Multiple ionization of the ${\mathrm{Ar}}^{+}\left(3s^{2}3p^5\right)$ ion by a single photon has been investigated in the photon-energy range 250–1800 eV employing the photon-ion merged-beams technique. Absolute partial cross sections were measured for all ${\mathrm{Ar}}^{(1+m)+}$ product-ion channels with $1\le m\le 6$ covering a size range from several tens of Mb down to a few b. Narrow $2p$-subshell excitation resonances were observed in all channels up to quadruple ionization at a photon-energy bandwidth of 52 meV. Double excitations involving a $2p$ and a $3s$ or $3p$ electron were also studied at high resolution and the measurements of the broad $2s$ excitation resonances directly showed their natural widths. Contributions of direct photo double ionization (PDI) to the production of the highest final Ar ion charge states are revealed, with PDI of the $2s$ subshell being mainly responsible for the production of ${\mathrm{Ar}}^{7+}$. The experiment made use of the PIPE setup installed at beamline P04 of the PETRA III synchrotron light source of DESY in Hamburg. The measurements were supported by theoretical calculations to identify the main contributions to the observed cross sections. Comparisons of theory and experiment show remarkable agreement but also hint to additional ionization mechanisms that are not considered in the theoretical models such as core ionization accompanied by excitations with subsequent Auger decays leading to net $m$-fold ionization with $m\ge 4$.

We investigate the liberation of an atomic electron by a linearly polarized single-cycle near-infrared laser pulse having a peak intensity that ensures tunneling. Based on phase space analysis and energy distribution in the instantaneous potential, we reveal the importance of quantum interference between tunneling and over-the-barrier pathways of escape. Tunneling is blurred both in space and time, and the contribution of tunneling at the mean energy is almost negligible. We suggest and justify improved initial conditions for a classical particle approximation of strong-field ionization, based on the quantum momentum function, and we show how to reconstruct them from the detected momentum of an escaped electron.

We study a class of multipartite open quantum dynamics for systems with an arbitrary number of qubits. The non-Markovian quantum master equation can involve arbitrary single or multipartite and time-dependent dissipative coupling mechanisms, expressed in terms of strings of Pauli operators. We formulate the general constraints that guarantee the complete positivity of this dynamics. We characterize in detail the underlying mechanisms that lead to memory effects, together with properties of the dynamics encoded in the associated system rates. We specifically derive multipartite “eternal” non-Markovian master equations that we term hyperbolic and trigonometric due to the time dependence of their rates. For these models we identify a transition between positive and periodically divergent rates. We also study non-Markovian effects through an operational (measurement-based) memory witness approach.

Preserving entanglement is a crucial dynamical process for entanglement-based quantum computation and quantum-information processes, such as one-way quantum computing and quantum key distribution. However, the problem of quantifying the ability of an experimental process to preserve two-qubit entanglement in experimentally feasible ways is not well understood. Accordingly, herein, we consider the use of two measures, namely composition and robustness, for quantitatively characterizing the ability of a process to preserve entanglement, referred to henceforth as entanglement preservability. A fidelity benchmark is additionally derived to identify the ability of a process to preserve entanglement. We show that the measures and introduced benchmark are experimentally feasible and require only local measurements on single qubits and preparations of separable states. Moreover, they are applicable to all physical processes that can be described using the general theory of quantum operations, e.g., qubit dynamics in photonic and superconducting systems. Our method extends the existing tools for analyzing channels, e.g., channel resource theory, to quantify entanglement preservability for non-trace-preserving quantum processes. The results are of significant interest for applications in quantum-information processing in which entanglement preservation is required.

We study quantum tomography from a continuous measurement record obtained by measuring expectation values of a set of Hermitian operators obtained from unitary evolution of an initial observable. For this purpose, we consider the application of a random unitary, diagonal in a fixed basis at each time step, and quantify the information gain in tomography using Fisher information of the measurement record and the Shannon entropy associated with the eigenvalues of covariance matrix of the estimation. Surprisingly, very high fidelity of reconstruction is obtained using random unitaries diagonal in a fixed basis even although the measurement record is not informationally complete. We then compare this with the information generated and fidelities obtained by application of a different Haar random unitary at each time step. We give an upper bound on the maximal information that can be obtained in tomography and show that a covariance matrix taken from the Wishart-Laguerre ensemble of random matrices and the associated Marchenko-Pastur distribution saturates this bound. We find that physically, this corresponds to an application of a different Haar random unitary at each time step. We show that repeated application of random diagonal unitaries gives a covariance matrix in tomographic estimation that corresponds to a new ensemble of random matrices. We analytically and numerically estimate eigenvalues of this ensemble and show the information gain to be bounded from below by the Porter-Thomas distribution.

