The self-energy of an electron confined between parallel surfaces with arbitrary dielectric properties is calculated. The mechanism for this effect is the surface-induced modification of the fluctuating quantized vacuum field to which the electron is coupled, thereby endowing it with a surface-dependent self-energy in broad analogy to the Casimir-Polder effect for an atom. We derive a general formula for this self-energy shift and find that its sign is different for two commonly used models of surface response, namely, the plasma model and the Drude model. We propose an experiment which could detect this difference in sign, shedding light on continuing uncertainty about the correct description of the interaction of low-frequency vacuum photons with media.

We observe 69 ${}^{41}\mathrm{K}{\text{−}}^6\mathrm{Li}$ interspecies Feshbach resonances including 13 elastic $p$-wave resonances and 6 broad $d$-wave resonances of ${}^{41}\mathrm{K}$ atoms in different spin-state combinations at fields up to 600 G. Multichannel quantum-defect theory calculation is performed to assign these resonances and the results show perfect agreement with experimental values after improving input parameters. The observed broad $p$- and $d$-wave resonances display a fully resolved multiplet structure. They may serve as important simulators to nonzero partial-wave-dominated physics.

In a recent paper [I. I. Arkhipov, Phys. Rev. A 98, 021803 (2018)], it was shown that one can completely identify the nonclassicality of single- and two-mode Gaussian states by means of certain nonclassicality witnesses which are based on intensity moments up to the third order of optical fields, provided that an appropriate coherent displacement is applied to a given Gaussian state. Here, we utilize a mathematical equivalence between the description of the coherent displaced Gaussian states generated in the spontaneous parametric processes and the Gaussian states generated in the corresponding stimulated parametric processes. Resorting to that equivalence, we study and compare the power of those nonclassicality witnesses in the detection of the nonclassicality of the two-mode Gaussian states generated in both the spontaneous and stimulated second subharmonic and down-conversion processes and which are subsequently subject to a beam splitter. We demonstrate that by means of an appropriate induced stimulated emission, one can completely identify the nonclassicality of the considered Gaussian states in comparison to the case of the spontaneous emission. This is important from the experimental point of view, as the stimulated emission can be easily implemented in running optical experiments, and such method can exploit just simple linear detectors.

The ability to characterize the complete quantum state of light is essential for both fundamental and applied science. For single photons the quantum state is provided by the mode that it occupies. The spectral temporal mode structure of light has recently emerged as an essential means for quantum information science. Here we experimentally demonstrate a self-referencing technique to completely determine the pulse-mode structure of single photons by means of spectral shearing interferometry. We detail the calibration and resolution of the measurement and discuss challenges and critical requirements for future advances of this method.

Level repulsion—the opening of a gap between two degenerate modes due to coupling—is ubiquitous anywhere from solid-state theory to quantum chemistry. In contrast, if one mode has negative energy, the mode frequencies attract instead. They converge and develop imaginary components, leading to an instability; an exceptional point marks the transition. This only occurs if the dissipation rates of the two modes are comparable. Here we expose a theoretical framework for the general phenomenon and realize it experimentally through engineered dissipation in a multimode superconducting microwave optomechanical circuit. Level attraction is observed for a mechanical oscillator and a superconducting microwave cavity, while an auxiliary cavity is used for sideband cooling. Two exceptional points are demonstrated that could be exploited for their topological properties.

