We calculate the charge-transfer cross sections for the ${\mathrm{Ne}}^{2+}+\text{He}$ collision. To this end, we employ Ehrenfest molecular dynamics with time-dependent density-functional theory. The active electrons of the projectile are handled by applying an initial velocity to the Kohn-Sham orbitals via a Galilean boost. The dynamical calculations are performed in an inverse collision framework—the reference frame considers ${\mathrm{Ne}}^{2+}$ to be initially at rest, which ensures numerically converged final-time scattering states. The charge-transfer probabilities are extracted by extending the particle number projection technique to be able to handle the degenerate ${\mathrm{Ne}}^{2+}$ ion. Compared with experimental data available at 10–3000 keV, a fairly good agreement is found for the calculated single- and double-charge transfer cross sections, superior to other theoretical calculations for this ${\mathrm{Ne}}^{2+}+\text{He}$ collision. A time-resolved analysis of the charge-transfer probabilities finds that ionization to the continuum also takes place after the charge transfer has occurred. To account for it, the final scattering states should be followed for a long time, approximately 350 fs, until they stabilize.

We investigate the prospects of combining a standard momentum space approach for ultracold three-body scattering with efficient coordinate space schemes to solve the underlying two-body problem. In many of those schemes, the two-body problem is numerically restricted up to a finite interparticle distance $r_{\mathrm{b}}$. We analyze the effects of this two-body restriction on the two- and three-body level using pairwise square-well potentials that allow for analytic two-body solutions and more realistic Lennard-Jones van der Waals potentials to model atomic interactions. We find that the two-body $t$ operator converges exponentially in $r_{\mathrm{b}}$ for the square-well interaction. Setting $r_{\mathrm{b}}$ to 2000 times the range of the interaction, the three-body recombination rate can be determined accurately up to a few percent when the magnitude of the scattering length is small compared to $r_{\mathrm{b}}$, while the position of the lowest Efimov features is accurate up to the percent level. In addition, we find that with the introduction of a momentum cutoff, it is possible to determine the three-body parameter in good approximation even for deep van der Waals potentials.

Recording the fluorescence of a magneto-optical trap (MOT) is a standard tool for measuring atom numbers in experiments with ultracold atoms. When trapping few atoms in a small MOT, the emitted fluorescence increases with the atom number in discrete steps, which allows one to measure the atom number with single-particle resolution. Achieving such single-particle resolution requires stringent minimization of stray light from the MOT beams, which is very difficult to achieve in experimental setups that require in-vacuum components close to the atoms. Here, we present a modified scheme that addresses this issue: Instead of collecting the fluorescence on the MOT (D2) transition, we scatter light on an additional probing (D1) transition and collect this fluorescence with a high-resolution microscope while filtering out the intense MOT light. Using this scheme, we are able to reliably distinguish up to 17 ${}^{40}\mathrm{K}$ atoms with classification fidelities of $\sim 98\%$ for up to 5 atom numbers and fidelities of more than 85% for up to 17 atoms.

It is well known that the ground-state cooling of a single mechanical oscillator is limited by some strict conditions in the conventional sideband cooling method, such as small cavity decay, weak effective coupling, and minimum cooling phonon occupancy. Here we propose an effective scheme to greatly improve the ground-state cooling of two mechanical oscillators in a compound three-mode optomechanical system. By introducing the frequency modulation and the bias gate voltage modulation switch into the system, it is demonstrated that the scheme is well suitable for simultaneously cooling the two mechanical oscillators whether their frequencies are identical or largely different. Compared with previous schemes without modulation, our scheme can be achievable in a much wider parameter zone but shows a more ideal cooling effect, i.e., ranging from weak coupling to ultrastrong coupling and from the resolved-sideband regime to the unresolved-sideband regime. Moreover, the minimum cooling phonon occupancies obtained in our scheme for identical mechanical frequencies successfully break the quantum backaction limit defined in the absence of modulation. More importantly, even in the conventional unstable zone, our scheme is still workable and the steady ground-state cooling can be achieved perfectly. Our scheme opens up an alternative method for simultaneously cooling multiple mechanical oscillators with lower cooling phonon occupancy even breaking the quantum backaction limit but with fewer parametric limitations.

This work reports on an investigation of the dynamics of $1.3\mu \text{m}$ epitaxial quantum dot (QD) lasers on silicon subject to delayed optical feedback. Operating the device under different feedback conditions, we experimentally identify various dynamical states of periodic oscillations. In the long-cavity feedback regime, the device remains chaos-free up to $\approx 70\% (-1.55\mathrm{dB})$ feedback strength. This remarkable result is in agreement with prior studies and is attributed to the particular design and properties of the QD-based active region. Shortening the external cavity length to the short-cavity regime being on the scale of photonics integrated circuits (PICs), the onset of periodic oscillations only takes place under extremely high feedback strength, which is much higher than those in PICs. The devices studied exhibit strong resistance to chip-scale back reflections in absence of any unstable oscillation. Our results also demonstrate that p doping is an efficient technique to further improve the feedback tolerance. These results point out the potential of QD lasers as an on-chip light source not requiring an optical isolator and gives insights for developing ultrastable silicon transmitters for PIC applications.