Benchmark variational calculations of the lowest ten Rydberg ${}^{2}S$ states of two stable isotopes of the boron atom $({}^{10}\mathrm{B} \mathrm{and} {}^{11}\mathrm{B})$ are reported. The nonrelativistic wave functions of this five-electron system are expanded in terms of 16 000 all-particle explicitly correlated Gaussians (ECGs). The ECG nonlinear exponential parameters are extensively optimized using a procedure that employs the analytic gradient of the energy with respect to these parameters. A finite nuclear mass value is used in the calculations and the motion of the nucleus is explicitly represented in the nonrelativistic Hamiltonian. The leading relativistic corrections to the energy levels are computed in the framework of the perturbation theory. The lowest-order quantum electrodynamics corrections are also estimated. The results obtained for the energy levels enable determination of interstate transition frequencies with accuracy that approaches the available experimental spectroscopic data.

Radiative double-electron capture (RDEC), in which two-electron capture is accompanied by simultaneous emission of a single photon, was investigated for fully stripped and one-electron projectiles colliding with gaseous and thin-foil targets. RDEC can be considered the inverse of double photoionization by a single photon. For the gaseous targets, measurements were done for 2.11 MeV/u ${\mathrm{F}}^{9+}$ and ${\mathrm{F}}^{8+}$ ions interacting with ${\mathrm{N}}_2$ and Ne, while for the thin-foil target the measurements were done for 2.11 MeV/u ${\mathrm{F}}^{9+}$ and ${\mathrm{F}}^{8+}$ and 2.19 MeV/u ${\mathrm{O}}^{8+}$ and ${\mathrm{O}}^{7+}$ ions striking thin C targets. Reports on this work were already published separately in shorter accounts by La Mantia et al. [Phys. Rev. Lett. 124, 133401 (2020) for the gas targets and Phys. Rev. A 102, 060801(R) (2020) for the thin-foil targets]. The gas targets were studied under single-collision conditions, while the foil targets suffered unavoidable multiple collisions. The measurements were carried out by detecting x-ray emission from the target at ${90}^{\circ }$ to the beam direction in coincidence with outgoing ions undergoing double, single, and, in the case of the foil targets, no charge change inside the target. Striking differences between the gaseous and foil targets were found from these measurements, with RDEC for the gaseous targets occurring only in coincidence with q-2 outgoing projectiles as expected, while RDEC for the foil targets was seen in each of the outgoing q-2, q-1, and no charge-change states. The no charge-change result was totally unexpected. The cross sections for RDEC for the fully stripped ions on gas targets were found to be about six times larger than those for the one-electron projectiles. For the foil targets, the RDEC cross sections for the fully stripped and one-electron projectiles differ somewhat from one another but not to the the extent they did for the gas targets. In this work the cross sections for all of the projectiles for the foil targets were adjusted due to the target contaminant background from potassium and calcium atoms that existed in the spectra. Also, the cross sections for the incident one-electron projectiles were modified due to a correction for the fraction of these ions that becomes fully stripped in passage through the foil. These differences are attributed to the effects of the multiple collisions that occur for the foil targets. The differential cross sections at ${90}^{\circ }$ determined for each of the projectiles interacting with each of the targets are compared with each other and with the previous measurements. To the extent that the cross sections follow a ${sin}^2\theta$ dependence, the total cross sections are compared with theoretical calculations [E. A. Mistonova and O. Yu. Andreev, Phys. Rev. A 87, 034702 (2013)], for which the agreement is poor, with the measured cross section exceeding the predicted ones by about an order of magnitude. Possible reasons for this discrepancy will be discussed.

We have used the Cryogenic Storage Ring (CSR) at the Max Planck Institute for Nuclear Physics to study long-lived metastable states of the silicon anion. A ${\mathrm{Si}}^{-}$ beam of 58 keV kinetic energy was stored in the ultrahigh cryogenic vacuum of the CSR, employing only electrostatic deflection elements. We used laser systems at various wavelengths to infer information on the decay of the metastable anionic states by selective photodetachment. Our results give evidence of an excited anionic state for which we determine the extremely long lower lifetime limit of 5.7 h at 90% confidence level, consistent with theoretical predictions for the ${}^{2}D$ term. Furthermore, we find an average lifetime of $\tau =(22.2\pm 2.5)$ s for the weakly bound ${}^{2}P$ states, employing coincidence counting with a pulsed nanosecond laser at 2.45 $\mu \mathrm{m}$. Using a laser depletion technique, we produce a pure ground term ${}^{4}S_{3/2}{\mathrm{Si}}^{-}$ beam, and we quantify the fraction of ions in metastable states in our initial ion sample. We combine our experimental efforts with state-of-the-art multiconfiguration Dirac-Hartree-Fock calculations for the radiative lifetimes of all metastable levels of ${\mathrm{Si}}^{-}$. We find these calculations to be in excellent agreement with our measurements and to improve previous efforts considerably.

