We study the effect of Efimov physics (in the few-body sector) on the spectral response of the Bose polaron, a many-body system consisting of an impurity immersed in a bath of bosonic atoms. We find that the Efimov correlation can be greatly enhanced by increasing the mass ratio between the bosons and the impurity, which results in visible signatures in the rf spectrum of the polaron. Using a diagrammatic approach up to the third-order virial expansion, we show how the mass imbalance and the enhanced three-body effect modify the line shape and linewidth of the polaron spectrum. Moreover, we study the effect of a finite boson-boson interaction on the spectrum. Taking the realistic system of Li impurities immersed in Cs bosons with a positive Cs-Cs scattering length, we find a visible Efimov branch, which is associated with the second lowest Efimov trimer, in the polaron spectrum. In particular, by adjusting the boson density the Efimov branch can greatly hybridize with the attractive polaron branch, leading to spectrum broadening near their avoided level crossing. Our results can be directly probed in the cold atoms experiments on Li-Cs and Li-Rb Bose polarons.

Nanoparticle assisted high-order-harmonic generation by low-intensity ultrashort laser pulses in hydrogen atomic gas is studied. This work is based on surface plasmon-polariton coupling in metal-insulator-metal structures. The necessary laser intensity is provided by enhancement of the incident laser power in the vicinity of bowtie nanoparticles installed on an insulator-metal structure. The inhomogeneous electric field distribution in the Au nanobowtie gap region is investigated. Simulations show that the insulator layer installed on the Au metal film that supports the plasmon-polariton interactions has a dramatic effect on the field enhancement factor. High-order-harmonic generation cutoffs for different arrangements are calculated and results show that the metal-insulator-metal structure is an excellent device for high-order-harmonic generation purposes. Also, the harmonic cutoff order is extended to more than 170, which is a considerable value and will be an efficient source for extreme ultraviolet radiation.

Trapping cold, chemically important molecules with electromagnetic fields is a useful technique to study small molecules and their interactions. Traps provide long interaction times, which are needed to precisely examine these low-density molecular samples. However, the trapping fields lead to nonuniform molecular density distributions in these systems. Therefore, it is important to be able to experimentally characterize the spatial density distribution in the trap. Ionizing molecules at different locations in the trap using resonance-enhanced multiphoton ionization (REMPI) and detecting the resulting ions can be used to probe the density distribution even at the low density present in these experiments because of the extremely high efficiency of detection. Until recently, one of the most chemically important molecules, OH, did not have a convenient REMPI scheme identified. Here, we use a newly developed $1+1^{\prime }$ REMPI scheme to detect trapped cold OH molecules. We use this capability to measure the trap dynamics of the central density of the cloud and the density distribution. These types of measurements can be used to optimize loading of molecules into traps, as well as to help characterize the energy distribution, which is critical knowledge for interpreting molecular collision experiments.

We measure photoelectron momentum spectra from above-threshold ionization of Xe atoms in a linearly polarized two-color laser field with comparable intensities. The spectra show distinct forward-backward asymmetries along the laser polarization direction, which depend sensitively on the electron emission angle and the relative phase between the two-color components. We find that with increasing the emission angle relative to the laser polarization direction, there is a phase shift in the asymmetry curve as a function of the two-color relative phase. Using a semiclassical model, we disentangle the relative contributions of the nonscattering and the rescattering trajectories on the photoelectron angular distributions of the above-threshold ionization. We show that the angular-dependent asymmetry comes from different contributions of the nonscattering and rescattering trajectories at different emission angles. By tuning the relative phase of the parallel two-color laser fields, the relative contributions of the nonscattering and the rescattering trajectories can be precisely controlled.

We investigate a procedure to generate turbulence in a trapped Bose-Einstein condensate which takes advantage of the decay of multicharged vortices to reduce surface oscillations. We show that the resulting singly charged vortices twist around each other, intertwined in the shape of helical Kelvin waves, which collide and undergo vortex reconnections, creating a disordered vortex state. By examining the velocity statistics, the energy spectrum, the correlation functions, and the temporal decay and comparing these properties with the properties of classical turbulence and observations in superfluid helium, we conclude that this disordered vortex state can be identified with the Vinen regime of turbulence which has been discovered in the context of superfluid helium.

The wave-front aberration of Raman beams induces phase noise in atom interferometers and thus limits the performance of atom interferometers. We theoretically analyze the wave-front-aberration phase noise in weak-equivalence-principle tests using dual-species atom interferometers, and propose an expansion-rate-selection method to suppress the wave-front-aberration phase noise in both isotopic and nonisotopic atom interferometers. The suppression ratio is about 10 at the expense of half the number of atoms. The simulation based on appropriate experimental parameters shows that the wave-front aberration caused standard deviations to the Eötvös coefficient in weak-equivalence-principle tests using ${}^{85}\mathrm{Rb}\text{−}{}^{87}\mathrm{Rb}$ and ${}^{41}\mathrm{K}\text{−}{}^{87}\mathrm{Rb}$ are $1.3\times {10}^{-14}$ and $3.0\times {10}^{-13}$. Better results can be obtained by improving the wave-front quality and utilizing atom cloud with lower temperature. The analysis and suppression of wave-front-aberration phase noise will be useful for the design of high-precision atom interferometer–based weak-equivalence-principle tests both on ground and in space.

