We study quantum-state transfer (QST) through a strongly coupled antiferromagnetic spin chain (acting as a spin bus), between weakly coupled external qubits. By treating the weak coupling as a perturbation, we find that QST is enabled specifically by the second-order terms in the perturbative expansion. We show that QST is robust against disorder in the couplings, either within the bus or to the external qubits. We find that the protocol works when the qubits are attached to any node on an even-size bus or to the antiferromagnetic nodes on an odd-size bus. The optimal time for QST is found to depend nonmonotonically on qubit separation.

We present a number of schemes that use quantum mechanics to preserve privacy, in particular, we show that entangled quantum states can be useful in maintaining privacy. We further develop our original proposal [see M. Hillery, M. Ziman, V. Bužek, and M. Bieliková, Phys. Lett. A 349, 75 (2006)] for protecting privacy in voting, and examine its security under certain types of attacks, in particular dishonest voters and external eavesdroppers. A variation of these quantum-based schemes can be used for multiparty function evaluation. We consider functions corresponding to group multiplication of $N$ group elements, with each element chosen by a different party. We show how quantum mechanics can be useful in maintaining the privacy of the choices group elements.

The two-photon absorption of few-electron ions has been studied by using second-order perturbation theory and Dirac's relativistic equation. Within this framework, the general expressions for the excitation cross sections and rates are derived including a full account of the higher-order multipole terms in the expansion of the electron-photon interaction. While these expressions can be applied to any ion, independent of its particular shell structure, detailed computations are carried out for the two-photon absorption of hydrogen-, helium-, and berylliumlike ions and are compared with the available theoretical and experimental data. The importance of relativistic and nondipole effects in the analysis and computation of induced two-photon transitions is pointed out. Moreover, we discuss the potential of these transitions for atomic parity-violation studies in the high-$Z$ domain.

We examine the dynamics of ultracold atoms held in optical-lattice potentials. By controlling the switching of a periodic driving potential we show how a phase-induced renormalization of the intersite tunneling can be used to produce directed motion and control wave-packet spreading. We further show how this generation of a synthetic gauge potential can be used to split and recombine wave packets, providing an attractive route to implementing quantum computing tasks.

In the field of ultracold atoms in optical lattices a plethora of phenomena governed by the hopping energy $J$ and the interaction energy $U$ have been studied in recent years. However, the trapping potential typically present in these systems sets another energy scale and the effects of the corresponding time scale on the quantum dynamics have rarely been considered. Here we study the quantum collapse and revival of a lattice Bose-Einstein condensate (BEC) in an arbitrary spatial potential, focusing on the special case of harmonic confinement. Analyzing the time evolution of the single-particle density matrix, we show that the physics arising at the (temporally) recurrent quantum phase revivals is essentially captured by an effective single-particle theory. This opens the possibility of preparing exotic nonequilibrium condensate states with a large degree of freedom by engineering the underlying spatial lattice potential.

We obtain in both the scalar and vector photon models the analytical expressions for the initial cooperative decay rate and the cooperative Lamb shift for an ensemble of resonant atoms distributed uniformly in an infinite cylindrical geometry for the case that the initial state of the system is prepared in a phased state modulated in the direction of the cylindrical axis. We find that qualitatively the scalar and vector theories give different results.

In this brief report we study thermodynamic properties of noninteracting quantum gases with isotropic spin-orbit coupling. At high temperature, coefficients of virial expansion depend on both temperature $T$ and spin-orbit coupling strength $\kappa$. For strong coupling, virial expansion is applicable to the temperature region below the conventional degenerate temperature $T_{\mathrm{F}}$. At low temperature, specific heat is proportional to $\sqrt{T}$ in Bose gases and $T$ in Fermi gases. Temperature dependence of the chemical potential of fermions shows a different behavior when the Fermi surface is above and below the Dirac point.

