In order to understand the bounds of utilization of Grover's search algorithm for large unstructured data in the presence of quantum computer noise, we undertake a series of simulations by inflicting various types of noise, modeled by the IBM qiskit. We apply three forms of Grover's algorithm: (i) the standard one, with 4–10 qubits; (ii) a recently published modified Grover's algorithm, set to reduce the circuit depth; and (iii) the algorithms in (i) and (ii) with multicontrol Toffoli's gates modified by the addition of an ancilla qubit. Based on these simulations, we find the upper bound of noise for these cases, establish its dependence on the quantum depth of the circuit, and provide comparison among them. By extrapolation of the fitted thresholds, we predict what would be the typical gate error bounds when applying Grover's algorithms for the search of data in a data set as large as 32 000.

The efficient detection of a single spin is a significant goal of improving the sensitivity of quantum magnetic-field sensors. Recent results show that a specific type of entanglement such as Greenberger-Horne-Zeilinger (GHZ) states can be used as a resource to improve the performance of single spin detection. However, scalable generation of the GHZ states is experimentally difficult to realize. It is desirable to use a practical entangled state that can be easily generated. In this paper, we propose the efficient detection of a single spin with Dicke states. We show a way to prepare and measure Dicke states via a global control. Moreover, we investigate how dephasing due to unwanted coupling with the environment affects the performance of our proposal, and show that single spin detection with Dicke states with dephasing has a significant advantage over the classical strategy with separable states. Our results are important toward realizing entanglement enhanced single spin detection.

Large-scale ab initio QED calculations are performed for the $2p_{3/2}\text{–}2p_{1/2}$ fine-structure interval of Li-like ions with nuclear charges $Z=5\text{–}92$. Improved theoretical predictions are obtained by combining together two complementary theoretical methods, namely, the approach that accounts for all orders in the binding nuclear strength and the nonrelativistic QED approach that accounts for all orders in the nonrelativistic electron-electron interaction. The resulting unified approach provides theoretical predictions which are more accurate than the available experimental results across the interval of the nuclear charges considered.

Due to the conservation of the canonical angular momentum, charged particle beams which are generated inside a solenoid field acquire a kinetic angular momentum outside of the solenoid field. The relation of kinetic orbital angular momentum to the field strength and the beam size on the cathode is called the Busch theorem. We formulate the Busch theorem in quantum mechanical form and discuss the generation of quantized vortex beams, i.e., beams carrying a quantized orbital angular momentum. Immersing a cathode in a solenoid field presents an efficient and flexible method for the generation of electron vortex beams, while, e.g., vortex ions can be generated by immersing a charge stripping foil in a solenoid field. Both techniques are utilized at accelerators for the production of nonquantized vortex beams. As a highly relevant use case we discuss in detail the conditions for the generation of quantized vortex beams from an immersed cathode in an electron microscope. General possibilities of this technique for the production of vortex beams of other charged particles are pointed out.

Optical coupling of two identical dielectric particles gives rise to bonding and antibonding resonances. The latter is featured by significant narrowing of the resonant width and strong enhancement of the $Q$ factor for the high-index micron-size particles in subwavelength range. We consider particles shaped as spheres and disks under coaxial illumination of dual incoherent counterpropagating Bessel beams. In the case of spheres we derive analytical expressions for the optical binding (OB) force which decays and displays two periods of oscillations. For close distances the OB force enormously increases in the resonant regime. The case of two coaxial disks is featured by extremal enhancement of the $Q$ factor owing to the twofold variation over the distance between disks and the aspect ratio of each disk compared to the case of two spheres. In that case we demonstrate enhancement of the OB force up to several tens of nanonewtons. We show that the magnitude and sign of the OB force strongly depend on the longitudinal wave vector of the Bessel beams.

