We make direct measurements of the duration of ultrafast cold-electron bunches produced by photoionization of laser-cooled atoms. We show that the bunch duration can vary by up to six orders of magnitude for relatively small changes in laser wavelength that enhance or inhibit specific photoexcitation pathways and below-threshold tunneling. By selecting a two-color multiphoton excitation process, bunches with durations as low as the measurement resolution limit of $130\mathrm{ps}$ are measured using a streak technique. Verification that ultrafast cold-electron bunches can be generated by photoionization of cold atoms is an important step towards their application in high-brightness ultrafast electron diffraction and injectors for particle accelerators.

Motivated by a recent experiment by Meinert et al. [arXiv:1608.08200], we study the dynamics of an impurity moving in the background of a harmonically trapped one-dimensional Bose gas in the hard-core limit. We show that due to the hidden “lattice” structure of background bosons, the impurity effectively feels a quasiperiodic potential via impurity-boson interactions that can drive the Bloch oscillation under an external force, even in the absence of real lattice potentials. Meanwhile, the inhomogeneous density of trapped bosons imposes an additional harmonic potential to the impurity, resulting in a similar oscillation dynamics but with a different period and amplitude. We show that the sign and strength of the impurity-boson coupling can significantly affect the above two potentials in determining the impurity dynamics.

We theoretically analyze antibunching of the phonon field in an optomechanical oscillator employing the membrane-in-the-middle geometry. More specifically, a single-mode mechanical oscillator is quadratically coupled to a single-mode cavity field in the regime in which the cavity dissipation is a dominant source of damping, and adiabatic elimination of the cavity field leads to an effective cubic nonlinearity for the mechanics. We show analytically in the weak-coupling regime that the mechanics displays a chaotic phonon field for small optomechanical cooperativity, whereas an antibunched single-phonon field appears for large optomechanical cooperativity. This opens the door to control of the second-order correlation function of a mechanical oscillator in the weak-coupling regime.

High-order-harmonic generation (HHG) is a tabletop and tunable source of extreme ultraviolet (XUV) light. Its flexibility was lately evidenced by the production of Laguerre-Gaussian (LG) modes in the XUV with a known azimuthal index. Here we investigate the role of the radial index of LG modes in HHG. We show both numerically and experimentally that the mode content of the emitted XUV radiation can be tuned by controlling the weight of the different quantum trajectories involved in the process. The appearance of high-order radial modes is finally linked to the atomic dipole phase of HHG. These results extend the capabilities of shaping the spatial mode of ultrashort XUV pulses of light.

Arrays of qubits encoded in the ground-state manifold of neutral atoms trapped in optical (or magnetic) lattices appear to be a promising platform for the realization of a scalable quantum computer. Two-qubit conditional gates between nearest-neighbor qubits in the array can be implemented by exploiting the Rydberg blockade mechanism, as was shown by D. Jaksch et al. [Phys. Rev. Lett. 85, 2208 (2000)]. However, the energy shift due to dipole-dipole interactions causing the blockade falls off rapidly with the interatomic distance, and protocols based on direct Rydberg blockade typically fail to operate between atoms separated by more than one lattice site. In this work, we propose an extension of the protocol of Jaksch et al. for controlled-$Z$ and controlled-not gates which works in the general case where the qubits are not nearest neighbors in the array. Our proposal relies on the Rydberg excitation hopping along a chain of ancilla noncoding atoms connecting the qubits on which the gate is to be applied. The dependence of the gate fidelity on the number of ancilla atoms, the blockade strength, and the decay rates of the Rydberg states is investigated. A comparison between our implementation of a distant controlled-not gate and one based on a sequence of nearest-neighbor two-qubit gates is also provided.

We consider an instance of “black-box” quantum metrology in the Gaussian framework, where we aim to estimate the amount of squeezing applied on an input probe, without previous knowledge on the phase of the applied squeezing. By taking the quantum Fisher information (QFI) as the figure of merit, we evaluate its average and variance with respect to this phase in order to identify probe states that yield good precision for many different squeezing directions. We first consider the case of single-mode Gaussian probes with the same energy, and find that pure squeezed states maximize the average quantum Fisher information (AvQFI) at the cost of a performance that oscillates strongly as the squeezing direction is changed. Although the variance can be brought to zero by correlating the probing system with a reference mode, the maximum AvQFI cannot be increased in the same way. A different scenario opens if one takes into account the effects of photon losses: coherent states represent the optimal single-mode choice when losses exceed a certain threshold and, moreover, correlated probes can now yield larger AvQFI values than all single-mode states, on top of having zero variance.

