The interactions between polarized Laguerre-Gaussian beams and clusters of metallic nanoparticles have been investigated by means of the nonlinear multiple scattering method. The near-field phase singularities and vorticity in the longitudinal component of the second-harmonic field have been discussed. We have found that the phase profile of the second-harmonic field is twice as many helicities as that of the fundamental frequency field. The spin-based second-harmonic generations—spatial splitting of spin states and spin-Hall effect of the second-harmonic field—have been observed. A class of spin-based second-harmonic generation devices is anticipated.

The amount of nonlocality in quantum theory is limited compared to that allowed in generalized no-signaling theory [S. Popescu and D. Rohrlich, Found. Phys. 24, 379 (1994)]. This feature, for example, gets manifested in the amount of Bell inequality violation as well as in the degree of success probability of Hardy's (Cabello's) nonlocality argument. Physical principles like information causality and macroscopic locality have been proposed for analyzing restricted nonlocality in quantum mechanics, viz. explaining the Cirel'son bound. However, these principles are not very successful in explaining the maximum success probability of Hardy's as well as Cabello's argument in quantum theory. Here we show that a recently proposed physical principle, namely local orthogonality, does better by providing a tighter upper bound on the success probability for Hardy's nonlocality. This bound is relatively closer to the corresponding quantum value compared to the bounds achieved from other principles.

We propose a scheme for enhanced probing of an interaction between two single fermions based on weak-value amplification. The scheme is applied to measuring the anisotropic electron-hole exchange interaction strength in semiconductor quantum dots where both spin and energy are mapped onto emitted photons. We study the effect of dephasing of the probe on the weak-value-enhanced measurement. We find that, in the limit of slow noise, weak-value amplification provides a unique tool for enhanced-precision measurement of few-fermion systems.

Quantum key distribution protocols typically make use of a one-way quantum channel to distribute a shared secret string to two distant users. However, protocols exploiting a two-way quantum channel have been proposed as an alternative route to the same goal, with the potential advantage of outperforming one-way protocols. Here we provide a strategy to prove security for two-way quantum key distribution protocols against the most general quantum attack possible by an eavesdropper. We utilize an entropic uncertainty relation, and only a few assumptions need to be made about the devices used in the protocol. We also show that a two-way protocol can outperform comparable one-way protocols.

We explore the implementation of pseudorandom single-qubit rotations and multiqubit pseudorandom circuits constructed only from Clifford gates and the $T$ gate, a phase rotation of $\pi /4$. Such a gate set would be appropriate for computations performed in a fault tolerant setting. For single-qubit rotations the distribution of parameters found for unitaries constructed from Clifford plus $T$ quickly approaches that of random rotations and requires significantly fewer gates than the construction of arbitrary single-qubit rotations. For Clifford-plus-$T$ pseudorandom circuits we find an exponential convergence to a random matrix element distribution and a Gaussian convergence to the higher-order moments of the matrix element distribution. In addition, the nearest-neighbor eigenangle statistics distribution almost immediately converges to that of random unitary matrices. All of these convergence rates are found to be insensitive to the number of qubits.

We propose a linear-optical setup for heralded qubit amplification with tunable output qubit fidelity. We study its success probability as a function of output qubit fidelity, showing that at the expense of lower fidelity, the setup can considerably increase probability of successful operation. These results are subsequently applied in a proposal for state-dependent qubit amplification. Similar to state-dependent quantum cloning, the a priori information about the input state allows us to optimize the qubit amplification procedure to obtain a better fidelity versus success probability trade-off.

We evaluate the electrostatic interaction energy between two surfaces, one flat and the other slightly curved, in terms of the two-point autocorrelation functions for patch potentials on each one of them, and of a single function $\psi$ which defines the curved surface. The resulting interaction energy, a functional of $\psi$, is evaluated up to the second order in a derivative expansion approach. We derive explicit formulas for the coefficients of that expansion as simple integrals involving the autocorrelation functions, and evaluate them for some relevant patch-potential profiles and geometries.

The Ge $3p$ core excitation spectrum of the $n$-butylgermane molecule only reveals two peaks, whereas the rest of the fine structure is obscured due to the large lifetime broadenings of core-excited states. A two-dimensional presentation of resonant photoemission spectra allows us to observe some other resonances. The interpretation of experimental results is supported by ab initio calculations conducted at the four-component relativistic level of theory with full account made for spin-orbit interactions already in the zeroth-order Hamiltonian.

We report on measurements of the differential polarizability between the nearly degenerate, opposite parity states in atomic dysprosium at 19 797.96 cm${}^{-1}$, and the differential blackbody radiation induced Stark shift of these states. The differential scalar and tensor polarizabilities due to additional states were measured for the $\left|M\right|=7,\cdots ,10$ sublevels in ${}^{164}$Dy and ${}^{162}$Dy and determined to be ${\overline{\alpha }}_{BA}^{\left(0\right)}=180{\left(45\right)}_{\mathrm{stat}}{\left(8\right)}_{\mathrm{sys}}$ $\text{mHz}/{(\mathrm{V}/\mathrm{cm})}^2$ and ${\overline{\alpha }}_{BA}^{\left(2\right)}=-163{\left(65\right)}_{\mathrm{stat}}{\left(5\right)}_{\mathrm{sys}}$ $\text{mHz}/{(\mathrm{V}/\mathrm{cm})}^2$, respectively. The average blackbody radiation induced Stark shift of the Zeeman spectrum was measured around 300 K and found to be $-34\left(4\right)$ mHz/K and $+29\left(4\right)$ mHz/K for ${}^{164}$Dy and ${}^{162}$Dy, respectively. We conclude that ac Stark related systematics will not limit a search for variation of the fine-structure constant, using dysprosium, down to the level of $|\dot{\alpha }/\alpha |=2.6\times {10}^{-17}$ yr${}^{-1}$, for two measurements of the transition frequency one year apart.

