Efforts to improve enantioselective detection of chiral molecules have initiated great interest in the fields of chiral light and chiral plasmonics. While superchiral light had been reported as highly sensitive enantiomer probes, optical response of chiral molecules can also be boosted at the vicinity of chiral plasmonic structures due to enhanced light-molecule coupling. Here, we apply the fundamental description of chiral light, known as the chirality density and flux, to the local density of states (LDOS) which rules the processes of light-matter interactions. In spiral plasmonic structures, we experimentally demonstrate the mapping of far-field chiral LDOS properties by means of polarimetry analysis. We establish the link between the degree of circular polarization of freely propagating cavity modes and the chirality density and flux. Using the reciprocity theorem, we show that this measure can be used to probe to the near-field chirality associated with surface plasmon field. By mapping the chiral features of the LDOS, we report a helpful method for guiding the design of superchiral light and for the investigation of chiral light-matter interactions, opening doors to improved chiral molecular sensing.

We propose and analyze an efficient scheme for the one- and two-dimensional atomic gratings based on periodic spatial modulation of the Raman gain and dispersion, which we name the Raman-induced diffraction gratings (RIDGs). There are fundamentally different from those based on electromagnetically induced transparency. As the probe field propagates along the direction normal to the standing pump wave, it can be effectively diffracted into high-order directions. The grating is a hybrid grating, i.e. it represents a mixture of amplitude and phase gratings. We identify the conditions when all high-order diffractions are amplified. In addition, we also show that diffraction of a probe field could be dynamically controlled using an additional laser field. With its help, it is possible to suppress or amplify diffraction beams. The RIDGs can be considered as all-optical multibeam splitters with amplification.

This Rapid Communication provides an exact formula for the signal $n$-point correlation functions of detectors continuously measuring an arbitrary quantum system, in the presence of detection imperfections. The derivation uses only continuous stochastic calculus techniques, but the final result is easily understood from a discrete picture of repeated interactions with qubits or from a parallel with continuous matrix product states. This result provides a crude yet efficient a way to estimate system parameters directly from experimental data, without requiring nonlinear state reconstruction.

Quantum systems are prone to decoherence due to both intrinsic interactions as well as random fluctuations from the environment. Using the Pechukas-Yukawa formalism, we investigate the influence of noise on the dynamics of an adiabatically evolving Hamiltonian which can describe a quantum computer. Under this description, the level dynamics of a parametrically perturbed quantum Hamiltonian are mapped to the dynamics of one-dimensional classical gas. We show that our framework coincides with the results of the classical Landau-Zener transitions upon linearization. Furthermore, we determine the effects of external noise on the level dynamics and its impact on Landau-Zener transitions.

Spin cat states are promising candidates for quantum-enhanced measurement. Here, we analytically show that the ultimate measurement precision of spin cat states approaches the Heisenberg limit, where the uncertainty is inversely proportional to the total particle number. In order to fully exploit their metrological ability, we propose to use the interaction-based readout for implementing phase estimation. It is demonstrated that the interaction-based readout enables spin cat states to saturate their ultimate precision bounds. The interaction-based readout comprises a one-axis twisting, two $\frac{\pi }{2}$ pulses, and a population measurement, which can be realized via current experimental techniques. Compared with the twisting echo scheme on spin squeezed states, our scheme with spin cat states is more robust against detection noise. Our scheme may pave an experimentally feasible way to achieve Heisenberg-limited metrology with non-Gaussian entangled states.

We study the relationship between (non-)Markovian evolutions, established correlations, and the entropy production rate. We consider a system qubit in contact with a thermal bath and in addition the system is strongly coupled to an ancillary qubit. We examine the steady-state properties finding that the coupling leads to effective temperatures emerging in the composite system, and show that this is related to the creation of correlations between the qubits. By establishing the conditions under which the system reaches thermal equilibrium with the bath despite undergoing a non-Markovian evolution, we examine the entropy production rate, showing that its transient negativity is a sufficient sign of non-Markovianity.

We investigate the zero-temperature limit of thermodynamic quantum master equations that govern the time evolution of density matrices for dissipative quantum systems. The quantum master equations for $T=0$ and for $T>0$ possess completely different structures: (i) the equation for $T=0$ is linear in the deviation from the ground-state density matrix, whereas the equation for $T>0$, in general, is seriously nonlinear, and (ii) the Gibbs state is obtained as the steady-state solution of the nonlinear equation for $T>0$, whereas the ground state cannot be found from the equation for $T=0$. Nevertheless, the equation for $T=0$ can reproduce the behavior for $T\gtrsim 0$ remarkably well. We discuss some implications of that observation for dissipative quantum field theory.

