We study a general family of quantum protocols for position verification and present a class of attacks based on the Clifford hierarchy. These attacks outperform current strategies based on port-based teleportation for a large class of practical protocols. We then introduce the interleaved product protocol, a scheme for position verification involving only the preparation and measurement of single qubit states for which the best available attacks have a complexity exponential in the number of classical bits transmitted.

We systematically study the localization effect in discrete-time quantum walks on a honeycomb network and establish the mathematical framework. We focus on the Grover walk first and rigorously derive the limit form of the walker's state, showing it has a certain probability to be localized at the starting position. The relationship between localization and the initial coin state is concisely represented by a linear map. We also define and calculate the average probability of localization by generating random initial states. Further, coin operators varying with positions are considered and the sufficient condition for localization is discussed. We also similarly analyze another four-state Grover walk. Theoretical predictions are all in accord with numerical simulation results. Finally, our results are compared with previous works to demonstrate the unusual trapping effect of quantum walks on a honeycomb network, as well as the advantages of our method.

In modern circuit QED architectures, superconducting transmon qubits are measured via the state-dependent phase and amplitude shift of a microwave field leaking from a coupled resonator. Determining this shift requires integrating the field quadratures for a nonzero duration, which can permit unwanted concurrent evolution. Here we investigate such dynamical degradation of the measurement fidelity caused by a detuned neighboring qubit. We find that in realistic parameter regimes, where the qubit ensemble-dephasing rate is slower than the qubit-qubit detuning, the joint qubit-qubit eigenstates are better discriminated by measurement than the bare states. Furthermore, we show that when the resonator leaks much more slowly than the qubit-qubit detuning, the measurement tracks the joint eigenstates nearly adiabatically. However, the measurement process also causes rare quantum jumps between the eigenstates. The rate of these jumps becomes significant if the resonator decay is comparable to or faster than the qubit-qubit detuning, thus significantly degrading the measurement fidelity in a manner reminiscent of energy relaxation processes.

In order to understand why carbon dioxide $\left({\mathrm{CO}}_2\right)$ and methane $\left({\mathrm{CH}}_4\right)$ molecules interact differently with surfaces, we investigate the Casimir-Polder energy of a linearly polarizable ${\mathrm{CO}}_2$ molecule and an isotropically polarizable ${\mathrm{CH}}_4$ molecule in front of an atomically thin gold film and an amorphous silica slab. We quantitatively analyze how the anisotropy in the polarizability of the molecule influences the van der Waals contribution to the binding energy of the molecule.

We investigate the two-photon double ionization of beryllium atom induced by ultrashort pulses. We use a time-dependent formalism to evaluate the ionization amplitudes and generalized cross sections for the ejection of the $2s^2$ valence shell electrons in the presence of a fully occupied $1s^2$ frozen core shell. The relative contributions of the two-photon direct and sequential process are systematically explored by varying both pulse duration and central frequency. The energy and angular differential ionization yields reveal the signatures of both mechanisms, as well as the role of electron correlation in both the single and double ionization continua. In contrast with previous results on the helium atom, the presence of an electronic core strongly affects the final state leading to back-to-back electron emission even in the a priori less correlated two-photon sequential mechanism. In particular, a dominant pathway via excitation ionization through the ${\mathrm{Be}}^{+}\left(2p\right)$ determines the profiles and pulse-duration dependencies of the energy and angle differential yields.

The influence of wavelength on nonadiabatic effects in an intensive, circularly polarized laser field has been studied by solving the time-dependent Schrödinger equation of a single active electron of the argon atom in a three-dimensional spherical coordinate system. The nonadiabatic process considering the nonzero initial velocity of the electron is very vital to reproducing the experimental observation. Our calculated photoelectron angular distribution in the directions perpendicular to the polarization plane shows nonadiabatic effects in strong laser ionization. The analysis of angular distribution on the “fast” time scale corresponding to wavelength indicates that as the wavelength gets shorter, the nonadiabatic effects get stronger. While the analysis on the “slow” time scale corresponding to the pulse envelope shows that the short pulse duration comes to play an important role for the nonadiabatic effects. When the pulse duration is more than 15 cycles, the influence of pulse duration on nonadiabatic effects fades away and the effects approach stabilization.

We design fast bias inversions of an asymmetric double well so that the lowest states in each well stay in the same well they started, free from residual motional excitation. This cannot be done adiabatically, and a sudden bias switch produces in general motional excitation. The residual excitation is suppressed by complementing a predetermined fast bias change with a linear ramp whose time-dependent slope compensates for the displacement of the wells. The process, combined with vibrational multiplexing and demultiplexing, can produce vibrational state inversion without exciting internal states, just by deforming the trap.

We discuss the behavior of the Larmor frequency shift and the longitudinal relaxation rate due to nonuniform electromagnetic fields on an assembly of spin-1/2 particles, in adiabatic and nonadiabatic regimes. We also show some general relations between the various frequency shifts and between the frequency shifts and relaxation rates. The remarkable feature of all our results is that they are obtained without any specific assumptions on the explicit form of the correlation functions of the fields. Hence, we expect that our results are valid both for a diffusive and ballistic regime of motion and for arbitrary cell shapes and surface scattering. These results can then be applied to a wide variety of realistic systems.

