The reconstruction of attosecond beating by interference of two-photon transitions is one of the most widely used techniques for obtaining both the relative phases of harmonics forming an attosecond pulse train and the phase of atomic radiative transitions. If the latter is computed by theory, it allows one to reconstruct the attosecond pulse train; if the former is known experimentally, it allows for reconstruction of the electronic dynamics of photoionization in atomic and molecular systems with attosecond temporal resolution. As it relies on the interference of photoelectrons in vacuum, similar interference has never been contemplated inside crystals. Here we explore the applicability of this scheme to solid-state systems using a one-dimensional model and a density-functional theory–calculated structure of two-dimensional hexagonal boron nitride. We discuss the possibility of (i) reconstructing the relative phases between harmonics with trivial influence of the “atomic phase” and (ii) retrieving the relative phases of two-photon transitions through different bands, which are generally challenging to obtain both experimentally and numerically. These phases are recorded in the beating of the population signal arising from interfering two-photon pathways, and can be read out with angle-resolved photoemission spectroscopy. Furthermore, the amplitude of the population beating decays as the pump and probe pulses are separated in time due to electron-hole decoherence, providing a simple interferometric method to extract dephasing times.

We present a theoretical analysis of the spatial shape of two symmetric signals of degenerate four-wave mixing induced by Gaussian beams in a thin sample of two-level atoms. Our calculations take into account the full spatial and spectral dependences of the relevant nonlinear susceptibilities that govern the two processes. This reveals two interesting effects. The first one is that the total power of incident beams affects the transverse profile of the four-wave-mixing signals at the medium exit and their free propagation. The second one is the influence of the spectral characteristics of the medium on the longitudinal profile of both generated signals upon free propagation. We argue that the first effect can be seen as the saturation of the medium in regions of higher intensity, while the second can be understood as the result of a nonlinear contribution to the refractive index inside the atomic medium. These effects can be symmetric between the two signals, with asymmetries induced by different detunings from the resonance of the incident fields.

The interplay between thermal machines and quantum correlations is of great interest in both quantum thermodynamics and quantum information science. Recently, a quantum Szilárd engine has been proposed, showing that the quantum steerability between a Maxwell's demon and a work medium can be beneficial to a work extraction task. Nevertheless, this type of quantum-fueled machine is usually fragile in the presence of decoherence effects. Therefore, in this work we tackle this question by introducing a second demon who can access a control system and make the work medium pass through two dephasing channels in a manner of quantum superposition. Furthermore, we provide a quantum circuit to simulate our proposed concept and test it on IBMQ and IonQ quantum computers.

A family of skew information quantities is obtained, in which the well-known Wigner-Yanase skew information and quantum Fisher information stand as special cases. A transparent proof of convexity of the generalized skew information is given, implying a simple proof of the Wigner-Yanase-Dyson conjecture. We find in this work an exact skew information inequality for a qubit system, which may be regarded as the information counterpart of the uncertainty relation. A lower bound for the generalized skew information of a pair of incompatible observables in arbitrary dimension and also the upper bound for a qubit system are achieved.

The distribution of typical bipartite pure states is studied within the framework of state transformation via local operations and classical communication (LOCC). We report the statistics of comparable and incomparable states in different dimensions for single- and multicopy regimes, and we establish a connection between state transformation and the difference between the entanglement contents of the initial and the target states. From the analysis of catalyst resources, required to further otherwise impossible LOCC transformations between pairs, we demonstrate a universal pattern in the average and minimum entanglement of the randomly generated catalysts. Furthermore, we introduce a concept of hierarchy between different kinds of catalysts, and we show how they can not only aid in the conversion of incomparable states, but they can also act as a less costly resource toward this goal. We confirm the existence of catalysts, referred to as strong catalysts, which can activate LOCC transformation between pairs at the single-copy level, when it is initially impossible even with multiple copies.

We develop a time-dependent Schrieffer-Wolff-Lindblad perturbation theory to study effective interactions for driven open quantum systems. The starting point of our analysis is a given Lindblad equation, based on which we obtain an effective (averaged) map that describes the renormalization of both the Hamiltonian and collapse operators due to the drive. As a case study, we apply this method to the dispersive readout of a transmon qubit and derive an effective dispersive map that describes measurement-induced dephasing and Stark shift for the transmon. The effective map we derive is completely positive and trace preserving under adiabatic resonator response. To benchmark our method, we demonstrate good agreement with a numerical computation of the effective rates via the Lindbladian spectrum. Our results are also in agreement with, and extend upon, an earlier derivation of such effects by Gambetta et al. [Phys. Rev. A 74, 042318 (2006)] using the positive P representation for the resonator field.

