Quantum coin tossing (QCT) is an important primitive of quantum cryptography and has received continuous interest. However, in practical QCT, Bob's detectors can be subjected to detector-side channel attacks launched by dishonest Alice, which will possibly make the protocol completely insecure. Here, we report a simple strategy of a detector-blinding attack based on a recent experiment. To remove all the detector side channels, we present a solution of measurement-device-independent QCT (MDI-QCT). This method is similar to the idea of MDI quantum key distribution (QKD). MDI-QCT is loss tolerant with single-photon sources and has the same bias as the original loss-tolerant QCT under a coherent attack. Moreover, it provides the potential advantage of doubling the secure distance for some special cases. Finally, MDI-QCT can also be modified to fit the weak coherent-state sources. Thus, based on the rapid development of practical MDI-QKD, our proposal can be implemented easily.

A recent experiment [T. Lanting et al., Phys. Rev. X 4, 021041 (2014)] claimed to provide evidence of up to eight-qubit entanglement in a D-Wave quantum annealing device. However, entanglement was measured using qubit tunneling spectroscopy, a technique that provides indirect access to the state of the system at intermediate times during the anneal by performing measurements at the end of the anneal with a probe qubit. In addition, an underlying assumption was that the quantum transverse-field Ising Hamiltonian, whose ground states are already highly entangled, is an appropriate model of the device and not some other (possibly classical) model. This begs the question of whether alternative classical or semiclassical models would be equally effective at predicting the observed spectrum and thermal state populations. To check this, we consider a recently proposed classical rotor model with classical Monte Carlo updates, which has been successfully employed in describing features of earlier experiments involving the device. We also consider simulated quantum annealing with quantum Monte Carlo updates, an algorithm that samples from the instantaneous Gibbs state of the device Hamiltonian. Finally, we use the quantum adiabatic master equation, which cannot be efficiently simulated classically and which has previously been used to successfully capture the open-system quantum dynamics of the device. We find that only the master equation is able to reproduce the features of the tunneling spectroscopy experiment, while both the classical rotor model and simulated quantum annealing fail to reproduce the experimental results. We argue that this bolsters the evidence for the reported entanglement.

The one-clean-qubit model of quantum computation (DQC1) efficiently implements a computational task that is not known to have a classical alternative. During the computation, there is never more than a small but finite amount of entanglement present, and it is typically vanishingly small in the system size. In this paper, we demonstrate that there is nothing unexpected hidden within the DQC1 model—Grover's search, when acting on a mixed state, provably exhibits a speedup over classical, with guarantees as to the presence of only vanishingly small amounts of quantum correlations (entanglement and quantum discord)—while arguing that this is not an artifact of the oracle-based construction. We also present some important refinements in the evaluation of how much entanglement may be present in the DQC1 and how the typical entanglement of the system must be evaluated.

We study the adiabatic passage for a pair of interacting cold atoms driven into the ladder configuration with one ground state and two Rydberg states. We find, with proper single-photon and two-photon detunings, that it is viable to (i) achieve efficient population transfer from the ground state to either Rydberg state by fully overcoming the dipole blockade effect and (ii) implement maximal entangled states by partially overcoming the dipole blockade effect. These entangled atomic states are very stable and have purities and fidelities approaching 100%, among which one is of particular interest since it involves the simultaneous excitation of two different Rydberg states.

We study the Anderson localization of atomic gases exposed to simple-cubic optical lattices with a superimposed disordered speckle pattern. The two mobility edges in the first band and the corresponding critical filling factors are determined as a function of the disorder strength, ranging from vanishing disorder up to the critical disorder intensity where the two mobility edges merge and the whole band becomes localized. Our theoretical analysis is based both on continuous-space models that take into account the details of the spatial correlation of the speckle pattern, and also on a simplified tight-binding model with an uncorrelated distribution of the on-site energies. The mobility edges are computed via the analysis of the energy-level statistics, and we determine the universal value of the ratio between consecutive level spacings at the mobility edge. We analyze the role of the spatial correlation of the disorder, and we also discuss a qualitative comparison with available experimental data for interacting atomic Fermi gases obtained in the moderate interaction regime.

We study experimentally the energy-temperature relationship of a harmonically trapped Bose-Einstein condensate by transferring a known quantity of energy to the condensate and measuring the resulting temperature change. We consider two methods of heat transfer, the first using a free expansion under gravity and the second using an optical standing wave to diffract the atoms in the potential. We investigate the effect of interactions on the thermodynamics and compare our results to various finite-temperature theories.