Quantum state transfer (QST) is a fundamental problem in quantum mechanics and is important for quantum information processing. In this paper we explore a fast, robust, and efficient QST in spin-based quantum dots. Differing from the usual method that only uses the exchange interactions to control the evolution of the spin system, here we treat the non-negligible Dzyaloshinskii-Moriya (DM) interactions that can be electrically and thermally controlled in quantum dots as an assisted driving to accelerate the population transfer. We demonstrate that the DM interactions are not an error source, but are beneficial for population evolution under certain condition because they can sufficiently eliminate the undesirable quantum transitions among adiabatic passages and strictly maintain the population evolution in a designed passage with high efficiency. We give a generic $N$-spin anisotropic Heisenberg spin model to describe the population evolution and take specific two- and three-spin systems as examples to demonstrate the advantages of the DM interactions for QST.

The Auger cascades following the resonant $1s\to np$ ($n=3,4$) excitation of neutral neon are studied theoretically. In contrast to previous investigations, we here model the complete cascade from the initially core-excited $1s^{-1}3p{}^{1}P_1$ and $1s^{-1}4p{}^{1}P_1$ levels of Ne up to the doubly ionized ${\mathrm{Ne}}^{2+}$ ions. Extensive multiconfiguration Dirac-Fock calculations are carried out, combined with a proper cascades model, to incorporate as many decay branches as possible, including all major single-electron shake-up and shake-down processes. We simulate the electron spectra and predict shake probabilities, ion yields, as well as the relative population of the intermediate and final states. Experimentally known level energies for neutral, singly, and doubly ionized neon are utilized whenever possible in order to improve the predictions. Most features from experiment can be reproduced with quite good agreement if a sufficiently large basis is taken into account. These simulations therefore demonstrate not only the required computational effort, but also that it is nowadays possible to predict whole Auger spectra of decay cascades, a central feature for further exploring electron coincidence maps as obtained at synchrotrons and free-electron lasers.

A Chern insulator is a building block of many topological quantum matters. Here, we discuss a nonsymmorphic Chern insulator, which preserves the nonsymmorphic symmetry and breaks the time-reversal symmetry. It consists of two half-filled Chern insulators, and the bulk energy spectrum is obtained from folding that of either Chern insulator. Such folding gives rise to a nodal boundary of the Brillouin zone, which is protected by the nonsymmorphic symmetries and other lattice symmetries of the system. It also provides one a natural platform to explore the non-Abelian Berry curvature and the resultant quantum phenomena. Moreover, the interplay between nonsymmorphic symmetries and the broken time-reversal symmetry leads to intriguing properties absent in ordinary Chern insulators. An additional degree of freedom, the parity of the nonsymmorphic symmetry, needs to be introduced for describing the topological pumping.

This article investigates the properties of a few interacting particles trapped in a few wells and how these properties change under adiabatic tuning of interaction strength and interwell tunneling. While some system properties are dependent on the specific shapes of the traps and the interactions, this article applies symmetry analysis to identify generic features in the spectrum of stationary states of few-particle, few-well systems. Extended attention is given to a simple but flexible three-parameter model of two particles in two wells in one dimension. A key insight is that two limiting cases, hard-core repulsion and no interwell tunneling, can both be treated as emergent symmetries of the few-particle Hamiltonian. These symmetries are the mathematical consequences of infinite barriers in configuration space. They are necessary to explain the pattern of degeneracies in the energy spectrum, to understand how degeneracies are broken for models away from limiting cases, and to explain separability and integrability. These symmetry methods are extendable to more complicated models and the results have practical consequences for stable state control in few-particle, few-well systems with ultracold atoms in optical traps.

