We derive a master equation for a superradiant medium which includes multilevel interference between the individual scatterers. The derivation relies on the Born-Markov approximation and implements the coarse-graining formalism. The master equation fulfills the Lindblad form and contains terms describing multilevel interference between parallel transitions of a single atom, multiatom interference between identical transitions, and multiatom interference between different electronic transitions with parallel dipoles. This formalism is then applied to determine the excitation spectrum of two emitters using the parameters of the hydrogen transitions ${2S}_{1/2}\to {4P}_{1/2}$ and ${2S}_{1/2}\to {4P}_{3/2}$, where the gap between the parallel dipoles is of the order of GHz. The distortion of the signal due to the interplay of multilevel and multiemitter interference is analyzed as a function of their distance. These results suggest that interference between parallel dipolar transition can significantly affect the spectroscopic properties of optically dense media.

We present a kinetic model for optical pumping in ${\mathrm{Lu}}^{+}$ and ${\mathrm{Lr}}^{+}$ ions as well as a theoretical approach to calculate the transport properties of ${\mathrm{Lu}}^{+}$ in its ground ${}^{1}S_0$ and metastable ${}^{3}D_1$ states in helium background gas. Calculations of the initial ion state populations, the field and temperature dependence of the mobilities and diffusion coefficients, and the ion arrival time distributions demonstrate that the ground- and metastable-state ions can be collected and discriminated efficiently under realistic macroscopic conditions.

Macrorealism (MR) is the view that a system evolving in time possesses definite properties independent of past or future measurements and is traditionally tested for systems described at each time by a single dichotomic variable $Q$. A number of necessary and sufficient conditions for macrorealism have been derived for a dichotomic variable using sets of Leggett-Garg (LG) inequalities, or the stronger no signaling in time (NSIT) conditions, or a combination thereof. Here we extend this framework by establishing necessary and sufficient conditions for macrorealism for measurements made at two and three times for systems described by variables taking three or more values at each time. Our results include a generalization of Fine's theorem to many-valued variables for measurements at three pairs of times and we derive the corresponding complete set of LG inequalities. We find that LG inequalities and NSIT conditions for many-valued variables do not enjoy the simple hierarchical relationship exhibited by the dichotomic case. This sheds light on some recent experiments on three-level systems which exhibit a LG inequality violation even though certain NSIT conditions are satisfied. Under measurements of dichotomic variables using the Lüders projection rule the three-time LG inequalities cannot be violated beyond the Lüders bound (which coincides numerically with the Tsirelson bound obeyed by correlators in Bell experiments), but this bound can be violated in LG tests using degeneracy-breaking (von Neumann) measurements. We identify precisely which MR conditions are violated under these circumstances.

Amplified spontaneous emission (ASE) causes fluctuations of pulse energy, of the optical phase and of the timing of the pulse intensity envelope in a mode-locked laser or frequency comb. Starting from the assumption of one ASE photon per longitudinal laser mode and round trip, we rederive analytic equations for the three fundamental types of quantum noise in a laser. To this end, we analyze the interference of the coherent intracavity field and a spectrally localized ASE photon as a function of wavelength and phase of the latter. Performing an integration over all wavelengths and phases and taking stochastic noise into account, we compute ASE-induced jitters for all quantities considered. Continuing this approach, we then derive an expression for the resulting carrier-envelope phase noise of the comb, for which so far only numerical estimates exist. We further compute analytical estimates for ASE-induced pulse chirp and duration variations and address the issue of resulting pulse contrast in a mode-locked laser and the resulting coherence properties. Considering three example cases, we finally compute estimates for all quantities analyzed. Taken together, our analysis provides a comprehensive view of ASE effects in a mode-locked laser, which unites numerous scattered reports across the literature.

In this work, we irradiate a superconducting artificial molecule composed of two coupled tunable transmons with microwave light while monitoring its state via joint dispersive readout. Performing high-power spectroscopy, we observe and identify a variety of single- and multiphoton transitions. We also find that at certain fluxes, the measured spectrum of the system deviates significantly from the solution of the stationary Schrödinger equation with no driving. We reproduce these unusual spectral features by solving numerically the full master equation for a steady state and attribute them to an Autler-Townes-like effect in which a single tone is simultaneously dressing the system and probing the transitions between new eigenstates. We show that it is possible to find analytically the exact frequencies at which the satellite spectral lines appear by solving self-consistent equations in the rotating frame. Our approach agrees well with both the experiment and the numerical simulation.

