The theory of a frequency shift in primary frequency standards due to microwave lensing in Gibble [Phys. Rev. A 90, 015601 (2014)] contains a number of problems that undermine its validity. Furthermore, because the exposition of the theory has multiple errors and because the shift has never been experimentally observed, we believe this possible shift should not be included as a correction to primary frequency standards contributing to international atomic time. Although the theory may describe the basic mechanisms of a possible frequency shift, we argue it is not possible to use this theory to make reliable corrections to a primary frequency standard at the $\delta f/f\sim {10}^{-16}$ level.

The Comment by Jefferts et al. [Phys. Rev. A 91, 067601 (2015)] discusses the microwave-lensing frequency shift's possible dependence on the initial wave-packet size and two effects of wall interactions, the frequency shifts that they produce, and the nature of how dressed states are clipped by apertures. I identify conceptual errors in their criticisms, some of which are related to fundamental problems in their lensing treatment [Ashby et al., Phys. Rev. A 91, 033624 (2015)] for the NIST-F1 and NIST-F2 atomic clocks [K. Gibble, arXiv:1505.00691]. Aside from typesetting errors that they note, the criticisms in the Comment are shown to be incorrect.

Recent experiments with increasingly larger numbers of qubits have sparked renewed interest in adiabatic quantum computation, and in particular quantum annealing. A central question that is repeatedly asked is whether quantum features of the evolution can survive over the long time scales used for quantum annealing relative to standard measures of the decoherence time. We reconsider the role of decoherence in adiabatic quantum computation and quantum annealing using the adiabatic quantum master-equation formalism. We restrict ourselves to the weak-coupling and singular-coupling limits, which correspond to decoherence in the energy eigenbasis and in the computational basis, respectively. We demonstrate that decoherence in the instantaneous energy eigenbasis does not necessarily detrimentally affect adiabatic quantum computation, and in particular that a short single-qubit $T_2$ time need not imply adverse consequences for the success of the quantum adiabatic algorithm. We further demonstrate that boundary cancellation methods, designed to improve the fidelity of adiabatic quantum computing in the closed-system setting, remain beneficial in the open-system setting. To address the high computational cost of master-equation simulations, we also demonstrate that a quantum Monte Carlo algorithm that explicitly accounts for a thermal bosonic bath can be used to interpolate between classical and quantum annealing. Our study highlights and clarifies the significantly different role played by decoherence in the adiabatic and circuit models of quantum computing.

We present two efficient schemes for the deterministic generation and the complete nondestructive analysis of hyperentangled Bell states in both the polarization and spatial-mode degrees of freedom (DOF) of two-photon systems, assisted by the nitrogen-vacancy (NV) centers in diamonds coupled to microtoroidal resonators as a result of cavity quantum electrodynamics. With the input-output process of photons, two-photon polarization-spatial hyperentangled Bell states can be generated in a deterministic way and their complete nondestructive analysis can be achieved. These schemes can be generalized to generate and analyze hyperentangled Greenberger-Horne-Zeilinger states of multiphoton systems as well. Compared with previous works, these two schemes relax the difficulty of their implementation in experiment as it is not difficult to obtain the $\pi$ phase shift in single-sided NV-cavity systems. Moreover, our schemes do not require that the transmission for the uncoupled cavity is balanceable with the reflectance for the coupled cavity. Our calculations show that these schemes can reach a high fidelity and efficiency with current technology, which may be a benefit to long-distance high-capacity quantum communication with two DOF of photon systems.

We propose a scheme for translating metrological precision bounds into lower bounds on query complexity of quantum search algorithms. Within the scheme the link between quadratic performance enhancement in idealized quantum metrological and quantum computing schemes becomes clear. More importantly, we utilize results from the field of quantum metrology on a generic loss of quadratic quantum precision enhancement in the presence of decoherence to infer an analogous generic loss of quadratic speedup in oracle based quantum computing. While most of our reasoning is rigorous, at one of the final steps, we need to make use of an unproven technical conjecture. We hope that we will be able to amend this deficiency in the near future, but we are convinced that even without the conjecture proven our results provide a deep insight into the relationship between quantum algorithms and quantum metrology protocols.

