In this article, we demonstrate that the angles at which second-harmonic (SH) waves are generated at ferroelectric domain walls satisfy the Snell law for nonlinear media. Nonlinear reflection and refraction are observed experimentally and the relation is found to be in good agreement with theoretical predictions. The ratio of the intensities of refracted and reflected waves has been measured. Under an anomalous-dispersion-like condition, the forbidden nonlinear reflection and refraction is analyzed and found to have a behavior similar to that of the total internal reflection in linear optics. In the periodic domain structure, the coherent superposition of SH waves has been observed, on the basis of which we have proposed a comprehensive theory to explain nonlinear effects in multilayered structures.

The one-clean qubit model (or the DQC1 model) is a restricted model of quantum computing where only a single qubit of the initial state is pure and others are maximally mixed. Although the model is not universal, it can efficiently solve several problems whose classical efficient solutions are not known. Furthermore, it was recently shown that if the one-clean qubit model is classically efficiently simulated, the polynomial hierarchy collapses to the second level. A disadvantage of the one-clean qubit model is, however, that the clean qubit is too clean: for example, in realistic NMR experiments, polarizations are not high enough to have the perfectly pure qubit. In this paper, we consider a more realistic one-clean qubit model, where the clean qubit is not clean, but depolarized. We first show that, for any polarization, a multiplicative-error calculation of the output probability distribution of the model is possible in a classical polynomial time if we take an appropriately large multiplicative error. The result is in strong contrast with that of the ideal one-clean qubit model where the classical efficient multiplicative-error calculation (or even the sampling) with the same amount of error causes the collapse of the polynomial hierarchy. We next show that, for any polarization lower-bounded by an inverse polynomial, a classical efficient sampling (in terms of a sufficiently small multiplicative error or an exponentially small additive error) of the output probability distribution of the model is impossible unless BQP (bounded error quantum polynomial time) is contained in the second level of the polynomial hierarchy, which suggests the hardness of the classical efficient simulation of the one nonclean qubit model.

Quantifying coherence is a key task in both quantum-mechanical theory and practical applications. Here, a reliable quantum coherence measure is presented by utilizing the quantum skew information of the state of interest subject to a certain broken observable. This coherence measure is proven to fulfill all the criteria (especially the strong monotonicity) recently introduced in the resource theories of quantum coherence. The coherence measure has an analytic expression and an obvious operational meaning related to quantum metrology. In terms of this coherence measure, the distribution of the quantum coherence, i.e., how the quantum coherence is distributed among the multiple parties, is studied and a corresponding polygamy relation is proposed. As a further application, it is found that the coherence measure forms the natural upper bounds for quantum correlations prepared by incoherent operations. The experimental measurements of our coherence measure as well as the relative-entropy coherence and $l_p$-norm coherence are studied finally.

Quantum digital signatures (QDSs) provide a means for signing electronic communications with information-theoretic security. However, all previous demonstrations of quantum digital signatures assume trusted measurement devices. This renders them vulnerable against detector side-channel attacks, just like quantum key distribution. Here we exploit a measurement-device-independent (MDI) quantum network, over a metropolitan area, to perform a field test of a three-party MDI QDS scheme that is secure against any detector side-channel attack. In so doing, we are able to successfully sign a binary message with a security level of about ${10}^{-7}$. Remarkably, our work demonstrates the feasibility of MDI QDSs for practical applications.

We present calculations of the one-loop vacuum polarization correction (Uehling potential) for the three-body problem in the NRQED formalism. The case of one-electron molecular systems is considered. Numerical results of the vacuum polarization contribution at $m{\alpha }^7$ and higher orders for the fundamental transitions $(v=0, L=0)\to (v^{\prime }=1, L^{\prime }=0)$ in the ${\mathrm{H}}_2^{+}$ and ${\mathrm{HD}}^{+}$ molecular ions are presented and compared with calculations performed in the adiabatic approximation. The residual uncertainty from this contribution in the transition frequencies is shown to be a few tens of Hz.

