Building on work by J. Meixner [Z. Naturforschung 3a, 506 (1948)], we show how to compute the exact scattering amplitude (or $\mathrm{T}$-matrix) for electromagnetic scattering from a perfectly conducting disk. This calculation is a rare example of a nondiagonal $\mathrm{T}$-matrix that can nonetheless be obtained in a semianalytic form. We then use this result to compute the electromagnetic Casimir interaction energy for a disk opposite a plane, for arbitrary orientation angle of the disk, for separations greater than the disk radius. We find that the proximity force approximation (PFA) significantly overestimates the Casimir energy, in the case of both the ordinary PFA, which applies when the disk is parallel to the plane, and the “edge PFA”, which applies when the disk is perpendicular to the plane.

Photoionization from energetically distinct electronic states may have a relative time delay of tens of attoseconds. Here we demonstrate that pulses of optical vortices allow the measurement of such attosecond delays from magnetic sublevels, even from a spherically symmetric target. The difference in the time delay is substantial and exhibits a strong angular dependence. Furthermore, we find an atomic scale variation in the time delays depending on the target orbital position in the laser spot. The findings thus offer a method for spatiotemporal sensing of the magnetic states from which photoelectrons originate, with a spatial resolution far below the diffraction limit of the vortex beam. Our conclusions follow from analytical considerations based on symmetry, complemented and confirmed by full numerical simulations of the quantum dynamics.

We show that, when $N$ is a multiple of 6 ($N=6m$, where $m$ is an integer), the $\mathrm{SU}{\left(N\right)}_{}$ Heisenberg model on the honeycomb lattice with $m$ particles per site has a clear tendency toward chiral order as soon as $m\ge 2$. This conclusion has been reached by a systematic variational Monte Carlo investigation of Gutzwiller projected wave functions as a function of $m$ between the case of one particle per site ($m=1$), for which the ground state has recently been shown to be in a plaquette singlet state, and the $m\to \infty$ limit, where a mean-field approach has established that the ground state has chiral order. This demonstrates that the chiral phase can indeed be stabilized for not too large values of $m$, opening the way to its experimental realizations in other lattices.

We consider the ground state properties of a trapped dipolar condensate under the influence of quantum fluctuations. We show that this system can undergo a phase transition from a low density condensate state to a high density droplet state, which is stabilized by quantum fluctuations. The energetically favored state depends on the geometry of the confining potential, the number of atoms, and the two-body interactions. We develop a simple variational ansatz and validate it against full numerical solutions. We produce a phase diagram for the system and present results relevant to current experiments with dysprosium and erbium condensates.

We investigate electromagnetic scattering of normally irradiated gyrotropic, magneto-optical core-shell cylinders using Lorenz-Mie theory. A general expression for time-averaged electromagnetic energy inside a coated gyroelectric and gyromagnetic scatterer is derived. Using realistic material parameters for a silica core and InSb shell, we calculate the stored electromagnetic energy and the scattering anisotropy. We show that the application of an external magnetic field along the cylinder axis induces a drastic decrease in electromagnetic absorption in a frequency range in the terahertz, where absorption is maximal in the absence of the magnetic field. We demonstrate not only that the scattering anisotropy can be externally tuned by applying a magnetic field, but also that it reaches negative values in the terahertz range even in the dipolar regime. We also show that this preferential backscattering response results in an anomalous regime of multiple light scattering from a collection of magneto-optical core-shell cylinders, in which the extinction mean free path is longer than the transport mean free path. By additionally calculating the energy-transport velocity and diffusion coefficient, we demonstrate an unprecedented degree of external control of multiple light scattering, which can be achieved by either applying an external magnetic field or varying the temperature.

We derive analytical expressions for the coherence in the onset of modulation instability, in excellent agreement with thorough numerical simulations. As usual, we start by a linear perturbation analysis, where broadband noise is added to a cw pump; then, we investigate the effect of adding a deterministic seed to the cw pump, a case of singular interest as it is commonly encountered in parametric amplification schemes. Results for the dependence of coherence on parameters such as fiber type, pump power, propagated distance, and seed signal-to-noise ratio are presented. Finally, we show the importance of including higher-order linear and nonlinear dispersion when looking at longer-wavelength regions (mid IR).

We investigate the photon emission properties of the driven-dissipative Rabi model in the so-called ultrastrong- and deep strong-coupling regimes where the atom-cavity coupling rate $g$ becomes comparable or larger than the cavity frequency ${\omega }_c$. By solving numerically the master equation in the dressed-state basis, we compute the output field correlation functions in the steady-state for a wide range of coupling rates. We find that, as the atom-cavity coupling strength increases, the system undergoes multiple transitions in the photon statistics. In particular, a first sharp antibunching-to-bunching transition, occurring at $g\sim 0.45{\omega }_c$, leading to the breakdown of the photon blockade due to the counter-rotating terms, is shown to be the consequence of a parity shift in the energy spectrum. A subsequent revival of the photon blockade and the emergence of the quasicoherent statistics for even larger coupling rates are attributed to an interplay between the nonlinearity in the energy spectrum and the transition rates between the dressed states.

