Using a time-dependent density-functional-theory (TDDFT) method that incorporates the exact exchange, we reproduce the measured ionization suppression for vanadium in 1500-nm lasers of 1.4 to $2.8\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. The calculated ionization yields are 0.07 to 0.5 in 100 fs ${sin}^2$ pulses. For weaker laser intensities a method with more configurations is needed to properly describe the multiphoton, rather than tunneling, ionization of a transition-metal atom. Our calculations show that the isotropic component of the induced potential increases the binding energy of the electron while the dipole component elevates the potential barrier of tunneling ionization. Both effects suppress the tunneling ionization.

Mean-field energy-level shifts arising as a result of strong electrostatic dipole interactions within dilute gases of polarized helium Rydberg atoms have been probed by microwave spectroscopy. The Rydberg states studied had principal quantum numbers $n=70$ and 72, and electric dipole moments of up to 14 050 D, and were prepared in pulsed supersonic beams at particle number densities on the order of ${10}^8 {\mathrm{cm}}^{-3}$. Comparisons of the experimental data with the results of Monte Carlo calculations highlight effects of the distribution of nearest-neighbor spacings in the pulsed supersonic beams, and the dielectric properties of the strongly polarized Rydberg gases, on the microwave spectra. These observations reflect the emergence of macroscopic electrical properties of the atomic samples when strongly polarized.

Topological Lifshitz phase transition characterizes an abrupt change of the topology of the Fermi surface through a continuous deformation of parameters. Recently, Lifshitz transition has been predicted to separate two types of Weyl points: type-I and type-II (or called structured Weyl points), which has attracted considerable attention in various fields. Although recent experimental investigation has seen a rapid progress on type-II Weyl points, it still remains a significant challenge to observe their characteristic Lifshitz transition. Here, we propose a scheme to realize both type-I and type-II Weyl points in three-dimensional ultracold atomic gases by introducing an experimentally feasible configuration based on current spin-orbit coupling technology. In the resultant Hamiltonian, we find three degenerate points: two Weyl points carrying a Chern number $-1$ and a fourfold degenerate point carrying a Chern number 2. Remarkably, by continuous tuning of a convenient experimental knob, all these degenerate points can transition from type-I to type-II, thereby providing an ideal platform to study different types of Weyl points and directly probe their Lifshitz phase transition.

In this article we introduce an interesting quantum state, the perfect quantum optical vortex state which exhibits a highly localized distribution along a ring in the quadrature space. We examine its nonclassical properties using the Wigner function and the negativity volume. Such a quantum state can be a useful resource for quantum information processing and communication.

We analyzed the optical band gaps for axially propagating electromagnetic waves throughout a nanocomposite structurally chiral medium. This medium is made of metallic nanoballs (silver) randomly dispersed in a structurally chiral material whose dielectric properties can be represented by a resonant effective uniaxial tensor. The structurally chiral material is taken to possess locally a $\overline{4}2m$ point group symmetry. We found that the band-gap properties of the periodic system depends strongly on the volume fraction of nanoparticles in the chiral matrix. Particularly, we observed splitting of the bands and the creation of new sub band gaps when the resonance frequency of the composite medium lies within the band gap without inclusions.

We theoretically investigate the interaction of light and a collection of emitters in a subwavelength one-dimensional medium (nanoguide), where enhanced emitter-photon coupling leads to efficient multiple scattering of photons. We show that the spectrum of the transmitted light undergoes normal-mode splitting even though no external cavity resonance is employed. By considering densities much higher than those encountered in cold atom experiments, we study the influence of the near-field dipole coupling and disorder on the resulting complex super-radiant and subradiant polaritonic states. In particular, we provide evidence for the longitudinal localization of light in a one-dimensional open system and provide a polaritonic phase diagram. Our results motivate a number of experiments, where new coherent superposition states of light and matter can be realized in the solid state.

We present a technique, based on atoms coupled to an optical cavity, for nondestructive detection of trapped ions. We demonstrate the vacuum-Rabi splitting (VRS), arising from collective strong coupling of ultracold Rb atoms and a cavity, to change in the presence of trapped $\mathrm{R}{\mathrm{b}}^{+}$ ions. The $\mathrm{R}{\mathrm{b}}^{+}$ ions are optically dark and the Rb atoms are prepared in a dark magneto-optical trap. The VRS is measured on an optically open transition of the initially dark Rb atoms. The measurement itself is fast, nondestructive, and has sufficient fidelity to permit the measurement of the atomic-state-selective ion-atom collision rate. This demonstration illustrates a method based on atom-cavity coupling to measure two-particle interactions generically and nondestructively.

