An ultracold atom above a horizontal mirror experiences quantum reflection from the attractive Casimir-Polder interaction, which holds it against gravity and leads to quantum levitation states. We analyze this system by using a Liouville transformation of the Schrödinger equation and a Langer coordinate adapted to problems with a classical turning point. Reflection on the Casimir-Polder attractive well is replaced by reflection on a repulsive wall, and the problem is then viewed as an ultracold atom trapped inside a cavity with gravity and Casimir-Polder potentials acting, respectively, as top and bottom mirrors. We calculate numerically Casimir-Polder shifts of the energies of the cavity resonances and propose an approximate treatment which is precise enough to discuss spectroscopy experiments aimed at tests of the weak-equivalence principle on antihydrogen. We also discuss the lifetimes by calculating complex energies associated with cavity resonances.

Ultrarelativistic photoelectron spectra exhibit unexpected characteristics in a paraxial laser focus. The photoelectron energy scales superponderomotively, and the usual parabolic momentum distribution is distorted into a variety of intricate patterns, depending on the location of the ion. These patterns include discrete contours, which in some cases can be easily identified with a particular subcycle burst of ionization current. An analytical formula for the maximum photoelectron energy in a paraxial radiation field is given, and high-resolution momentum distributions with narrowly peaked features are presented. These narrowly peaked features suggest application to electron injection into plasma-based accelerators.

We report on the experimental observation of photon energy sharing among two electrons and two ions ejected from a doubly ionized molecule exposed to an intense ultraviolet femtosecond laser pulse. Although two electrons are successively released one after the other, bridged by the nuclear motion via their interactions, photon energy sharing among four particles is observed as multiple energy conservation lines in their joint energy spectrum. For sequential double ionization of ${\mathrm{H}}_2$, the electron-nuclear joint energy spectrum allows us to identify three pathways towards the charge-resonance enhanced ionization of the stretching ${\mathrm{H}}_2{}^{+}$ in strong laser fields. By counting the photon number absorbed by the molecule, we trace the accessibility, enhancement, and suppression of various pathways. The correlated electron-nuclear motion provides profound insights of the complicated strong-field dynamics of molecules.

We study the stability of a spin-orbit-coupled binary Bose-Einstein condensate. It is known that a trapped binary Bose-Einstein condensate will collapse when the interatomic attraction is beyond the threshold. We demonstrate both analytically and numerically that instead of collapse such a system can be stabilized by spin-orbit coupling and Raman coupling. The stability criteria and stability diagram in full parameter space are presented analytically. The stabilizing mechanism is revealed explicitly: the spin-orbit coupling and Raman coupling generate an effective interatomic repulsion, which changes the stability criteria, neutralizes the mean-field attraction, and stabilizes the system against collapse. Interestingly, the generated effective interatomic repulsion is determined by spin-orbit coupling and Raman coupling. That is, a quantitative theoretical evidence for controlling the stability of the system is provided, which can be realized easily in current experiments.

The existence of a self-localization transition in the polaron problem has been under an active debate ever since Landau suggested it 84 years ago. Here we reveal the self-localization transition for the rotational analog of the polaron—the angulon quasiparticle. We show that, unlike for the polarons, self-localization of angulons occurs at finite impurity-bath coupling already at the mean-field level. The transition is accompanied by the spherical-symmetry breaking of the angulon ground state and a discontinuity in the first derivative of the ground-state energy. Moreover, the type of symmetry breaking is dictated by the symmetry of the microscopic impurity-bath interaction, which leads to a number of distinct self-localized states. The predicted effects can potentially be addressed in experiments on cold molecules trapped in superfluid helium droplets and ultracold quantum gases, as well as on electronic excitations in solids and Bose-Einstein condensates.

We analyze theoretically the features of the output field of a quadratically coupled optomechanical system, which is driven by a strong coupling field and a weak signal field, and in which the membrane (treated as a mechanical resonator) is excited by a weak coherent driving field with two-phonon resonance. We show that the system exhibits complex quantum coherent and interference effects resulting in transmission of the signal field from opacity to remarkable amplification. We also find that the total phase of the applied fields can significantly adjust the signal field's transmission spectrum. The study of the propagation of the signal field in such a quadratically coupled optomechanical system proves that the proposed device can operate as an optical transistor.

Hamiltonian quantum computing, such as the adiabatic and holonomic models, can be protected against decoherence using an encoding into stabilizer subspace codes for error detection and the addition of energy penalty terms. This method has been widely studied since it was first introduced by Jordan, Farhi, and Shor (JFS) in the context of adiabatic quantum computing. Here, we extend the original result to general Markovian environments, not necessarily in Lindblad form. We show that the main conclusion of the original JFS study holds under these general circumstances: Assuming a physically reasonable bath model, it is possible to suppress the initial decay out of the encoded ground state with an energy penalty strength that grows only logarithmically in the system size, at a fixed temperature.

