Implementing variational quantum algorithms with noisy intermediate-scale quantum machines of up to a hundred qubits is nowadays considered as one of the most promising routes towards achieving a quantum practical advantage. In multiqubit circuits, running advanced quantum algorithms is hampered by the noise inherent to quantum gates which distances us from the idea of universal quantum computing. Based on a one-dimensional quantum spin chain with competing symmetric and asymmetric pairwise exchange interactions, herein we discuss the capabilities of quantum algorithms with special attention paid to a hardware-efficient variational eigensolver. A delicate interplay between magnetic interactions allows one to stabilize a chiral state that destroys the homogeneity of magnetic ordering, thus making this solution highly entangled. Quantifying entanglement in terms of quantum concurrence, we argue that, while being capable of correctly reproducing a uniform magnetic configuration, the hardware-efficient Ansatz meets difficulties in providing a detailed description to a noncollinear magnetic structure. The latter naturally limits the application range of variational quantum computing to solve quantum simulation tasks.

Investigations of isotope shifts of atomic spectral lines provide insights into nuclear properties. Deviations from the linear dependence of the isotope shifts of two atomic transitions on nuclear parameters, leading to a nonlinearity of the so-called King plot, are actively studied as a possible way of searching for new physics. In the present Letter we calculate the King-plot nonlinearities originating from the standard-model atomic theory. The calculation is performed both analytically, for a model example applicable for an arbitrary atom, and numerically, for one-electron ions. It is demonstrated that the standard-model predictions of the King-plot nonlinearities are hypersensitive to experimental errors of nuclear charge radii. This effect significantly complicates identifications of possible King-plot nonlinearities originating from the new physics.

When noninteracting fermions are confined in a $D$-dimensional region of volume $O\left(L^D\right)$ and subjected to a continuous (or piecewise-continuous) potential $V$ which decays sufficiently fast with distance, in the thermodynamic limit, the ground-state energy of the system does not depend on $V$. Here, we discuss this theorem from several perspectives and derive a proof for radially symmetric potentials valid in $D$ dimensions. We find that this universality property holds under a quite mild condition on $V$, with or without bounded states, and extends to thermal states. Moreover, it leads to an interesting analogy between Anderson's orthogonality catastrophe and first-order quantum phase transitions.

We systematically study the effects of higher-order quantum and thermal fluctuations on the stabilization of self-bound droplets in Bose mixtures employing the time-dependent Hartree-Fock-Bogoliubov theory. We calculate the ground-state energy, the droplet equilibrium density, the depletion and anomalous density of the droplets as well as the critical temperature as a function of the relevant parameters. Our findings are compared with previous analytical predictions and diffusion Monte Carlo simulations. We employ our theory together with the local density approximation for quantum and thermal fluctuations to obtain an extended finite-temperature Gross-Pitaevskii equation. The density profiles and breathing modes of the droplet are deeply examined in terms of the interaction strength and the temperature by numerically solving the developed generalized Gross-Pitaevskii equation.

Squeezing of quantum states is of great interest due to its application in high-precision measurement that can exceed standard quantum noise limit described by Heisenberg uncertainty relations. Here, we study the squeezing of quantum systems with $\text{SU}\left(3\right)$ symmetry via a one-axis twisting Hamiltonian and examine the intriguing connections between the squeezing of spin-1/2 and spin-1 systems. There are seven subalgebras of $\text{su}\left(3\right)$ Lie algebra. All these subalgebras are identical with $\text{su}\left(2\right)$ Lie algebra but not all of them have the same anticommutation relations as $\text{su}\left(2\right)$. Interestingly, squeezing parameters corresponding to spin-1 subalgebras depend not only on structure constants but also on anticommutation relations of the subalgebras. Our results are reported for the subalgebras with vanishing anticommutators and nematic squeezing, while in other cases our first-principle calculation recovers known results.