We develop a general dynamical theory for studying a single-photon transport in a one-dimensional (1D) waveguide coupled to multiple emitters which can be either identical or nonidentical. In this theory, both the effects of the waveguide and non-waveguide vacuum modes are included. This theory enables us to investigate the propagation of an emitter excitation or an arbitrary single-photon pulse along an array of emitters coupled to a 1D waveguide. The dipole-dipole interaction induced by the non-waveguide modes, which is usually neglected in the literature, can significantly modify the dynamics of the emitter system as well as the characteristics of the output field if the emitter separation is much smaller than the resonance wavelength. Nonidentical emitters can also strongly couple to each other if their energy difference is less than or of the order of the dipole-dipole energy shift. Interestingly, if their energy difference is close but nonzero, a very narrow transparency window around the resonance frequency can appear which does not occur for identical emitters. This phenomenon may find important applications in quantum waveguide devices such as optical switches and ultranarrow single-photon frequency comb generator.

We report the existence of a universal trimer state induced by an impurity interacting with a two-component spin-orbit-coupled Fermi gas in two dimensions. In the zero-density limit with a vanishing Fermi sea, the trimer is stabilized by the symmetry of the single-particle spectrum under spin-orbit coupling, and is therefore universal against the short-range details of the interaction potential. When the Fermi energy increases, we show that the trimer is further stabilized by particle-hole fluctuations over a considerable parameter region. We map out the phase diagram consisting of trimers, dimers, and polarons, and discuss the detection of these states using radio-frequency spectroscopy. The universal trimer revealed in our work is a direct manifestation of intriguing three-body correlations emerging from a many-body environment, which, in our case, is cooperatively supported by the single-particle spectral symmetry and the collective particle-hole excitations.

Time-energy high-dimensional quantum key distribution (HD-QKD) leverages the high-dimensional nature of time-energy entangled biphotons and the loss tolerance of single-photon detection to achieve long-distance key distribution with high photon information efficiency. To date, the general-attack security of HD-QKD has only been proven in the asymptotic regime, while HD-QKD's finite-key security has only been established for a limited set of attacks. Here we fill this gap by providing a rigorous HD-QKD security proof for general attacks in the finite-key regime. Our proof relies on an entropic uncertainty relation that we derive for time and conjugate-time measurements that use dispersive optics, and our analysis includes an efficient decoy-state protocol in its parameter estimation. We present numerically evaluated secret-key rates illustrating the feasibility of secure and composable HD-QKD over metropolitan-area distances when the system is subjected to the most powerful eavesdropping attack.

Quantum computers are poised to radically outperform their classical counterparts by manipulating coherent quantum systems. A realistic quantum computer will experience errors due to the environment and imperfect control. When these errors are even partially coherent, they present a major obstacle to performing robust computations. Here, we propose a method for introducing independent random single-qubit gates into the logical circuit in such a way that the effective logical circuit remains unchanged. We prove that this randomization tailors the noise into stochastic Pauli errors, which can dramatically reduce error rates while introducing little or no experimental overhead. Moreover, we prove that our technique is robust to the inevitable variation in errors over the randomizing gates and numerically illustrate the dramatic reductions in worst-case error that are achievable. Given such tailored noise, gates with significantly lower fidelity—comparable to fidelities realized in current experiments—are sufficient to achieve fault-tolerant quantum computation. Furthermore, the worst-case error rate of the tailored noise can be directly and efficiently measured through randomized benchmarking protocols, enabling a rigorous certification of the performance of a quantum computer.

Establishing a quantum interface between different physical systems is of special importance for developing the practical versatile quantum networks. Entanglement between low- and high-lying atomic spin waves is essential for building up Rydberg-based quantum information engineering, which is also helpful to study the dynamics behavior of entanglement under external perturbations. Here, we report on the successful storage of a single photon as a high-lying atomic spin wave in a quantum regime. By storing a K-vector entanglement between a single photon and low-lying spin wave, we experimentally realize the entanglement between low- and high-lying atomic spin waves in two separated atomic systems. This makes our experiment a primary demonstration of Rydberg quantum memory of entanglement, representing a primary step toward the construction of a hybrid quantum interface.