We propose quantum control protocols for the high-fidelity preparation of target states in systems with Autler-Townes splitting. We investigate an approximated three-level system obtained from a four-level one by adiabatically eliminating a state that does not participate in the evolution. In our work we use linear, arctan, and Roland-Cerf functions for transferring population between two eigenstates of the system obtaining a high fidelity for long evolution times. Additionally, in order to overcome the restriction given by the lifetimes of the experimental setup, we propose an accelerated adiabatic evolution with a shortcut to adiabaticity protocol, which allows us to reach fidelities close to one but much faster.

Phase and time delays of atomic above-threshold ionization were recently experimentally explored in an $\omega -2\omega$ setting [Zipp et al., Optica 1, 361 (2014)]. The phases of wave packets ejected from argon by a strong $2\omega$ pulse were probed as a function of the relative phase of a weaker $\omega$ probe pulse. Numerical simulations solving the time-dependent Schrödinger equation (TDSE) displayed a sensitive dependence of the doubly differential momentum distribution on the relative phase between the $\omega$ and $2\omega$ fields. Moreover, a surprisingly strong variation of the extracted phase delays on the intensity of the probe pulse was found. We present a semiclassical strong-field description of the phase delays in the emission of electrons in an $\omega -2\omega$ setting and apply it to atomic hydrogen. Nonperturbative effects in both the $2\omega$ pump and the $\omega$ probe field are included. The semiclassical description allows tracing phase delays to path interferences between emission during different points in time of emission within the temporal unit cell of the two-color laser field. We find good agreement between the semiclassical saddle-point approximation, the full strong-field approximation (SFA), and previous results applicable in the perturbative limit of probe fields. We show that the RABBIT-like perturbative description of phase delays breaks down for stronger fields and higher-energy electron emission. In this regime, characterization of the ionization signal requires an entire ensemble of phase delays ${\delta }_i\left(E\right)$ with $i=1,2,...$ the difference in photon numbers of the strong $2\omega$ field involved in the interfering paths. Comparison between SFA and TDSE calculations reveals the influence of the Coulomb field even in this strong-field scenario.

We investigate the subcycle temporal property of multiphoton ionization of Ar atoms in a linearly polarized laser pulse at 400 nm by employing the two-color phase-of-the-phase photoelectron spectroscopy. A perturbative weak 800-nm circularly polarized field is used to clock the intrinsic multiphoton ionization dynamics. The resulting phase-of-the-phase spectrum under this field configuration reveals a prominent angular-dependent feature. We show that the angular-dependent phase-of-the-phase is associated with the phase difference of photoelectron intracycle interference. This enables us to access the complex phase of the photoelectron wave packet of multiphoton ionization. The intrinsic complex ionization time of strong-field approximation has been retrieved for above-threshold ionization.

Owing to the replication of Floquet bands and the presence of additional gaps in the quasienergy dimension, the topological phases in a periodically driven system cannot be fully characterized by the conventional topological invariants used in static systems. In particular, an anomalous strong topological phase can be a host in driven systems, featured by nontrivial counterpropagating edge modes which cannot be characterized by the bulk band structure. In this paper, we propose a scheme to obtain a complete characterization of Floquet topological phases using only information about bulk dispersions under the condition that both the location and chirality of Floquet band-touching points are not changed by the periodical driving. A set of topological invariants associated with the band-touching points are formulated to establish a one-to-one correspondence to the number of edge modes. Finally, we discuss the experimental realization and detection scheme using cold atomic gases.

The phase difference of the macroscopic wave function is a unique structure of the soliton in an atomic Bose-Einstein condensate (BEC). However, experiments on ultracold atoms so far have observed the valley of the density profile to study the dynamics of solitons. We propose a method to observe the phase difference of a soliton in a BEC by using an interference technique with Raman and rf pulses. We introduce a phase jump factor, which is an indicator to measure the phase difference between two points. It is demonstrated by using the projected Gross-Pitaevskii equation that an interference density ratio, the density ratio of two-component BECs after the Raman and rf pulses, reproduces the phase jump factor well. This technique will become an alternative method to study the decay and breakdown of a phase-imprinted soliton in atomic BECs.