We investigate our ability to determine the mean position, or centroid, of a linear array of equally bright incoherent point sources of light whose continuum limit is the problem of estimating the center of a uniformly radiating object. We consider two receivers, an image-plane ideal direct-detection imager and a receiver that employs Hermite-Gaussian (HG) spatial-mode demultiplexing in the image plane, prior to shot-noise-limited photon detection. We compare the Fisher information (FI) for estimating the centroid achieved by these two receivers, which quantifies the information-accrual rate per photon, and compare those with the quantum Fisher information (QFI): the maximum attainable FI by any choice of measurement on the collected light allowed by physics. We find that focal-plane direct imaging is strictly suboptimal, although not by a large margin. We also find that not only is the HG mode sorter, which is the optimal measurement for estimating the separation between point sources (or the length of a line object), suboptimal, but it performs worse than direct imaging. We study the scaling behavior of the QFI and direct imaging's FI for a continuous uniformly bright object in terms of its length and find that both are inversely proportional to the object's length when it is sufficiently larger than the Rayleigh length. Finally, we propose a two-stage adaptive modal receiver design that attains the QFI for centroid estimation.

Relativistic multiconfiguration Dirac-Hartree-Fock calculations with configuration interaction are carried out for the ${}^{1}S_0$ and ${}^{3}P_{0,1}^o$ states in neutral ytterbium by use of the available grasp2018 package. From the resultant atomic state functions and the ris4 program, we evaluate the mass- and field-shift parameters for the ${}^{1}S_0-{}^{3}P_0^o$ (clock) and ${}^{1}S_0-{}^{3}P_1^o$ (intercombination) lines. We present improved estimates of the nuclear charge parameters ${\lambda }^{A,A^{\prime }}$ and differences in mean-square charge radii $\delta {\langle r^2\rangle }^{A,A^{\prime }}$ and examine the second-order hyperfine interaction for the ${}^{3}P_{0,1}^o$ states. Isotope shifts for the clock transition have been estimated by three largely independent means from which we predict the unknown clock-line frequencies in bosonic Yb isotopes. Knowledge of these line frequencies has implications for King-plot nonlinearity tests and the search for beyond-standard-model signatures.

We report a theoretical study on the long-range additive and nonadditive potentials for a three-body hybrid atom-atom-ion system composed of one ground-$S$-state Li atom, one excited-$P$-state Li atom, and one ground-$S$-state ${\mathrm{Li}}^{+}$ ion, $\mathrm{Li}\left(2{}^{2}S\right)\text{−}\mathrm{Li}\left(2{}^{2}P\right)\text{−}{\mathrm{Li}}^{+}(1{}^{1}S$). The interaction coefficients are evaluated with highly accurate wave functions calculated variationally in Hylleraas coordinates. For this hybrid system the three-body nonadditive interactions (appearing in second order) induced by the energy degeneracy and enhanced by the induction effect of the ${\mathrm{Li}}^{+}$ ion through the internal electric field can be strong and even stronger than the two-body additive interactions at the same order. We find that for particular geometries the two-body additive interactions of the system sum to zero, leaving only three-body nonadditive interactions, thus making the present system potentially a platform to explore quantum three-body effects. We also extract by first principles the leading coefficients of the long-range electrostatic, induction, and dispersion energies of ${\text{Li}}_2{}^{+}$ electronic states correlating to ${\text{Li}}^{+}\left(1{}^{1}S\right)-\text{Li}\left(2{}^{2}P\right)$. The results should be especially valuable for the exploration of schemes to create trimers with ultracold atoms and ions in optical lattices.