We present a technique for empirically determining the photon scattering rate of atoms in a magneto-optical trap (MOT). An accurate measurement of the scattering rate provides a way to accurately determine the number of atoms in the MOT, the excited-state fraction, and the in situ light intensity experienced by the atoms in the trap. We validate this technique for determining the scattering rate by comparing the atom number determined by the fluorescence emitted by atoms in a MOT with an independent and unrelated measure of the number determined by an absorption measurement of an optical pumping beam. We also observe minor deviations from the two-level model for the single-photon scattering rate and extend our analysis to a four-level model. The main advantage of the method described here is that it provides a simple way of determining the photon scattering rate of the MOT empirically and, thus, of evaluating the atom number from fluorescence measurements more accurately.

The density matrix renormalization group algorithm is used to characterize the ground states of one-dimensional coupled cavities in the regime of low photon densities. Numerical results for photon and spin excitation densities, one- and two-body correlation functions, superfluid and condensate fractions, as well as the entanglement entropy and localizable entanglement are obtained for the Jaynes-Cummings-Hubbard (JCH) model and are compared with those for the Bose-Hubbard (BH) model where applicable. The results indicate that a Tonks-Girardeau phase, in which the photons are strongly fermionized, appears between the Mott-insulating and superfluid phases as a function of the intercavity coupling. In fact, the superfluid density is found to be zero in a wide region outside the Mott-insulator phase boundary. The presence of two different species of excitation (spin and photon) in the JCH model gives rise to properties with no analog in the BH model, such as the (quasi)condensation of spin excitations and the spontaneous generation of entanglement between the atoms confined to each cavity.

We propose and analyze a probabilistic but heralded scheme to generate pure, entangled, non-Gaussian states of collective spin in large atomic ensembles by means of single-photon detection. One photon announces the preparation of a Dicke state, while two or more photons announce Schrödinger cat states. The method produces pure states even for finite photon detection efficiency and weak atom-photon coupling. The entanglement generation can be made quasideterministic by means of repeated trial and feedback, enabling metrology beyond the standard quantum limit.

We advocate a simple multipole expansion of the polarization density matrix. The resulting multipoles appear as successive moments of the Stokes variables and can be obtained from feasible measurements. In terms of these multipoles we construct a whole hierarchy of measures that accurately assess higher-order polarization fluctuations.

Two-color photoionization produces a cycle-averaged current driving broadband, conically emitted THz radiation. We investigate, through simulation, the processes determining the angle of conical emission. We find that the emission angle is determined by an optical Cherenkov effect, where the front velocity of the current source is faster than the THz phase velocity.

The scattering properties of plasmonic nanostructures arranged in two-dimensional periodic arrays are analyzed by expanding their response into perfectly emitting and absorbing modes. It is shown that the frequencies of these modes determine the shape of the reflection and transmission spectra in the same way that the positions of point-like charges determine the electric field around them. This helps to develop a visual interpretation of many resonant effects which occur due to overlapping resonances, and the Fano interference effect is considered as a basic example. This approach works naturally in the domain of complex frequencies and provides a systematic tool to analyze, optimize, and engineer the resonant properties of nanostructures for various applications.

The system-environment interaction is simulated by light propagating in coupled photonic waveguides. The profile of the electromagnetic field provides intuitive physical insight to study the Markovian and non-Markovian dynamics. The transition from non-Markovian to Markovian process is demonstrated by increasing the size of the environment, as the energy evolution changes from oscillation to exponential decay and the revival period increases. Moreover, the dynamical decoupling with a sequence of phase modulations is introduced to such a photonic system to form a band structure in the time dimension, where the energy dissipation can be significantly accelerated or inhibited. It opens the possibility to tune the dissipation in a photonic system, similar to the dynamic decoupling of spins.

We present a robust tool to analyze nonclassical properties of multimode twin-beam states in the mesoscopic photon-number domain. The measurements are performed by direct detection. The analysis exploits three different nonclassicality criteria for detected photons exhibiting complementary behavior in the explored intensity regime. Joint signal-idler photon-number distributions and quasidistributions of integrated intensities are determined and compared with the corresponding distributions of detected photons. Experimental conditions optimal for nonclassical properties of twin-beam states are identified.

Comparison of the transmission of 27-keV Ar${}^{9+}$ ions through insulating funnel- and conical-shaped glass capillaries of outlet diameters of ∼22 μm is reported. Beam intensities of 1, 5, and 10 pA were injected into both capillaries. Transmission at the untilted angle of 0° was measured as well as at a tilt angle of ∼0.5° for the funnel capillary and a tilt angle of ∼1.1° for the conical capillary. For the funnel capillary, blocking of transmission was observed, whereas, the transmission was continuous for the conical capillary. These measurements suggest that conical-shaped capillaries have transport properties that are different than funnel-shaped capillaries for slow highly charged ions.