We present numerical simulations of ultrafast multiphoton ionization dynamics in a two-dimensional atomic model driven by co- and counter-rotating circularly polarized single-color and bichromatic carrier envelope phase (CEP) stable laser pulse sequences. Taking into account phase variations due to CEP fluctuations and the Gouy phase, our results accurately reproduce recently measured photoelectron momentum distributions [Phys. Rev. Lett. 118, 053003 (2017); Phys. Rev. A 96, 043426 (2017); Nat. Commun. 10, 658 (2019); Adv. Phys.: X 4, 1672583 (2019)]. The time evolution of the complex-valued electron wave function in coordinate and momentum space is calculated to study the bound-state and vortex formation dynamics. The nonvanishing azimuthal probability current density proves the vortex nature of electron wave packets with odd-numbered rotational symmetry. Their angular momentum expectation value assumes half-integer values of 3.5 (corotating) and 0.5 (counter-rotating). Knowledge of the wave function allows us to analyze the photoionization dynamics and to validate the physical pictures proposed in previous experimental studies. As an outlook, we investigate how electron vortices develop from the multiphoton to the tunneling ionization regime with increasing laser intensity.

Time-domain interferometry (TDI) is a method to probe space-time correlations among particles in condensed matter systems. Applying TDI to quantum systems raises the general question of whether two-time correlations can be reliably measured without an adverse impact of the measurement backaction on the dynamics of the system. Here, we show that a recently developed quantum version of TDI (QTDI) indeed can access the full quantum-mechanical two-time correlations without backaction. We further generalize QTDI to weak classical continuous-mode coherent input states, alleviating the need for single-photon input fields. Finally, we interpret our results by splitting the space-time correlations into two parts. While the first one is associated with projective measurements and thus insensitive to backaction, we identify the second contribution as arising from the coherence properties of the state of the probed target system, such that it is perturbed or even destroyed by measurements on the system.

We demonstrate that a two-dimensional atomic array can be used as a platform for quantum optomechanics. Such arrays feature both nearly perfect reflectivity and ultralight mass, leading to significantly enhanced optomechanical phenomena. Considering the collective atom-array motion under continuous laser illumination, we study the nonlinear optical response of the array. We find that the spectrum of light scattered by the array develops multiple sidebands, corresponding to collective mechanical resonances, and exhibits nearly perfect quantum-noise squeezing. Possible extensions and applications for quantum nonlinear optomechanics are discussed.

We report the creation of heteronuclear ultralong-range Rydberg-molecule dimers by excitation of minority ${}^{88}\mathrm{Sr}$ atoms to $5sns{}^3{S}_1$ Rydberg states ($n=$ 31–39) in a dense background of ${}^{84}\mathrm{Sr}$. We observe an isotope shift of the $\nu =0$ vibrational state over this range of $n$ and compare our measurements with a theoretical prediction and a simple scaling argument. At low principal quantum number, the isotope shift is sufficiently large to produce heteronuclear dimers with almost perfect fidelity. When the spectral selectivity is limited, we obtain a lower bound on the ratio of heteronuclear to homonuclear excitation probability of 30 to 1 by measuring the scaling of the molecular excitation rate with varying relative densities of ${}^{88}\mathrm{Sr}$ and ${}^{84}\mathrm{Sr}$ in the ultracold mixture.

Noncontextuality inequalities are usually derived from the distinguishability properties of quantum states, i.e., their orthogonality. Here, we show that antidistinguishability can also be used to derive noncontextuality inequalities. The Yu-Oh 13-ray noncontextuality inequality can be rederived and generalized as an instance of our antidistinguishability method. For some sets of states, the antidistinguishability method gives tighter bounds on noncontextual models than just considering orthogonality, and the Hadamard states provide an example of this. We also derive noncontextuality inequalities based on mutually unbiased bases and symmetric informationally complete positive operator-valued measures. Antidistinguishability based inequalities were initially discovered as overlap bounds for the reality of the quantum state. Our main contribution here is to show that they are also noncontextuality inequalities.

