Quantum imaging techniques offer enhanced resolution, contrast, and precision at ultralow illumination levels compared to traditional imaging approaches. Relying on the unique properties of entangled photon pairs, two of these techniques stand out: the correlation-based quantum imaging technique provides visibility enhancement in imaging of a low-reflectivity object which is subject to excessive noise and losses, while the interaction-free ghost imaging allows for probing the presence of an object with an ultimately low number of photons. Here we propose a quantum imaging scheme that combines the unique advantages of these two approaches. We show that this scheme offers high-contrast imaging of objects with a minimal number of photons that can minimize thermal noise efficiently and create background-free images. We anticipate that this approach can find application in the imaging of photosensitive biological tissues in a noninvasive and harm-free fashion.

Until large-scale fault-tolerant quantum devices become available, one has to find ways to make the most of current noisy intermediate-scale quantum devices. One possibility is to seek smaller repetitive hybrid quantum-classical tasks with higher fidelity, rather than directly pursuing large complex tasks. We present an approach in this direction in which quantum computation is supplemented by a classical result. While the presence of the supplementary classical information is helpful in and of itself, taking advantage of its anticipation also leads to a distinct type of quantum measurement, which we call anticipative. Anticipative quantum measurements lead to an improved success rate compared with cases in which quantum measurements are optimized without assuming the subsequent arrival of supplementary information. Importantly, in an anticipative quantum measurement, we do not combine the results from classical and quantum computations until the end of the process, and there is no need for feedback from one computation to the other, thus both computations can be run in parallel. We demonstrate the method using an IBMQ device, and we show that it leads to an improved success rate even in a noisy setting.

We apply an extension of the Pontryagin maximum principle to derive time-optimal controls of two-level quantum systems by means of piecewise constant pulses. Global optimal solutions are obtained for state-to-state transfer in the cases with one and two controls. Exact quantum speed limits are established as a function of the sampling period. We observe numerically an exponential convergence towards the minimum time in the continuous limit when this period goes to zero. We show that this convergence is only polynomial for a linearized quantum system. We discuss the experimental impact of this result.

We report on the extension of the single-center convergent close-coupling method to positron scattering on multielectron atoms, along with the development of a technique for separating the direct ionization and positronium-formation channels in single-center calculations. We have performed calculations of positron scattering on carbon and present total elastic, momentum transfer, excitation, direct ionization, total ionization, total inelastic, positronium-formation, stopping power, and total cross sections from threshold to 5000 eV. We also present oscillator strengths, the scattering length, the hidden Ramsauer-Townsend minimum, the energy of the positron-carbon virtual state, and the mean excitation energy. Agreement with electron-scattering experiment and positron-scattering theory for several cross sections has been demonstrated for high energies. However, discrepancies exist between different theoretical methods at lower energies and for the ionization and positronium-formation processes.

Pump-probe experiments are ubiquitous in atomic, molecular, and optical physics, where a strong pump field induces some change in a quantum system that is probed by a much weaker field. Often, high intensity of the pump field is at a premium with all other characteristics of the strong field subordinate. As a particular example, of relevance to atomic clocks and atomic magnetometers, a strong broad-linewidth laser optically pumps an atomic vapor, which is then probed by a much weaker narrow-linewidth laser. Though the broad-linewidth field suffers laser phase noise (PM) to amplitude noise (AM) conversion, the narrow-linewidth probe by itself produces a much quieter signal. Here, we consider the question of whether the noisy pump field maps its (PM-to-AM) absorption cross-section fluctuations onto the quiet probe field's transmission. Our results show that PM-to-AM noise does not transfer directly: atoms interacting with the pump field are instantaneously distinct from those interacting with the probe field. However, the broad-linewidth laser can influence the quiet field's transmission through the vapor due to optical pumping, resulting in fluctuations in the number density of atoms in the absorbing states. Nevertheless, there is a saving grace for this “PM-to-AM induced optical pumping” noise transfer: for very high noisy-field intensities (where optical pumping saturates) this type of noise on the probe field becomes negligible.

