We describe how microwave spectroscopy of cold fermions in quasi-1D traps can be used to detect, manipulate, and entangle exotic nonlocal qubits associated with “Majorana” edge modes. We present different approaches to generate the $p$-wave superfluidity which is responsible for these topological zero-energy edge modes. We find that the edge modes have clear signatures in the microwave spectrum and that the line shape distinguishes between the degenerate states of a qubit encoded in these edge modes. Moreover, the microwaves rotate the system in its degenerate ground-state manifold. We use these rotations to implement a set of universal quantum gates, allowing the system to be used as a universal quantum computer.

We discuss how large amounts of steady-state quantum squeezing (beyond 3 dB) of a mechanical resonator can be obtained by driving an optomechanical cavity with two control lasers with differing amplitudes. The scheme does not rely on any explicit measurement or feedback, nor does it simply involve a modulation of an optical spring constant. Instead, it uses a dissipative mechanism with the driven cavity acting as an engineered reservoir. It can equivalently be viewed as a coherent feedback process, obtained by minimally perturbing the quantum nondemolition measurement of a single mechanical quadrature. This shows that in general the concepts of coherent feedback schemes and reservoir engineering are closely related. We analyze how to optimize the scheme, how the squeezing scales with system parameters, and how it may be directly detected from the cavity output. Our scheme is extremely general, and could also be implemented with, e.g., superconducting circuits.

An imaginary-time nonuniform mesh method for diagonalizing multidimensional quantum Hamiltonians is proposed and used to find the first 50 eigenstates and energies of up to $D=5$ strongly interacting spinless quantum Lennard-Jones particles trapped in a one-dimensional harmonic potential. We show that the use of tailored grids allows exploitation of the symmetries of the system (in our case the $D!$ degeneracy derived from all possible permutations of distinguishable particles), reducing drastically the computational effort needed to diagonalize the Hamiltonian. This leads to a favorable scaling with dimensionality, requiring for the five-dimensional system four orders of magnitude fewer grid points than the equivalent regular grid. Solutions to both bosonic and fermionic counterparts of this strongly interacting system are constructed, the bosonic case clustering as a Tonks-Girardeau crystal exhibiting the phenomenon of fermionization. The numerically exact excited states are used to describe the melting of this crystal at finite temperature.

Measurement-device-independent quantum key distribution (MDIQKD) is proposed to be secure against any possible detection attacks. The security of the original proposal relies on the assumption that the legitimate users can fully characterize the encoding systems including sources. Here, we propose a MDIQKD protocol where we allow uncharacterized encoding systems as long as qubit sources are used. A security proof of the MDIQKD protocol is presented that does not need the knowledge of the encoding states. Simulation results show that the scheme is practical.

This paper reports on the excitation of surface plasmons on gold-coated nanofiber tips by side illumination with a laser beam and the coupling of the surface plasmons to the optical fiber. The measurements show a strong dependence of the coupling efficiency on the incidence angle with a maximum coupling efficiency on the order of a few percent. Moreover, the fiber tip was used as a scanning probe device for measuring the beam waist of a focused laser beam. This work is motivated by the goal to use such plasmonic nanofiber tips in quantum optics experiments with cold atoms.

A model for off-resonant microscopic cascading of (hyper)polarizabilities is developed using a self-consistent field approach to study mesoscopic systems of nonlinear polarizable atoms and molecules. We find enhancements in the higher-order susceptibilities resulting from geometrical and boundary orientation effects. We include an example of the dependence on excitation beam cross sectional structure and a simplified derivation of the microscopic cascading of the nonlinear-optical response in guest-host systems.

We study an ensemble of cold atoms driven to the four-level double-$\Lambda$ configuration and trapped in the one-dimensional optical lattices for achieving dynamically controlled photonic band gaps (PBGs) via the balanced four-wave mixing interaction. Numerical results show that a pair of PBGs characterized by high platforms of probe reflectivities and reduced density of photonic states are established, simultaneously, on both probe transitions in one symmetric driving scheme. Such two-color PBGs can be easily manipulated in position and width by modulating, e.g., the balanced Rabi frequencies of two coupling fields and the Gaussian widths of atomic density distributions, to change the equal probe detunings fulfilling both Bragg conditions. Each two-color PBG as predicted in the coherent atomic lattices may be explored to attain robust light entanglement, even for single-probe photons, via an efficient frequency conversion during the nonlinearly correlated reflection.

We investigate the one-photon double ionization of H${}_2$ by numerically solving the time-dependent Schrödinger equation (TDSE) in the prolate spheroidal coordinates. The triple differential cross sections (TDCS) are extracted from the differential probability by three different methods. The dependence of the TDCS on the orientation of H${}_2$ is analyzed by a configuration interference model. The model perfectly reproduces the TDSE results. The obvious interference patterns are observed when the angle between the laser polarization and the molecular axis is properly small. In addition, we revisit the heliumlike model by including the phase difference, which gives a much better fitting of the TDSE results than the previous model without the phase difference taken into account.