We investigate the cooperative behavior of regular monolayers of driven two-level dipoles, using classical electrodynamics simulations. The dipolar response results from the interference of many cooperative eigenmodes, each frequency-shifted from the single resonant dipole case, and with a modified lifetime, due to the interactions between dipoles. Of particular interest is the kagome lattice, where the semiregular geometry permits simultaneous excitation of two dominant modes, one strongly subradiant, leading to an electromagnetically induced transparencylike interference in a two-level system. The interfering modes are associated with ferroelectric and antiferroelectric ordering in alternate lattice rows with long-range interactions.

In this article we present a concrete proposal for spin squeezing the cold ground-state polar paramagnetic molecule OH, a system currently under fine control in the laboratory. In contrast to existing work, we consider a single, noninteracting molecule with angular momentum greater than $1/2$. Starting from an experimentally relevant effective Hamiltonian, we identify an adiabatic regime where different combinations of static electric and magnetic fields can be used to realize the single-axis twisting Hamiltonian of Kitagawa and Ueda [M. Kitagawa and M. Ueda, Phys. Rev. A 47, 5138 (1993)], the uniform field Hamiltonian proposed by Law et al. [C. K. Law, H. T. Ng, and P. T. Leung, Phys. Rev. A 63, 055601 (2001)], and a model of field propagation in a Kerr medium considered by Agarwal and Puri [G. S. Agarwal and R. R. Puri, Phys. Rev. A 39, 2969 (1989)]. We then consider the situation in which nonadiabatic effects are quite large and show that the effective Hamiltonian supports spin squeezing even in this case. We provide analytical expressions as well as numerical calculations, including optimization of field strengths and accounting for the effects of field misalignment. Our results have consequences for applications such as precision spectroscopy, techniques such as magnetometry, and stereochemical effects such as the orientation-to-alignment transition.

We study the lensing effect experienced by a weak probe field under conditions of electromagnetically induced transparency (EIT) involving a Rydberg state. A Gaussian coupling beam tightly focused on a laser-cooled atomic cloud produces an inhomogeneity in the coupling Rabi frequency along the transverse direction and makes the EIT area acting like a gradient-index medium. We image the probe beam at the position where it exits the atomic cloud and observe that a red-detuned probe light is strongly focused with a greatly enhanced intensity whereas a blue-detuned one is defocused with a reduced intensity. Our experimental results agree very well with the numerical solutions of Maxwell-Bloch equations.

The year 1982 is often credited as the year that theoretical quantum computing was started with a keynote speech by Richard Feynman, who proposed a universal quantum simulator, the idea being that if you had such a machine you could in principle “imitate any quantum system, including the physical world.” With that in mind, we present an algorithm for a continuous-variable quantum computing architecture which gives an exponential speedup over the best-known classical methods. Specifically, this relates to efficiently calculating the scattering amplitudes in scalar bosonic quantum field theory, a problem that is believed to be hard using a classical computer. Building on this, we give an experimental implementation based on continuous-variable states that is feasible with today's technology.

We demonstrate the possibility to produce a blazed grating for electron waves using Kapitza-Dirac diffraction on a standing wave created by several harmonics of a fundamental wavelength. We show that an optimized multicolor standing wave can lead to strongly asymmetric diffraction patterns. Using a bichromatic standing wave, we predict a diffraction probability larger than 50% in a specific order. This order can be chosen with the spatial relative phase between the fundamental and second harmonic. Since the Kapitza-Dirac diffraction grating is experimentally more feasible than Bragg diffraction, this could be an interesting alternative to be used in electron interferometry.

We propose a selective dynamical decoupling scheme on a chain of permanently coupled qubits with XX-type interactions, which is capable of dynamically suppressing any coupling in the chain by applying sequences of local pulses to the individual qubits. We demonstrate that high-fidelity single- and two-qubit gates can be achieved by this procedure and that sequences of gates can be implemented by this pulse control alone. We discuss the applicability and physical limitations of our model specifically for strongly coupled superconducting flux qubits. Since dynamically modifying the couplings between flux qubits is challenging, they are a natural candidate for our approach.

We consider a two-way quantum cryptographic protocol with coherent states assuming direct reconciliation. A detailed security analysis is performed considering a two-mode coherent attack, which represents the residual eavesdropping once the parties have reduced the general attack by applying symmetric random permutations. In this context we provide a general analytical expression for the key rate, discussing the impact of the residual two-mode correlations on the security of the scheme. In particular, we identify the optimal eavesdropping against two-way quantum communication, which is given by a two-mode coherent attack with symmetric and separable correlations.