The interaction between a Rydberg electron and a neutral atom situated inside its extended orbit is described via contact interactions for each atom-electron scattering channel. In ultracold environments, these interactions lead to long-range molecules with binding energies typically ranging from 10 to $1\times {10}^4$ MHz. These energies are comparable to the relativistic and hyperfine structure of the separate atomic components. Studies of molecular formation aiming to reproduce observations with spectroscopic accuracy must therefore include the hyperfine splitting of the neutral atom and the spin-orbit splittings of both the Rydberg atom and the electron-atom interaction. Adiabatic potential energy curves and permanent electric dipole moments are presented for ${\mathrm{Rb}}_2$ and ${\mathrm{Cs}}_2$. The influence of spin degrees of freedom on the potential energy curves and multipole moments probed in recent experimental work is elucidated, and the observed dipole moments of butterfly molecules are explained by the generalized ${}^{3}P_J$ pseudopotential derived here.

The coherent nuclear motion in photoexcited 2,4-difluorophenol (2,4-DFP) has been visualized in real time using femtosecond time-resolved ion-yield spectroscopy and time-resolved photoelectron imaging. A coherent vibrational packet is created by the simultaneous excitation of the out-of-plane bending modes in the first excited electronic state $\left(S_1\right)$ in 2,4-DFP. By virtue of the geometry change upon photoionization, the wave packets around the planar geometry are ionized exclusively with selective probe wavelengths, exhibiting the pronounced quantum beats superimposed on the parent-ion transients. Furthermore, photoionization signals of the planar and nonplanar geometries are both acquired in the photoelectron spectra but dispersed with respect to kinetic energy via resonances with different intermediate Rydberg states. The time dependences of the photoelectron peaks originating from the planar and nonplanar geometries exhibit the clear beats with similar periodicities but a phase shift of π rad, offering an unambiguous picture of the oscillating nuclear motion between the planar geometry and the nonplanar minimum.

By two-color photoassociation of ${}^{40}\mathrm{Ca}$ four weakly bound vibrational levels in the ${\mathrm{Ca}}_2 X{}^1{\mathrm{\Sigma }}_g^{+}$ ground-state potential were measured, using highly spin-forbidden transitions to intermediate states of the coupled system ${{}^3\mathrm{\Pi }}_u$ and ${{}^3\mathrm{\Sigma }}_u^{+}$ near the ${{}^{3}P}_1+{{}^{1}S}_0$ asymptote. From the observed binding energies, including the least bound state, the long-range dispersion coefficients ${\mathrm{C}}_6,{\mathrm{C}}_8,{\mathrm{C}}_{10}$ and a precise value for the $s$-wave scattering length of 308.5(50)$a_0$ were derived. From mass scaling we also calculated the corresponding scattering length for other natural isotopes. From the Autler-Townes splitting of the spectra, the molecular Rabi frequency has been determined as a function of the laser intensity for one bound-bound transition. The observed value for the Rabi frequency is in good agreement with calculated transition moments based on the derived potentials, assuming a dipole moment being independent of internuclear separation for the atomic pair model.

Streaked photoemission from nanostructured surfaces and nanoparticles by attosecond extreme ultraviolet pulses into an infrared (IR) or visible streaking pulse allows for sub-fs-resolution of the plasmonically enhanced streaking-pulse electric field. It thus holds promise for the time-resolved imaging of the dielectric response in and plasmonic fields near nanostructures. After calculating the plasmonic field induced by IR and visible streaking pulses in 10- to 200-nm diameter Au, Ag, and Cu nanospheres, we numerically simulated streaked photoelectron spectra within a quantum-mechanical model. Our spectra show significant oscillation-amplitude enhancements and phase shifts relative to calculations that neglect the induced plasmonic field. We trace these observable effects to the distinct dielectric properties of the three investigated metals, demonstrating the applicability of streaking spectroscopy to the element-specific investigation of induced time-dependent electric fields near nanoparticle surfaces.