We propose a theoretical scheme for implementing nonleaky and accelerated population transfer with a transmon qutrit. For a transmon-regime artificial atom with weak level anharmonicity, the leakage effects on the target population transfer are considerable. Allowed by the level-transition rule, a two-photon resonant interaction can be obtained between the qutrit and microwave drivings. In the regime of large detuning, by adopting the technique of invariant-based inverse engineering, the population transfer with no leakage errors can be sped up drastically when compared with the adiabatic operation. Moreover, the accelerated operation is highly robust against decoherence effects. Thanks to these distinctive advantages, the proposed protocol could offer a promising avenue to optimal quantum operations on transmon artificial atoms.

We present a simulation study of terahertz radiation from a gas driven by two-color laser pulses in a broad range of frequency ratios ${\omega }_1/{\omega }_0$. Our particle-in-cell simulation results show that there are three series with ${\omega }_1/{\omega }_0=2n, n+1/2, n\pm 1/3$ ($n$ is a positive integer) for high-efficiency and stable radiation generation. The radiation strength basically decreases with the increasing ${\omega }_1$ and scales linearly with the laser wavelength. These rules are broken when ${\omega }_1/{\omega }_0<1$ and much stronger radiation may be generated at any ${\omega }_1/{\omega }_0$. These results can be explained with a model based on gas ionization by two linear-superposition laser fields, rather than a multiwave mixing model.

By using unbiased continuous-space quantum Monte Carlo simulations, we investigate the ground-state properties of a one-dimensional repulsive Fermi gas subjected to a commensurate periodic optical lattice (OL) of arbitrary intensity. The equation of state and the magnetic structure factor are determined as a function of the interaction strength and of the OL intensity. In the weak OL limit, Yang's theory for the energy of a homogeneous Fermi gas [C.-N. Yang, Phys. Rev. Lett. 19, 1312 (1967)] is recovered. In the opposite limit (deep OL), we analyze the convergence to the Lieb-Wu theory for the Hubbard model [E. H. Lieb and F. Y. Wu, Phys. Rev. Lett. 20, 1445 (1968)], comparing two approaches to map the continuous space to the discrete-lattice model: The first is based on (noninteracting) Wannier functions and the second effectively takes into account strong-interaction effects within a parabolic approximation of the OL wells. We find that strong antiferromagnetic correlations emerge in deep OLs and also in very shallow OLs if the interaction strength approaches the Tonks-Girardeau limit. In deep OLs we find quantitative agreement with density-matrix renormalization-group calculations for the Hubbard model. The spatial decay of the antiferromagnetic correlations is consistent with quasi-long-range order even in shallow OLs, in agreement with previous theories for the half-filled Hubbard model.

We herein report the experimental demonstration of adaptive quantum state estimation for totally unknown photonic qubits. Similar to our previous study [R. Okamoto et al., Phys. Rev. Lett. 109, 130404 (2012)], the measurement configuration is updated using the results of each photon detection event so that our method does not require prior knowledge of the total number of samples. The experimental results obtained herein demonstrate both strong consistency and asymptotic efficiency through several rigorous statistical tests. Furthermore, we demonstrate that the experimentally obtained distribution of the states estimated using an adaptive quantum state estimation is significantly different from that obtained by conventional state tomography and agrees well with theoretical predictions.

Multiplexing is a strategy to augment the transmission capacity of a communication system. It consists of combining multiple signals over the same data channel and it has been very successful in classical communications. However, the use of enhanced channels has only reached limited practicality in quantum communications (QC) as it requires the manipulation of quantum systems of higher dimensions. Considerable effort is being made towards QC using high-dimensional quantum systems encoded into the transverse momentum of single photons, but so far no approach has been proven to be fully compatible with the existing telecommunication fibers. Here we overcome such a challenge and demonstrate a secure high-dimensional decoy-state quantum key distribution session over a 300-m-long multicore optical fiber. The high-dimensional quantum states are defined in terms of the transverse core modes available for the photon transmission over the fiber, and theoretical analyses show that positive secret key rates can be achieved through metropolitan distances.

Recently, sequential state discrimination, as a quantum-key distribution protocol, has been proposed for multiple receivers. A previous study [J. A. Bergou et al., Phys. Rev. Lett. 111, 100501 (2013)] showed that every receiver could successfully perform a sequential state discrimination of two pure states with identical prior probabilities. In this study, we extend the sequential state discrimination to mixed states with arbitrary prior probability. First, we analytically obtain the condition of the receiver's optimal measurement. In addition, we show that the optimal probability for every receiver to share the mixed state prepared by the sender is not zero. Furthermore, we compare the sequential state discrimination to the strategies of quantum reproducing and quantum broadcasting. We find that there are cases in which, unlike that of the pure state, the sequential state discrimination of mixed states shows a better performance than the other strategies.