We provide an in-depth discussion of a theoretical framework recently introduced [Lindel et al., Phys. Rev. A 102, 041701(R) (2020)] which is capable of predicting the electromagnetic field emerging from a nonlinear crystal through which a coherent laser pulse is shone. This framework is based on macroscopic quantum electrodynamics and includes dispersion and absorption effects inside the crystal and allows for arbitrary optical environments through the classical Green's tensor. We introduce a diagrammatic approach with which the nonlinear processes contributing to the electric field operator up to certain orders in the perturbation series can be represented in a convenient way. Applying this framework to the setup of electro-optic sampling experiments of the polaritonic quantum vacuum, we derive analytical results for the electro-optic sampling between distinct spatiotemporal regions. Also, we discuss different approximations and the parameter ranges in which they apply including angled or diverging beams, thermal fluctuations, as well as (linear) absorption effects upon the polaritonic quantum vacuum. Finally, we compare these theoretical results to experimental data.

Accurate quantum chemistry simulations remain challenging on classical computers for problems of industrially relevant sizes and there is reason for hope that quantum computing may help push the boundaries of what is technically feasible. While variational quantum eigensolver algorithms may already turn noisy intermediate scale quantum (NISQ) devices into useful machines, one has to make all efforts to use the scarce quantum resources as efficiently as possible. We combine the so-called seniority-zero, or paired-electron, approximation of computational quantum chemistry with techniques for simulating molecular chemistry on gate-based quantum computers and obtain a very resource efficient quantum simulation algorithm. While some accuracy is lost through the paired-electron approximation, we show that using the freed-up quantum resources for increasing the basis set can lead to more accurate results and reductions in the necessary number of quantum computing runs by several orders of magnitude, already for a simple system like lithium hydride. We also discuss an error mitigation scheme based on postselection which shows an attractive scaling when the given Hamiltonian format is considered, increasing the viability of its NISQ implementation.

We use kinetic theory to model the dynamics of a small Bose condensed cloud of heavy particles moving through a larger degenerate Fermi gas of light particles. Varying the Bose-Fermi interaction, we find a crossover between bulk- and surface-dominated regimes—where scattering occurs throughout the Bose cloud or solely on the surface, respectively. We calculate the damping and frequency shift of the dipole mode in a harmonic trap as a function of the magnetic field controlling an interspecies Feshbach resonance. We find excellent agreement between our stochastic model and the experimental studies of Cs-Li mixtures.

Robust transmission in topological insulators makes it possible to steer waves without attenuation along bent paths within imperfectly fabricated photonic devices. But the absence of reflection does not guarantee the fidelity of pulsed transmission which is essential for core photonic functionalities. Pulse transmission is disrupted by localized modes in the bulk of topological insulators which coexist with the continuum edge mode and are pushed deeper into the band gap with increasing disorder. Here we show in simulations of the Haldane model that pulse propagation in disordered topological insulators is robust throughout the central portion of the band gap where localized modes do not arise. Since transmission is robust in topological insulators, the essential field variable is the phase of the transmitted field, or, equivalently, its spectral derivative, which is the transmission time. Except near resonances with bulk localized modes that couple the upper and lower edges of a topological insulator, the transmission time in a topological insulator is proportional to the density of states and to the energy excited within the sample. The variance of the transmission time at the band edge for a random ensemble with moderate disorder is dominated by fluctuations at resonances with localized states, and initially scales quadratically. When modes are absent, such as in the center of the band gap, the transmission time self-averages and its variance scales linearly. This leads to significant sample-to-sample fluctuations in the transmission time. However, because the transmission time is the sum of contributions from the continuum edge mode, which stretches across the band gap, and far-off-resonance modes of the passband and band edge, there are no sharp features in the spectrum of transmission time in the center of the band gap. Instead of creating obstacles to be circumvented with inevitable increase of the transmission time in the band edge mode, the transmission time of the band edge mode is decreased by disorder. As a result, ultrashort, broadband pulses are faithfully transmitted in the center of the band gap of topological insulators with moderate disorder and bent paths. This allows for robust signal propagation in complex topological metawaveguides for applications in high-speed optoelectronics and telecommunications.

We propose to employ a recurrent neural network to estimate a fluctuating magnetic field from continuous optical Faraday rotation measurement on an atomic ensemble. We show that an encoder-decoder architecture neural network can process measurement data and learn an accurate map between recorded signals and the time-dependent magnetic field. The performance of this method is comparable to Kalman filters while it is free of the theory assumptions that restrict their application to particular measurements and physical systems.