We present angle-differential and angle-integrated cross sections for electron-impact excitation of the ($4d^{10}5s){}^{2}S_{1/2}\to \left(4d^{9}5s^2\right){}^{2}D_{3/2}$ and $\left(4d^{10}5s\right){}^{2}S_{1/2}\to \left(4d^{10}6s\right){}^{2}S_{1/2}$ transitions in atomic silver. Experimental data for four incident electron energies between 10 and 60 eV are compared with predictions from our relativistic distorted wave (RDW) and nonrelativistic atomic optical potential models. Agreement between our measured and calculated data is only fair, although in the case of the RDW it is seen to improve with increasing incident electron energy. However, only for the $\left(4d^{10}6s\right){}^{2}S_{1/2}$ excitation process, agreement of our measured data with earlier relativistic convergent close coupling results from McNamara et al. [J. Phys. B 51, 085203 (2018)] was, with a few exceptions, typically observed to be very good, to within the uncertainties on the data.

We present a systematic study of the anisotropy and polarization of x-ray line emissions in $KLL$ dielectronic recombination of hydrogenlike ${\mathrm{Fe}}^{25+}$ ions. For the $KLL$ dielectronic recombination considered, it consists of a resonant electron capture plus a two-step radiative decay. While the anisotropy and polarization of the first-step decay photons are calculated by using the flexible atomic code, those of the second-step decay photons are obtained with two methods, i.e., the formalism of the deorientation factor and density matrix theory. Resonance strengths are presented as well to synthesize the dielectronic satellite line spectra. The synthetic spectra including the correction of anisotropy and polarization effects show better agreement with existing experiments than previous calculations. This indicates that these effects are important for the intensity distribution of the dielectronic satellite lines.

The $K$ x-ray spectra of different targets (Cu, Zn, and Ge) induced by 3–5 MeV/u Si projectile ions have been measured to determine the $K$-shell ionization cross section. A significant difference is observed between the measurements and theoretical estimates, with the latter being about 50% below the experimental results. This underestimation is attributed to the charge exchange from the target $K$ shell to projectile $K$ and $L$ shells. Such an observation can only be possible if the projectile ions attain up to H- and He-like charge states. Corresponding projectile charge state fractions have been evaluated from the Lorentzian charge state distribution, where the mean charge state is taken from the Fermi gas model [W. Brandt et al., Phys. Rev. Lett. 30, 358 (1973)] and the width from the Novikov and Teplova approach [Phys. Lett. A 378, 1286 (2014)]. The sum of the direct ionization cross section and $K\text{−}K+K\text{−}L$ capture cross sections gives a good agreement with the measured cross sections. Furthermore, we have validated this methodology with available data for a Si ion on Ti target. Such results may be useful in many solid target-based applications.

According to Heisenberg's uncertainty relation, there is an ultimate limit to how precisely we may predict the outcome of position and momentum measurements on a quantum system. We show that this limit may be violated by an arbitrarily large factor if one aims, instead, to guess the unknown value of a past measurement. For experiments on a single quantum system, the precise assignment of past position and momentum measurement outcomes is accompanied by large uncertainty about their linear combinations, while we show that entanglement with an ancillary system permits accurate retrodiction of any such linear combination. Finally, we show that the outcomes of experiments that jointly measure multiple linear combinations of position and momentum observables by means of ancillary probe particles can also be guessed with no formal lower limit. We present quantitative results for projective measurements and for generalized measurements where all components are prepared in Gaussian states.

We show that a perfectly genuine tripartite steerable assemblage can be distilled from partially genuine tripartite EPR steerable assemblages. In particular, we consider two types of hybrid scenarios: one-sided device-independent scenario (where one observer is untrusted, and other two observers are trusted) and two-sided device-independent scenario (where two observers are untrusted, and one observer is trusted). In both scenarios, we show distillation of perfectly genuine steerable assemblage of three-qubit Greenberger-Horne-Zeilinger states or three-qubit W states from many copies of initial partially genuine steerable assemblages of the corresponding states. In each of these cases, we demonstrate that at least one copy of a perfectly genuine steerable assemblage can be distilled with certainty from infinitely many copies of initial assemblages. In case of practical scenarios employing finite copies, we show that the efficiency of our distillation protocols reaches near perfect levels using only a few number of initial assemblages.

