Digital signatures are widely used in modern communication to guarantee authenticity and transferability of messages. The security of currently used classical schemes relies on computational assumptions. We present a quantum signature scheme that does not require trusted quantum channels. We prove that it is unconditionally secure against the most general coherent attacks, and show that it requires the transmission of significantly fewer quantum states than previous schemes. We also show that the quantum channel noise threshold for our scheme is less strict than for distilling a secure key using quantum key distribution. This shows that “direct” quantum signature schemes can be preferable to signature schemes relying on secret shared keys generated using quantum key distribution.

We describe a simple experimental apparatus for laser ablation of barium monohydride (BaH) molecules and the study of their rovibrational spectra relevant to direct laser cooling. BaH is a promising candidate for laser cooling and ultracold fragmentation, both of which are precursors to novel experiments in many-body physics and precision measurement. We present a detailed analysis of the properties of ablation plumes that can improve the understanding of surface ablation and deposition technologies. A range of absorption spectroscopy and collisional thermalization regimes has been studied. We directly measured the Franck-Condon factor of the $B{}^2{\mathrm{\Sigma }}^{+}(v^{\prime }=0)\leftarrow X{}^2{\mathrm{\Sigma }}^{+}(v^{\prime \prime }=1)$ cycling transition. Prospects for production of a high luminosity cryogenic BaH beam are outlined.

We present a detailed experimental-theoretical analysis of O $K$-edge resonant $1\sigma -2\pi$ inelastic x-ray scattering (RIXS) from carbon monoxide with unprecedented energy resolution. We employ high-level ab initio calculations to compute the potential energy curves of the states involved in the RIXS process and simulate the measured RIXS spectra using the wave-packet-propagation formalism, including Coulomb coupling in the final-state manifold. The theoretical analysis allows us to explain all the key features of the experimental spectra, including some that were not seen before. First, we clearly show the interference effect between different RIXS channels corresponding to the transition via orthogonal ${}^1\mathrm{\Pi }_x$ and ${}^1\mathrm{\Pi }_y$ core-excited states of CO. Second, the RIXS region of 13 eV energy loss presents a triple structure, revealed only by the high-resolution measurement. In previous studies, this region was attributed solely to a valence state. Here we show a strong Coulomb mixing of the Rydberg and valence final states, which opens the forbidden RIXS channels to the “dark” final Rydberg states and drastically changes the RIXS profile. Third, using a combination of high-resolution experiment and high-level theory, we improve the $|4{\sigma }^{-1}2{\pi }^1\rangle$ final-state potential-energy curve by fitting its bottom part with the experiment. Also, the coupling constants between Rydberg and valence states were refined via comparison with the experiment. Our results illustrate the large potential of the RIXS technique for advanced studies of highly excited states of neutral molecules.

We theoretically study the phenomenon of propulsion through Casimir forces in thermal nonequilibrium. Using fluctuational electrodynamics, we derive a formula for the propulsion force for an arbitrary small object in two scenarios: (i) for the object being isolated, and (ii) for the object being close to a planar surface. In the latter case, the propulsion force (i.e., the force parallel to the surface) increases with decreasing distance, i.e., it couples to the near field. We numerically calculate the lateral force acting on a hot spheroid near a surface and show that it can be as large as the gravitational force, thus being potentially measurable in fly-by experiments. We close by linking our results to well-known relations of linear-response theory in fluctuational electrodynamics: Looking at the friction of the anisotropic object for constant velocity, we identify a correction term that is additional to the typically used approach.

The effect of the ionic Coulomb field in laser-induced above-threshold ionization is analyzed in terms of the classical mapping of the initial conditions (time of ionization and momentum transverse to the laser field) on the plane of the final momenta. This mapping brings out the Coulomb effect especially clearly because changes of the transverse momentum are only possible via the action of the Coulomb field. We formulate a model that restricts the action of the Coulomb field to a sequence of successive interactions in hard and soft recollisions and find an analytical approximation to this model. It allows us to trace the low-energy structure to the detailed behavior of the Coulomb corrections to the longitudinal and the transverse momentum and their interplay, which causes a caustic in the plane of the final momentum.

