The observation of significant dipolar effects in gases of ultracold polar molecules typically demands a strong external electric field to polarize the molecules. We show that, even in the absence of a significant polarization, dipolar effects may play a crucial role in the physics of polar molecules in bilayers, provided that the molecules in each layer are initially prepared in a different rotational state. Then, interlayer dipolar interactions result in a nonlocal swap of the rotational state between molecules in different layers, even for weak applied electric fields. The interlayer scattering due to the dipole-dipole interaction leads to a nontrivial dependence of the swapping rate on density, temperature, interlayer spacing, and population imbalance. For reactive molecules such as KRb, chemical recombination immediately follows a nonlocal swap and dominates the losses even for temperatures well above quantum degeneracy, and hence could be observed under current experimental conditions.

We report on the higher-order photon correlations of a high-$\beta$ nanolaser under pulsed excitation at room temperature. Using a multiplexed four-element superconducting single-photon detector we measured $g^{\left(n\right)}\left(\vec{0}\right)$ with $n=2,3,4$. All orders of correlation display partially chaotic statistics, even at four times the threshold excitation power. We show that this departure from coherence and Poisson statistics is due to the quantum fluctuations associated with the small number of photons at the lasing threshold.

The controlled-$Z$ (cz) operations acting separately on pairs of qubits are commonly adopted in the schemes of generating graph states, the multipartite entangled states for the one-way quantum computing. For this purpose, we propose a setup of cascade cz operation on a whole group of qubits in sequence. The operation of the setup starts with entangling an ancilla photon with the first photon as qubit, and this ancilla automatically moves from one entanglement link to another in assisting the formation of a string in graph states. The generation of some special types of graph states, such as the three-dimensional ones, can be greatly simplified in this approach. The setup presented uses weak nonlinearities, but an implementation using probabilistic linear optics is also possible.

The impact of both confinement and electron correlation on generalized oscillator strengths (GOS's) of endohedral atoms, $A$@C${}_{60}$, is theoretically studied choosing the Xe@C${}_{60}$ $4d$, $5s$, and $5p$ fast electron impact ionization as the case study. Calculations are performed in the transferred to the atom energy region beyond the $4d$ threshold, $\omega =75$–175 eV. The calculation methodology combines the plane-wave Born approximation, Hartree-Fock approximation, and random-phase approximation with exchange in the presence of the C${}_{60}$ confinement. The confinement is modeled by a spherical $\delta$-function-like potential as well as by a square well potential to evaluate the effect of the finite thickness of the C${}_{60}$ cage on the Xe@C${}_{60}$ GOS's. Dramatic distortion of the $4d$, $5p$, and $5s$ GOS's by the confinement is demonstrated, compared to the free atom. Considerable contributions of multipolar transitions beyond dipole transitions in the calculated GOS's are revealed, in some instances. The vitality of accounting for electron correlation in calculation of the Xe@C${}_{60}$ $5s$ and $5p$ GOS's is shown.

We consider selectivity of strong-field ionization in circularly polarized laser fields to the sense of electron rotation in the laser polarization plane in the initial state. We show that, in contrast to the textbook examples of one-photon ionization and bound-state excitations with increase in the electron angular momentum, and also in contrast to the well-studied ionization of Rydberg atoms in microwave fields, which all prefer corotating electrons, optical tunneling selectively depletes states where the electron initially rotates against the laser field. We also show that key assumptions regarding adiabaticity of optical tunneling may quickly become inaccurate in typical experimental conditions.

The response of spin-2 small condensates to an external magnetic field $B$ is studied. The parameters of the interaction are considered as variable. The emphasis is placed on clarifying the modes of excitation caused by the quadratic Zeeman term. The theoretical method used is beyond the mean-field theory. A set of eigenstates with the $\text{U}\left(5\right)\supset \text{SO}\left(5\right)\supset \text{SO}\left(3\right)$ symmetry is introduced to facilitate the analysis. To obtain a quantitative evaluation on the response, the fidelity susceptibility and the $B$-dependent average populations of spin components have been calculated. Mostly the particle number $N=30$ is assumed. The effect with a larger or smaller $N$ is also considered. It was found that the sensitivity of the response depends strongly both on the interaction and on the inherent symmetry.

We report a quantum Monte Carlo calculation of the phase diagram of bosons interacting with a repulsive inverse sixth power pair potential, a model for assemblies of Rydberg atoms in the local van der Waals blockade regime. The model can be parametrized in terms of just two parameters, the reduced density and temperature. Solidification happens to the fcc phase. At zero temperature, the transition density is found with the diffusion Monte Carlo method at density $\rho =3.9{({\hslash }^2/m{C}_6)}^{3/4}$, where $C_6$ is the strength of the interaction. The solidification curve at nonzero temperature is studied with the path-integral Monte Carlo approach and is compared with transitions in corresponding harmonic and classical crystals. Relaxation mechanisms are considered in relation to present experiments.