Fast and efficient state preparation of molecules can be accomplished by optical pumping. Molecular structure that most obviously facilitates cycling involves a strong electronic transition, with favorable vibrational branching [diagonal Franck-Condon factors (FCFs)] and without any intervening electronic states. Here, we propose important adjustments to those criteria, based on our experience optically pumping ${\mathrm{SiO}}^{+}$. Specifically, the preference for no intervening electronic states should be revised, and over-reliance on FCFs can miss important features. The intervening electronic state in ${\mathrm{SiO}}^{+}$ is actually found to be beneficial in ground rotational state preparation, by providing a pathway for population to undergo a parity flip. This contribution demonstrates the possibility that decay through intervening states may help state preparation of nondiagonal or polyatomic molecules. We also expand upon the definition of favorable branching. In ${\mathrm{SiO}}^{+}$, we find that the off-diagonal FCFs fail to reflect the vibrational heating versus cooling rates. Since the branching rates are determined by transition dipole moments (TDMs) we introduce a simple model to approximate the TDMs for off-diagonal decays. We find that two terms, set primarily by the slope of the dipole moment function ($d\mu /dx$) and offset in equilibrium bond lengths ($\mathrm{\Delta }x=r_e^g-r_e^e$), can add (subtract) to increase (decrease) the magnitude of a given TDM. Applying the model to ${\mathrm{SiO}}^{+}$, we find there is a fortuitous cancellation, where decay leading to vibrational excitation is reduced, causing optical cycling to lead naturally to vibrational cooling.

In frequently studied two-band models for solid-state high-order harmonic generation, interband harmonics, in principle, can range from the minimum to the maximum band gap. However, it is known that a laser-intensity-dependent cutoff exists that may be well below the maximum band gap unless the laser intensity is so high that the electrons explore the entire Brillouin zone. We show that this laser-intensity-dependent cutoff is formed by destructive interference of the emission of electrons starting at different initial states in the Brillouin zone. The calculations in this work are for Su-Schrieffer-Heeger chains, but our findings apply to other two-band systems as well. The destructive interference is complete and forms the cutoff only when the sampling of the Brillouin zone is fine enough or, equivalently, a finite chain is long enough in position space. For coarser sampling and shorter chains all harmonics between minimum and maximum band gap are emitted. A time-frequency analysis shows how certain trajectories are responsible for the formation of the cutoff.

We study the ground-state properties of the SU($N$) generalization of the Kondo lattice model in one dimension when the Kondo coupling $J_{\text{K}}$ (both ferromagnetic and antiferromagnetic) is sufficiently strong. Both cases can be realized using alkaline-earth-metal-like cold gases in optical lattices. Specifically, we first carry out the strong-coupling expansion and identify two insulating phases [one of which is the SU($N$) analog of the well-known gapped Kondo singlet phase]. We then rigorously establish that the ground state in the low-density (for $J_{\text{K}}<0$) or the high-density (for $J_{\text{K}}>0$) region is ferromagnetic. The results are accounted for by generalizing the double-exchange mechanism to SU($N$) “spins.” Possible realizations of Bose-Fermi supersymmetry SU($N|1$) in the (generalized) SU($N$) Kondo lattice model are discussed as well.

In distributed quantum sensing the correlations between multiple modes, typically of a photonic system, are utilized to enhance the measurement precision of an unknown parameter. In this work, we investigate the metrological potential of a multimode, tilted Bose-Hubbard system and show that it can allow for parameter estimation at the Heisenberg limit of ${\left[N(M-1)T\right]}^2$, where $N$ is the number of particles, $M$ is the number of modes, and $T$ is the measurement time. The quadratic dependence on the number of modes can be used to increase the precision compared to typical metrological systems with only two atomic modes and does not require correlations between different modes. We show that the limit can be reached by using an optimized initial state given as the superposition of all the atoms occupying the first and last sites. Subsequently, we present strategies that would allow us to obtain quadratic dependence on $M$ of the Fisher information in a more realistic experimental setup.

