{H}}^{+}$.

We perform a systematic analysis of how ultrafast photoelectron holography is influenced by an elliptically polarized field, with emphasis on quantum interference effects. We find that the interplay of the external field and the binding potential leads to twisted holographic patterns for low ellipticities and recover well-known angular offsets for high ellipticities. Using the Coulomb quantum-orbit strong-field approximation, we assess how the field ellipticity affects specific holographic patterns, such as the fan and the spider. The interplay of the external field and the binding potential leads to twisted holographic patterns in the fan and to loss of contrast in the spider. This behavior can be traced back to interfering electron trajectories and unequal changes in tunneling probability due to nonvanishing ellipticity. We also derive tunneling times analytically using the strong-field approximation, provide estimates for ellipticity ranges for which interference is expected to be prominent, and discuss how to construct continuous electron momentum distributions exploring the rotation symmetry around the origin.

We present an analytical pulse design protocol for controlling the vibrational dynamics of polar diatomic molecules within a given electronic state. Altering the potential energy function via the position-dependent electric permanent dipole moment, the vibrational state population dynamics is directly controlled using appropriately shaped laser pulses in the midinfrared regime. The optimal pulse shapes—that are expected to drive the molecule along user-defined quantum pathways—are obtained by reverse engineering, that is, solving the Schrödinger equation of the nuclei inversely in a relevant subspace. The proposed control scheme is validated by accurately solving the full time-dependent Schrödinger equation of the ${\mathrm{HeH}}^{+}$ molecular ion with two completely different methods: (1) propagating the complex population amplitudes of many field-free eigenstates or (2) propagating directly the nuclear wave packet on a grid. We find that besides smooth transitions, arbitrary Rabi oscillations as well as vibrational ladder climbing can be efficiently controlled with the present scheme. As a result, the molecule is successively excited beyond the potential barrier, leading to enhanced dissociation in the ground electronic state. Rotational effects and possible extensions of the presented control are also briefly discussed.

A vortex molecule is a topological excitation in two coherently coupled superfluids consisting of a vortex in each superfluid connected by a domain wall of the relative phase, also known as a Josephson vortex. We investigate the dynamics of this excitation in a quasi-two-dimensional geometry with slab or channel boundary conditions using an extended point-vortex framework complemented by Gross-Pitaevskii simulations. Apart from translational motion along the channel, the vortex molecule is found to exhibit intriguing internal dynamics including rotation and rotational-pendulum-like dynamics. Trajectories leading to a boundary-induced breakup of the vortex molecule are also described qualitatively by the simplified model. We classify the stable and unstable fixed points as well as separatrices that characterize the vortex molecule dynamics.

Ultracold potassium is an interesting candidate for quantum technology applications and fundamental research as it allows controlling intra-atomic interactions via low-field magnetic Feshbach resonances. However, the realization of high-flux sources of Bose-Einstein condensates remains challenging due to the necessity of optical trapping to use magnetic fields as free parameters. We investigate the production of all-optical ${}^{39}\mathrm{K}$ Bose-Einstein condensates with different scattering lengths using a Feshbach resonance near 33 G. By tuning the scattering length in a range between $75a_0$ and $300a_0$ we demonstrate a tradeoff between evaporation speed and final atom number and decrease our evaporation time by a factor of 5 while approximately doubling the evaporation flux. To this end, we are able to produce fully condensed ensembles with $5.8\times {10}^4$ atoms within $850\text{−}\mathrm{ms}$ evaporation time at a scattering length of $232a_0$ and $1.6\times {10}^5$ atoms within $3.9\mathrm{s}$ at $158a_0$, respectively. We deploy a numerical model to analyze the flux and atom number scaling with respect to scattering length, identify current limitations, and simulate the optimal performance of our setup. Based on our findings we describe routes towards high-flux sources of ultracold potassium for inertial sensing.

We consider a Bose-Josephson junction (BJJ) formed by a binary mixture of ultracold atoms to investigate the manifestation of coherent collective dynamics on ergodicity and quantum scars, unfolding the connection between them. By tuning the inter- and intraspecies interaction, we demonstrate a rich variety of Josephson dynamics and transitions between them, which plays a crucial role in controlling the overall ergodic behavior. The signature of underlying classicality is revealed from the entanglement spectrum, which also elucidates the formation of quantum scars of unstable steady states and of periodic orbits leading to athermal behavior. The degree of ergodicity across the energy band and scarring phenomena can be probed from the autocorrelation function as well from the phase fluctuation of the condensates, which has relevance in cold atom experiments.

Loss-induced transparency (LIT), featuring the revival of optical intensity by adding loss, has been demonstrated in classical optics. However, a fundamental question has remained unexplored, i.e., during the process of LIT, whether quantum correlations of the photons can also be revived or even tuned by increasing the loss. Here we find that, accompanying classical LIT in a nonlinear optical-molecule system, a purely quantum effect as photon blockade (PB) indeed can be revived with the help of loss. In particular, a quantum critical point emerges in the system: below the point, adding loss leads to the suppression of optical intensity and its correlations; in contrast, by surpassing the point, PB is revived and enhanced with more losses. Also, a quantum switch between single-PB and two-PB can be realized by simply tuning the loss. Our work provides a counterintuitive strategy to engineer quantum devices in a practical lossy environment.