Unpolarized light is ubiquitous in nature. It does not carry spin angular momentum but it can be converted to partially polarized light with spin angular momentum after scattering. In this work, we demonstrate that the unpolarized light not only possesses a definite spin state after scattering but also induces lateral separation of scattered light with opposite chiralities. Such spin-locked scattering of unpolarized light stems from the inherent property of magnetoelectric coupling of the scatterer. Moreover, the spin-locked scattering for arbitrary linearly polarized light is also investigated and compared with the case of unpolarized light. Our findings can broaden the application range of unpolarized light and open other avenues for spin-related phenomena induced by unpolarized light.

Ensembles of Mössbauer nuclei embedded in thin-film cavities form a promising platform for x-ray quantum optics. A key feature is that the joint nuclei-cavity system can be considered as an artificial x-ray multilevel scheme in the low-excitation regime. Using the cavity environment, the structure and parameters of such level schemes can be tailored beyond those offered by the bare nuclei. However, so far, the direct determination of a cavity structure providing a desired quantum optical functionality has remained an open challenge. Here we address this challenge using an inverse design methodology. As a first qualitative result, we show that the established fitting approach based on scattering observables in general is not unique, since the analysis may lead to different multilevel systems for the same cavity if based on observables in different scattering channels. Motivated by this, we distinguish between scattering signatures and the microscopic level scheme as separate design objectives, with the latter being uniquely determined by an ab initio approach. We find that both design objectives are of practical relevance and that they complement each other regarding potential applications. We demonstrate the inverse design for both objectives using example tasks, such as realizing electromagnetically induced transparency. Our results pave the way for future applications in nuclear quantum optics involving more complex x-ray cavity designs.

Coherent interactions between quantum emitters in tailored photonic structures is a fundamental building block for future quantum technologies, but remains challenging to observe in complex solid-state environments, where the role of decoherence must be considered. Here, we investigate the optical interaction between two quantum emitters mediated by one-dimensional waveguides in a realistic solid-state environment, focusing on the creation, population, and detection of a subradiant state, in the presence of dephasing. We show that as dephasing increases, the signatures of subradiance quickly vanish in intensity measurements, yet remain pronounced in photon correlation measurements, particularly when the two emitters are pumped separately so as to populate the subradiant state efficiently. The applied Green's tensor approach is used to model a photonic crystal waveguide, including the dependence on the spatial position of the integrated emitter. The work lays out a route to the experimental realization of subradiant states in nanophotonic waveguides containing solid-state emitters.

We exploit fluctuational electrodynamics to present trace expressions for the torque experienced by arbitrary objects in a passive, nonabsorbing, rotationally invariant background environment. Specializing to a single object, this formalism, together with recently developed techniques for calculating bounds via Lagrange duality, is then used to derive limits on the maximum Casimir torque that a single object with an isotropic electric susceptibility can experience when out of equilibrium with its surrounding environment. The maximum torque achievable at any wavelength is shown to scale in proportion to body volumes in both subwavelength (quasistatics) and macroscopic (ray optics) settings, and come within an order of magnitude of achievable torques on topology optimized bodies. Finally, we discuss how to extend the formalism to multiple bodies, deriving expressions for the torque experienced by two subwavelength particles in proximity to one another.

We present a simple and effective approach for extracting the bath information in open-quantum-system dynamics under the framework of the numerically exact hierarchical equations-of-motion method, for which it is usually not straightforward to adjust the boundary between the system and bath. The key component of our approach is an additional bath mode that weakly interacts with the system through the same coupling channel as the mode of interest, which we call the probe mode. By combining the probe mode with the system and calculating its expectation value, we can deduce the corresponding expectation value of the mode of interest via simple relations. The effectiveness of our approach is demonstrated by exact decomposition of dissipation by a continuous spectral density.

We study the Bell's inequality violation by dynamical Casimir radiation with pseudospin measurement. We consider a circuit quantum electrodynamical setup where a relativistically moving mirror is simulated by variable external magnetic flux in a superconducting quantum interference device terminating a superconducting-microwave waveguide. We analytically obtain expectation values of the Bell operator optimized with respect to channel orientations in terms of the system parameters. We consider the effects of local noise in the microwave field modes, asymmetry between the field modes resulting from nonzero detuning, and signal loss. Our analysis provides ranges of the above experimental parameters for which the Bell violation can be observed. We show that the Bell violation can be observed in this setup up to $40\text{mK}$ temperature as well as up to 65% signal loss.

The situation is studied when an ensemble of pure quantum states is mapped onto another ensemble of pure quantum states. In general, this operation may not be done with unit probability. In this work, the bounds for success probability of such operation are provided, with closed-form expressions involving the Gram matrices and quantum max-relative entropy. We also discuss a partial version of this operation which has arbitrary success probability.