Motivated by fundamental molecular physics and by atmospheric and planetary sciences, the valence excitations of N${}_2$ gas have seen several decades of intensive study, especially by electron-energy-loss spectroscopy (EELS). It was consequently surprising when a comparison of nonresonant inelastic x-ray scattering (NIXS) and nonresonant EELS found strong evidence for violations of the first Born approximation for EELS when leaving the dipole scattering limit. Here we reassess the relative strengths of the constituent resonances of the lowest-energy excitations of N${}_2$, encompassed by the so-called Lyman-Birge-Hopfield (LBH) band, expanding on the prior, qualitative interpretation of the NIXS results for N${}_2$ by both quantifying the generalized oscillator strength of the lowest-energy excitations and also presenting a time-dependent density functional theory (TDDFT) calculation of the $q$ dependence of the entire low-energy electronic excitation spectrum. At high $q$, we find that the LBH band has an unexpectedly large contribution from the octupolar $w{}^1{\Delta }_u$ resonance exactly in the regime where theory and EELS experiment for the presumed-dominant $a{}^1{\Pi }_g$ resonance have previously had substantial disagreement, and also where the EELS results must now be expected to show violations of the Born approximation. After correcting for this contamination, the $a{}^1{\Pi }_g$ generalized oscillator strength from the NIXS results is in good agreement with prior theory. The NIXS spectra, over their entire $q$ range, also find satisfactory agreement with the TDDFT calculations for both bound and continuum excitations.

We consider a two-component Bose-Einstein condensate in a double-well potential, where the atoms are magnetically coupled to a single mode of the microwave field inside a superconducting resonator. We find that the system has different dark-state subspaces in the strong- and weak-tunneling regimes. In the limit of weak tunnel coupling, steady-state entanglement between the two spatially separated condensates can be generated by evolving to a mixture of dark states via the dissipation of the photon field. We show that the entanglement can be faithfully indicated by an entanglement witness. Long-lived entangled states are useful for quantum-information processing with atom-chip devices.

Frequency-resolved observations of heavy Rydberg states in molecular hydrogen are reported in the ungerade manifold of states. Double-resonance spectroscopy via the $E,F$ ${}^1{\Sigma }_g^{+}$, $v^{\prime }=6$ state has been used to probe the energy region above the H($1s$) $+$ H(3$l$) dissociation threshold. Resonances are observed by ionizing H(3$l$) to produce H${}^{+}$, which is then detected by using a time-of-flight mass spectrometer. The kinetic energy of the H${}^{+}$ ion confirms that the observed signal is due to the photoionization of neutral H(3$l$) atoms, indicating that dissociation is a significant decay channel for the ion-pair states. The pattern of energies of the resonances agree well with the predictions of a mass-scaled Rydberg formula for bound quantum states of the H${}^{+}$H${}^{-}$ ion pair. Energies and quantum defects have been determined for principal quantum numbers in the range of $n=130$ to 207.

We observe the coherence of an interacting two-component Bose-Einstein condensate (BEC) surviving for seconds in a trapped Ramsey interferometer. Mean-field-driven collective oscillations of two components lead to periodic dephasing and rephasing of condensate wave functions with a slow decay of the interference fringe visibility. We apply spin echo synchronous with the self-rephasing of the condensate to reduce the influence of state-dependent atom losses, significantly enhancing the visibility up to $0.75$ at the evolution time of $1.5$ s. Mean-field theory consistently predicts higher visibility than experimentally observed values. We quantify the effects of classical and quantum noise and infer a coherence time of $2.8$ s for a trapped condensate of $5.5\times {10}^4$ interacting atoms.

We demonstrate the existence of an additional class of stationary accelerating Airy wave forms that exist in the presence of third-order (Kerr) nonlinearity and nonlinear losses. Numerical simulations and experiments, in agreement with the analytical model, highlight how these stationary solutions sustain the nonlinear evolution of Airy beams. The generic nature of the Airy solution allows extension of these results to other settings, and a variety of applications are suggested.

In this work, we describe in detail the chopped random basis (CRAB) optimal control technique recently introduced to optimize time-dependent density matrix renormalization group simulations [P. Doria, T. Calarco, and S. Montangero, Phys. Rev. Lett. 106, 190501 (2011)]. Here, we study the efficiency of this control technique in optimizing different quantum processes and we show that in the considered cases we obtain results equivalent to those obtained via different optimal control methods while using less resources. We propose the CRAB optimization as a general and versatile optimal control technique.