In photoexcitation processes, if the motion of excited electrons can be precisely steered by the instantaneous electric field of an arbitrary waveform of a Fourier-synthesized laser field, the resultant matter response can be achieved within one optical cycle, usually within the attosecond $(1\mathrm{as}={10}^{-18}\mathrm{s})$ regime. Fourier synthesis of laser fields has been achieved in various ways. However, the general use of Fourier-synthesized laser fields for the control of matter is extremely limited. Here, we report the quantum control of a nonlinear response of a molecular ionization process by using Fourier-synthesized laser fields. The directionally asymmetric molecular tunneling ionization induced by intense $(5.0\times {10}^{12}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2)$ Fourier-synthesized laser fields consisting of fundamental, second-, third-, and fourth-harmonic light achieves the orientation-selective ionization; we utilized the orientation-selective ionization for measurement of the relative phase differences between the fundamental and each harmonic light. Our findings impact not only light-wave engineering but also the control of matter, possibly triggering the creation and establishment of a new methodology that uses Fourier-synthesized laser fields.

We have realized a magnetic tensor gradiometer by interferometrically measuring the relative phase between two spatially separated Bose-Einstein condensates (BECs). We perform simultaneous Ramsey interferometry of the proximate ${}^{87}\mathrm{Rb}$ spin-1 condensates in free fall and infer their relative Larmor phase, and thus the differential magnetic-field strength, with a common-mode phase noise suppression exceeding $50\mathrm{dB}$. By appropriately biasing the magnetic field and separating the BECs along orthogonal directions, we measure in vacuo the magnetic-field-gradient tensor of ambient and applied magnetic fields with a nominal precision of $0.30\mathrm{nT}{\mathrm{mm}}^{-1}$ and a sensor volume of $2\times {10}^{-5}{\mathrm{mm}}^3$. We predict a spin-projection noise-limited magnetic energy resolution of order $\sim 10\hslash$ for typical Zeeman coherence times of trapped condensates with this scheme, even with the low measurement duty cycle of current BEC experiments.

We study excitonic states of an atomic impurity in a Fermi gas, i.e., bound states consisting of the impurity and a hole. Previous studies considered bound states of the impurity with particles from the Fermi sea where the holes formed only part of the particle-hole dressing. Within a two-channel model, we find that, for a wide range of parameters, excitonic states are not ground but metastable states. We further calculate the decay rates of the excitonic states to polaronic and dimeronic states and find they are long-lived, scaling as ${\mathrm{\Gamma }}_{\mathrm{Pol}}^{\mathrm{Exc}}\propto {\left(\mathrm{\Delta }\omega \right)}^{5.5}$ and ${\mathrm{\Gamma }}_{\mathrm{Dim}}^{\mathrm{Exc}}\propto {\left(\mathrm{\Delta }\omega \right)}^4$. We also find that a new continuum of exciton-particle states should be considered alongside the previously known dimeron-hole continuum in spectroscopic measurements. Excitons must therefore be considered a new ingredient in the study of metastable physics currently being explored experimentally.

We describe a surface structure consisting of a metal-air interface where the metallic part consists of two metallic segments with a periodic modulation of the interface between them. Such a structure possesses a different transmissivity for a surface plasmon polariton incident on it from one side of it than it has for a surface plasmon polariton incident on it from the opposite side. This asymmetric transmission of a surface plasmon polariton is based on the suppression of the zero-order Bragg beam which, for a certain value of the modulation depth, is not transmitted through the structure, while the diffraction efficiencies of the +1 and $-1$ Bragg beams can be modified by varying the period of the grating and/or the angle of incidence. For a certain range of the incidence angle one can observe asymmetry in transmittance for the $-1$ mode, while the +1 mode is completely suppressed. By varying the material and geometrical parameters of the diffractive structure one can control the contrast transmission that characterizes the degree of the asymmetry. This property of the structure is demonstrated by the results of computer simulation calculations.

We propose a technique of a simultaneous excitation of disordered qubits providing an effective suppression of inhomogeneous broadening in their spectral density. The technique is based on applying of an optimally chosen $\pi$ pulse with a smooth nonrectangular shape. We study excitation dynamics of an off-resonant qubit subjected to a strong classical electromagnetic driving field with a large reference frequency and slow envelope. Within this solution we optimize the envelope to achieve a preassigned accuracy in qubit synchronization.

In addition to the implementation of parity-time–($\mathcal{PT}$-) symmetric optical systems by carefully and actively controlling the gain and loss, we show that a $2\times 2 \mathcal{PT}$-symmetric Hamiltonian has a unitarily equivalent representation without complex optical potentials in the resulting optical coupler. Through the Naimark dilation in operator algebra, passive $\mathcal{PT}$-symmetric couplers can thus be implemented with a refractive index of real values and asymmetric coupling coefficients. This opens up the possibility to implement general $\mathcal{PT}$-symmetric systems with state-of-the-art asymmetric slab waveguides, dissimilar optical fibers, or cavities with chiral mirrors.