We show how to emulate a conventional pump-probe scheme using a single frequency-chirped ultrashort UV pulse to obtain a time-resolved image of molecular ultrafast dynamics. The chirp introduces a spectral phase in time that encodes the delay between the pump and the probe frequencies contained in the pulse. By comparing the results of full dimensional ab initio calculations for the ${\mathrm{H}}_2{}^{+}$ molecule with those of a simple sequential model, we demonstrate that, by tuning the chirp parameter, two-photon energy-differential ionization probabilities directly map the wave-packet dynamics generated in the molecule. As a result, one can also achieve a significant amount of control of the total ionization yields, with a possible enhancement by more than an order of magnitude.

We study the effects of synthetic spin-orbit coupling on the pairing physics in quasi-one-dimensional ultracold Fermi gases of alkaline-earth-metal-like atoms near an orbital Feshbach resonance (OFR). The interplay between spin-orbit coupling and pairing interactions near the OFR leads to an interesting topological Fulde-Ferrell state, where the nontrivial topology of the state is solely encoded in the closed channel with a topologically trivial Fulde-Ferrell pairing in the open channel. We confirm the topological property of the system by characterizing the Zak phase and the edge states. The topological Fulde-Ferrell state can be identified by the momentum-space density distribution obtained from time-of-flight images.

Recent experimental advances in the cooling and manipulation of bialkali-metal dimer molecules have enabled the production of gases of ultracold molecules that are not chemically reactive. It has been presumed in the literature that in the absence of an electric field the low-energy scattering of such nonreactive molecules (NRMs) will be similar to atoms, in which a single $s$-wave scattering length governs the collisional physics. However, Doçaj et al. [Phys. Rev. Lett. 116, 135301 (2016)] argued that the short-range collisional physics of NRMs is much more complex than for atoms and that this leads to a many-body description in terms of a multichannel Hubbard model. In this work we show that this multichannel Hubbard model description of NRMs in an optical lattice is robust against the approximations employed by Doçaj et al. to estimate its parameters. We do so via an exact, albeit formal, derivation of a multichannel resonance model for two NRMs from an ab initio description of the molecules in terms of their constituent atoms. We discuss the regularization of this two-body multichannel resonance model in the presence of a harmonic trap and how its solutions form the basis for the many-body model of Doçaj et al.. We also generalize the derivation of the effective lattice model to include multiple internal states (e.g., rotational or hyperfine). We end with an outlook to future research.

We calculate the parameters of the recently derived many-channel Hubbard model that is predicted to describe ultracold nonreactive molecules in an optical lattice, going beyond the approximations used by Doçaj et al. [A. Doçaj et al., Phys. Rev. Lett. 116, 135301 (2016)]. Although those approximations are expected to capture the qualitative structure of the model parameters, finer details and quantitative values are less certain. To set expectations for experiments, whose results depend on the model parameters, we describe the approximations' regime of validity and the likelihood that experiments will be in this regime, discuss the impact that the failure of these approximations would have on the predicted model, and develop theories going beyond these approximations. Not only is it necessary to know the model parameters in order to describe experiments, but the connection that we elucidate between these parameters and the underlying assumptions that are used to derive them will allow molecule experiments to probe new physics. For example, transition state theory, which is used across chemistry and chemical physics, plays a key role in our determination of lattice parameters, thus connecting its physical assumptions to highly accurate experimental investigation.

We consider the propagation of Gaussian spatiotemporal wave packets in arrays of parallel optical waveguides, assuming linear and nondispersive coupling between the adjacent guides. The numerical analysis is based on a discrete version of the modified Korteweg–de Vries equation that adequately describes the propagation of ultrashort (few-cycle) spatiotemporal solitons in waveguide arrays. Two kinds of such discrete-continuous localized wave forms, which are discrete solitons in the transverse direction, and few-cycle solitons in the longitudinal one, are put forward, namely breathing solitons and single-humped ones. The conditions of formation of these localized spatiotemporal structures, their time duration and spatial width, as well as their energies, are also investigated.