Topological phases of matter arise in distinct fermionic and bosonic flavors. The fundamental differences between them are encapsulated in their rotational symmetries—the spin. Although spin quantization is routinely encountered in fermionic topological edge states, analogous quantization for bosons has proven elusive. To this end, we develop the complete electromagnetic continuum theory characterizing 2+1D topological bosons, taking into account their intrinsic spin and orbital angular momentum degrees of freedom. We demonstrate that spatiotemporal dispersion (momentum and frequency dependence of linear response) captures the matter-mediated interactions between bosons and is a necessary ingredient for topological phases. We prove that the bulk topology of these 2+1D phases is manifested in transverse spin-1 quantization of the photon. From this insight, we predict two unique bosonic phases—one with even parity $C=\pm 2$ and one with odd $C=\pm 1$. To understand the even parity phase $C=\pm 2$, we introduce an exactly solvable model utilizing nonlocal optical Hall conductivity and reveal a single gapless photon at the edge. This unidirectional photon is spin-1 helically quantized, immune to backscattering, defects, and exists at the boundary of the $C=\pm 2$ bosonic phase and any interface-even vacuum. The contrasting phenomena of transverse quantization in the bulk, but longitudinal (helical) quantization on the edge is addressed as the quantum gyroelectric effect. We also validate our bosonic Maxwell theory by direct comparison with the supersymmetric Dirac theory of fermions. To accelerate the discovery of such bosonic phases, we suggest two probes of topological matter with broken time-reversal symmetry: momentum-resolved electron energy-loss spectroscopy and cold atom near-field measurement of nonlocal optical Hall conductivity.

The thorium nucleus with a mass number $A=229$ has attracted much interest because its extremely low-lying first excited isomeric state at about 8 eV opens the possibility for the development of a nuclear clock. Both the energy of this state as well as the nuclear magnetic dipole and electric quadrupole moment of the ${}^{229m}\mathrm{Th}$ isomer are subjects of intense research. The latter can be determined by investigating the hyperfine structure of thorium atoms or ions. Due to its electronic structure and the long lifetime of the nuclear isomeric state, ${\mathrm{Th}}^{2+}$ is especially suitable for such kinds of studies. In this Rapid Communication, we present a combined experimental and theoretical investigation of the hyperfine structure of the ${}^{229}\mathrm{Th}^{2+}$ ion in the nuclear ground state, where a good agreement between theory and experiment is found. For the nuclear excited state we use our calculations in combination with recent measurements [J. Thielking et al., Nature (London) 556, 321 (2018)] to obtain the nuclear dipole moment of the isomeric state ${\mu }_{\text{iso}}=-0.35{\mu }_{\text{N}}$, which is in contradiction to the theoretically predicted value of ${\mu }_{\text{iso}}=-0.076{\mu }_{\text{N}}$ [A. M. Dykhne and E. V. Tkalya, JETP Lett. 67, 251 (1998)].

We study in detail the decoherence process of a quantum register, coupled to a composite environment. We use recently developed methods of information transfer study in open quantum systems to analyze information flow between the register and its environment. We show that there are regimes when not only the register decoheres effectively to a classical bit string, but this bit string is redundantly encoded in the environment, making it available to multiple observers. This process is more subtle and in general qualitatively different than in the case of a single qubit due to the presence of various protected subspaces: decoherence-free subspaces and so-called orthogonalization-free subspaces. We show that this leads to a rich structure of coherence loss or protection in the asymptotic state of the register and a part of its environment. We formulate a series of examples illustrating these structures.

Entanglement is an essential resource in quantum information science and it is often sensitive to temperature fluctuation. Quantifying and understanding the detailed influence of temperature on entanglement is important for both the fundamental interest and the application value. Here we theoretically study the entanglement distribution in a many-body system subjected to a temperature gradient and identify an entanglement difference on two sides of the system. This phenomenon is named the entanglement Seebeck effect, which is similar to the traditional Seebeck effect where an electric voltage can be generated on the two sides of a metal with different temperatures. (i) In contrast to general wisdom, the spins on the hotter side can have larger entanglement than those on the colder side of a magnetic system. This anomalous phenomenon is well explained by the increase of magnon density with the temperature in a magnetic system. (ii) The entanglement length (the critical distance beyond which the entanglement of electrons disappears) in such a nonequilibrium system is larger than that in an equilibrium system with equal temperature as a particular local temperature chosen from the nonequilibrium system. This enhancement is ascertained to result from the particle number fluctuation of a nonequilibrium state. Our findings provide a possible way to quantify the temperature influence on the spin entanglement in nonequilibrium many-body systems and may further benefit the experimental measurements of entanglement where the entanglement length is an important asset.