We present a theoretical study of vibrationally resolved and unresolved molecular-frame photoelectron angular distributions (MFPADs) resulting from $K$-shell photoionization of N${}_2$, CO, C${}_2$H${}_2$, NH${}_3$, CH${}_4$, CF${}_4$, BF${}_3$, and SF${}_6$ in the range of photoelectron energies 0–500 eV. We show that the MFPADs of NH${}_3$ and CH${}_4$, averaged over the polarization direction, image the molecular geometry at very low energies but also at selected higher energies. For all other molecules, the MFPADs do not image the system's geometry. However, for molecules containing heavy atoms in the periphery, CF${}_4$, BF${}_3$, and SF${}_6$, and for N${}_2$ and CO, the polarization-averaged MFPADs reflect the partial accumulation of the photoelectron density in the region surrounded by the peripheral atoms. For energies at which this accumulation occurs, the MFPADs encode information about the three dimensional arrangement of the system. In general, the polarization averaged MFPADs remain quite anisotropic even at photoelectron energies as high as 500 eV.

We study the extracavity quantum vacuum radiation generated by a sudden switch on and off of the vacuum Rabi frequency of a single two-level system coupled to a single cavity mode in the strong and ultrastrong coupling regime. Dissipation is described by including the qubit-resonator coupling in the derivation of the master equation. The output stream of photons and their second-order correlation functions are calculated by exploiting a recently developed input-output formulation of resonators valid for arbitrary light-matter couplings. The obtained output photon correlations provide direct information on the quantum correlations among the virtual excitations of the dressed vacuum.

We consider the composability of quantum channels from a limited amount of entanglement via local oper-ations and classical communication (LOCC). We show that any $k$-partial-entanglement-breaking channel can be composed from an entangled state with a Schmidt number of $k$ via one-way LOCC. From the entanglement-assisted construction we can reach an alternative definition of partial-entanglement-breaking channels.

We report on a spectroscopic method to coherently drive a selective transition between two levels in a ladder system of equally spaced energy levels. A typical example of such a ladder system is a set of Zeeman sublevels in the same hyperfine level of a ground-state alkali-metal atom. The scheme is based on a stimulated Raman transition with a Zeeman sublevel belonging to the other hyperfine level as an intermediate state. We identify quantization conditions on a single-photon detuning and Rabi frequencies for either a $\pi /2$ or a $\pi$ transition between adjacent Zeeman sublevels without populating the intermediate state. We experimentally demonstrate the scheme using lithium atoms in an optical trap. Application of the scheme includes manipulating a pair of Zeeman sublevels as a quantum bit or probing Zeeman coherence established by coherent population trapping.

In a previous paper [J. S. Briggs and A. Eisfeld, Phys. Rev. A 85, 052111 (2012)] we showed that the time development of the complex amplitudes of $N$ coupled quantum states can be mapped by the time development of positions and velocities of $N$ coupled classical oscillators. Here we examine to what extent this mapping can be realized to simulate the “quantum,” properties of entanglement and qubit manipulation. By working through specific examples, e.g., of quantum gate operation, we seek to illuminate quantum and classical differences which hitherto have been treated more mathematically. In addition, we show that important quantum coupled phenomena, such as the Landau-Zener transition and the occurrence of Fano resonances can be simulated by classical oscillators.

We develop a dynamical symmetry approach to path integrals for general interacting quantum spin systems. The time-ordered exponential obtained after the Hubbard-Stratonovich transformation can be disentangled into the product of a finite number of the usual exponentials. This procedure leads to a set of stochastic differential equations on the group manifold, which can be further formulated in terms of the supersymmetric effective action. This action has the form of the Witten topological field theory in the continuum limit. As a consequence, we show how it can be used to obtain the exact results for a specific quantum many-body system which can be otherwise solved only by the Bethe ansatz. This represents an example of a many-body system treated exactly using the path-integral formulation. Moreover, our method can deal with time-dependent parameters, which we demonstrate explicitly.

A local exclusion principle is observed for identical particles obeying intermediate and fractional exchange statistics in one and two dimensions, leading to bounds for the kinetic energy in terms of the density. This has implications for models of Lieb-Liniger and Calogero-Sutherland type and implies a nontrivial lower bound for the energy of the anyon gas whenever the statistics parameter is an odd numerator fraction. We discuss whether this is actually a necessary requirement.