We study the fragmentation of very large dimers into two singly charged ions caused by absorption of a photon. In this process a photoelectron emitted from one atom of the dimer has certain chances to hit the other atom, knocking out one of its electrons. This results in the production of two singly charged ions and the consequent Coulomb explosion of the residual doubly charged system. We develop a theory of this process and apply it to calculate the fragmentation of ${}^4\mathrm{He}_2, {}^7\mathrm{Li}\text{−}{}^4\mathrm{He}$, and ${}^6\mathrm{Li}\text{−}{}^4\mathrm{He}$ dimers. Our results for the helium dimer are in good agreement with available experimental data. For the Li-He dimers (where experimental data are still absent) our results predict that this fragmentation mechanism becomes much more efficient than in the case of ${\mathrm{He}}_2$. We also show that, provided the recoil effects are of minor importance, the so-called reflection approximation is very accurate also for large dimers.

We propose and demonstrate an experimental method to measure by absorption imaging the size and local column density of a cloud of atoms, even when its smallest dimension is smaller than the resolution of the imaging system. To do this, we take advantage of the fact that, for a given total number of atoms, a smaller and denser cloud scatters less photons when the gas is optically thick. The method relies on making an ansatz on the cloud shape along the unresolved dimension(s) and on providing an additional information, such as the total number of atoms. We demonstrate the method on in situ absorption images of elongated ${}^{87}\mathrm{Sr}$ Fermi gases. We find significant nonlinear corrections to the estimated size and local density of the cloud compared to a standard analysis. This allows us to recover an undistorted longitudinal density profile and to measure transverse sizes as small as one-fourth of our imaging resolution. The ultimate limit of our method is the wavelength that is used for imaging.

We study enhanced magneto-association of atoms into weakly bound molecules near a Feshbach resonance using a quench preparatory stage. In anticipation of experiments with NASA's Cold Atom Laboratory aboard the International Space Station, we assume as a baseline a dual-species $({}^{87}\mathrm{Rb} \mathrm{and} {}^{41}\mathrm{K})$ gas in a parameter regime enabled by a microgravity environment. It includes subnanokelvin temperatures and dual-species gases at densities as low as ${10}^8/{\mathrm{cm}}^3$. Our studies indicate that, in such a regime, traditional magneto-association schemes are inefficient due to the weak coupling between atomic and molecular states at low densities, thus requiring extremely long magnetic field sweeps. To address this issue we propose a modified scheme in which atoms are quenched to unitarity before proceeding with magneto-association. This modified scheme substantially improves molecular formation, allowing for up to $80\%$ efficiency within timescales much shorter than those associated with atomic and molecular losses. We show that this scheme also applies at higher densities, therefore proving to be of interest in ground-based experiments as well.

A self-consistent theoretical treatment of a triple-well atomtronic transistor circuit reveals the mechanism of gain, conditions of oscillation, and properties of the subsequent coherent matter waves emitted by the circuit. A Bose-condensed reservoir of atoms in a large source well provides a chemical potential that drives circuit dynamics. The theory is based on the ansatz that a condensate arises in the transistor gate well as a displaced ground state, that is, one that undergoes dipole oscillation in the well. That gate atoms remain condensed and oscillating is shown to be a consequence of the cooling induced by the emission of a matter wave into the vacuum. Key circuit parameters such as the transistor transconductance and output current are derived by transitioning to a classical equivalent circuit model. Voltage-like and current-like matter-wave circuit wave fields are introduced in analogy with microwave circuits, as well as an impedance relationship between the two. This leads to a notion of a classically coherent matter wave that is the dual of a coherent electromagnetic wave and which is distinct from a de Broglie matter wave associated with cold atoms. Subjecting the emitted atom flux to an atomic potential that will reduce the de Broglie wavelength, for example, will increase the classical matter-wave wavelength. Quantization of the classical matter-wave fields leads to the dual of the photon that is identified not as an atom but as something else, which is here dubbed a “matteron.”