A nonrelativistic quantum mechanical particle moving freely on a curved surface feels the effect of the nontrivial geometry of the surface through the kinetic part of the Hamiltonian, which is proportional to the Laplace-Beltrami operator, and a geometric potential, which is a linear combination of the mean and Gaussian curvatures of the surface. The coefficients of these terms cannot be uniquely determined by general principles of quantum mechanics but enter the calculation of various physical quantities. We examine their contribution to the geometric scattering of a scalar particle moving on an asymptotically flat embedded surface. In particular, having in mind the possibility of an experimental realization of the geometric scattering in a low-density electron gas formed on a bumped surface, we determine the scattering amplitude for arbitrary choices of the curvature coefficients for a surface with global or local cylindrical symmetry. We also examine the effect of perturbations that violate this symmetry and consider surfaces involving bumps that form a lattice.

This work presents a quantum algorithm for the Monte Carlo pricing of financial derivatives. We show how the relevant probability distributions can be prepared in quantum superposition, the payoff functions can be implemented via quantum circuits, and the price of financial derivatives can be extracted via quantum measurements. We show how the amplitude estimation algorithm can be applied to achieve a quadratic quantum speedup in the number of steps required to obtain an estimate for the price with high confidence. This work provides a starting point for further research at the interface of quantum computing and finance.

In this work we investigate methods to improve the efficiency and scalability of quantum algorithms for quantum chemistry applications. We propose a transformation of the electronic structure Hamiltonian in the second quantization framework into the particle-hole picture, which offers a better starting point for the expansion of the system wave function. The state of the molecular system at study is parametrized so as to constrain the sampling of the corresponding wave function within the sector of the molecular Fock space that contains the desired solution. To this end, we explore different mapping schemes to encode the molecular ground state wave function in a quantum register. Taking advantage of known post-Hartree-Fock quantum chemistry approaches and heuristic Hilbert space search quantum algorithms, we propose a new family of quantum circuits based on exchange-type gates that enable accurate calculations while keeping the gate count (i.e., the circuit depth) low. The particle-hole implementation of the unitary coupled-cluster (UCC) method within the variational quantum eigensolver approach gives rise to an efficient quantum algorithm, named q-UCC, with important advantages compared to the straightforward translation of the classical coupled-cluster counterpart. In particular, we show how a single Trotter step in the expansion of the system wave function can accurately and efficiently reproduce the ground-state energy of simple molecular systems.

Solving the fundamental and optical gap problems, which yield information about charged and neutral excitations in electronic systems, is one of the biggest challenges in density-functional theory. Despite their intrinsic differences, we show that the two problems can be made formally identical by introducing a universal and canonical ensemble weight-dependent exchange-correlation (xc) density functional. The weight dependence of the xc energy turns out to be the key ingredient for describing the infamous derivative discontinuity and represents an alternative path for its approximation.

We investigate theoretically the Casimir-Polder potential of an atom which is driven by a laser field close to a surface. This problem is addressed in the framework of macroscopic quantum electrodynamics using the Green's tensor formalism and we distinguish between two different approaches: a perturbative ansatz and a method based on Bloch equations. We apply our results to a concrete example, namely an atom close to a perfectly conducting mirror, and create a scenario where the tunable Casimir-Polder potential becomes similar to the respective potential of an undriven atom due to fluctuating field modes. Whereas the perturbative approach is restricted to large detunings, the ansatz based on Bloch equations is exact and yields an expression for the potential which does not exceed $1/2$ of the undriven Casimir-Polder potential.

Radiative polarization of electrons and positrons through the Sokolov-Ternov effect is important for applications in high-energy physics. Radiative spin polarization is a manifestation of quantum radiation reaction affecting the spin dynamics of electrons. We recently proposed that an analog of the Sokolov-Ternov effect could occur in the strong electromagnetic fields of ultra-high-intensity lasers, which would result in a buildup of spin polarization in femtoseconds. In this paper, we develop a density matrix formalism for describing beam polarization in strong electromagnetic fields. We start by using the density matrix formalism to study spin flips in nonlinear Compton scattering and its dependence on the initial polarization state of the electrons. Numerical calculations show a radial polarization of the scattered electron beam in a circularly polarized laser, and we find azimuthal asymmetries in the polarization patterns for ultrashort laser pulses. A degree of polarization approaching 9% is achieved after emitting just a single photon. We develop the theory by deriving a local constant crossed-field approximation (LCFA) for the polarization density matrix, which is a generalization of the well-known LCFA scattering rates. We find spin-dependent expressions that may be included in electromagnetic charged-particle simulation codes, such as particle-in-cell plasma simulation codes, using Monte Carlo modules. In particular, these expressions include the spin-flip rates for arbitrary initial polarization of the electrons. The validity of the LCFA is confirmed by explicit comparison with an exact QED calculation of electron polarization in an ultrashort laser pulse.