We study the mean-field BCS-BEC evolution of a uniform Fermi gas on a single-band triangular lattice and construct its ground-state phase diagrams, showing a wealth of topological quantum phase transitions between gapped and gapless superfluids that are induced by the interplay of an out-of-plane Zeeman field and a generic non-Abelian gauge field.

Gaussian quantum systems exhibit many explicitly quantum effects but can be simulated classically. By using both the Hilbert space (Koopman) and the phase-space (Moyal) formalisms we investigate how robust this classicality is. We find failures of consistency of the dynamics of hybrid classical-quantum systems from both perspectives. By demanding that no unobservable operators couple to the quantum sector in the Koopmanian formalism, we show that the classical equations of motion act on their quantum counterparts without experiencing any back reaction, resulting in nonconservation of energy in the quantum system. By using the phase-space formalism we study the short-time evolution of the moment equations of a hybrid classical-Gaussian quantum system and observe violations of the Heisenberg uncertainty relation in the quantum sector for a broad range of initial conditions. We estimate the timescale for these violations, which is generically rather short. This inconsistency indicates that while many explicitly quantum effects can be represented classically, quantum aspects of the system cannot be fully masked. We comment on the implications of our results for quantum gravity.

We determine the optimal parameters for a simple and efficient scheme of dispersive readout of a qubit. Depending on the qubit state (ground or excited), the resonance of a cavity is shifted either to the red or to the blue side. Qubit state is inferred by detecting the photon number transmitted through the cavity. It turns out that this kind of detection provides better measurement fidelity than the detection of the presence or absence of photons only. We show that radiating the cavity on either of the frequencies it shifts to results in a suboptimal measurement. The optimal frequency of the probe photons is determined, as well as the optimal ratio of the shift to the resonator leakage. It is shown that to maximize the fidelity of a long-lasting measurement, it is sufficient to use the parameters optimizing the signal-to-noise ratio in the photon count. One can reach 99% fidelity for a single-shot measurement in various physical realizations of the scheme.

We develop a dynamical approach to study the Casimir-Polder force between an initially bare molecule and a magnetodielectric body at finite temperature, valid for arbitrary magnetodielectric properties and also in the presence of chiral effects. Switching on the interaction between the molecule and the field at a particular time, we study the resulting temporal evolution of the Casimir-Polder interaction. The dynamical self-dressing of the molecule and its population-induced dynamics are accounted for and discussed. In particular, we find that the Casimir-Polder force between a molecule and a surface oscillates in time with a frequency related to the molecular transition frequency. We verify that the dynamical force converges to the static result for time much larger than the inverse of the transition frequency, and it is particularly strong around the back-reaction time $t=2d/c$, the time needed for the molecule to emit and reabsorb a photon reflected by the surface.

We report elastic differential cross sections (DCSs) for electron interactions with acetone, ${\mathrm{C}}_3{\mathrm{H}}_6\mathrm{O}$. The incident electron energy range was 7.0–50 eV, and the scattered electron angular range for the differential measurements varied from 10° to 120°. The calculated cross sections were obtained with two different methodologies, the Schwinger multichannel method with pseudopotentials (SMCPP), and the independent-atom method with screening-corrected additivity rule (IAM-SCAR). The present elastic DCSs have been found to agree well with the results of IAM-SCAR calculations above 20 eV, and also with the SMC calculations below 30 eV. Although some discrepancies were found for lower energies, the agreement between the SMCPP data and the DCSs obtained with the IAM-SCAR method improves, as expected, as the impact energy increases. Comparison with previous DCSs shows good agreement albeit the present data is extended down to lower electron impact energies. We find a low-lying ${\pi }^{*}$ shape resonance located at 2.6 eV, in agreement with recent results on electron collisions with acetone [M. G. P. Homem et al., Phys. Rev. A 92, 032711 (2015)]. The presence of a ${\sigma }^{*}$ resonance is also discussed.