We propose a scheme to generate coherent radiation with a tunable frequency spectrum based on the excitation of an inhomogeneous thin layer of a resonant medium by an ultrashort pulse. The Cherenkov-type radiation emitted in a transient stage, i.e., while the excitation wavefront is still propagating over the medium, is shown to contain additional frequency components, which are determined by the spatial frequency components of the medium's density distribution and the excitation geometry. We demonstrate that the spectral content of this transient radiation can be adjusted in wide limits by controlling the spatial density distribution of the resonant medium and the shape of the excitation wavefront. Our theoretical results pave the way towards simple and compact swept-frequency or frequency-tunable optical oscillators as well as sources of the frequency-modulated optical radiation.

Based on the reciprocity theorem, we put forward a generalized parametric space for arbitrary transfer matrix with parity-time (PT) symmetry. Through this parametric space, one can extract complete information involving PT-symmetric phases, reflectances, transmittances, and known extraordinary scattering phenomena. We demonstrate a PT-symmetric heterostructure with coherent perfect absorption-lasing, anisotropic transmission resonance, and parity symmetry of reflection coefficients at the frequencies of interest. In addition, with the parametric space and the analytical formula, the corresponding complex dielectric permittivities for a simple PT-symmetric system made of a gain, an air gap, and a loss media in deeply subwavelength is derived to achieve various exotic PT-symmetric functionality. This work could offer an alternative route to design versatile optical and photonic PT-symmetric devices.

We study the photon-blockade effect in a loop-coupled optomechanical system consisting of two cavity modes and one mechanical mode. Here, the mechanical mode is optomechanically coupled to the two cavity modes, which are coupled with each other via a photon-hopping interaction. By treating the photon-hopping interaction as a perturbation, we obtain the analytical results of the eigenvalues and eigenstates of the system in the subspaces associated with zero, one, and two photons. We find a phenomenon of optical normal-mode-induced phonon-sideband splitting in the photon-blockade effect by analytically and numerically calculating the second-order correlation functions of the two cavity modes. This work not only presents a method to choose optimal driving frequency for photon blockade by tuning the photon-hopping interaction, but also provides a means to characterize the normal-mode splitting with cavity photon statistics.

Atomic inner-shell x-ray lasers pumped by an x-ray free-electron laser generate ultrashort, intense, monochromatic pulses with temporal coherence. Coherent control of lasing in monovalent magnesium ions is investigated theoretically, where valence-shell transitions are driven by optical lasers. Modulated high-order sidebands and attosecond pulses are generated with quasi-continuous-wave and few-cycle control lasers, respectively. The general manipulation scheme involves fast Auger decay, limited pump coherence, diverse coupling channels, and collisions in the plasma. Our approach shows potential applications in high-resolution spectroscopy, nonlinear x-ray studies, metrology, and ultrafast science in keV regime.

Multiphoton interference is at the very heart of quantum foundations and applications in quantum sensing and information processing. In particular, boson-sampling experiments have the potential to demonstrate quantum computational supremacy while relying on only multiphoton interference in linear optical interferometers. However, even when photonic losses are negligible, scalable experiments are challenged by the rapid decrease of the probability of the success of current schemes with probabilistic sources for a large number of single photons in each experimental sample. Remarkably, we show a boson sampling scheme in which the probability of success increases instead of decreasing with the number of input photons, eventually approaching a unit value even with nondeterministic sources. It is achieved by sampling measurements not only at the interferometer output but also over all the possible numbers of photons at each input port and, different from scattershot boson sampling, allowing a random number of occupied input ports from one experimental run to another. Therefore, these results provide an exciting route toward future demonstrations of quantum computational supremacy with scalable experimental resources.

Bell's theorem asserts that no model that satisfies all of the plausible physical assumptions of outcome independence, measurement independence, and parameter independence can reproduce quantum mechanics. Here I find the optimum model that saturates the Clauser-Horne-Shimony-Holt inequality for the case that outcome independence and measurement independence hold but parameter dependence is allowed. I find the bound as a function of a measure of parameter dependence and show that the model proposed in New J. Phys. 12, 083051 (2010) is optimal for one-way parameter dependence. The symbolic optimizations to find the saturating models are performed using the open source software analytical optimizer v1.0.

The ability to precisely discriminate multiple quantum channels is fundamental to achieving quantum enhancements in data-readout, target detection, pattern recognition, and more. Optimal discrimination protocols often rely on entanglement shared between an incident probe and a protected idler-mode. While these protocols can be highly advantageous over classical ones, the storage of idler-modes is extremely challenging in practice. In this work, we investigate idler-free block protocols based on the use of multipartite entangled probe states. In particular, we focus on a class of idler-free protocol which uses nondisjoint distributions of multipartite probe states irradiated over multichannels, known as dynamic discrimination protocols. We derive analytical bounds for the average error probability of such protocols in a bosonic Gaussian channel setting, revealing idler-free strategies that display performance close to idler-assistance for powerful, near-term quantum sensing applications.