We demonstrate a simple and robust technique for improving the signal-to-noise ratio (SNR) of heterodyne beats between a low-power frequency comb and a continuous-wave (cw) laser. By exploiting the pulsed nature of the comb, a SNR better than the shot-noise limit for the beating of a single mode can be obtained. This technique relies on electrical gating of the beat signal to remove as much shot noise as possible from the measurement. This technique can also be understood as efficiently using the beating of the cw laser with many comb modes simultaneously. We demonstrate a simple experimental setup which achieves a 50-fold improvement in the shot-noise limited SNR of a conventional heterodyne setup and is in long-term agreement with a conventional heterodyne setup, at least to better than 1 part in ${10}^{17}$. The resulting analog signal is completely compatible with existing heterodyne setups and can be used as an upgrade to an existing setup which would benefit from the SNR improvement.

We discuss fluctuations in a dilute two-dimensional Bose-condensed dipolar gas, which has a roton-maxon character of the excitation spectrum. We calculate the density-density correlation function, fluctuation corrections to the chemical potential, compressibility, and normal (superfluid) fraction. It is shown that the presence of the roton strongly enhances fluctuations of the density, and we establish the validity criterion of the Bogoliubov approach. At $T=0$ the condensate depletion becomes significant if the roton minimum is sufficiently close to 0. At finite temperatures exceeding the roton energy, the effect of thermal fluctuations is stronger and it may lead to a large normal fraction of the gas and compressibility.

Quantum channel teleporting one party of a bipartite entangled state is considered. It is assumed that resource is coupled to thermal decohering (dissipative and/or pure dephasing) environment. We investigate both the fidelity of the channel and its ability to swap the initial correlation to the final state of the system in which subsystems did not interact directly. We qualify the correlations by negativity and quantum discord. We show that the correlation loss defined as a difference between values of the correlation measure for the teleported state and the resource vanishes for certain choice of parameters at certain time instants. Moreover, the perfect transmission of entanglement takes place in the case when the resource qubits have the same energy splitting.

We investigate the schemes of quantum network teleportation for quantum information distribution and concentration, which are essential in quantum cloud computation and the quantum internet. In those schemes, with the prior shared entanglement in the quantum network, the cloud can send simultaneously identical unknown quantum states to clients located in different places. Additionally, with the same entanglement resource, these clients can concentrate their states to the cloud to reconstruct the original state. The number of clients can be beyond the number of identical quantum states intentionally being sent; this quantum network teleportation can make sure that the quantum states' distribution is optimal in the sense that the fidelity achieves the upper bound. These schemes facilitate the quantum information distribution and concentration in quantum networks in the framework of quantum cloud computation. Potential applications in time synchronization and the photonic implementation of those schemes are discussed.

We report a study on the ultrafast coherent population oscillations (UCPOs) in two-level atoms induced by a frequency-modulated few-cycle optical pulse train. The phenomenon of UCPOs is investigated by numerically solving the optical Bloch equations beyond the rotating wave approximation. We demonstrate that the quantum state of the atoms and the frequency of the UCPOs may be controlled by controlling the number of pulses in the pulse trains and the pulse repetition time, respectively. Moreover, the robustness of the population inversion against the variation of the laser pulse parameters is also investigated. The proposed scheme may be useful for the creation of atoms in selected quantum states for desired time duration and may have potential applications in ultrafast optical switching. The scheme may also be used to measure pulse repetition rate.

We study the motion of an undamped single-ion harmonic oscillator, resonantly driven with a pulsed radiation pressure force. We demonstrate that a barium ion, initially cooled to the Doppler limit, quickly phase locks to the drive and builds up coherent oscillations above the thermal distribution after scattering of order 100 photons. In our experiment, this seeded motion is subsequently amplified and then analyzed by Doppler velocimetry. Since the coherent oscillation is conditional upon the internal quantum state of the ion, this motional excitation technique could be useful in atomic or molecular single-ion spectroscopy experiments, providing a simple protocol for state readout of nonfluorescing ions with partially closed-cycle transitions.

Unsharp positive operator-valued measurements allow a variety of measurement applications which minimally disrupt the state of the quantum system. Experimental schemes are proposed for implementing unsharp measurements on the qubit levels of a trapped ion. The schemes rely on introducing weak entanglement between the state of a target ion and that of an auxiliary ion, using standard ion-trap quantum logic operations, and then realizing an unsharp measurement through projective measurement on the auxiliary atom. We analyze common sources of error and their effect on different applications of unsharp measurements.

In this paper, we study the tunneling and traversal of ultracold two-level atoms through the potential induced by the vacuum cavity mode. In particular, we discuss the effects of off-resonant interaction between the cavity mode and atomic transition on tunneling time of the ultracold atoms through a high-$Q$ mazer cavity. The phase time which may be considered as an appropriate measure of the time required for the atom to cross the cavity, exhibits some interesting features in the presence of off-resonant interaction. For example, switching between the sub and superclassical behaviors in phase time occurs for proper choice of detuning. Similarly, negative phase time appears for the transmission of atoms in both excited and ground states in the presence of off-resonant interaction.