Digital quantum simulation of fermionic systems is important in the context of chemistry and physics. Simulating fermionic models on general purpose quantum computers requires imposing a fermionic algebra on qubits. The previously studied Jordan-Wigner and Bravyi-Kitaev transformations are two techniques for accomplishing this task. Here, we reexamine an auxiliary fermion construction which maps fermionic operators to local operators on qubits. The local simulation is performed by relaxing the requirement that the number of qubits should match the number of single-particle states. Instead, auxiliary sites are introduced to enable nonconsecutive fermionic couplings to be simulated with constant low-rank tensor products on qubits. The additional number of auxiliary qubits required per fermionic degree of freedom depends only on the degree of connectivity of the Hamiltonian. We connect the auxiliary fermion construction to topological models and give examples of the construction.

Complex mesoscopic systems play increasingly important roles in modern science, from understanding biological functions at the molecular level to designing solid-state information processing devices. The operation of these systems typically depends on their energetic structure, yet probing their energy landscape can be extremely challenging; they have many degrees of freedom, which may be hard to isolate and measure independently. Here, we show that a qubit (a two-level quantum system) with a biased energy splitting can directly probe the spectral properties of a complex system, without knowledge of how they couple. Our work is based on the completely positive and trace-preserving map formalism, which treats any unknown dynamics as a “black-box” process. This black box contains information about the system with which the probe interacts, which we access by measuring the survival probability of the initial state of the probe as function of the energy splitting and the process time. Fourier transforming the results yields the energy spectrum of the complex system. Without making assumptions about the strength or form of its coupling, our probe could determine aspects of a complex molecule's energy landscape as well as, in many cases, test for coherent superposition of its energy eigenstates.

Negative entropy in connection with the Casimir effect at uniform temperature is a phenomenon rooted in the circumstance that one is describing a nonclosed system, or only part of a closed system. In this paper we show that the phenomenon is not necessarily restricted to electromagnetic theory but can be derived from the quantum theory of interacting harmonic oscillators, most typically two oscillators interacting not directly but indirectly via a third one. There are two such models, actually analogous to the transverse magnetic (TM) and transverse electric (TE) modes in electrodynamics. These mechanical models in their simplest version were presented some years ago, by Høye et al. [Phys. Rev. E 67, 056116 (2003)]. In the present paper we reemphasize the physical significance of the mechanical picture and extend the theory so as to include the case where there are several mediating oscillators instead of only one. The TE oscillator exhibits negative entropy. Finally, we show explicitly how the interactions via the electromagnetic field contain the two oscillator models.

In this work we present a generalization of the recently developed Hardy-like logical proof of contextuality and of the so-called KCBS contextuality inequality for any qudit of dimension greater than three. Our approach uses compatibility graphs that can only be satisfied by qudits. We find a construction for states and measurements that satisfy these graphs and demonstrate both logical and inequality-based contextuality for qudits. Interestingly, the quantum violation of the inequality is constant as dimension increases. We also discuss the issue of imprecision in experimental implementations of contextuality tests and a way of addressing this problem using the notion of ontological faithfulness.

We present a scheme for generating robust and persistent entanglement between qubits that do not interact and that are separated by a long and lossy transmission channel, using Markovian reservoir engineering. The proposal uses only the correlated decay into the common channel of remotely separated, driven single-photon qubit transitions. This simple scheme is generic and applicable to various experimental implementations, including circuit and cavity QED, with little experimental overhead compared with methods requiring dynamic control, initialization, measurement, or feedback. In addition to avoiding these inefficiencies, the simple protocol is highly robust against noise, miscalibration, and loss in the channel. We find high-quality solutions over a wide range of parameters and show that the optimal strategy reflects a transition from ballistic to diffusive photon transmission, going from symmetrically and coherently driving a common steady state to asymmetrically absorbing photons that are emitted from one qubit by the second. Detailed analysis of the role of the transmission channel shows that allowing bidirectional decay drastically increases indistinguishability and thereby quadratically suppresses infidelity.

The uncertainty principle can be understood as constraining the probability of winning a game in which Alice measures one of two conjugate observables, such as position or momentum, on a system provided by Bob, and he is to guess the outcome. Two variants are possible: either Alice tells Bob which observable she measured, or he has to furnish guesses for both cases. Here I derive uncertainty relations for both, formulated directly in terms of Bob's guessing probabilities. For the former these relate to the entanglement that can be recovered by action on Bob's system alone. This gives an explicit quantum circuit for approximate quantum error correction using the guessing measurements for “amplitude” and “phase” information, implicitly used in the recent construction of efficient quantum polar codes. I also find a relation on the guessing probabilities for the latter game, which has application to wave-particle duality relations.

