Two ultracold atom clouds, each separately in a dipole-blockade regime, realize a source of entangled atom pairs that can be ejected on demand. Entanglement generation and ejection is due to resonant dipole-dipole interactions, while van der Waals interactions are predominantly responsible for the blockade that ensures the ejection of a single atom per cloud. A source of entangled atoms using these effects can operate with a 10 kHz repetition rate producing ejected atoms with velocities of about $0.5$ m/s. Using spatially resolved Rydberg state coupling and detection, a violation of Bell's inequalities could be measured in our setup.

We calculate the force of a quasicircularly polarized guided light field of a nanofiber on a dielectric spherical particle. We show that the orbital parts of the axial and azimuthal components of the Poynting vector are always positive, while the spin parts can be either positive or negative. We find that, for appropriate values of the size parameter of the particle, the azimuthal component of the force is directed oppositely to the circulation direction of the energy flow around the nanofiber. The occurrence of such a negative azimuthal force indicates that the particle undergoes a negative torque.

Due to their central role in our classical intuition of the physical world and their potential for interacting with the gravitational field, mechanical degrees of freedom are of special interest in testing the nonclassical predictions of quantum theory at ever larger scales. The projection postulate of quantum theory predicts that, for certain types of measurements, continuously measuring a system induces a stochastic collapse of the state of the system toward a random eigenstate. Here we propose an optomechanical scheme to observe this fundamental effect in a vibrational mode of a mechanical membrane. The observation in the scheme is direct (it is not inferred via an a priori assumption of the projection postulate for the mechanical mode) and is made possible through an in situ probe of the mechanical energy variance. In the scheme, quantum theory predicts that a steady state is reached as the measurement-induced collapse is counteracted by dissipation to the unmonitored environment. Numerical simulations show this to result in a monotonic decrease in the time-averaged energy variance as the ratio of continuous measurement strength to dissipation is increased. The measurement strength in the proposed scheme is tunable in situ, and the behavior predicted by the simulations therefore implies a way to verifiably control the time-averaged variance of a mechanical wave function over the course of a single quantum trajectory. The scheme's ability to directly probe the energy variance of the mechanical mode may also enable further investigations of the effects on the mechanical state of coupling the mechanical mode to other quantum systems.

We determine the magnetic dipole $\mu =0.360\left(7\right){\mu }_N$ and the electric quadrupole $Q=3.11\left(6\right)$$e$b moments of the ${}^{229}$Th nucleus by combining our high-precision calculations of the hyperfine constants with measurements reported in Campbell et al. [Phys. Rev. Lett. 106, 223001 (2011)]. We find that the previous value $\mu =0.46\left(4\right){\mu }_N$ [Gerstenkorn et al., J. Phys. (Paris) 35, 483 (1974)] is incorrect by 25%. We report a method for determining the accuracy of theoretical hyperfine constants $B/Q$ and demonstrate that it can be used to extract the electric quadrupole moment $Q$ with a 1%–2% uncertainty for a large number of nuclei. This approach allowed us to identify 40% inconsistencies in measurements of Ra${}^{+}$ hyperfine constants $B$.

We consider pair creation by collision of a couple of counterpropagating electromagnetic pulses with arbitrary frequency ratio, mostly in the context of a setup with collision of optical and coherent hard x-ray (or $\gamma$-ray) pulses. This problem is nonperturbative and in general does not admit exact analytical solutions. We discuss several known approaches according to the ratio of the parameters. Certain regions of the parameter space are not covered by the existing approximations or models. We present a simplified exactly solvable model with one of the pulses being a $\delta$ pulse. This model partially fills the yet unexplored gap in the space of parameters. The shape of the momentum distribution of the created particles in such a model is discussed. Even though the model may not have immediate implications for the forthcoming experiments, it can still provide some hints for better understanding of the fully nonperturbative regime and vacuum instability in QED interactions of extremely strong and short laser pulses.

The inspiration for this theoretical paper comes from recent experiments on a $\mathcal{PT}$-symmetric system of two coupled optical whispering galleries (optical resonators). The optical system can be modeled as a pair of coupled linear oscillators, one with gain and the other with loss. If the coupled oscillators have a balanced loss and gain, the system is described by a Hamiltonian and the energy is conserved. This theoretical model exhibits two $\mathcal{PT}$ transitions depending on the size of the coupling parameter $\epsilon$. For small $\epsilon$, the $\mathcal{PT}$ symmetry is broken and the system is not in equilibrium, but when $\epsilon$ becomes sufficiently large, the system undergoes a transition to an equilibrium phase in which the $\mathcal{PT}$ symmetry is unbroken. For very large $\epsilon$, the system undergoes a second transition and is no longer in equilibrium. The principal result presented here is that the classical and quantized versions of the system exhibit transitions at exactly the same values of $\epsilon$.