Floodlight quantum key distribution (FL-QKD) has realized a 1.3 Gbit/s secret-key rate (SKR) over a 10-dB-loss channel against a frequency-domain collective attack [Quantum Sci. Technol. 3, 025007 (2018)]. It achieved this remarkable SKR by means of binary phase-shift keying (BPSK) of multiple optical modes. Moreover, it did so with available technology, and without space-division or wavelength-division multiplexing. In this paper we explore whether replacing FL-QKD's BPSK modulation with a high-order encoding can further increase that protocol's SKR. First, we show that going to $K$-ary phase-shift keying with $K=32$ doubles—from 2.0 to 4.5 Gbit/s—the theoretical prediction from [Phys. Rev. A 94, 012322 (2016)] for FL-QKD's BPSK SKR on a 50-km-long fiber link. Second, we show that $2d\times 2d$ quadrature amplitude modulation does not offer any SKR improvement beyond what its $d=1$ case—which is equivalent to quadrature phase-shift keying—provides.

Quantum machine learning is expected to be one of the first potential general-purpose applications of near-term quantum devices. A major recent breakthrough in classical machine learning is the notion of generative adversarial training, where the gradients of a discriminator model are used to train a separate generative model. In this work and a companion paper, we extend adversarial training to the quantum domain and show how to construct generative adversarial networks using quantum circuits. Furthermore, we also show how to compute gradients—a key element in generative adversarial network training—using another quantum circuit. We give an example of a simple practical circuit ansatz to parametrize quantum machine learning models and perform a simple numerical experiment to demonstrate that quantum generative adversarial networks can be trained successfully.

Nonrelativistic and relativistic coupled-cluster calculations extrapolated to the complete basis set limit including excitations up to the quintuple level were carried out to accurately determine the static electric dipole polarizability of the closed-shell palladium atom. The resulting value of $\alpha$ = 26.14(10) a.u. implies that palladium has the smallest dipole polarizability of all known elemental metal atoms due to its unique $4d^{10}5s^0$ configuration. Relativistic effects are found to be already sizable (${\mathrm{\Delta }}_R\alpha$ = +1.86 a.u.) compared to electron correlation (${\mathrm{\Delta }}_C\alpha$ = +5.06 a.u.) and need to be included for the accurate determination of the dipole polarizability. We also report a value of the second hyperpolarizability to be $\gamma \approx$ 40 000 a.u., but here the coupled-cluster contributions are not yet converged with respect to higher than quintuple excitations.

We evaluate a number of systematic effects that are important for an experimental microwave measurement of the $n=2$ $S$-to-$P$ intervals in atomic hydrogen. The analysis is important both for reevaluating the best existing measurement [S. R. Lundeen and F. M. Pipkin, Phys. Rev. Lett. 46, 232 (1981)] of the $2S_{1/2}-\mathrm{to}-2P_{1/2}$ Lamb shift and for a measurement that is ongoing in our laboratory. This work is part of a larger program to understand the several-standard-deviation discrepancies between various methods for determining the proton charge radius.

Electron scattering and dielectronic recombination with an ion in the presence of a neighboring atom is studied. The incident electron is assumed to be captured by the ion, leading to resonant excitation of the atom which afterwards may stabilize either by electron reemission or radiative decay. We show that the participation of the atom can strongly affect, both quantitatively and qualitatively, the corresponding processes of electron scattering and recombination. Various ion-atom systems and electronic transitions are considered. In particular we show that electron scattering under backward angles may be strongly enhanced, provided the incident energy lies very close to the resonance. Besides, we derive the scaling behavior of two-center dielectronic recombination with the principal quantum numbers of the participating atomic states. The resonant enhancement in this process is shown to be so strong that it can compete with ordinary single-center recombination even after integrating over a rather broad distribution of incident electron energies.

We study the population transfer between resonance states for a time-dependent loop around the exceptional points in the spectra of the hydrogen atom in parallel electric and magnetic fields. The exceptional points are well suited for population transfer mechanisms, since a closed loop around these in parameter space commutes eigenstates. We address the question of how shape and duration of the dynamical parameter loop affect the transferred population, in order to optimize the latter. Since the full quantum dynamics of the expansion coefficients is time consuming, we furthermore present an approximation method, based on a $2\times 2$ matrix.