Atom interferometers provide a powerful means of realizing quantum coherent systems with increasingly macroscopic extent in space and time. These systems provide an opportunity for a variety of novel tests of fundamental physics, including ultralight dark matter searches and tests of modifications of gravity, using long drop times and microgravity environments. However, as experiments operate with longer periods of free fall and become sensitive to smaller background effects, key questions start to emerge about the fundamental limits to future atom interferometry experiments. We study the effects on atomic coherence from hard-to-screen backgrounds due to baths of ambient particles with long-range forces, such as gravitating baths and charged cosmic rays. Our approach—working in the Heisenberg picture for the atomic motion—makes proper inclusion of the experimental apparatus feasible and clearly shows how to handle long-range forces. We find that these potential backgrounds are likely negligible for the next generation of interferometers, as aggressive estimates for the gravitational decoherence from a background bath of dark matter particles gives a decoherence timescale on the order of years.

Placing a large mass in a large spatial superposition, such as a Schrödinger cat state, is a significant and important challenge. In particular, the large spatial superposition $\left[O\right(10–100\left)\mu \mathrm{m}\right]$ of mesoscopic masses $[m\sim O({10}^{-14}–{10}^{-15})\mathrm{kg}]$ makes it possible to test the quantum nature of gravity via entanglement in the laboratory. To date, the proposed methods of achieving this spatial delocalization are to use wave-packet expansions or quantum ancilla- (for example, spin-) dependent forces, all of whose efficacy reduces with mass. Thus increasing the spatial splitting independently of the mass is an important open challenge. In this paper we present a method of achieving a mass-independent enhancement of superposition via diamagnetic repulsion from current-carrying wires. We analyze an example system which uses the Stern-Gerlach effect to creating a small initial splitting and then apply our diamagnetic repulsion method to enhance the superposition size $O(400–600)\mu \mathrm{m}$ from an initial modest split of the wave function. We provide an analytical and numerical analysis of our scheme.

The Aharonov-Bohm effect is a fundamental topological phenomenon with a wide range of applications. It consists of a charge encircling a region with a magnetic flux in a superposition of wave packets having their relative phase affected by the flux. In this work we analyze this effect using an entropic measure known as realism, originally introduced as a quantifier of a system's degree of reality and mathematically related to notions of global and local quantum coherence. More precisely, we look for observables that lead to gauge-invariant realism associated with the charge before it completes its loop. We find that the realism of these operators has a sudden change when the line connecting the center of both wave packets crosses the solenoid. Moreover, we consider the case of a quantized magnetic-field source, pointing out similarities and differences between the two cases. Finally, we discuss some consequences of these results.

Variational quantum algorithms are a class of techniques intended to be used on near-term quantum computers. The goal of these algorithms is to perform large quantum computations by breaking the problem down into a large number of shallow quantum circuits, complemented by classical optimization and feedback between each circuit execution. One path for improving the performance of these algorithms is to enhance the classical optimization technique. Given the relative ease and abundance of classical computing resources, there is ample opportunity to do so. In this work, we introduce the idea of learning surrogate models for variational circuits using a few experimental measurements, and then performing parameter optimization using these models as opposed to the original data. We demonstrate this idea using a surrogate model based on kernel approximations, through which we reconstruct local patches of variational cost functions using batches of noisy quantum circuit results. Through application to the quantum approximate optimization algorithm and preparation of ground states for molecules, we demonstrate the superiority of surrogate-based optimization over commonly used optimization techniques for variational algorithms.

The implementation of quantum repeaters needed for long-distance quantum communication requires the generation of quantum entanglement distributed among the elementary links. These entanglements must be swapped among the quantum repeaters through Bell-state measurements. This study aims to improve the entanglement generation rate by frequency multiplexing the Bell-state measurements. As a preliminary step of the frequency-multiplexed Bell-state measurements, three frequency modes are mapped to a temporal mode by an atomic frequency comb prepared in ${\mathrm{Pr}}^{3+}$ ion-doped ${\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystals using a weak coherent state, and Hong-Ou-Mandel interference, which is a measure of the indistinguishability of two inputs, is observed in each frequency mode by coincidence detection. The visibility for all the modes was 40%–42% (theoretically up to 50%). Furthermore, we show that a mixture of different modes is avoided. The present results are connected to frequency-selective Bell-state measurements and therefore frequency-multiplexed quantum repeaters.