We present polarization-controlled multiphoton two-color above-threshold ionization (TCATI) of molecules. The intensity modulations of valence photoelectron intensities of molecules arising from varying the relative orientation of the linear polarization vectors of femtosecond infrared (IR) and vacuum-ultraviolet (VUV) radiation in TCATI of the highest occupied molecular orbitals of ${\mathrm{H}}_2\mathrm{O}, {\mathrm{O}}_2$, and ${\mathrm{N}}_2$ are reported. The results on the molecular systems are compared to the $3p$ photoionization of atomic Ar, which serves as a reference system. Modeling the large differences of the modulation amplitudes within the soft-photon approximation enables us to extract the one-photon-ionization anisotropy parameter ${\beta }_2$. Accounting only for the first sideband due to two-photon TCATI by one VUV and one IR photon we find satisfactory agreement between experiment and simulation for ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{O}}_2$. However, the model fails for ${\mathrm{N}}_2$ and possible reasons are discussed. We discuss that the described approach may represent an alternative way of determining photoelectron angular distributions from valence shells of molecules and indicate future directions for modeling TCATI of molecules.

We study the propagation of electromagnetic waves in layered photonic crystals formed by materials with a spontaneous nonreciprocal response, such as Tellegen (axion) media or topological insulators. Surprisingly, it is proven that stratified Tellegen photonic crystals that break simultaneously the space inversion and time-reversal symmetries have always symmetric dispersion diagrams. Interestingly, we show that by combining chiral and nonreciprocal materials the photonic band diagrams can exhibit a spectral asymmetry such that $\omega \left(\mathbf{k}\right)\ne \omega \left(-\mathbf{k}\right)$. Furthermore, it is demonstrated that in some conditions two juxtaposed Tellegen medium layers have an electromagnetic response analogous to that of a biased ferrite slab.

We observed experimentally nonlinear variations in the three-vertex geometric phase in a two-photon polarization qutrit. The three-vertex geometric phase is defined by three quantum states, which generally form a three-state (qutrit) system. By changing one of the three constituent states, we observed two rapid increases in the three-vertex geometric phase. The observed variations are inherent in a three-state system and cannot be observed in a two-state system. We used a time-reversed two-photon interferometer to measure the geometric phase with much more intense signals than those of a typical two-photon interferometer.

We provide necessary and sufficient conditions for freezing of quantum correlations as measured by quantum discord and quantum work deficit in the case of bipartite as well as multipartite states subjected to local noisy channels. We recognize that inhomogeneity of the magnetizations of the shared quantum states plays an important role in the freezing phenomena. We show that the frozen value of the quantum correlation and the time interval for freezing follow a complementarity relation. For states which do not exhibit “exact” freezing, but can be frozen “effectively,” by having a very slow decay rate with suitable tuning of the state parameters, we introduce an index—the freezing index—to quantify the goodness of freezing. We find that the freezing index can be used to detect quantum phase transitions and discuss the corresponding scaling behavior.

The role of measurement-induced disturbance in weak measurements is of central importance for the interpretation of the weak value. Uncontrolled disturbance can interfere with the postselection process and make the weak value dependent on the details of the measurement process. Here we develop the concept of a generalized weak measurement for classical and quantum mechanics. The two cases appear remarkably similar, but we point out some important differences. A priori it is not clear what the correct notion of disturbance should be in the context of weak measurements. We consider three different notions and get three different results: (1) For a “strong” definition of disturbance, we find that weak measurements are disturbing. (2) For a weaker definition we find that a general class of weak measurements is nondisturbing, but that one gets weak values which depend on the measurement process. (3) Finally, with respect to an operational definition of the “degree of disturbance,” we find that the AAV weak measurements are the least disturbing, but that the disturbance is always nonzero.