Laser-induced dissociation of a photoionized oxygen molecule is studied employing an extreme-ultraviolet-pump–near-infrared-probe (EUV-NIR pump-probe) technique. A combination of a narrow-band 11th harmonic pump centered at 17.3 eV and a moderate-intensity NIR probe restricts the dissociation dynamics to the pair of low-lying cationic states, $a{}^4{\mathrm{\Pi }}_u$ and $f{}^4{\mathrm{\Pi }}_g$. The measured kinetic energies of the ${\mathrm{O}}^{+}$ fragments reveal contributions from one-, two-, and three-photon dissociation pathways ($1\omega$, $2\omega$, and $3\omega$) involving these two states. While the yields of the two- and three-photon channels initially rise and then decrease as a function of EUV-NIR delay, the yield of the single-photon pathway rises slower but keeps increasing over the whole delay range studied. This behavior reflects the evolving probability density of the ionic nuclear wave packet at the internuclear distances, where it can undergo resonant $3\omega$ and $1\omega$ transitions from the $a{}^4{\mathrm{\Pi }}_u$ to the $f{}^4{\mathrm{\Pi }}_g$ state of ${\mathrm{O}}_2{}^{+}$.

By measuring the ejected neutral and ionic fragments as well as the freed electron in coincidence, we track the laser induced dissociative frustrated double ionization (FDI) of ${\mathrm{D}}_2$, in which process one of the two tunneled electrons is eventually recaptured by the outgoing nuclei of the breaking molecule after the conclusion of the femtosecond laser fields. The dynamics are investigated in a pump-probe scheme by using an elliptically and linearly polarized few-cycle laser pulses with tunable time delay. The distinct momentum distributions of the escaped electrons governed by the light polarizations allow us to identify from which laser fields, i.e., the pump or the probe fields, the electron is released and thus deduce the origin of the recaptured one. Our results show that the electron released in the second ionization step after the stretching of molecular bond is favored to be recaptured in the dissociative FDI of molecules.

By analytically continuing the eigenvalue problem of a system of two coupled harmonic oscillators in the complex coupling constant $g$, we have found a continuation structure through which the conventional ground state of the decoupled system is connected to three other lower unconventional ground states that describe the different combinations of the two constituent oscillators, taking all possible spectral phases of these oscillators into account [Bender et al., Phys. Scr. 92, 015201 (2017)]. In this work we calculate the connecting structures for the higher excitation states of the system and argue that, in contrast to the fourfold Riemann surface identified for the ground state, the general structure is eightfold instead. Furthermore we show that this structure in principle remains valid for equal oscillator frequencies as well and comment on the similarity of the connection structure to that of the single complex harmonic oscillator.

We study the two-dimensional photoelectron momentum distribution in a synthesized field of an attosecond pulse and a weak infrared femtosecond pulse by solving the time-dependent Schrödinger equation of He atoms. The photoelectron momentum spectra reveal fruitful interference structures. With a semiclassical model, we show that some of the photoelectron interference structures come from the interference between the direct and backward-scattering electron trajectories. Those backward-scattering electrons are freed by the attosecond pulse and then are hard rescattered with parent ions by the streaking infrared field. By changing the phase delay between the attosecond pulse and the infrared laser field, we find that two kinds of backward-scattering electron trajectories will contribute to the holographic pattern. We show that backward-scattering holography can be explored with attosecond streaking experiments.

We present a theoretical study of He $2{}^{1}P–2{}^{1}S$ emission from a helium gas, where the upper atomic state is prepared by a 100 fs laser pulse resonant with the $1{}^{1}S–2{}^{1}P$ transition. The fluorescence signal emitted in the forward direction at 2059 nm and the transmitted signal at $58.4\mathrm{nm}$ are followed in time starting with the low pump intensity regime where the single-atom spontaneous emission results in a characteristic exponential decay. By increasing the gas density and pump intensity, collective effects such as self-amplified spontaneous emission and superfluorescence modify the temporal dependence of the radiation field by amplifying the radiation pattern as well as changing its delay and intensity distribution. The extended Maxwell-Bloch simulation of the corresponding three-level $\mathrm{\Lambda }$ system in one spatial dimension covers emission delays of up to 2.4 ns, reaching beyond the natural lifetime of the $2{}^{1}P$ state.

The presence of superradiance and subradiance in microscopic and mean-field approaches to light scattering in atomic media is investigated. We show that these phenomena are present in both descriptions, with only minor quantitative differences, so neither rely on disorder. In particular, they are most prominent in media with high resonant optical depth yet far-detuned light, i.e., in the single-scattering regime.