We study a nonlinear oscillator, which is parametrically driven at a frequency close to twice its eigenfrequency. By judiciously choosing the frequency detuning and linearly increasing the driving amplitude, one can prepare any even quasienergy state starting from the oscillator ground state. Such state preparation is effectively adiabatic. We find the Wigner distribution of the prepared states. For a different choice of the frequency detuning, the adiabaticity breaks down, which allows one to prepare on demand a superposition of quasienergy states using Landau-Zener-type transitions. We find the characteristic spectrum of the transient radiation emitted by the oscillator after it has been prepared in a given quasienergy state.

Einstein-Podolsky-Rosen (EPR) steering is a quantum nonlocal effect which is intrinsically distinct from Bell nonlocality and quantum entanglement. In this paper, we investigate in detail the properties of steady-state light-mechanical Gaussian steerable correlations in a generic cavity optomechanical system. When considering the steering between the intracavity field and the mechanical oscillator, we find that under blue-detuned driving, the steady-state steering via optomechanical parametric downconversion is present in only one direction and, moreover, the steering direction is determined merely by the relative dissipation strength of the cavity to the mechanics. Furthermore, when considering the steering between the cavity output field and the mechanical oscillator, we reveal that under red-detuned driving, strong steering can be achieved in the sideband-unresolved regime for a filtered output field with given central frequency and bandwidth. This steering with the output field can also be present in one way by adjusting the driving strength and exists up to the environment temperature $T\approx 10$ K for the parameters close to those in the recent experiments. Finally, we show that the achieved strong light-mechanical correlations can be explored to realize macroscopic EPR steering of two distant optomechanical oscillators in the regime of unresolved sidebands via entanglement swapping.

Angular spectra of reflectance of light from one-dimensional periodic gratings of V-shaped grooves in a metal film have been theoretically studied at a fixed wavelength. The exciting light wave was incident in the plane parallel to grooves. For the case of $s$-polarized incident wave there are two types of features (dips) in reflection spectra. The first type is associated with the excitation of channel plasmons in every single groove of a grating. Angular positions of features of the second type are determined by a period of a grating. We have shown that the features of the second type are Wood anomalies associated with the excitation of surface waves on a metal film by means of diffraction of the incident wave on a periodic grating. For certain periods of a grating and angles of incidence of the exciting wave the resonances of two types can be coupled.

In 2001, Zurek derived the generic minimum scale $a_Z$ for the area of structures of Wigner's quantum phase distribution. Here we show by construction, using superoscillatory functions, that the Wigner distribution can locally show regular spotty structures on scales much below Zurek's scale $a_Z$. The price to pay for the presence of such structures is their exponential smallness. For the case we construct, there is no increased interferometric sensitivity from the presence of patches with superoscillatory structure in phase space.

$\mathcal{PT}$-symmetric quantum mechanics began with a study of the Hamiltonian $H=p^2+x^2{\left(ix\right)}^{\varepsilon }$. When $\varepsilon \ge 0$, the eigenvalues of this non-Hermitian Hamiltonian are discrete, real, and positive. This portion of parameter space is known as the region of unbroken $\mathcal{PT}$ symmetry. In the region of broken $\mathcal{PT}$ symmetry, $\varepsilon <0$, only a finite number of eigenvalues are real and the remaining eigenvalues appear as complex-conjugate pairs. The region of unbroken $\mathcal{PT}$ symmetry has been studied but the region of broken $\mathcal{PT}$ symmetry has thus far been unexplored. This paper presents a detailed numerical and analytical examination of the behavior of the eigenvalues for $-4<\varepsilon <0$. In particular, it reports the discovery of an infinite-order exceptional point at $\varepsilon =-1$, a transition from a discrete spectrum to a partially continuous spectrum at $\varepsilon =-2$, a transition at the Coulomb value $\varepsilon =-3$, and the behavior of the eigenvalues as $\varepsilon$ approaches the conformal limit $\varepsilon =-4$.

Einstein-Podolsky-Rosen (EPR) steering exhibits an inherent asymmetric feature that differs from both entanglement and Bell nonlocality, which leads to one-way EPR steering. Although this one-way EPR steering phenomenon has been experimentally observed, the schemes to manipulate the direction of EPR steering have not been investigated thoroughly. In this paper, we propose and experimentally demonstrate three schemes to manipulate the direction of EPR steering, either by varying the noise on one party of a two-mode squeezed state (TMSS) or transmitting the TMSS in a noisy channel. The dependence of the direction of EPR steering on the noise and transmission efficiency in the quantum channel is analyzed. The experimental results show that the direction of EPR steering of the TMSS can be changed in the presented schemes. Our work is helpful in understanding the fundamental asymmetry of quantum nonlocality and has potential applications in future asymmetric quantum information processing.