Motivated by a recent experiment at MIT, we consider the collision of two clouds of spin-polarized atomic Fermi gases close to a Feshbach resonance. We explain why two dilute gas clouds, with underlying attractive interactions between their constituents, bounce off each other in the strongly interacting regime. Our hydrodynamic analysis, in excellent agreement with experiment, gives strong evidence for a metastable many-body state with effective repulsive interactions.

Three-mode optomechanical interactions have been predicted to allow the creation of very high sensitivity transducers in which very strong optical self-cooling and strong optomechanical quantum entanglement are predicted. Strong coupling is achieved by engineering a transducer in which both the pump laser and a single signal sideband frequency are resonantly enhanced. Here we demonstrate that very high sensitivity can be achieved in a very simple system consisting of a Fabry-Perot cavity with CO${}_2$ laser thermal tuning. We demonstrate a displacement sensitivity of $\sim$$1\times {10}^{-17}\text{m}/\sqrt{\text{Hz}}$, which is sufficient to observe a thermally excited acoustic mode in a 5.6 kg sapphire mirror with a signal-to-noise ratio of more than 20 dB. It is shown that a measurement sensitivity of $\sim$$2\times {10}^{-20}\mathrm{m}/\sqrt{\mathrm{Hz}}$ limited by the quantum shot noise is achievable with optimization of the cavity parameters.

We show that complex coordinates used in conjunction with transformation optics offer an extra degree of freedom that allows control over not only the propagation direction of electromagnetic waves but also their amplitude. We illustrate this idea in two applications. First, we show that in an $n$-dimensional space one can manipulate the field amplitude for up to $n$ different amplitude distributions in regions that are critical to the performance of the device under consideration, and thus reduce the device's sensitivity to design imperfections in these regions. Second, we expand previous work on reflectionless perfectly matched layers and show how complex coordinates and transformation optics are a natural choice for designing perfectly matched layers of arbitrary shape.

We present the details of our previously formulated model [Saleh et al., Phys. Rev. Lett. 107, 203902 (2011)] that governs pulse propagation in hollow-core photonic crystal fibers filled by an ionizable gas. By using perturbative methods, we find that the photoionization process induces the opposite phenomenon of the well-known Raman self-frequency redshift of solitons in solid-core glass fibers, as was recently experimentally demonstrated [Hölzer et al., Phys. Rev. Lett. 107, 203901 (2011)]. This process is only limited by ionization losses, and leads to a constant acceleration of solitons in the time domain with a continuous blueshift in the frequency domain. By applying the Gagnon-Bélanger gauge transformation, multipeak “inverted gravitylike” solitary waves are predicted. We also demonstrate that the pulse dynamics shows the ejection of solitons during propagation in such fibers, analogous to what happens in conventional solid-core fibers. Moreover, unconventional long-range nonlocal interactions between temporally distant solitons, unique of gas plasma systems, are predicted and studied. Finally, the effects of higher-order dispersion coefficients and the shock operator on the pulse dynamics are investigated, showing that the conversion efficiency of resonant radiation into the deep UV can be improved via plasma formation.

The hyperspherical method based on infinite angular expansions is introduced. We approximate the cusp behavior of a wave function using $B$-spline techniques. Calculations for the ground-state energies of the atomic helium and the $e^{+}$Li system are presented as two examples for testing this method. The computed ground-state energy of He is $-2.903724$ a.u. with single particle orbitals $l_{\text{max}}=8$. For the $e^{+}$Li system, with $l_{\text{max}}=9$, the ground-state energy is $-0.25083$ a.u., which is better than the configuration interaction result of $-0.25010782$ a.u. with $l_{\text{max}}=30$.

The $R$-matrix incorporating time (RMT) method is a method developed recently for solving the time-dependent Schrödinger equation for multielectron atomic systems exposed to intense short-pulse laser light. We have employed the RMT method to investigate the time delay in the photoemission of an electron liberated from a $2p$ orbital in a neon atom with respect to one released from a $2s$ orbital following absorption of an attosecond xuv pulse. Time delays due to xuv pulses in the range 76–105 eV are presented. For an xuv pulse at the experimentally relevant energy of 105.2 eV, we calculate the time delay to be $10.2\pm 1.3$ attoseconds (as), somewhat larger than estimated by other theoretical calculations, but still a factor of 2 smaller than experiment. We repeated the calculation for a photon energy of 89.8 eV with a larger basis set capable of modeling correlated-electron dynamics within the neon atom and the residual Ne${}^{+}$ ion. A time delay of $14.5\pm 1.5$ as was observed, compared to a $16.7\pm 1.5$ as result using a single-configuration representation of the residual Ne${}^{+}$ ion.