We theoretically investigate the spectral entanglement of a multiphoton source generated from the cascade emissions in cascaded cold atomic ensembles. This photon source is highly directional, which is guaranteed under the four-wave mixing condition, and is also highly frequency correlated due to finite driving pulse durations and superradiant decay constants. We utilize Schmidt decomposition to study the bipartite entanglement of the biphoton states projected from the multiphoton ones. This entropy of entanglement can be manipulated by controlling the driving parameters and superradiant decay rates. Moreover, the projected biphoton states in the cascaded scheme can have larger entanglement than the one produced from only one atomic ensemble, which results from larger capacity in multipartite entanglement. This cascaded scheme enables a multiphoton source useful in quantum-information processing. It also allows for potential applications in multimode quantum communication and spectral shaping of high-dimensional continuous frequency entanglement.

We analyze entanglement and nonlocal properties of the convex set of symmetric $N$-qubit states which are diagonal in the Dicke basis. First, we demonstrate that within this set, semidefinite positivity of partial transposition (PPT) is necessary and sufficient for separability—which has also been reported recently by Yu [Phys. Rev. A 94, 060101(R) (2016)]. Furthermore, we show which states among the entangled diagonal symmetric are nonlocal under two-body Bell inequalities. The diagonal symmetric convex set contains a simple and extended family of states that violate the weak Peres conjecture, being PPT with respect to one partition but violating a Bell inequality in such partition. Our method opens directions to address entanglement and nonlocality on higher dimensional symmetric states, where presently very few results are available.

In which-way double-slit experiments with perfect detectors, it is assumed that having a second detector at the slits is redundant because it will not change the interference pattern. We, however, show that if higher-order or nonclassical paths are accounted for, the presence of the second detector will have an effect on the interference pattern. Accounting for these nonclassical paths also means that the Sorkin parameter in triple-slit experiments is only an approximate measure of Born-rule violation. Using the difference between single and double which-way detectors, we give an alternative parameter which is an exact measure of Born-rule violation.

We consider the problem of determining the presence of genuine tripartite entanglement through the violation of Mermin inequality. Although the violation of Mermin inequality cannot certify the presence of genuine nonlocality, it can certify the presence of genuine tripartite entanglement whenever the violation is strictly greater than $2\sqrt{2}$. Here we show that Mermin inequality suffices as a genuine entanglement witness even when its value is $2\sqrt{2}$ if at least two of the local marginal distributions are not completely random, provided the local Hilbert space dimension of at least one of the subsystems is 2. Thus local marginals suffice as a semi-device-independent genuine entanglement witness. This is intriguing in a sense, as the local properties of a composite system can help to identify its global property. Furthermore, analyzing another quantity constructed from Mermin polynomials, we show that the genuine entanglement certification task for the correlations with Mermin inequality violation equal to $2\sqrt{2}$ can actually be made fully device independent.

We provide a large class of quantum evolutions governed by the memory kernel master equation. This class defines a quantum analog of so-called semi-Markov classical stochastic dynamics. In this paper we provide a precise definition of quantum semi-Markov evolution, and using the appropriate gauge freedom we propose a suitable generalization which contains a majority of examples considered so far in the literature. The key concepts are quantum counterparts of classical waiting time distribution and survival probability—a quantum waiting time operator and a quantum survival operator, respectively. In particular collision models and their generalizations considered recently are special examples of generalized semi-Markov evolution. This approach allows for an interesting generalization of the trajectory description of the quantum dynamics in terms of positive operator-valued measure densities.

We investigate the quantum Zeno and anti-Zeno effects in quantum dissipative systems by employing a hierarchical equations of motion approach which is beyond the usual Markovian approximation, the rotating wave approximation, and the perturbative approximation. The quantum Zeno and anti-Zeno dynamics of a biased qubit-boson model and a biased qutrit-boson model are provided as illustrative examples. It is found that (i) there exists multiple Zeno–anti-Zeno crossover phenomena, (ii) the non-Markovian characteristic of the bath may be favorable for the accessibility of the Zeno dynamics, and (iii) high bath temperature may add the difficulty in observing the quantum Zeno effect in quantum dissipative systems.