The local convertibility of quantum states, measured by the Rényi entropy, is concerned with whether a state can be transformed into another state, using only local operations and classical communications. We found that in the one-dimensional Kitaev chain with quenched chemical potential $\mu$, the convertibility between the state for $\mu$ and that for $\mu +\delta \mu$ depends on the quantum phases of the system ($\delta \mu$ is a perturbation). This is similar to the adiabatic case where the ground state is considered. Specifically, when the quenched system has edge modes and the subsystem size for the partition is much larger than the correlation length of the Majorana fermions which forms the edge modes, the quenched state is locally inconvertible. We give a physical interpretation for the result, based on analyzing the interactions between the two subsystems for various partitions. Our work should help to better understand the many-body phenomena in topological systems and also the entanglement properties in the Majorana fermionic quantum computation.

To avoid the combinatorial computational cost of configuration interaction (CI), we previously introduced the symmetric tensor decomposition CI (STD-CI) method, which takes advantage of the antisymmetric nature of the electronic wave function and expresses the CI coefficients compactly as a series of Kronecker product states (STD series) [W. Uemura and O. Sugino, Phys. Rev. Lett. 109, 253001 (2012)]. Here we extend the variational degrees of freedom by using different molecular orbitals for different terms in the STD series. This scheme is equivalent to the linear combination of the Hartree-Fock-Bogoliubov state or the antisymmetrized geminal powers (AGPs). The total energy converges very rapidly within 0.72 $\mu \mathrm{hartree}$ taking only 10 terms for the water molecule, and the convergence is likewise fast for Hubbard tetramers. The computational cost scales as the fifth power of the number of electrons and the square of the number of terms in the STD series, indicating the promise of this AGP-based scheme for highly accurate and efficient computation of quantum systems.

We study the positron–nitrogen-molecule scattering process under the rovibrational coupling method to include the effect of rotational and vibrational motion of the nuclei dynamically during the scattering process. Here we compute the angle integrated elastic and (state-to-state) rotational excitation, elastic and (state-to-state) vibrational (summed over rotational) excitation, and total (summed over rotational and vibrational) cross sections for the incident positron energy between 0.0 and 10 eV. However, in present paper we concentrate our discussion on the results in the lower-energy region, especially below 3.0 eV. To calculate the cross sections we use the model correlation polarization potential to include the distortion effect of the target electronic state in the presence of the positron. The calculated total cross sections are compared with the theoretical calculations and experimental results. The present theoretical results agree quite well with the recent theoretical and measured values. The vibrational and rotational elastic and excitation cross sections are also compared with the existing theoretical results. The state-to-state potential coupling and dynamical coupling effects on different cross sections are studied.

It is shown that excitations from the ground configuration to high-$nl$ shells of the ${\text{W}}^{26+}$ ion have a large contribution to the electron-impact ionization process. Large-scale calculations for the excitations to over 900 configurations are performed using the Dirac-Fock-Slater approximation. The investigation includes subconfiguration-average and detailed level-to-level approaches. Our findings illustrate how important it is to study convergence of cross sections for the excitations to the high-$nl$ shells. The obtained results show that the excitations from the $4s$, $4p$, and $4d$ shells to the shells with the orbital quantum number $l=4$ dominate in the indirect process of the ionization. The excitation-autoionization cross sections make about 40% of the total ionization cross sections. Comparison between our data and previous configuration-average distorted-wave calculations for Maxwellian rate coefficients reveals large discrepancies for the direct and indirect parts of the ionization process.

We present half-cycle cutoff features in the water window using high-order-harmonic generation (HHG) with two-cycle, carrier-envelope-phase-controlled pulses at 1.85 $\mu \mathrm{m}$ in a high-pressure neon target. A relative carrier-envelope-phase (CEP) shift of the acquired spectra at different pressures is related to an in situ slip of the CEP in the HHG target. A simple model based on the pressure-dependent dispersion of the target gas reveals the importance of the free electrons for this effect. Our analysis also reveals that the effect is relaxed at high target pressures due to plasma defocusing of the driving pulse. We show that as a consequence of the CEP slip mediated by free electrons, moderate-intensity fluctuations can have a detrimental influence on the generation and application of attosecond pulses. Our findings emphasize the potential of combining high target pressures and long driver wavelengths to reveal subtle details of the complex light-matter interaction of HHG.