We have carried out an extensive investigation of various spectroscopic properties of highly charged inert-gas ions using a relativistic coupled-cluster method through a one-electron detachment procedure. In particular, we have calculated the atomic states $2s^{2}2p^5{}^{2}P_{3/2}$, $2s^{2}2p^5{}^{2}P_{1/2}$, and $2s2p^6{}^{2}S_{1/2}$ in F-like inert-gas ions; $3s^{2}3p^5{}^{2}P_{3/2}$, $3s^{2}3p^5{}^{2}P_{1/2}$, and $3s3p^6{}^{2}S_{1/2}$ states in Cl-like Kr, Xe, and Rn; and $4s^{2}4p^5{}^{2}P_{3/2}$, $4s^{2}4p^5{}^{2}P_{1/2}$, and $4s4p^6{}^{2}S_{1/2}$ states in Br-like Xe and Rn. Starting from a single-reference Dirac-Hartree-Fock wave function, we construct our exact atomic states by including the dynamic correlation effects in an all-order perturbative fashion. Employing this method, we estimate the ionization potential energies of three low-lying orbitals present in their respective closed-shell configurations. Since the considered highly charged inert-gas ions exhibit huge relativistic effects, we have taken into account the corrections due to Breit interaction as well as from the dominant quantum electrodynamic correction such as vacuum polarization and self-energy effects in these systems. Using our calculated relativistic atomic wave functions and energies, we accurately determine various transition properties such as wavelengths, line strengths, oscillator strengths, transition probabilities, and lifetimes of the excited states.

Multicomponent Bose-Einstein condensates exhibit an intriguing variety of nonlinear structures. In recent theoretical work [C. Qu, L. P. Pitaevskii, and S. Stringari, Phys. Rev. Lett. 116, 160402 (2016)], the notion of magnetic solitons has been introduced. Here we examine a variant of this concept in the form of vector dark-antidark solitary waves in multicomponent Bose-Einstein condensates (BECs). We first provide concrete experimental evidence for such states in an atomic BEC and subsequently illustrate the broader concept of these states, which are based on the interplay between miscibility and intercomponent repulsion. Armed with this more general conceptual framework, we expand the notion of such states to higher dimensions presenting the possibility of both vortex-antidark states and ring-antidark-ring (dark soliton) states. We perform numerical continuation studies, investigate the existence of these states, and examine their stability using the method of Bogoliubov–de Gennes analysis. Dark-antidark and vortex-antidark states are found to be stable for broad parametric regimes. In the case of ring dark solitons, where the single-component ring state is known to be unstable, the vector entity appears to bear a progressively more and more stabilizing role as the intercomponent coupling is increased.

Bragg diffraction has been used in atom interferometers because it allows signal enhancement through multiphoton momentum transfer and suppression of systematics by not changing the internal state of atoms. Its multiport nature, however, can lead to parasitic interferometers, allows for intensity-dependent phase shifts in the primary interferometers, and distorts the ellipses used for phase extraction. We study and suppress these unwanted effects. Specifically, phase extraction by ellipse fitting and the resulting systematic phase shifts are calculated by Monte Carlo simulations. Phase shifts arising from the thermal motion of the atoms are controlled by spatial selection of atoms and an appropriate choice of Bragg intensity. In these simulations, we found that Gaussian Bragg pulse shapes yield the smallest systematic shifts. Parasitic interferometers are suppressed by a “magic” Bragg pulse duration. The sensitivity of the apparatus was improved by the addition of ac Stark shift compensation, which permits direct experimental study of sub-part-per-billion (ppb) systematics. This upgrade allows for a $310\hslash k$ momentum transfer, giving 6.6 Mrad measured in a Ramsey-Bordé interferometer.

We consider a mechanical resonator made of diamond, which contains a nitrogen-vacancy center (NV center) locating at the end of the oscillator. A second-order magnetic gradient is applied and induces coupling between mechanical modes and the NV center. By applying a proper external magnetic field, the energy difference between NV center electron spin levels can be tuned to match the energy difference between two mechanical modes $a$ and $b$. A laser is used for continuously initializing the NV center electron spin. The mode $a$ with lower frequency is driven by a thermal bath. We find that the temperature of the mode $b$ is significantly cooled when the heating bath temperature is increased. We discuss the conditions that quantum regime cooling requires and confirm the results by numerical simulation. Finally we give the intuitive physical explanation of this unusual effect.

In characterization of quantum systems, adapting measurement settings based on data while it is collected can generally outperform in efficiency conventional measurements that are carried out independently of data. The existing methods for choosing measurement settings adaptively assume that the model, or the number of unknown parameters, is known. We introduce simultaneous adaptive model selection and parameter estimation. We apply our technique for characterization of a superconducting qubit and a bath of incoherent two-level systems, a leading decoherence mechanism in the state-of-the-art superconducting qubits.