Numerous nonclassicality criteria based on the probabilities of experimental photocount or theoretical photon-number distributions are derived using several approaches. Relations among the derived criteria are revealed and the fundamental criteria are identified. They are grouped into parametric systems that allow the analysis of the nonclassicality from different points of view (“local” nonclassicality, pairwise character of photon correlations, etc.). Considering their structure, the criteria may be divided into groups that differ in the power to resolve the nonclassicality. Quantification of the nonclassicality using the Lee nonclassicality depth and the nonclassicality counting parameter is discussed. The number of field modes used is identified as an important parameter that may cause unexpected results. An appropriate linear transformation of a photocount (photon-number) distribution into its $s$-ordered form, needed for the determination of the nonclassicality depth, is derived. For comparison, the derived criteria are applied both to an experimental photocount histogram of a twin beam and the reconstructed photon-number distributions with different levels of the noise.

Recent progress in electro-optic sampling has allowed direct access to the fluctuations of the electromagnetic ground state. Here, we present a theoretical formalism that allows for an in-depth characterization and interpretation of such quantum-vacuum detection experiments by relating their output statistics to the quantum statistics of the electromagnetic vacuum probed. In particular, we include the effects of absorption and dispersion. Our results agree with available experimental data while leading to significant corrections to previous theoretical predictions and generalizing them to new parameter regimes. We show that transverse (free-field) as well as longitudinal (matter or near-field) fluctuations can be accessed individually by tuning the experimental parameters.

Within the standard weak-coupling limit, the reduced dynamics of open quantum spin chains with their two end spins coupled to two distinct heat baths at different temperatures are mainly derived using the so-called global and local approaches, in which, respectively, the spin self-interaction is and is not taken into account. In order to compare the differences between the two regimes, we concentrate on an open three-site $XX$ spin chain, provide systematic techniques to address the global and local asymptotic states, and then compare the asymptotic spin-transport features by studying the spin flux through the middle site. Based on the analytical expressions of the stationary states in the two regimes, we find that the local approach misses important global effects emerging as spin sink and source terms that can only be due to nonnegligible interspin interactions. Moreover, we show that the local asympotic transport features cannot be recovered from the global ones by letting the interspin coupling vanish, thus pointing to the existence of different coupling ranges where only one approach is physically tenable and possibly a region where the two descriptions may coexist.

Due to its unique scalability potential, continuous-variable quantum optics is a promising platform for large-scale quantum computing. In particular, very large cluster states with a two-dimensional topology that are suitable for universal quantum computing and quantum simulation can be readily generated in a deterministic manner, and routes towards fault tolerance via bosonic quantum error correction are known. In this article we propose a complete measurement-based quantum computing architecture for the implementation of a universal set of gates on the recently generated two-dimensional cluster states [M. V. Larsen et al., Science 366, 369 (2019); W. Asavanant et al., Science 366, 373 (2019)]. We analyze the performance of the various quantum gates that are executed in these cluster states as well as in other two-dimensional cluster states (the bilayer-square lattice and quad-rail lattice cluster states [R. N. Alexander et al., Phys. Rev. A 94, 032327 (2016); N. C. Menicucci, Phys. Rev. A 83, 062314 (2011)]) by estimating and minimizing the associated stochastic noise addition as well as the resulting gate error probability. We compare the four different states and find that, although they all allow for universal computation, the quad-rail lattice cluster state performs better than the other three states, which all exhibit similar performance.

The process of the elastic scattering of photons on atoms, known as the Rayleigh scattering, is investigated. Expressing the scattering observables in terms of the electric and magnetic complex scattering amplitudes, we work over the scenarios for their independent benchmarking by experiments. In particular, the combination of the cross section and polarization transfer of initially linearly polarized light provides an opportunity for a complete experiment for any but fixed scattering angle. It allows us to deduce the modulus of the electric and magnetic amplitudes together with the phase difference individually. The findings are illustrated by the theoretical calculations of the scattering of 150–250 keV photons on lead atoms.