Beyond the adiabatic approximation, we develop a quantum theory for optical probe pulses propagating in electromagnetically-induced-transparency (EIT) media by including Langevin noise operators and asking the field operator to satisfy bosonic commutation relation. Influences on the degradation of quantum noise squeezing from optical depth of atomic ensemble, strength of control field, and ground-state decoherence are studied in the slow light, as well as storage and retrieval, for a squeezed probe pulse. Moreover, to give guidelines for realization of quantum interfaces based on EIT media, we demonstrate that the quantum squeezing of output probe pulses could be preserved with a stronger classical control field.

We have theoretically and experimentally investigated surface dispersive shock waves (SDSWs) at the interface between a self-defocusing medium and a linear medium. We demonstrate that SDSWs can form when the linear refractive index of the self-defocusing medium is much greater than that of the linear medium, and the initial nonlinearity far outweighs diffraction. SDSWs have been observed at the interface between air and a weakly absorbing liquid when the power of the input beam far exceeds that needed to trap a surface dark soliton. We also observed the formation of SDSWs when an input beam was projected away from the interface, and observed these patterns at the curved surface.

We explore the formation of bright dissipative solitons in cavities with built-in nonlinear resonators. The solitons in such systems are studied both analytically and numerically. We have found a different kind of bright soliton which emerges due to an intrinsic bistability of the system. The solitonic structures have discontinuity in the plasmonic part of the polaritonic field and exist due to an intrinsic multiplicity of the solution for the plasmonic resonators density. The solutions may have two profoundly different localization scales, referred to as single- and dual-core solitons. We found that the bifurcation diagrams associated with dissipative solitons reveal the existence either a single- or a dual-core soliton depending on the intensity of the fields in a cavity. It was established that the solitons of both kinds can be dynamically stable and thus can be observed experimentally. A physical system for which the predicted phenomena may occur is described in terms of the effective medium model and verified numerically.

We consider a quantum particle, moving on a lattice with a tight-binding Hamiltonian, which is subjected to measurements to detect its arrival at a particular chosen set of sites. The projective measurements are made at regular time intervals $\tau$, and we consider the evolution of the wave function until the time a detection occurs. We study the probabilities of its first detection at some time and, conversely, the probability of it not being detected (i.e., surviving) up to that time. We propose a general perturbative approach for understanding the dynamics which maps the evolution operator, which consists of unitary transformations followed by projections, to one described by a non-Hermitian Hamiltonian. For some examples of a particle moving on one- and two-dimensional lattices with one or more detection sites, we use this approach to find exact expressions for the survival probability and find excellent agreement with direct numerical results. A mean-field model with hopping between all pairs of sites and detection at one site is solved exactly. For the one- and two-dimensional systems, the survival probability is shown to have a power-law decay with time, where the power depends on the initial position of the particle. Finally, we show an interesting and nontrivial connection between the dynamics of the particle in our model and the evolution of a particle under a non-Hermitian Hamiltonian with a large absorbing potential at some sites.

A quantum system can be monitored through repeated interactions with meter systems. The state of the system at time $t$, represented by the density matrix $\rho \left(t\right)$, then becomes conditioned on the information obtained by the meters until that time. More insight in the state of the system at any time $t$ is provided, however, by taking into account the full detection of all meters interacting with the system both in the past and in the future of $t$. We present experiments that use near-resonant atomic probes to monitor the evolution of the quantized field in a cavity. The application of the forward-backward smoothing method to this quantum problem, justified by the past quantum state formalism [S. Gammelmark et al., Phys. Rev. Lett. 111, 160401 (2013)], allows us to resolve a posteriori dynamics, which is not reflected by the usual quantum state $\rho \left(t\right)$.