In this paper we study the effects of a topological Weyl semimetal (WSM) upon the ground state and polarization of a hydrogen-like atom near its surface. The WSM is assumed to be in the equilibrium state and at the neutrality point, such that the interaction between the atomic charges and the material is fully described (in the nonretarded regime) by axion electrodynamics, which is an experimentally observable signature of the anomalous Hall effect in the bulk of the WSM. The atom-WSM interaction provides additional contributions to the Casimir-Polder potential thus modifying the energy spectra and wave function, which now becomes distance dependent. Using variational methods, we solve the corresponding Schrödinger equation for the atomic electron. The ground state and the polarization are analyzed as a function of the atom-surface distance, and we directly observe the effects of the nontrivial topology of the material by comparing our results with that of a topologically trivial sample. We also study the impact of the medium's permittivity by assuming a hydrogen atom in vacuum, and a donor impurity in the semiconductors gallium arsenide (GaAs) and gallium phosphide (GaP). We find that the topological interaction behaves as an effective-attractive charge so that the electronic cloud tends to be polarized to the interface of materials. Moreover, the loss of wave-function normalization is interpreted as a critical location from below which the bound state is broken.

We present the systematic QED treatment of the electron correlation effects on the $g$ factor of lithiumlike ions for the wide range of nuclear charge number $Z=14$–82. The one- and two-photon exchange corrections are evaluated rigorously within the QED formalism. The electron-correlation contributions of the third and higher orders are accounted for within the Breit approximation employing the recursive perturbation theory. The calculations are performed in the framework of the extended Furry picture, i.e., with inclusion of the effective local screening potential in the zeroth-order approximation. In comparison to the previous theoretical calculations, the accuracy of the interelectronic-interaction contributions to the bound electron $g$ factor in lithiumlike ions is substantially improved.

A method for highly accurate calculations of atomic electric quadrupole moments (EQM) is presented, using relativistic general-excitation-rank configuration interaction wave functions based on Dirac spinors. Application to the clock transition states of the thulium atom employing up to full Quadruple excitations for the atomic wave function yields a final value of $Q_{zz}\left({{}^{2}F}_{7/2}\right)=0.07\begin{array}{c}+0.07\\ -0.00\end{array}$ a.u., establishing that the thulium electronic ground state has an exceptionally small EQM. A detailed analysis of this result is presented which has implications for EQMs of other atoms with unpaired $f$ electrons.

Measurement-based cooling is a method by which a quantum system, initially in a thermal state, can be prepared probabilistically in its ground state through some sort of measurement. This is done by making a measurement that heralds the system being in the desired state. Here we demonstrate the application of a measurement-based cooling technique to a trapped atomic ion. The ion is precooled by Doppler laser cooling to a thermal state with a mean excitation of $\overline{n}\approx 18$ and the measurement-based cooling technique selects those occasions when the ion happens to be in the motional ground state. The fidelity of the heralding process is greater than 95%. This technique could be applied to other systems that are not as amenable to laser cooling as trapped ions.

We theoretically investigate a scheme to produce spin-polarized electrons based on the vortex interference of the photoelectrons in single-photon ionization. A pair of time-delayed counterrotating circularly polarized pulses are applied to ionize the rare-gas-atom krypton. The results show that the vortex-shaped photoelectron momentum distributions for the ionization channels of $J=1/2$ and $J=3/2$, as well as those associated with the initial $p_{+}$ and $p_{-}$ orbitals, can be well dislocated in momentum space, leading to the spin polarization exceeding $50\%$. Furthermore, by modifying the time delay, the wavelength, and the relative phase of the pulse pair, we show that the kinetic energy as well as the ejection angle of the spin-polarized electrons can be flexibly controlled in the present scheme.

The variational theorem for the scattering length in the presence of the dipole-dipole interaction is developed. The theorem is applied to the spinless dipolar Bose gas in three dimensions. We calculated analytically the long-range tails of the single-particle momentum distribution and static structure factor, and the pair distribution function at short distances. The momentum distribution is inversely proportional to $q^4$ with the anisotropic prefactor. In the absence of the dipole-dipole interaction, Tan's adiabatic sweep theorem is reproduced as a particular case. For the homogeneous dilute Bose gas, all the relations are calculated analytically.