Quantum walks are very useful tools in designing quantum algorithms. Amplitude amplification is a key technique to increase the success probability of a quantum-walk-based algorithm, and it is quadratically faster than classical probabilistic amplification. However, amplitude amplification only applies to quantum walks with one-shot hitting time, where no measurements except a final one are performed, and not to quantum walks with concurrent hitting time, where measurements happen or absorbing boundaries exist at each step. In this paper, we propose a procedure to modify quantum walks with concurrent hitting time by removing measurements from them. This procedure enables us to use amplitude amplification to design algorithms based on the modified quantum walks which are faster than those based on the original walks with a concurrent hitting time and more robust than those based on the corresponding walks with a one-shot hitting time.

In this work, inspired by the study of semidefinite programming for block-diagonalizing matrix *-algebras, we propose an algorithm that can find the algebraic structure of decoherence-free subspaces (DFS's) for a given noisy quantum channel. We prove that this algorithm will work for all cases with probability 1, and it is more efficient than the algorithm proposed by J. A. Holbrook, D. W. Kribs, and R. Laflamme, [Quantum. Inf. Proc. 80, 381 (2003)]. In fact, our results reveal that this previous algorithm only works for special cases. As an application, we discuss how this method can be applied to increase the efficiency of an optimization procedure for finding an approximate DFS.

In this paper, we investigate a high-dimensional quantum state transfer protocol. An arbitrary unknown high-dimensional state can be transferred with high fidelity between two remote registers through an $XX$ coupling spin chain of arbitrary length. The evolution of the state transfer is determined by the natural dynamics of the chain without external modulation and coupling strength engineering. As a consequence, entanglement distribution with a high efficiency can be achieved. Also the strong field and high spin quantum number can partly counteract the effect of finite temperature to ensure the high fidelity of the protocol when the quantum data bus is in the thermal equilibrium state under an external magnetic field.

We critically analyze the commonly maintained statement that entanglement is necessary to beat the shot-noise limit or standard quantum limit (SQL) in the sensitivity with which certain parameters can be measured in interferometric experiments. We consider in detail three different physical realizations of a beam splitter and a Mach-Zehnder interferometer using photons, massive bosons, and distinguishable spins or qubits. We study several input states and the corresponding definitions of entanglement. We show that mode entanglement is not required for photons or massive bosons for beating the SQL. In particular, with a fluctuating number of two-mode bosons, the shot-noise limit can be beaten by nonentangled bosonic states with all bosons in one mode. As a consequence, the trivially present entanglement due to symmetrization that appears when writing bosonic states in terms of individual particles is a useful resource for precision measurements.

We measure hyperfine structure in the metastable ${{}^{3}P}_2$ state of ${}^{173}$Yb and extract the nuclear magnetic octupole moment. We populate the state using dipole-allowed transitions through the ${{}^{3}P}_1$ and ${{}^{3}S}_1$ states. We measure frequencies of hyperfine transitions of the ${{}^{3}P}_2\to {{}^{3}S}_1$ line at 770 nm using a Rb-stabilized ring cavity resonator with a precision of 200 kHz. Second-order corrections due to perturbations from the nearby ${{}^{3}P}_1$ and ${{}^{1}P}_1$ states are below 30 kHz. We obtain the hyperfine coefficients as $A=-742.11\left(2\right)$ MHz and $B=1339.2\left(2\right)$ MHz, which represent a two orders-of-magnitude improvement in precision, and $C=0.54\left(2\right)$ MHz. From atomic structure calculations, we obtain the nuclear moments quadrupole $Q=2.46\left(12\right)$ b and octupole $\Omega =-34.4\left(21\right)$ $\mathrm{b}\times {\mu }_N$.

Cross sections for the formation of positronium (Ps) in collisions of positrons with methanol and ethanol molecules, $Q_{\mathrm{Ps}}$, have been measured for positrons of incident energies from 3 to 40 eV. The threshold energies for Ps formation were found to be 4.0 ± 0.2 eV and 3.5 ± 0.5 eV, respectively. $Q_{\mathrm{Ps}}$ for the two molecules are very similar in size and energy dependence, suggesting that Ps formation is associated preferentially with the OH group, whereas ($Q_{\mathrm{tot}}$–$Q_{\mathrm{Ps}}$), where $Q_{\mathrm{tot}}$ is the total scattering cross section, is related more to the physical size of the molecules.

Reactive or elastic two-body collisions in an ultracold gas are affected by quantum statistics. In this paper, we study ensemble-averaged collision rates for a two-species fermionic gas. The two species may have different masses, densities, and temperatures. We investigate how averaged collision rates are affected by the presence of Fermi spheres in the initial states; Pauli blocking of final states is not considered. It is shown that, independently on the details of the collision, Fermi-averaged collision rates deviate from Boltzmann-averaged ones, particularly for a gas with strong imbalance of masses or densities.