We report on the measurement of the isotope shifts of the $3d^{7}4sa{}^5{F}_5\text{–}3d^{7}4pz{}^5{G}_6^o$ Fe i line at 358 nm between all four stable isotopes ${}^{54}\text{Fe},{}^{56}\text{Fe},{}^{57}\text{Fe}$, and ${}^{58}\text{Fe}$, as well as the hyperfine structure of that line for ${}^{57}\text{Fe}$, the only stable isotope having a nonzero nuclear spin. This line is of primary importance for laser-cooling applications. In addition, an experimental value of the field and specific mass shift coefficients of the transition is reported as well as the hyperfine structure magnetic dipole coupling constant $A$ of the transition excited state in ${}^{57}\text{Fe}$, namely $A\left(3d^{7}4pz{}^5{G}_6^o\right)=31.241\left(48\right)$ MHz. The measurements were carried out by means of laser-induced fluorescence spectroscopy performed on an isotope-enriched iron atomic beam. All measured frequency shifts are reported with relative uncertainties below one third percent.

It has recently been demonstrated experimentally that $200\text{–}300$ keV electrons with the unusual spatial profiles can be produced and even focused to a subnanometer scale—namely, electrons carrying nonzero orbital angular momentum and also the so-called Airy beams. Since the wave functions of such electrons do not represent plane waves, the standard Born formula for scattering of them off a potential field is no longer applicable and, hence, needs modification. In the present paper, we address the generic problem of elastic scattering of a wave packet of a fast nonrelativistic particle off a potential field. We obtain simple and convenient formulas for a number of events and an effective cross section in such a scattering, which represent generalization of the Born formula for a case when finite sizes and spatial inhomogeneity of the initial packet should be taken into account. As a benchmark, we consider two simple models corresponding to scattering of a Gaussian wave packet on a Gaussian potential and on a hydrogen atom, and perform a detailed analysis of the effects brought about by the limited sizes of the incident beam and by the finite impact parameter between the potential center and the packet's axis.

We show theoretically that a quantum system in a nonequilibrium state interacting with a set of laser pulses in a four-wave-mixing setup leads to signal emission in directions opposite to the ones usually considered. When combined with a pump mechanism which sets a time origin for the nonequilibrium state creation, this particular optical response can be utilized to directly follow decoherence processes in real time. By varying the time delays within the probe sequence, signals in these unconventional directions can also be used to detect two-dimensional spectra determined by the dynamics of up to three-quantum coherences, revealing energetical anharmonicities and environmental influences. As a numerical example, these findings are demonstrated by considering a model of vibrational decoherence of carbon monoxide after photolysis from a hemeprotein.

We investigate the influence of the attosecond electron dynamics of photoionization on the femtosecond fragmentation of the molecular ion left behind. We consider the dissociative photoionization dynamics of the ${\mathrm{N}}_2$ molecule, induced by an attosecond extreme-ultraviolet (XUV) pulse in the presence of a moderately strong infrared (IR) laser field. We show that the kinetic energy spectrum of ${\mathrm{N}}^{+}$ fragments depends on (i) the phases between the different electronic states of ${{\mathrm{N}}_2}^{+}$ established by the photoionization process and (ii) phases associated with the vibrational dynamics in the dissociating molecular ion. We show that the phase acquired during the photoionization can be obtained from the dependence of the ${\mathrm{N}}^{+}$ ion kinetic energy release spectra on the time delay between the XUV and IR pulses.

We present the production of dual-species Bose-Einstein condensates (BECs) of ${}^{39}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$. Preparation of both species in the $\left|F=1,m_F=-1\right.〉$ state enabled us to exploit a total of three Feshbach resonances, which allows for simultaneous Feshbach tuning of the ${}^{39}\mathrm{K}$ intraspecies and the ${}^{39}\mathrm{K}\text{−}{}^{87}\mathrm{Rb}$ interspecies scattering length. Thus dual-species Bose-Einstein condensates were produced by sympathetic cooling of ${}^{39}\mathrm{K}$ with ${}^{87}\mathrm{Rb}$. A dark spontaneous force optical trap was used for ${}^{87}\mathrm{Rb}$ to reduce the losses in ${}^{39}\mathrm{K}$ due to light-assisted collisions in the optical trapping phase, which can be of benefit for other dual-species experiments. The tunability of the scattering length was used to perform precision spectroscopy of the interspecies Feshbach resonance located at 117.56(2) G and to determine the width of the resonance to 1.21(5) G by rethermalization measurements. The transition region from miscible to immiscible dual-species condensates was investigated and the interspecies background scattering length was determined to $28.5a_0$ using an empirical model. This paves the way for dual-species experiments with ${}^{39}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$ BECs ranging from molecular physics to precision metrology.