We investigate the spectral and dynamical properties of a quantum particle constrained on a ring threaded by a magnetic flux in the presence of a complex (non-Hermitian) potential. For a static magnetic flux, the quantum states of the particle on the ring can be mapped onto the Bloch states of a complex crystal, and magnetic flux tuning enables us to probe the spectral features of the complex crystal, including the appearance of exceptional points. For a time-varying (linearly ramped) magnetic flux, Zener tunneling among energy states is realized owing to the induced electromotive force. Compared to the Hermitian case, striking effects are observed in the non-Hermitian case, such as a highly asymmetric behavior of particle motion when reversing the direction of the magnetic flux and field-induced delayed transparency.

I analyze a metrological strategy for improving the precision of frequency estimation via Ramsey interferometry with strings of atoms in the presence of correlated dephasing. This strategy does not employ entangled states but rather a product state which evolves into a stationary state under the influence of correlated dephasing. It is shown that by using this state an improvement in precision compared to standard Ramsey interferometry can be gained. This improvement is not an improvement in scaling; i.e., the estimation precision has the same scaling with the number of atoms as the standard quantum limit but gains an improvement proportional to the free evolution time in the Ramsey interferometer. Since a stationary state is used, this evolution time can be substantially larger than in standard Ramsey interferometry which is limited by the coherence time of the atoms.

We introduce a measure for the genuinely N-partite (all-party) entanglement of N-qubit states using the trace distance metric and find an algebraic formula for the Greenberger-Horne-Zeilinger (GHZ)-diagonal states. We then use this formula to show how the all-party entanglement of experimentally produced GHZ states of an arbitrary number of qubits may be bounded with only four measurements.

We propose a scheme to quickly create a conditional quantum phase shift and quantum entanglement of two qubits interacting with a common heat bath. Dynamic decoupling is applied in the scheme so that it works even in the regime of strong interaction between the qubits and the environment. Our scheme does not require any direct interaction between the two qubits.

We investigate the influence of environmental noise on spin networks and spin chains. In addition to the common model of an independent bath for each spin in the system we also consider noise with a finite spatial correlation length. We present the emergence of decoherence-free subspaces and different dynamics with increasing correlation length for both dephasing and dissipating environments. This leads to relaxation blocking of one spin by uncoupled surrounding spins. We then consider perfect state transfer through a spin chain in the presence of decoherence and discuss the dependence of the transfer quality on spatial noise correlation length. We identify qualitatively different features for dephasing and dissipative environments in spin-transfer problems.

In quantum error-correcting code (QECC), many quantum operations and measurements are necessary to correct errors in logical qubits. In the stabilizer formalism, which is widely used in QECC, generators $G_i(i=1,2,...)$ consist of multiples of Pauli matrices and perform encoding, decoding, and measurement. In order to maintain encoding states, the stabilizer Hamiltonian $H_{\mathrm{stab}}=-{\sum }_i{G}_i$ is suitable because its ground state corresponds to the code space. On the other hand, Hamiltonians of most solid-state qubits have two-body interactions and show their own dynamics. In addition solid-state qubits are fixed on substrate and qubit-qubit operation is restricted in their neighborhood. The main purpose of this paper is to show how to directly generate the stabilizer Hamiltonian $H_{\mathrm{stab}}$ from conventional two-body Hamiltonians with Ising interaction and $XY$ interaction by applying a pulse control method such as an NMR technique. We show that generation times of $H_{\mathrm{stab}}$ for nine-qubit code, five-qubit code, and Steane code are estimated to be less than 300 ns when typical experimental data of superconducting qubits are used, and sufficient pulse control is assumed. We also show how to prepare encoded states from an initial state $|0,...,0\rangle$. In addition, we discuss an appropriate arrangement of two- or three-dimensional arrayed qubits.