We investigate the feasibility of performing quantum nondemolition (QND) measurements in the one-dimensional Dirac oscillator, a frequently used toy model to investigate relativistic extensions of nonrelativistic quantum effects. We derive general expressions for its QND observables and find that they are in general complex combinations of the position, momentum, and spin operators, which makes them challenging to realize experimentally. However, the situation is considerably simplified in both the weakly and strongly relativistic limits, where they should be amenable to an experimental demonstration. This suggests exploiting continuous measurements of small departures from the QND character of these variables in the intermediate regime as probes of relativistic effects.

In this paper we present the approach for description of quantum control attack based on combined protocol and hardware loopholes. It consolidates intercept-resend attack and detection node control (detector blinding attack). In the basic version of B92 protocol detection control is not that crucial; however, when one scales the number of states the state imposing plays a significant role. Protocols that operate with arbitrary even symmetric linearly independent nonorthogonal (e.g., coherent) states are of interest. The cornerstone of the considered approach is that we combine both state discrimination by eavesdropper and different methods of state imposing. In principle, detection control allows one to exclude any bit correlations between legitimate users, which are unknown to Eve, and can be considered as the necessary part of most intercept-resend attacks, including a faked-state attack impossible without a hardware loophole. Moreover, the issue related to unified quantum description of the intercept-resend attack was solved by combining the concepts of von Neumann's measurement scheme and ambiguity of square root extraction for operators. We also present a generalized countermeasure based on additional parameter estimation analysis. As an example, with some numerical estimations we investigate the attack on quantum key distribution systems based on utilization of symmetric coherent states and consider appropriate countermeasures.

We study the dynamics of bosonic and fermionic anyons defined on a one-dimensional lattice, under the effect of Hamiltonians quadratic in creation and annihilation operators, also called Gaussian Hamiltonians. These anyonic models are obtained from deformations of the standard bosonic or fermionic commutation relations via the introduction of a nontrivial exchange phase between different lattice sites. We study the effects of the anyonic exchange phase on the usual bosonic and fermionic bunching behaviors. We show how to exploit the inherent Aharonov-Bohm effect exhibited by these particles to build a deterministic, entangling two-qubit gate and prove quantum computational universality in these systems. We define coherent states for bosonic anyons and study their behavior under two-mode passive, Gaussian devices. In particular we prove that, for a particular value of the exchange factor, an anyonic mirror can generate cat states, an important resource in quantum information processing with continuous variables.

We study numerically the slow (subradiant) decay of the fluorescence of motionless atoms after a weak pulsed excitation. We show that, in the linear-optics regime and for an excitation detuned by several natural linewidths, the slow decay rate can be dominated by close pairs of atoms (dimers) forming superradiant and subradiant states. However, for a large-enough resonant optical depth and at later time, the dynamics is dominated by collective many-body effects. In this regime, we study the polarization and the spectrum of the emitted light, as well as the spatial distribution of excitation inside the sample, as a function of time during the decay dynamics. The behavior of these observables is consistent with what would be expected for radiation trapping of nearly resonant light. This finding sheds light on subradiance in dilute samples by providing an interpretation based on the light behavior of the system (multiple scattering) which is complementary to the more commonly used picture of the collective atomic Dicke state.

Decoherence is an unavoidable effect of the quantum system coupled to an environment, which leads to the system evolving from a pure state to a mixed one. A critical issue here is that the reduction of purity is quantum or classical, which corresponds to whether or not entanglement is generated between the system and environment. We report a proof-of-principle experimental investigation of such entanglement in a simple but paradigmatic case, in which a qubit system interacts with a pure dephasing channel. The pure dephasing process is realized in a linear photonic system, and the experiment is a validation in a very controlled setting. By adopting previous methods to detect qubit-environment entanglement, we prove that such entanglement can be detected by performing local measurements on only one subsystem, even when the other subsystem in the pair cannot be detected. Our experiment paves the way for investigating features of open quantum system dynamics.