The ultimate precision of any measurement of the temperature of a quantum system is the inverse of the local quantum thermal susceptibility [A. De Pasquale et al., Nat. Commun. 7, 12782 (2016)] of the subsystem with which the thermometer interacts. If this subsystem can be described with the canonical ensemble, such quantity reduces to the variance of the local Hamiltonian, which is proportional to the heat capacity of the subsystem. However, the canonical ensemble might not apply in the presence of interactions between the subsystem and the rest of the system. In this work, we address this problem in the framework of locally interacting quantum systems. We prove that the local quantum thermal susceptibility of any subsystem is close to the variance of its local Hamiltonian, provided the volume-to-surface ratio of the subsystem is much larger than the correlation length. This result greatly simplifies the determination of the ultimate precision of any local estimate of the temperature and rigorously determines the regime where interactions can affect this precision.

We study the consequences of superquantum nonlocal correlations as represented by the PR-box model of Popescu and Rohrlich, and show that PR boxes can enhance the capacity of noisy interference channels between two senders and two receivers. PR-box correlations violate Bell and CHSH inequalities and are thus stronger—more nonlocal—than quantum mechanics, yet weak enough to respect special relativity in prohibiting faster-than-light communication. Understanding their power will yield insight into the nonlocality of quantum mechanics. We exhibit two proof-of-concept channels: First, we show a channel between two sender-receiver pairs where the senders are not allowed to communicate, for which a shared superquantum bit (a PR box) allows perfect communication. This feat is not achievable with the best classical (senders share no resources) or quantum-entanglement-assisted (senders share entanglement) strategies. Second, we demonstrate a class of channels for which a tunable parameter $\epsilon$ achieves a double separation of capacities; for some range of $\epsilon$, the superquantum-assisted strategy does better than the entanglement-assisted strategy, which in turn does better than the classical one.

Lyman spectra and line ratios are reported for soft-x-ray emissions following the charge exchange process in 293, 414, 586, and 1256 km/s ${\mathrm{O}}^{8+}$ and Kr collisions. Lyman series from Ly-$\alpha$ to Ly-$\varepsilon$ were resolved for the ${\mathrm{O}}^{7+}$ ion using a high-resolution x-ray quantum microcalorimeter detector. It is found that the observed line ratios are dependent on the $nl$ distribution of the captured electron, and the Ly-$\alpha$ and Ly-$\beta$ x-ray emissions are enhanced. Moreover, by comparing the measured line ratios to the constructed theoretical single charge exchange line ratios for ${\mathrm{O}}^{8+}+\text{H}$, it is suggested that autoionizing double capture plays a significant role in the enhancement of Ly-$\alpha$ and Ly-$\beta$ emissions for the present system.

Attosecond time-resolved spectroscopy has been shown to be a powerful method for examining the electronic dynamics in atoms, and this technique is now being transferred to the investigation of elastic and inelastic scattering during electron transport and collective electronic (plasmonic) effects in solids. By sampling over classical photoelectron trajectories, we simulated streaked photoelectron energy spectra as a function of the time delay between ionizing isolated attosecond extreme ultraviolet (XUV) pulses and assisting infrared or visible streaking laser pulses. Our calculations comprise a sequence of four steps: XUV excitation, electron transport in matter, escape from the surface, and propagation to the photoelectron detector. Based on numerical applications to gold nanospheres of 5- and 50-nm radius, we investigate streaked photoemission spectra with regard to (i) the nanoparticle's dielectric response to the electric field of the streaking laser pulse, (ii) relative contributions to photoelectron release from different locations on the surface and inside the nanoparticle, (iii) contributions of photoemission from the Fermi level only versus emission from the entire occupied conduction band, and (iv) their fidelity in imaging the spatiotemporal distribution of the induced plasmonic field near the particle's surface.