The impact of the short-range interaction on the resonances' occurrence in the anisotropic dipolar scattering in a plane is numerically investigated for the arbitrarily oriented dipoles and for a wide range of collision energies. We reveal the strong dependence of the cross section of the two-dimensional (2D) dipolar scattering on the radius of short-range interaction, which is modeled by a hard wall potential and by the more realistic Lennard-Jones potential, and on the mutual orientations of the dipoles. We define the critical (magic) tilt angle of one of the dipoles, depending on the direction of the second dipole for arbitrarily oriented dipoles. We find that resonances arise only when this angle is exceeded. In contrast to the three-dimensional (3D) case, the energy dependencies of the boson (fermion) 2D scattering cross section grow (are reduced) with an energy decrease in the absence of the resonances. We show that the mutual orientation of dipoles strongly impacts the form of the energy dependencies, which begin to oscillate with the tilt angle increase, unlike the 3D dipolar scattering. The angular distributions of the differential cross section in the 2D dipolar scattering of both bosons and fermions are highly anisotropic at nonresonant points. The results of the accurate numerical calculations of the cross section agree well with the results obtained within the Born and eikonal approximations.

We investigate the interaction of Xe with isolated attosecond XUV pulses. Specifically, we calculate the ion yields and determine the pathways leading to the formation of ionic charged states up to ${\mathrm{Xe}}^{5+}$. To do so, in our formulation we account for single-photon absorption, sequential multiphoton absorption, direct two-photon absorption, single and double Auger decays, and shake-off. We compare our results for the ion yields and for ion yield ratios with recent experimental results obtained for 93 and 115 eV attosecond XUV pulses. In particular, we investigate the role that a sequence of two single-photon ionization processes plays in the formation of ${\mathrm{Xe}}^{4+}$. We find that each one of these two processes ionizes a core electron and thus leads to the formation of a double core-hole state. Remarkably, we find that the formation of ${\mathrm{Xe}}^{5+}$ involves a direct two-photon absorption process and the absorption of a total of three photons.

We study an atomic comagnetometer design based on the spin precessions of ${}^{129}\mathrm{Xe}$ and ${}^{131}\mathrm{Xe}$ atoms in glass cells. The quadrupole splittings in the precession spectrum of ${}^{131}\mathrm{Xe}$ are fully resolved, allowing a precise determination of the magnetic-dipole precession frequency. The transverse asymmetry of quadrupole interactions, due to both the geometry and surface properties of the cell, characterized by a nonzero asymmetry parameter $\eta$, modifies the dependence of the quadrupole splittings on the relative orientation between the cell axes and the bias magnetic field and lead to additional corrections in the precession frequencies of ${}^{131}\mathrm{Xe}$ atoms. We examine these effects both theoretically and experimentally and develop methods to quantify and control such shifts.

Trapped ions are a promising candidate for large scale quantum computation. Several systems have been built in both academic and industrial settings to implement modestly sized quantum algorithms. Efficient cooling of the motional degrees of freedom is a key requirement for high-fidelity quantum operations using trapped ions. Here, we present a technique whereby individual ions are used to cool individual motional modes in parallel, reducing the time required to bring an ion chain to its motional ground state. We demonstrate this technique experimentally and develop a model to understand the efficiency of our parallel-sideband-cooling technique compared to more traditional methods. This technique is applicable to any system using resolved sideband cooling of cotrapped atomic species and only requires individual addressing of the trapped particles.

We calculate the binding energy and closed-channel fraction of $p$-wave Feshbach molecules in quasi-one-dimension (quasi-1D) by examining the poles of the $p$-wave $S$ matrix. We show that under the right experimental conditions, the quasi-1D $p$-wave molecule behaves like a halo dimer with a closed-channel fraction approaching 0 at resonance and a binding energy following the universal relation $E_b\sim 1/a_{1\mathrm{D}}^2$, where $a_{1\mathrm{D}}$ is the 1D scattering length. We calculate these experimental conditions for both ${}^6\mathrm{Li}$ and ${}^{40}\mathrm{K}$ over a range of transverse confinements. We expect that in this halo dimer regime the three-body loss associated with the $p$-wave Feshbach resonance will be greatly suppressed, potentially allowing for a stable $p$-wave superfluid to be created. For an easy comparison between the three-dimensional and the quasi-1D cases, we provide the same pole analysis of the Feshbach molecules applied to the three-dimensional $p$-wave resonance and show that there is a qualitative difference between the two.