We demonstrate a scheme to produce coherent extreme ultraviolet and soft x-ray radiation with subpetahertz temporal helicity modulation, based on the high harmonic generation from current-carrying orbitals driven by intense linearly polarized laser fields. It is found that the electronic angular momentum of the orbitals oscillates periodically in femtosecond scale in the driving field. This so-far undescribed phenomenon is qualitatively interpreted and attributed to the laser-induced energy shift of orbitals. Consequently, the polarization of the harmonic radiation switches periodically in the temporal domain between left and right elliptical polarizations, and the frequency of the helicity modulation reaches subpetahertz. By varying the intensity of the laser field, the modulation frequency can be continuously controlled. This light source will serve as a potential tool to detect and manipulate the ultrafast dynamics in magnetic materials and chiral media.

A model is presented that mimics the nearest-neighbor-spacing (NNS) distribution of chaotic molecules such as ${\mathrm{Dy}}_2$ and ${\mathrm{Er}}_2$ just below their dissociation threshold. In this model the degree of chaos is controlled by choosing suitable Hamiltonian matrices from random ensembles. It is found that, in versions of the model that are not completely chaotic, the NNS of observable Fano-Feshbach resonances exhibits greater level repulsion, hence more chaos, than the corresponding NNS of a typical energy spectrum of the molecule at a fixed magnetic field.

We report the realization of correlated, density-dependent tunneling for fermionic ${}^{40}\mathrm{K}$ atoms trapped in an optical lattice. By appropriately tuning the frequency difference between a pair of Raman beams applied to a spin-polarized gas, simultaneous spin transitions and tunneling events are induced that depend on the relative occupations of neighboring lattice sites. This correlated spin-flip tunneling (CSFT) is spectroscopically resolved using gases prepared in opposite spin states, and the inferred Hubbard interaction energy is compared with a tight-binding prediction. We measure the doublons created by the laser-induced correlated tunneling process using loss induced by light-assisted collisions. Furthermore, by controllably introducing vacancies to a spin-polarized gas, we demonstrate that correlated tunneling is suppressed when neighboring lattice sites are unoccupied. We explain how the CSFT quench implemented here prepares and evolves a large number of resonating-valence-bond (RVB) singlets in a Hubbard model, thus allowing exploration of RVB dynamics.

Quantum cascade lasers (QCLs) are generally designed to avoid negative differential conductivity (NDC) in the vicinity of the operation point in order to prevent instabilities. We demonstrate that the threshold condition is possible under an inhomogeneous distribution of the electric field (domains) and leads to lasing at an operation point with a voltage bias normally attributed to the NDC region. For our example, a terahertz QCL operating up to the current maximum temperature of 199 K, the theoretical findings agree well with the experimental observations. In particular, we experimentally observe self-sustained oscillations with GHz frequency before and after threshold. These are attributed to traveling domains by our simulations. Overcoming the design paradigm to avoid NDC may allow for the further optimization of QCLs with less dissipation from stabilizing background currents.

We characterize a source of photon pairs based on cascade decay in a cold ${}^{87}\mathrm{Rb}$ ensemble. This source is particularly suited to generate photons for interaction with ${}^{87}\mathrm{Rb}$ based atomic systems. We experimentally investigate the dependence of pair generation rate, single photon heralding efficiency, and bandwidth as a function of the number of atoms, detuning, and intensity of the pump beams. The observed power and detuning behaviors can be explained by the steady-state solution of an established three-level model of an atom. Measurements presented here provide a useful insight on the optimization of this kind of photon-pair source.