Quantum position verification (QPV) is the art of verifying the geographical location of an untrusted party. Recently, it has been shown that the widely studied Bennett & Brassard 1984 (BB84) QPV protocol is insecure after the 3 dB loss point assuming local operations and classical communication (LOCC) adversaries. Here, we propose a time-reversed entanglement swapping QPV protocol (based on measurement-device-independent quantum cryptography) that is highly robust against quantum channel loss. First, assuming ideal qubit sources, we show that the protocol is secure against LOCC adversaries for any quantum channel loss, thereby overcoming the 3 dB loss limit. Then, we analyze the security of the protocol in a more practical setting involving weak laser sources and linear optics. In this setting, we find that the security only degrades by an additive constant and the protocol is able to verify positions up to 47 dB channel loss.

A simple and yet powerful approach for modeling the structure of endohedrally confined diatomic molecules is introduced. The theory, based on a $\mathrm{u}\left(4\right)\oplus \mathrm{u}\left(3\right)$ dynamical algebra, combines $\mathrm{u}\left(4\right)$, the vibron model dynamical algebra, with a $\mathrm{u}\left(3\right)$ dynamical algebra that models a spherically symmetric three-dimensional potential. The first algebra encompasses the internal rotovibration degrees of freedom of the molecule, while the second takes into account the confined molecule center-of-mass degrees of freedom. A resulting subalgebra chain is connected to the underlying physics and the model is applied to the prototypical case of ${\mathrm{H}}_2$ caged in a fullerene molecule. The spectrum of the supramolecular complex ${\mathrm{H}}_2@{\mathrm{C}}_{60}$ is described with a few parameters, and predictions for not yet detected levels are made. Our fits suggest that the quantum numbers of a few lines should be reassigned to obtain better agreement with data.

We investigate bond-rearrangement driven by photo-double-ionization (PDI) near and above the double-ionization threshold in a sequence of carbon-carbon double-bonded hydrocarbon molecules: ethylene, fluoroethylene, and 1,1-difluoroethylene. We employ the kinematically complete cold target recoil ion momentum spectroscopy method to resolve all photo-double-ionization events leading to two-ion fragments. We observe changes in the branching ratios of different dissociative ionization channels depending on the presence of no, one, or two fluorine atoms. The role of the fluorine atom in the bond-rearrangement channels is intriguing, as evident by the reordering of the threshold energies of the PDI in the fluorinated molecules. These effects offer a compelling argument that the electronegativity of the fluorine (or the polarity of the molecule) strongly influences the potential energy surfaces of the molecules and drives bond rearrangement during the dissociation process. The energy sharing and the relative angle between the three-dimensional momentum vectors of the two electrons enable us to distinguish between knockout and other ionization mechanisms of the PDI processes.

We report on the production of ultracold heteronuclear YbLi* molecules in a dual-species magneto-optical trap by photoassociation (PA). The formation of the electronically excited molecules close to dissociation was observed by trap loss spectroscopy. We find 4 rovibrational states within a range of $250\mathrm{GHz}$ below the $\mathrm{Yb}\left({}^{1}S_0\right)+\mathrm{Li}\left({}^{2}P_{1/2}\right)$ asymptote and observe isotopic PA line shifts in mixtures of ${}^6\mathrm{Li}$ with ${}^{174}\mathrm{Yb}$, ${}^{172}\mathrm{Yb}$, and ${}^{176}\mathrm{Yb}$. We also describe our theoretical ab initio calculation for the relevant electronic potentials and utilize it to analyze and identify the lines in the experimentally observed spectrum.

Experiments on quantum degenerate Fermi gases of magnetic atoms and dipolar molecules begin to probe their broken-symmetry phases dominated by the long-range, anisotropic dipole-dipole interaction. Several candidate phases including the $p$-wave superfluid, the stripe density wave, and a supersolid have been proposed theoretically for two-dimensional spinless dipolar Fermi gases. Yet the phase boundaries predicted by different approximations vary greatly and a definitive phase diagram is still lacking. Here we present a theory that treats all competing many-body instabilities in the particle-particle and particle-hole channel on equal footing. We obtain the low-temperature phase diagram by numerically solving the functional renormalization-group flow equations and find a nontrivial density wave phase at small dipolar tilting angles and strong interactions, but no evidence of the supersolid phase. We also estimate the critical temperatures of the ordered phases.

A gas of interacting ultracold fermions can be tuned into a strongly interacting regime using a Feshbach resonance. Here, we theoretically study quasiparticle transport in a system of two reservoirs of interacting ultracold fermions on the BCS side of the BCS-BEC crossover coupled weakly via a tunnel junction. Using the generalized BCS theory, we calculate the time evolution of the system that is assumed to be initially prepared in a nonequilibrium state characterized by a particle number imbalance or a temperature imbalance. A number of characteristic features like sharp peaks in quasiparticle currents or transitions between the normal and superconducting states are found. We discuss signatures of the Seebeck and the Peltier effects and the resulting temperature difference of the two reservoirs as a function of the interaction parameter ${\left(k_{F}a\right)}^{-1}$. The Peltier effect may lead to an additional cooling mechanism for ultracold fermionic atoms.