A collinear scattering of a structured particle from a hard wall is studied with consideration of vibrational transitions initiated by the collision. It is shown that this problem can be solved analytically in the framework of the source-function method. With the use of the continuum discretization technique we are able to take into account both discrete and continuum states. No approximations of the interatomic potential is required. We illustrate our approach for the case of a hydrogen molecule bound by the realistic Morse potential.

We provide detailed analysis of strong field ionization of degenerate valence $p$ orbitals by circularly polarized fields. Our analytical approach is conceptually equivalent to the Perelomov, Popov, and Terent'ev (PPT) theory and is virtually exact for short-range potentials. After benchmarking our results against the PPT theory for $s$ orbitals, we obtain the results for $p$ orbitals. We also show that, as long as the dipole approximation is valid, both the PPT method and our results are gauge invariant, in contrast with widely used strong field approximation (SFA). Our main result, which has already been briefly outlined in [I. Barth and O. Smirnova, Phys. Rev. A 84, 063415 (2011)], is that strong field ionization preferentially removes electrons counter-rotating to the circularly polarized laser field. The result is illustrated using the example of Kr atom. Strong, up to one order of magnitude, sensitivity of strong field ionization to the sense of electron rotation in the initial state is one of the key signatures of nonadiabatic regime of strong field ionization.

A minimum at $\sim$39 eV is observed in the high-harmonic-generation spectra of N${}_2$ for several laser intensities and frequencies. This minimum appears to be invariant for different molecular orientations. We reproduce this minimum for a set of laser parameters and orientations in time-dependent density-functional-theory calculations, which also render orientation-dependent maxima at 23–26 eV. Photon energies of these maxima overlap with ionization potentials of excited states observed in photoelectron spectra. Time profile analysis shows that these maxima are caused by resonance-enhanced multiphoton excitation. We propose a four-step mechanism, in which an additional excitation step is added to the well-accepted three-step model. Excitation to a linear combination of Rydberg states $c_4^{\prime }{}^1{\Sigma }_u^{+}$ and $c_3{{}^1\Pi }_u$ gives rise to an orientation-invariant minimum analogous to the “Cooper minimum” in argon. When the molecular axis is parallel to the polarization direction of the field, a radial node goes through the atomic centers, and hence the Cooper-like minimum coincides with the minimum predicted by a modified two-center interference model that considers the de-excitation of the ion and symmetry of the Rydberg orbital.

The laser-induced decay of ethylene oxide polymer ions of megadalton size has been studied in the multiphoton IR excitation regime, with fragmentation products of individual ions being monitored over long times by a trapping device. The experiment reveals several fragmentation pathways having distinct signatures at the single-molecule level that would not be accessible from studies based on statistically averaged reaction rates only. The observations are supported by dedicated molecular simulations based on a coarse-grained model, which further highlight the role played by continuous heating in such out-of-equilibrium conditions. In particular, both experiment and modeling indicate that the dissociation kinetics depends nonlinearly on heating rate.

We study interference effects that appear in the cross section due to detection of two indistinguishable electrons in a photoelectron–Auger-electron pair (intrachannel exchange interference), or due to detection of two indistinguishable photoelectron-Auger electron pairs that share the final state but are emitted via two different intermediate states (interchannel exchange interference). For photoinduced Kr $M_{4,5}$–$N_1{N}_{23}$ (Xe $N$${}_{4,5}$–$O_1{O}_{23}$) Auger decay, the intrachannel exchange interference is “switched on” at 131.3 eV (87.0 eV) and 133.8 eV (91.0 eV) photon impact energy for the $M_5$ ($N_5$) and $M_4$ ($N_4$) holes, respectively. The interchannel exchange interference is expected to appear at 132.6 eV (89.0 eV) when the photoelectron ejected from the $M_4$ ($N_4$) subshell is emitted with the same energy as the Auger electron emitted in the decay of the $M_5$ ($N_5$) hole, and vice versa, the energy of the photoelectron ejected from the $M_5$ ($N_5$) subshell equals the energy of the Auger electron emitted in the decay of the $M_4$ ($N_5$) hole. We use the approach of Vegh and Macek [Phys. Rev. A 50, 4031 (1994)], and Vegh [Phys. Rev. A 50, 4036 (1994)] to model these phenomena in a unified manner and examine their effects in an angle-resolved and angle-integrated coincidence spectra considering also the postcollision interaction (PCI). The previous and the present angle-resolved experimental results, together with the data obtained by a magnetic bottle time-of-flight spectrometer, indicate that in this particular case the exchange interference effects in the angle-integrated cross sections are considerably weaker than in the angle-resolved $(\gamma ,2e)$ cross sections.