Thanks to the high-speed self-differencing single-photon detector (SD-SPD), the secret key rate of quantum key distribution (QKD), which can, in principle, offer unconditionally secure private communications between two users (Alice and Bob), can exceed 1 Mbit/s. However, the SD-SPD may contain loopholes, which can be exploited by an eavesdropper (Eve) to hack into the unconditional security of the high-speed QKD systems. In this paper, we analyze the fact that the SD-SPD can be remotely controlled by Eve in order to spy on full information without being discovered, then proof-of-principle experiments are demonstrated. Here, we point out that this loophole is introduced directly by the operating principle of the SD-SPD, thus, it cannot be removed, except for the fact that some active countermeasures are applied by the legitimate parties.

Ultrafast electron diffraction from time-varying coherent electronic states of the H atom is analyzed theoretically. This theoretical analysis identifies the conditions necessary to obtain time-resolved measurements. Electron diffraction from coherent electronic states exhibiting breathing and wiggling modes of electronic motion are simulated numerically in order to demonstrate the capability of attosecond electron pulses to image electron dynamics. The scattering patterns and their temporal behaviors are shown to differentiate the two kinds of target electronic motion. Moreover, our simulations show that inelastic processes contribute significantly to the diffraction patterns. Thus, although the diffraction patterns clearly distinguish different modes of target electronic motion, they cannot be easily related to the time-dependent target charge density.

Using an almost complete set of electron impact cross sections for scattering from the important biomolecule tetrahydrofuran (THF), compiled as a part of this study, swarm transport coefficients are determined by solving the Boltzmann's equation over the range of applied reduced fields from 0.01 to 10 000 Td. The present investigation highlights the experimental issues associated with, and the real need for, measurements of the corresponding THF transport coefficients, so that the self-consistency of our proposed cross section set might be evaluated.

Two-color photoassociation of ground state ${}^6$Li${}^{87}$Rb molecules via the $B{}^1\Pi$ electronic state using short pulses near a magnetic Feshbach resonance is studied theoretically. A near-resonant magnetic field is applied to mix the hyperfine singlet and triplet components of the initial wave function and enhance the photoassociation rate, before the population is transferred to the ground state by a second pulse. We show that an increase of up to three orders of magnitude in the absolute number of molecules produced is attainable for deeply bound vibrational levels. This technique can be generalized to other molecules with accessible magnetic Feshbach resonances.

We study binary collisions of metastable calcium atoms (${}^{40}$Ca) in an optical dipole trap. Collisions between ${}^3{P}_0$ atoms and between ${}^3{P}_0$ and ${}^1{S}_0$ atoms are considered. In the former case, the elastic and inelastic collision parameters are found to be $5.4\times {10}^{-11}$ and $3.6\times {10}^{-11}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$, respectively. A fraction of the collisions between ${}^3{P}_0$ atoms is found to produce cold trapped atoms in the singlet ${}^1{S}_0$ state, suggesting that the internal energy for these collisions is dissipated by radiation. For collisions between ${}^3{P}_0$ and ${}^1{S}_0$ atoms, we find a two-body loss parameter of $8.5\times {10}^{-11}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$. Our observations show that metastable calcium samples in the ${}^3{P}_0$ state are not stable at high densities, which limits their usefulness in quantum computing or many-body quantum simulation schemes.

We use analytic soliton solutions of a two-component Bose gas in the ring geometry to analyze the mean-field yrast spectrum of the system. We find that the spectrum exhibits a surprisingly rich structure as a result of an intricate interplay of interparticle interactions and population imbalance. We discuss the implication of these results in regard to the possibility of persistent currents at higher angular momenta.

We construct a two-state theoretical model describing attractively interacting bosons in a cylindrical harmonic potential of varied anisotropy. These systems exhibit true many-body ground-state energy minima stable against variations in single-particle state radial and axial widths and occupation number. The many-body ground state undergoes a phase transition from pure to fragmented as the aspect ratio increases above an anisotropy threshold dependent on the interaction energy per particle. This fragmentation is robust and does not require careful tuning of experimental parameters.

We investigate the coherence dynamics of a two-mode Bose-Einstein condensate coupled with the environment in the mean-field approximation. We give an analytical result of the time-average coherence in the absence of system-environment coupling, and find that the time-average coherence attains its maximum value at the critical point which corresponds to the boundary between the self-trapping regime and the Josephson oscillation regime. The effect of noise on dynamical coherence is also considered by analyzing the couplings of the condensate to the environment. With the first kind of coupling, the coherence finally stabilizes at a fixed value. Meanwhile, we show that the presence of the noise can even enhance the coherence for some particular interaction. With the second kind of coupling as collisional dephasing, the noise leads a sudden transition of the coherence which then subjects to an exponential decay.