High-dimensional quantum systems are vital for quantum technologies and are essential in demonstrating practical quantum advantage in quantum computing, simulation, and sensing. Since dimensionality grows exponentially with the number of qubits, the potential power of noisy intermediate-scale quantum devices over classical resources also stems from entangled states in high dimensions. An important family of quantum protocols that can take advantage of high-dimensional Hilbert space is classification tasks. These include quantum machine learning algorithms, witnesses in quantum information processing, and certain decision problems. However, due to counterintuitive geometrical properties emergent in high dimensions, classification problems are vulnerable to adversarial attacks. We demonstrate that the amount of perturbation needed for an adversary to induce a misclassification scales inversely with dimensionality. This is shown to be a fundamental feature independent of the details of the classification protocol. Furthermore, this leads to a tradeoff between the security of the classification algorithm against adversarial attacks and quantum advantages that we expect for high-dimensional problems. In fact, protection against these adversarial attacks requires extra resources that can even scale polynomially with the Hilbert space dimension of the system, which can erase any significant quantum advantage that we might expect from a quantum protocol. This has wide-ranging implications in the use of both near-term and future quantum technologies for classification.

The problem of high-speed transport for cold atoms with minimal heating has received considerable attention in theory and experiment. Much theoretical work has focused on solutions of general problems, often assuming a harmonic trapping potential or a one-dimensional geometry. However, in the case of optical conveyor belts, these assumptions are not always valid. Here we present experimental and numerical studies of the effects of various motional parameters on heating and retention of atoms transported in an optical conveyor. Our numerical model is specialized to the geometry of a moving optical lattice and uses dephasing in the density-matrix formalism to account for the effects of motion in the transverse plane. We verify the model by a comparison with experimental measurements and use it to gain further insight into the relationship between the conveyor's performance and the various parameters of the system.

Focused short-wavelength free-electron laser (FEL) pulses interacting with gas phase samples can induce by inner-shell ionization a short-lived population inversion, followed by coherent collective emission of directed, short, and strong radiation bursts. We extend our studies into the warm-dense matter (WDM) regime by investigating the nanoplasmas produced in an ensemble of nanometer-sized clusters by FEL irradiation. Here, additional pathways can also lead to strong, laserlike emission: Electron-ion collisions can yield a long-lived population inversion, and subsequent amplified spontaneous emission. We observe amplified spontaneous emission (ASE) in the extreme ultraviolet in xenon clusters excited by soft x-ray FEL pulses, we diagnose the generated nanoplasmas by fluorescence spectroscopy, and we study under various cluster and FEL parameters the directed ASE from the ${\mathrm{Xe}}^{2+}$ 65 nm line. We show its exponential increase as a function of FEL irradiation power, and an accompanying collisional broadening of the emission spectra. These findings are corroborated by extensive numerical simulations based on theory, combining detailed hydrodynamic and kinetic simulations with time-dependent calculations of radiation transport, amplification, and collective emission in the WDM nanoplasma. Our theoretical findings underline that population inversion is due to electron-ion collisions and that the observed decoherence processes can be empirically characterized by a phenomenological decoherence time in the range of 100–200 fs.

Ultrafast laser excitation can trigger complex coherent dynamics in molecules. Here, we report attosecond transient-absorption experiments addressing simultaneous probing of electronic and vibrational dynamics in a prototype molecule, deuterium bromide (DBr), following its strong-field ionization. Electronic and vibrational coherences in the ionic $X{}^2\mathrm{\Pi }_{3/2}$ and $X{}^2\mathrm{\Pi }_{1/2}$ states are characterized in the Br-$3d$ core-level absorption spectra via quantum beats with 12.6-fs and 19.9-fs periodicities, respectively. Polarization scans reveal that the phase of the electronic quantum beats depends on the probe direction, experimentally showing that the coherent electronic motion corresponds to oscillation of the hole density along the ionization-field direction. The vibrational quantum beats are found to maintain a relatively constant amplitude, whereas the electronic quantum beats exhibit a partial decrease in time. Quantum wave-packet simulations show that decoherence from vibrational motion is insignificant because the $X{}^2\mathrm{\Pi }_{3/2}$ and $X{}^2\mathrm{\Pi }_{1/2}$ potentials are nearly parallel. A comparison between the DBr and HBr results suggests that rotational motion is responsible for the decoherence since it leads to initial alignment prepared by the strong-field ionization.