In this paper, we report the relativistic Fock space multireference coupled-cluster method for atomic structure calculations. We use the no-pair Dirac-Coulomb-Breit Hamiltonian, together with a finite $B$-spline basis set to expand the large and small components of the Dirac wave function. Our method is applied to calculate ionization energies, reduced matrix elements, lifetimes, and polarizabilities for many states of atomic francium. To evaluate uncertainties of our results and investigate the role of electron correlation effects, we carry out calculations using approximated models at different levels. The quality of our calculations is assessed by comparing with available experimental results, showing a good agreement. In addition, the tune-out wavelengths of the ground state in the range of 340–800 nm, the magic wavelengths for the transition $7s-8s$ in the range of 800–1500 nm and the transition $7s-7p$ in the range of 600–1500 nm are determined by evaluating the dynamic polarizabilities of the $7s, 8s$, and $7p$ states for a linearly polarized light. These tune-out and magic wavelengths may be useful for laser cooling and trapping of the Fr atom, and for related high-precision trapping measurements.

Magnetically tunable Feshbach resonances in ultracold atomic systems are chiefly identified and characterized through time-consuming atom loss spectroscopy. We describe an off-resonant dispersive optical probing technique to rapidly locate Feshbach resonances and demonstrate the method by locating four resonances of ${}^{87}\mathrm{Rb}$, between the $|\mathrm{F}=1,{\mathrm{m}}_{\mathrm{F}}=1\rangle$ and $|\mathrm{F}=2,{\mathrm{m}}_{\mathrm{F}}=0\rangle$ states. Despite the loss features being $\lesssim 0.1$ G wide, we require only 21 experimental runs to explore a magnetic field range $>18$ G, where $1\mathrm{G}={10}^{-4}\mathrm{T}$. The resonances consist of two known $s$-wave features in the vicinity of 9 G and 18 G and two previously unreported $p$-wave features near 5 G and 10 G. We further utilize the dispersive approach to directly characterize the two-body loss dynamics for each Feshbach resonance.

We calculate the kinetic-energy release distributions of fragments produced for electron-impact dissociation of the vibrationally excited molecular hydrogen ion ${{\mathrm{H}}_2}^{+}$ and its isotopologues ${{\mathrm{D}}_2}^{+}$ and ${{\mathrm{T}}_2}^{+}$. Here we apply the adiabatic-nuclei convergent close-coupling method and compare results with several different methods, including the $\delta$ approximation. Results are presented for a number of dissociative excitation transitions and dissociative ionization as a function of the initial vibrational state of the molecule. We confirm that the square root approximation is a good approximation for the adiabatic-nuclei kinetic-energy release cross sections of ${{\mathrm{H}}_2}^{+}$. Agreement with experiment, where available, is good.

We present an analytic solution of bound states in the continuum (BICs) for photons and atoms in a one-dimensional coupled-cavity array. These bound states are formed by two ensembles of two-level atoms confined in separated cavities of the array. We show that in the regime where the photon hopping rate between neighboring cavities is high compared with the collective Rabi frequency the BIC corresponds to a nonradiating collective atomic state in which the two ensembles of atoms are strongly entangled. On the other hand, in the low photon hopping rate regime, the BIC behaves as a quantum cavity in which photons can be trapped between the two ensembles of atoms.

We demonstrate control over the localization of high-lying Rydberg wave packets in argon atoms with phase-locked orthogonally polarized two-color laser fields. With a reaction microscope, we measure ionization signals of high-lying Rydberg states induced by a weak dc field and blackbody radiation as a function of the relative phase between the two-color fields. We find that the dc-field-ionization yield of high-lying Rydberg argon atoms oscillates with the relative two-color phase with a period of $2\pi$ while the photoionization signal by blackbody radiation shows a period of $\pi$. Accompanying simulations show that these observations are a clear signature of the asymmetric localization of electrons recaptured into very elongated (low angular momentum) high-lying Rydberg states after conclusion of the laser pulse. Our findings thus open an effective pathway to control the localization of high-lying Rydberg wave packets.

We construct ($d\times d$)-dimensional bound entangled states, which violate, for any $d>2$, a bipartite Bell inequality introduced in this paper. We conjecture that the proposed class of Bell inequalities acts as a dimension witness for bound entangled states: For any $d>2$ there exists a Bell inequality from this class that can be violated with bound entangled states only if their Hilbert space dimension is at least $d\times d$. Numerics supports this conjecture up to $d=8$.

Decoy-state quantum key distribution (QKD) is a standard tool for long-distance quantum communications. An important issue in this field is processing the decoy-state statistics taking into account statistical fluctuations (or “finite-key effects”). In this work, we propose and analyze an option for decoy statistics processing, which is based on the central limit theorem. We discuss such practical issues as inclusion of the failure probability of the decoy-state statistical estimates in the total failure probability of a QKD protocol and also taking into account the deviations of the binomially distributed random variables used in the estimations from the Gaussian distribution. The results of numerical simulations show that the obtained estimations are quite tight. The proposed technique can be used as a part of post-processing procedures for industrial quantum key distribution systems.