Coherence and nonlocal correlations are two fundamental features of quantum mechanics and the relations between them have received much attention recently. While the relations of coherence to Bell nonlocality and to entanglement have been established, an interesting question left open is how to quantitatively characterize the relation of coherence to steering, which is one kind of nonlocal correlation lying between Bell nonlocality and entanglement. In this paper, we address this question by considering the first-order coherence and the three-setting linear steering inequality for a two-qubit system. Interestingly, we find that the first-order coherence for both subsystems has a complementary relation with the maximum violation of the three-setting linear steering inequality. A weighted sum of their squares keeps invariant under unitary operations, although each of them changes in general. This suggests that the maximum violation of the three-setting linear steering inequality can be taken as a measure of hidden coherence unveiled by unitary operations.

Violation of a Bell inequality certifies that the underlying state must be entangled in a device-independent way, although there exist some entangled states which do not violate such an inequality. However, for every entangled state, it is possible to find a Hermitian operator called an entanglement witness that can detect entanglement through some local measurements in a device-dependent method, but implementation of wrong measurements may lead to fake detection of entanglement. To avoid such difficulties, measurement-device-independent entanglement witness (MDI-EW) based on a semiquantum nonlocal game was proposed, which is not only robust against wrong measurements but also against a specific kind of lossy detectors. We employ here a measurement-device-independent entanglement witness to detect entanglement in a scenario where half of an entangled pair is possessed by a single observer while the other half is with multiple observers performing unsharp measurements, sequentially, independently, and preserving entanglement as much as possible. Interestingly, we find that the numbers of successful observers who can detect entanglement, measurement-device-independently, both with equal and unequal sharpness parameters of the noisy measurements, are greater than that obtained with standard and Bell-inequality-based entanglement detection methods, reflecting its robustness. The entanglement contents of the sequentially shared states are also analyzed. Unlike other scenarios, our investigations also reveal that in this measurement-device-independent situation, states having entanglement in proximity to maximal remain entangled until there are two sequential observers, even if they measure sharply.

With the Gottesman-Kitaev-Preskill (GKP) encoding, Clifford gates and error correction can be carried out using simple Gaussian operations. Still, non-Clifford gates, required for universality, require non-Gaussian elements. In their original proposal, GKP suggested a particularly simple method of using a single application of the cubic phase gate to perform the logical non-Clifford $T$ gate. Here we show that this cubic phase gate approach performs extraordinarily poorly, even for arbitrarily large amounts of squeezing in the GKP state. Thus, contrary to common belief, the cubic phase gate is not suitable for achieving universal fault-tolerant quantum computation with GKP states.

We proposed a passive continuous-variable quantum key distribution (CVQKD) using a locally generated local oscillator (LLO) at reception. Different from the LLO-based Gaussian-modulated coherent-state quantum key distribution, our protocol is implemented by taking advantage of a multimode thermal source, beam splitters, optical attenuators, and homodyne detectors rather than the amplitude and phase modulators, which could be a cost-effective solution for LLO-based CVQKD. We adopt a general LLO noise model to analyze the performance of our protocol. That is to say, our consideration for noise in the proposed scheme contains the excess noise caused by passive state preparation, the phase noise, and the photon-leakage noise from the reference path. Because our protocol waives the necessity of the use of amplitude and phase modulators, there is no need to bear the effect of modulation noise caused by finite dynamics. Simulation results show that the improvement of the mode-overlap coefficient can suppress the excess noise caused by passive state preparation and thus enhance the performance of our protocol. Because lower relative phase drift and the photon-leakage noise can be obtained by using the multiplexing technique, we can still achieve reasonable performance of our protocol with the mode-overlap coefficient values which are very close to the experimental reference. Furthermore, we perform the finite-size analysis for the proposed protocol, which is more practical than that achieved in the asymptotic limit. This work confirms the feasibility of passive CVQKD implemented in the LLO configuration within the metropolitan area.

A critical set of two-loop quantum electrodynamics corrections to the $g$ factor of hydrogenlike ions is calculated in the Furry picture. These corrections are due to the polarization of the external magnetic field by the quantum vacuum, which is dressed by the binding field. The result obtained for the self-energy–magnetic-loop diagrams is compared with the current state-of-the-art result, derived through a perturbative expansion in the binding strength parameter $Z\alpha$, with $Z$ the atomic number and $\alpha$ the fine-structure constant. Agreement is found in the $Z\to 0$ limit. However, even for very light ions, the perturbative result fails to approximate the magnitude of the corresponding correction to the $g$ factor. The total correction to the $g$ factor coming from all diagrams considered in this work is found to be highly relevant for upcoming experimental tests of fundamental physics with highly charged ions.

We analyze the relationship between qubit-environment entanglement that can be created during the pure dephasing of the qubit and the effectiveness of the spin-echo protocol. We focus here on mixed states of the environment. We show that whereas the echo protocol can obviously counteract classical environmental noise, it can also undo dephasing associated with qubit-environment entanglement, and there is no obvious difference in its efficiency in these two cases. Additionally, we show that qubit-environment entanglement can be generated at the end of the echo protocol even when it is absent at the time of application of the local operation on the qubit (the $\pi$ pulse). We prove that this can occur only at isolated points in time, after fine-tuning of the echo protocol duration. Finally, we discuss the conditions under which the observation of specific features of the echo signal can serve as a witness of the entangling nature of the joint qubit-environment evolution.