Double-well systems loaded with one, two, or many quantum particles give rise to intriguing dynamics, ranging from Josephson oscillation to self-trapping. This work presents theoretical and experimental results for two distinct double-well systems, both created using dilute rubidium Bose-Einstein condensates with particular emphasis placed on the role of interaction in the systems. The first is realized by creating an effective two-level system through Raman coupling of hyperfine states. The second is an effective two-level system in momentum space generated through the coupling by an optical lattice. Even though the noninteracting systems can, for a wide parameter range, be described by the same model Hamiltonian, the dynamics for these two realizations differ in the presence of interactions. The difference is attributed to scattering diagrams that contribute in the lattice coupled system but vanish in the Raman coupled system. The internal dynamics of the Bose-Einstein condensates for both coupling scenarios is probed through a Ramsey-type interference pulse sequence, which constitutes a key building block of atom interferometers. These results have important implications in a variety of contexts including lattice calibration experiments and momentum-space lattices used for quantum analog simulations.

We study the one-shot distillation of general quantum resources, providing a unified quantitative description of the maximal fidelity achievable in this task, and revealing similarities shared by broad classes of resources. We establish fundamental quantitative and qualitative limitations on resource distillation applicable to all convex resource theories. We show that every convex quantum resource theory admits a meaningful notion of a pure maximally resourceful state, which maximizes several monotones of operational relevance and finds use in distillation. We endow the generalized robustness measure with an operational meaning as an exact quantifier of performance in distilling such maximal states in many classes of resources, including bi- and multipartite entanglement, multilevel coherence, as well as the whole family of affine resource theories, which encompasses important examples such as asymmetry, coherence, and thermodynamics.

Magnetic monopoles can appear as emergent structures in a wide range of physical settings, ranging from spin ice to Weyl points in semimetals. Here, a distribution of synthetic (Berry) monopoles in parameter space of a slowly changing external magnetic field is demonstrated in a system of interacting spin-$\frac{1}{2}$ particles with broken spherical symmetry. These monopoles can be found at points where the external field is nonzero. The spin-spin interaction provides a mechanism for splitting the synthetic local magnetic charges until their magnitude reaches the smallest allowed value $\frac{1}{2}$. For certain states, a nonzero net charge can be created in an arbitrarily large finite region of parameter space. The monopole field textures contain nonmonopolar contributions in the presence of spin-spin interaction.

Chiral quantum networks provide a promising route for realizing quantum information processing and quantum communication. Here we describe how two distant quantum nodes of chiral quantum network become dynamically entangled by a photon transfer through a common one-dimensional chiral waveguide. We harness the directional asymmetry in chirally coupled single-mode ring resonators to generate an entangled state between two atoms. We report a concurrence of up to 0.969, a huge improvement over 0.736, which was suggested and analyzed in great detail in Gonzalez-Ballestero et al. [Phys. Rev. B 92, 155304 (2015)]. This significant enhancement is achieved by introducing microresonators which serve as an efficient photonic interface between light and matter. The robustness of our protocol to experimental imperfections such as fluctuations in internodal distance, imperfect chirality, various detunings, and atomic spontaneous decay is demonstrated. Our proposal can be utilized for long-distance entanglement generation in quantum networks, which is a key ingredient for many applications in quantum computing and quantum information processing.

We investigate the excitation and probing of electronic coherences in atoms by a sequence of optical attosecond pulses. Wave packets representing the coherent superposition of bound states in atoms are generated by a strong optical attosecond pulse. Amplitudes and phases of induced coherences can be retrieved from quantum beats in the radiative emission signal induced by a time-delayed weaker optical attosecond probe pulse. Such an attosecond-pump attosecond-probe scenario promises access to the excitation amplitudes and the off-diagonal elements of the density matrix generated by strong-field multiphoton processes. We illustrate this attosecond quantum beat spectroscopy with simulations for atomic hydrogen.

The velocity distribution of a hot ionic beam can be filtered with a narrow stimulated Raman process to prepare a colder subensemble, as substantiated in this theoretical analysis. Using two counterpropagating far-detuned lasers, we can define a $\pi$ pulse for the resonant velocity to transfer atoms within the linewidth of the Raman resonance between the ground states of a $\mathrm{\Lambda }$ system. Spontaneous emission from the two single-photon resonances, as well as the ground-state decoherence induced by laser noise, diminishes the efficiency of the filter. From a comprehensive master equation, we obtain conditions for the optimal frequency pair of lasers and evaluate the filter performance numerically as well as analytically. If we apply this analysis to current ${}^{40}{\text{Ca}}^{+}$ ion experiments, we obtain a sensitivity for measuring high ion acceleration voltages on the ppm level or below.