Motivated by recent experimental observations [M. Kataoka et al., Phys. Rev. Lett. 102, 156801 (2009)], we propose here a theoretical approach to implement quantum computation with bound states of electrons in moving quantum dots generated by the driving of surface acoustic waves. Differing from static quantum dots defined by a series of static electrodes above the two-dimensional electron gas (2DEG), here a single electron is captured from a 2DEG reservoir by a surface acoustic wave and then trapped in a moving quantum dot (MQD) transported across a quasi-one-dimensional channel, wherein all the electrons have been excluded by the actions of the surface gates. The flying qubit introduced here is encoded by the two lowest levels of the electron in the MQD, and the Rabi oscillation between these two levels could be implemented by applying finely selected microwave pulses to the surface gates. By using the Coulomb interaction between the electrons in different MQDs, we show that a desirable two-qubit operation, i.e., iswap gate, could be realized. Readouts of the present flying qubits are also feasible with the current single-electron detected technique.

We propose a simple formalism to design unitary gates robust against given systematic errors. This formalism generalizes our previous observation [Y. Kondo and M. Bando, J. Phys. Soc. Jpn. 80, 054002 (2011)] that vanishing dynamical phase in some composite gates is essential to suppress pulse-length errors. By employing our formalism, we derive a composite unitary gate which can be seen as a concatenation of two known composite unitary operations. The obtained unitary gate has high fidelity over a wider range of error strengths compared to existing composite gates.

We present experimental results for the ionization of aniline and benzene molecules subjected to intense ultrashort laser pulses. Measured parent molecular ions yields were obtained using a recently developed technique capable of three-dimensional imaging of ion distributions within the focus of a laser beam. By selecting ions originating from the central region of the focus, where the spatial intensity distribution is nearly uniform, volumetric-free intensity-dependent ionization yields were obtained. The measured data revealed a previously unseen resonance-enhanced multiphoton ionization (REMPI)-like process. Comparison of benzene, aniline, and Xe ion yields demonstrates that the observed intensity-dependent structures are not due to geometric artifacts in the focus. Finally for intensities greater than $\sim 3\times {10}^{13}{\mathrm{W}/\mathrm{cm}}^2$, we attribute the ionization of aniline to a stepwise process going through the $\pi {\sigma }^{*}$ state which sits three photons above the ground state and two photons below the continuum.

Enhancement and control of the index of refraction in a mixture of two three-level atomic species that form a pair of far-detuned $\Lambda$ schemes under two-photon resonance has been studied. We employ the density-matrix approach to properly take population relaxation into account and to describe the interaction of each $\Lambda$ system with the electromagnetic fields. Both $\Lambda$ systems are driven by a corresponding far-detuned coherent field at one atomic transition and are probed by the same weak field. In the dressed-state basis, it represents a superposition of effective two-level subsystems with the positions, widths, and amplitudes of the resonances controlled by the driving fields and allows for efficient control of the susceptibility of the total system; leading to refractive index (RI) enhancement with vanishing absorption in the absence of amplification. We analyze the experimental implementation of such a system in a cell of Rb atoms with a natural abundance of isotopes. An upper limit estimate of the RI enhancement is obtained.

In this paper we compute the entanglement, as quantified by negativity, between two blocks of length $L_A$ and $L_B$, separated by $L$ sites in the one-dimensional spin-1 Affleck-Kennedy-Lieb-Tasaki (AKLT) model. We took the model with two different boundary conditions. We consider the case of $N$ spin-1 particles in the bulk and one spin-1/2 particle at each boundary, which constitute a unique ground state, and the case of just spin 1, even at the end of the chain, where the degeneracy of the ground state is 4. In both scenarios we made a partition consisting of two blocks $A$ and $B$, containing $L_A$ and $L_B$ sites, respectively. The separation of these two blocks is $L$. In both cases we explicitly obtain the reduced density matrix of the blocks $A$ and $B$. We prove that the negativity in the first case vanishes identically for $L⩾1$, while in the second scenario it may approach a constant value $N=1/2$ for each degenerate eigenstate depending on the way one constructs these eigenstates. However, as there is some freedom in constructing these eigenstates, vanishing entanglement is also possible in the latter case. Additionally, we also compute the entanglement between noncomplementary blocks in the case of periodic boundary conditions for the spin-1 AKLT model for which there is a unique ground state. Even in this case, we find that the negativity of separated blocks of spins is zero.

It is well known that the unconditional security of quantum-key distribution (QKD) can be guaranteed by quantum mechanics. However, practical QKD systems have some imperfections, which can be controlled by the eavesdropper to attack the secret key. With current experimental technology, a realistic beam splitter, made by fused biconical technology, has a wavelength-dependent property. Based on this fatal security loophole, we propose a wavelength-dependent attacking protocol, which can be applied to all practical QKD systems with passive state modulation. Moreover, we experimentally attack a practical polarization encoding QKD system to obtain all the secret key information at the cost of only increasing the quantum bit error rate from $1.3$ to $1.4\%$.

Entanglement distillation is a fundamental building block in long-distance quantum communication. Though known to be useless on their own for distilling Gaussian entangled states, local Gaussian operations may still help to improve non-Gaussian entanglement distillation schemes. Here we show that by applying local squeezing operations both the performance and the efficiency of existing distillation protocols can be enhanced. We find that such an enhancement through local Gaussian unitaries can be obtained even when the initially shared Gaussian entangled states are mixed, as, for instance, after their distribution through a lossy-fiber communication channel.