All known qudit-based prepare-and-measure quantum key distribution (PMQKD) schemes are more error resilient than their qubit-based counterparts. Their high error resiliency comes partly from the careful encoding of multiple bits of signals used to generate the raw key in each transmitted qudit so that the same eavesdropping attempt causes a higher bit error rate (BER) in the raw key. Here I show that highly-error-tolerant PMQKD schemes can be constructed simply by encoding one bit of classical information in each transmitted qudit in the form $\left(\right|i\rangle \pm |j\rangle )/\sqrt{2}$, where $|i\rangle$'s form an orthonormal basis of the $2^n$-dimensional Hilbert space. Moreover, I prove that these schemes can tolerate up to the theoretical maximum of a 50% BER for $n\ge 2$ provided the raw key is generated under a certain technical condition, making them extremely-error-tolerant PMQKD schemes involving the transmission of unentangled finite-dimensional qudits. This shows the potential of processing quantum information using lower-dimensional quantum signals encoded in a higher-dimensional quantum state.

A recently developed theoretical framework of performing self-consistent orbital-free (OF) density functional theory (DFT) calculations at Kohn-Sham DFT level accuracy is tested in practice. The framework is valid for spherically symmetric systems. Numerical results for the Beryllium atom are presented and compared to accurate Kohn-Sham data. These calculations make use of a differential equation that we have developed for the so called Pauli potential, a key quantity in OF-DFT. The Pauli potential differential equation and the OF Euler equation form a system of two coupled differential equations, which have to be solved simultaneously within the DFT self-consistent loop.

The influence of the collimating conditions of the incident beam on diffraction patterns produced by grazing scattering of fast atoms off crystal surfaces is studied within a semiquantum approach, called the surface initial value representation (SIVR) approximation. In this approach we incorporate a realistic description of the incident particle in terms of the collimating parameters, which determine the surface area that is coherently illuminated. The model is applied to He atoms colliding with a LiF(001) surface after passing through a rectangular aperture. As was experimentally observed [Nucl. Instrum. Methods Phys. Res., Sect. B 350, 99 (2015)], SIVR spectra as a function of the azimuthal angle are very sensitive to the width of the collimating slit. We also found that the length of the collimating aperture affects polar angle distributions, introducing additional interference structures for the longer collimating slits.

The x-ray emission during the penetration of 2–6 MeV ${\mathrm{Xe}}^{q+}$ ($q=20,22,26,30$) ions into thick solid targets of iron and nickel has been measured. An obvious charge-state and incident-energy dependence of the target-to-projectile vacancy-production cross-section ratios was found for the iron target but not for the nickel target. The results are supported by the vacancy-sharing model and direct ionization theory and imply the great importance of level matching for the inner-shell process. The charge equilibration time of ${\mathrm{Xe}}^{30+}$ in solid iron was derived to be around 9 fs from the measured x-ray yields.

The effects of short-range disorder on the polarization characteristics of light in photonic crystal waveguides were investigated using finite-difference time-domain simulations with a view to investigating the stability of polarization singularities. It was found that points of local circular polarization ($C$ points) and contours of linear polarization ($L$ lines) continued to appear even in the presence of high levels of disorder, and that they remained close to their positions in the ordered crystal. These results are a promising indication that devices exploiting polarization in these structures are viable given current fabrication standards.

The search for small violations of the validity of the symmetrization postulate and of the spin-statistics connection (SSC) has been addressed in the last four decades by experimental tests performed in different physical systems of identical fermions or bosons. In parallel and consequently, theories extending the quantum mechanics to a more general level have been proposed to explain such possible violations. In this paper, we present the most stringent test to a possible violation of the SSC under permutation of the bosonic ${}^{16}\mathrm{O}$ nuclei in the ${}^{12}\mathrm{C}{}^{16}\mathrm{O}_2$ molecule. An upper limit of $3.8\times {10}^{-12}$ for an SSC-anomalous ${\mathrm{CO}}_2$ molecule is obtained using saturated-absorption cavity ring-down spectroscopy in the SSC-forbidden $\left({00}^{0}1-{00}^{0}0\right) R$(25) rovibrational transition of ${}^{12}\mathrm{C}{}^{16}\mathrm{O}_2$ at a $4.25\text{−}\mu \mathrm{m}$ wavelength. Quantum mechanics implications of this result are discussed in the frame of the $q$-mutator theory. Finally, the perspective of stringent experimental tests of the symmetrization postulate in molecules that contain three or more identical nuclei is discussed.