We introduce random-matrix theory to study the tomographic efficiency of a wide class of measurements constructed out of weighted 2-designs, including symmetric informationally complete (SIC) probability operator measurements (POMs). In particular, we derive analytic formulas for the mean Hilbert-Schmidt distance and the mean trace distance between the estimator and the true state, which clearly show the difference between the scaling behaviors of the two error measures with the dimension of the Hilbert space. We then prove that the product SIC POMs, the multipartite analog of the SIC POMs, are optimal among all product measurements in the same sense as the SIC POMs are optimal among all joint measurements. We further show that, for bipartite systems, there is only a marginal efficiency advantage of the joint SIC POMs over the product SIC POMs. In marked contrast, for multipartite systems, the efficiency advantage of the joint SIC POMs increases exponentially with the number of parties.

A robust thermodynamic argument shows that a small reduction of the effective coupling constant $\alpha$ of QED greatly enhances the low-energy Compton-scattering cross section and that the Thomson scattering length is connected to a fundamental scale $\lambda$. A discussion provides a possible quantum interpretation of this enormous sensitivity to changes in the effective coupling constant $\alpha$.

We solve the three-dimensional time-dependent Schrödinger equation and present investigations of the imprint of the orbital angular node in photoelectron momentum distributions of an aligned atomic $p$-type orbital following ionization by an intense elliptically polarized laser pulse of femtosecond duration. We investigate the role of light ellipticity and the alignment angle of the major polarization axis of the external field relative to the probed orbital by studying radial and angular momentum distributions, the latter at a fixed narrow interval of final momenta close to the peak of the photoelectron momentum distribution. In general only the angular distributions carry a clear signature of the orbital symmetry. Our study shows that circular polarization gives the most clear imprints of orbital nodes. These findings are insensitive to pulse duration.

Motional heating of ions in microfabricated traps is one of the open challenges hindering experimental realizations of large-scale quantum processing devices. Recently, a series of measurements of the heating rates in surface-electrode ion traps characterized their frequency, distance, and temperature dependencies, but our understanding of the microscopic origin of this noise remains incomplete. In this work we develop a theoretical model for the electric field noise which is associated with a random distribution of adsorbed atoms on the trap electrode surface. By using first-principles calculations of the fluctuating dipole moments of the adsorbed atoms we evaluate the distance, frequency, and temperature dependence of the resulting electric field fluctuation spectrum. Our theory reproduces correctly the $d^{-4}$ dependence with distance of the ion from the electrode surface and calculates the noise spectrum beyond the standard scenario of two-level fluctuators by incorporating all the relevant vibrational states. Our model predicts a regime of $1/f$ noise which commences at roughly the frequency of the fundamental phonon transition rate and a thermally activated noise spectrum which for higher temperatures exhibits a crossover as a function of frequency.

We analyze the dynamics of two semiconductor lasers with so-called orthogonal time-delayed mutual coupling: the dominant TE ($x$) modes of each laser are rotated by 90${}^{\circ }$ (therefore, TM polarization or $y$) before being coupled to the other laser. Although this laser system allows for steady-state emission in either one or in both polarization modes, it may also exhibit stable time-periodic dynamics including square waveforms. A theoretical mapping of the switching dynamics unveils the region in parameter space where one expects to observe long-term time-periodic mode switching. Detailed numerical simulations illustrate the role played by the coupling strength, the mode frequency detuning, or the mode gain to loss difference. We complement our theoretical study with several experiments and measurements. We present time series and intensity spectra associated with the characteristics of the square waves and other waveforms observed as a function of the strength of the delay coupling. The experimental observations are in very good agreement with the analysis and the numerical results.

We apply gain and loss to honeycomb photonic lattices and show that the dispersion relation is identical to tachyons—particles with imaginary mass that travel faster than the speed of light. This is accompanied by -symmetry breaking in this structure. We further show that the -symmetry can be restored by deforming the lattice.

We present an efficient way of heralding photonic qubit signals using linear optics devices. First, we show that one can obtain asymptotically perfect heralding and unit success probability with growing resources. Second, we show that even using finite resources, we can improve qualitatively and quantitatively over earlier heralding results. In the latter scenario, we can obtain perfect heralded photonic qubits while maintaining a finite success probability. We demonstrate the advantage of our heralding scheme by predicting key rates for device-independent quantum key distribution, taking imperfections of sources and detectors into account.