Device-independent quantum key distribution (DIQKD) guarantees the security of a shared key without any assumptions on the apparatus used, provided that the observed data violates a Bell inequality. Such a violation is challenging experimentally due to photodetection inefficiencies and, especially, channel losses. Here we describe a realistic DIQKD implementation based on the interaction between light and spins stored in cavities, which allows a heralded mapping of polarization entanglement of light onto the spin. The spin state can subsequently be measured with near unit efficiency. Heralding alleviates the effect of channel loss, and as the protocol allows for local heralding, the spin decay is not affected by the communication time between the parties, making a Bell inequality violation over an arbitrary distance possible. We compute the achievable key rates for realistic parameter values and find significant improvements over entirely optical implementations in the key bits per source use and the attainable distance.

We propose a scheme for distillation of free bipartite entanglement from bipartite bound entangled states. The crucial element of our scheme is an ancillary system that is coupled to the initial bound entangled state via appropriate weak measurements. We show that in this protocol free entanglement can be always generated with nonzero probability by using a single copy of the bound entangled state. We also derive a lower bound on the entanglement cost of the protocol and conclude that, on average, applying weaker measurements results in relatively higher values of free entanglement as well as lower costs.

A photonic process called quantum-state joining has been recently experimentally demonstrated [Vitelli et al., Nat. Photonics 7, 521 (2013)] that corresponds to the transfer of the internal two-dimensional quantum states of two input photons, i.e., two photonic qubits, into the four-dimensional quantum state of a single photon, i.e., a photonic ququart. A scheme for the inverse process, namely, quantum state splitting, has also been proposed. Both processes can be iterated in a cascaded layout, to obtain the joining and/or splitting of more than two qubits, thus leading to a general scheme for varying the number of photons in the system while preserving its total quantum state, or quantum information content. Here, we revisit these processes from a theoretical point of view. After casting the theory of the joining and splitting processes in the more general occupation number notation, we introduce some modified schemes that are in principle unitary [not considering the implementation of the controlled-not (cnot) gates] and do not require projection and feed-forward steps. This can be particularly important in the quantum state splitting case, to obtain a scheme that does not rely on postselection. Moreover, we formally prove that the quantum joining of two photon states with linear optics requires the use of at least one ancilla photon. This is somewhat unexpected, given that the demonstrated joining scheme involves the sequential application of two cnot quantum gates, for which a linear optical scheme with just two photons and postselection is known to exist. Finally we explore the relationship between the joining scheme and the generation of clusters of multiparticle entangled states involving more than one qubit per particle.

A method of evaluating chemical shifts of x-ray emission lines for period four and heavier elements is developed. This method is based on the relativistic pseudopotential model and one-center restoration approach [Int. J. Quantum Chem. 104, 223 (2005)] to recover a proper electronic structure in heavy-atom cores after the pseudopotential simulation of chemical compounds. The approximations of instantaneous transition and frozen core are presently applied to derive an expression for chemical shift as a difference between mean values of certain effective operator. The method allows one to avoid evaluation of small quantities (chemical shifts $\sim 0.01$–1 eV) as differences of very large values (transition energies $\sim 1$–100 keV in various compounds). The results of our calculations of chemical shifts for the $K{\alpha }_1$, $K{\alpha }_2$, and $L$ transitions of group-14 metal cations with respect to neutral atoms are presented. Calculations of $K{\alpha }_1$-line chemical shifts for the Pb core transitions in PbO and PbF${}_2$ with respect to those in the Pb atom are also performed and discussed. The accuracy of approximations used is estimated and the quality of the calculations is analyzed.

We present a characterization technique for atomic vapor cells, combining time-domain measurements with absorption imaging to obtain spatially resolved information on decay times, atomic diffusion, and coherent dynamics. The technique is used to characterize a 5-mm-diameter, 2-mm-thick microfabricated Rb vapor cell, with N${}_2$ buffer gas, placed inside a microwave cavity. Time-domain Franzen and Ramsey measurements are used to produce high-resolution images of the population ($T_1$) and coherence ($T_2$) lifetimes in the cell, while Rabi measurements yield images of the ${\sigma }_{-}$, $\pi$, and ${\sigma }_{+}$ components of the applied microwave magnetic field. For a cell temperature of $90{}^{\circ }$C, the $T_1$ times across the cell center are found to be a roughly uniform $265\mu$s, while the $T_2$ times peak at around $350\mu$s. We observe a “skin” of reduced $T_1$ and $T_2$ times around the edge of the cell due to the depolarization of Rb after collisions with the silicon cell walls. Our observations suggest that these collisions are far from being 100$\%$ depolarizing, consistent with earlier observations made with Na and glass walls. Images of the microwave magnetic field reveal regions of optimal field homogeneity, and thus coherence. Our technique is useful for vapor cell characterization in atomic clocks, atomic sensors, and quantum information experiments.