Using exact diagonalization methods we study possible phases in a one-dimensional model of two differently populated fermionic species in a periodically driven optical lattice. The shaking amplitude and frequency are chosen to resonantly drive $s\text{−}p$ transition while minimizing the standard intraband tunnelings. We verify numerically the presence of an emergent density wave configuration of composites for appropriate filling fraction and minimized intraband tunnelings. The majority fermions moving in such a lattice mimic the celebrated Rice-Mele model. Far away from that region, the structure changes to a clustered phase, with the intermediate phase abundantly populated by defects of the density wave. These defects lead to localized modes carrying fractional particle charge. The results obtained are compared with earlier approximate predictions.

We show in this Comment that the nonclassicality indicator proposed in Sadeghi et al. [Phys. Rev. A 82, 012102 (2010)] is controversial. Defined in terms of the interferences displayed by the quasidistribution function of at least two superposition states, this indicator may logically lead to zero for any single quantum state irrespective to its nonclassicality, in contradiction with their results.

Sadeghi et al. [Phys. Rev. A 82, 012102 (2010)] presented a new nonclassicality indicator which works properly for Husimi and Rivier distribution functions as well as the Wigner distribution function. Some objections on the Sadeghi nonclassicality indicator are presented by Kenfack (preceding Comment, “Nonclassicality indicator for the real phase-space distribution functions”, Phys. Rev. A 93, 036101 (2016). In this Reply, some misconceptions on Sadeghi's nonclassicality indicator are discussed and clarified.

We prove that there exists a family of quantum systems that can be cooled to their ground states by a one-shot projective measurement on the ancillas coupled to these systems. Consequently, this proof gives rise to the conditions for achieving the one-shot measurement ground-state cooling (OSMGSC). We also propose a general procedure for finding unitary propagators and corresponding Hamiltonians to realize such cooling by means of inverse engineering techniques.

For a closed quantum system described by a time-periodic Hamiltonian, Floquet theorem says that the time evolution operator can be written as $U(t,0)\equiv P\left(t\right)e^{\frac{-i}{\hslash }{H}_{F}t}$ with $P(t+T)=P\left(t\right)$, and $H_F$ is Hermitian and time independent and called the Floquet Hamiltonian. In this work, we extend the Floquet theorem from closed to open systems described by a time-periodic Lindblad master equation with period $T=\frac{2\pi }{\omega }$. We derive an expansion for the Lindbladian in powers of $\frac{1}{\omega }$. Two examples are presented to illustrate the theorem. We find that the results given by the Lindbladian expansion based on the open system Floquet theorem agree well with the exact dynamics in the high-frequency limit. It may find applications in quantum engineering with systems subject to decoherence.

The vector vortex (VV) beam, originally introduced to exhibit a form of single-particle quantum entanglement between different degrees of freedom, has specific applications for quantum-information protocols. In this paper, by combining measurement-device-independent quantum key distribution (MDIQKD) with a spontaneous parametric-downconversion source (SPDCS), we present a modified MDIQKD scheme with pairs of VV beams, which shows a structure of hybrid entangled entanglement corresponding to intrasystem entanglement and intersystem entanglement. The former entanglement, which is entangled between polarization and orbit angular momentum within each VV beam, is adopted to overcome the polarization misalignment associated with random rotations in quantum key distribution. The latter entanglement, which is entangled between the two VV beams, is used to perform the MDIQKD protocol with SPDCS to inherit the merit of the heralded process. The numerical simulations show that our modified scheme has apparent advances both in transmission distance and key-generation rate compared to the original MDIQKD. Furthermore, our modified protocol only needs to insert $q$ plates in a practical experiment.

We analyze the asymptotic scaling of persistence of unvisited sites for quantum walks on a line. In contrast to the classical random walk, there is no connection between the behavior of persistence and the scaling of variance. In particular, we find that for a two-state quantum walk persistence follows an inverse power law where the exponent is determined solely by the coin parameter. Moreover, for a one-parameter family of three-state quantum walks containing the Grover walk, the scaling of persistence is given by two contributions. The first is the inverse power law. The second contribution to the asymptotic behavior of persistence is an exponential decay coming from the trapping nature of the studied family of quantum walks. In contrast to the two-state walks, both the exponent of the inverse power-law and the decay constant of the exponential decay depend also on the initial coin state and its coherence. Hence, one can achieve various regimes of persistence by altering the initial condition, ranging from purely exponential decay to purely inverse power-law behavior.