We formulate an exact space-time mapping between the $\mathcal{N}$-point correlation functions of two different experiments with open quantum gases. Our formalism extends a quantum-field mapping result for closed systems [Phys. Rev. A 94, 043628 (2016)] to the general case of open quantum systems with the Markovian property. For this, we consider an open many-body system consisting of a $D$-dimensional quantum gas of bosons or fermions that interacts with a bath under Born-Markov approximation and evolves according to a Lindblad master equation in a regime of loss or gain. Invoking the independence of expectation values on pictures of quantum mechanics and using the quantum fields that describe the gas dynamics, we derive the Heisenberg evolution of any arbitrary $\mathcal{N}$-point function of the system in the regime when the Lindblad generators feature a loss or a gain. Our quantum field mapping for closed quantum systems is rewritten in the Schrödinger picture and then extended to open quantum systems by relating onto each other two different evolutions of the $\mathcal{N}$-point functions of the open quantum system. As a concrete example of the mapping, we consider the mean-field dynamics of a simple dissipative quantum system that consists of a one-dimensional Bose-Einstein condensate being locally bombarded by a dissipating beam of electrons in both cases when the beam amplitude or the waist is steady and modulated.

In this work, we study the universal relations for one-dimensional spin-orbital-coupled fermions near both $s$- and $p$-wave resonances using effective field theory. Since the spin-orbital coupling mixes different partial waves, a contact matrix is introduced to capture the nontrivial correlation between dimers. We find the signature of the spin-orbital coupling appears at the leading order for the off-diagonal components of the momentum distribution matrix, which is proportional to $1/q^3$ ($q$ is the relative momentum). We further derive the large frequency behavior of the Raman spectroscopy, which serves as an independent measurable quantity for contacts. Finally, we give an explicit example of contacts by considering a two-body problem.

We present a phase-stable and selectable repetition-rate ($f_{\text{rep}}$) divider (PSRD) of an optical frequency comb (OFC) by down-picking a train of pulses from the fundamental OFC phase coherently. The PSRD operating at the integer pulse-picking order $p$ uses an acousto-optic modulator driven by a train of phase-synchronized single-cycle wave packets oscillating at the carrier frequency of $f_c\equiv f_{\text{rep}}$ and the gating frequency of $f_g^p\equiv f_{\text{rep}}/p$. The demonstrated PSRD can selectively pick a train of pulses with $p=2$ to 25 from the fundamental OFC with $f_{\text{rep}}=250$ MHz at 530 nm. Integrated phase noises from 10 Hz to 25 kHz offset frequency from the optical beat note between each down-picked OFC and the fundamental OFC are measured to be about ($59\pm 6$) mrad through the whole pulse picking order. The phase-coherent down-picked OFC could be of use for various applications where phase-stable and repetition-rate selectable OFCs are the essential radiation sources.

We investigate the performance of ptychography with noisy data by analyzing the Cramér-Rao lower bound. The lower bound of ptychography is derived and numerically computed for both top-hat plane wave and structured illumination. The influence of Poisson noise on the ptychography reconstruction is discussed. The computation result shows that, if the estimator is unbiased, the minimum variance for Poisson noise is mostly determined by the illumination power and the transmission function of the object. Monte Carlo analysis is conducted to validate our calculation results for different photon flux numbers. Furthermore, the performance of the maximum-likelihood method and the approach of amplitude-based cost-function minimization is studied in the Monte Carlo analysis.

In a recent article [R. Bombin, F. Mazzanti, and J. Boronat, Phys. Rev. A 100, 063614 (2019)], it is contended that a two-dimensional system of dipolar bosons, with dipole moments aligned at particular angles with respect to the direction perpendicular to the plane of motion, featuring a “striped” crystalline ground state, in turn undergoes a Berezinskii-Kosterlitz-Thouless superfluid transition at low temperature, making it a two-dimensional supersolid. We show here that the results provided therein, obtained by means of quantum Monte Carlo simulations, do not actually support such a conclusion. Rather, they are consistent with that expounded in our work [J. Low Temp. Phys. 196, 413 (2019)], namely that the striped ground state is insulating (i.e., nonsuperfluid in the conventional sense), essentially behaving like a system of quasi-one-dimensional, parallel independent chains. We attribute the incorrectness of the conclusion reached by Bombin, Mazzanti, and Boronat to the very small sizes of their simulated system, which do not allow for a reliable extrapolation to the thermodynamic limit.