A solid state spin is an attractive system with which to realize an ultrasensitive magnetic field sensor. A spin superposition state will acquire a phase induced by the target field, and we can estimate the field strength from this phase. Recent studies have aimed at improving sensitivity through the use of quantum error correction (QEC) to detect and correct any bit-flip errors that may occur during the sensing period. Here we investigate the performance of a two-qubit sensor employing QEC and under the effect of energy relaxation. Surprisingly, we find that the standard QEC technique to detect and recover from an error does not improve the sensitivity compared with the single-qubit sensors. This is a consequence of the fact that the energy relaxation induces both a phase-flip and a bit-flip noise where the former noise cannot be distinguished from the relative phase induced from the target fields. However, we have found that we can improve the sensitivity if we adopt postselection to discard the state when error is detected. Even when quantum error detection is moderately noisy, and allowing for the cost of the postselection technique, we find that this two-qubit system shows an advantage in sensing over a single qubit in the same conditions.

A suitable photon-subtraction operation can be exploited to improve the maximal transmission of continuous-variable quantum key distribution (CVQKD) in point-to-point quantum communication. Unfortunately, the photon-subtraction operation faces solving the improvement transmission problem of practical quantum networks, where the entangled source is located in the third part, which may be controlled by a malicious eavesdropper, instead of in one of the trusted parts, controlled by Alice or Bob. In this paper, we show that a solution can come from using a non-Gaussian operation, in particular, the photon-subtraction operation, which provides a method to enhance the performance of entanglement-based (EB) CVQKD. Photon subtraction not only can lengthen the maximal transmission distance by increasing the signal-to-noise rate but also can be easily implemented with existing technologies. Security analysis shows that CVQKD with an entangled source in the middle (ESIM) from applying photon subtraction can well increase the secure transmission distance in both direct and reverse reconciliations of the EB-CVQKD scheme, even if the entangled source originates from an untrusted part. Moreover, it can defend against the inner-source attack, which is a specific attack by an untrusted entangled source in the framework of ESIM.

The electron emission produced by the grazing incidence of ultrashort laser pulses on two faces of aluminum, Al(100) and Al(111), is investigated using the band-structure-based-Volkov (BSB-V) approximation. The present version of the BSB-V approach includes not only a realistic description of the surface interaction, accounting for band-structure effects, but also effects due to the induced potential that originates from the collective response of valence electrons to the external electromagnetic field. For both crystallographic orientations we found that the induced potential contributes to the emergence of band-structure signatures in the near-threshold region of photoelectron spectra. This result opens a window to scrutinize band-structure effects in metal surfaces via ultrashort-laser interactions.

The interaction of a helium atom with an intense, short, 800-nm, laser pulse is studied theoretically beyond the single-active-electron approximation. For this purpose, the time-dependent Schrödinger equation for a two-electron wave packet driven by a linearly polarized infrared pulse is solved by the time-dependent restricted-active-space configuration-interaction method in the dipole velocity gauge. By systematically extending the space of active configurations, we investigate the role of the collective two-electron dynamics in the strong-field ionization and high-order-harmonic generation processes. Our numerical results demonstrate that allowing both electrons in He to be dynamically active results in a considerable extension of the computed high-order-harmonic generation spectrum.

We explore the ground-state properties of cold atomic gases focusing on the cases of noninteracting fermions and hard-core (Tonks-Girardeau) bosons, trapped by the combination of two potentials (bichromatic lattice) with incommensurate periods. For such systems, two limiting cases have been thoroughly established. In the tight-binding limit, the single-particle states in the lowest occupied band show a localization transition, as the strength of the second potential is increased above a certain threshold. In the continuous limit, when the tight-binding approximation does not hold, a mobility edge is found, instead, whose position in energy depends upon the strength of the second potential. Here, we study how the crossover from the discrete to the continuum behavior occurs, and prove that signatures of the localization transition and mobility edge clearly appear in the generic many-body properties of the systems. Specifically, we evaluate the momentum distribution, which is a routinely measured quantity in experiments with cold atoms, and demonstrate that, even in the presence of strong boson-boson interactions (infinite in the Tonks-Girardeau limit), the single-particle mobility edge can be observed in the ground-state properties.

Motivated by recent experiments on Bose-Einstein condensed atoms which rotate in annular and/or toroidal traps, we study the effect of the finiteness of the atom number $N$ on the states of lowest energy for a fixed expectation value of the angular momentum, under periodic boundary conditions. To attack this problem, we develop a general strategy, considering a linear superposition of the eigenstates of the many-body Hamiltonian, with amplitudes that we extract from the mean-field approximation. This many-body state breaks the symmetry of the Hamiltonian; it has the same energy to leading order in $N$ as the mean-field state and the corresponding eigenstate of the Hamiltonian, however, it has a lower energy to subleading order in $N$ and thus it is energetically favorable.

We investigate schemes for conversion between qubits encoded into superpositions of vacuum and single-photon states and qubits encoded into states of single photons. The analyzed schemes combine linear optics, homodyne or heterodyne detection, single-photon detection, and electro-optical feedforward, and are designed to be suitable for signals with arbitrary temporal profiles. To achieve perfect conversion, quantum filtering is utilized based on conditioning on no-clicks of single-photon detectors, which makes the conversion probabilistic. High-quality conditional conversion is nevertheless possible even without single-photon detectors, by conditioning on outcomes of a homodyne or heterodyne detector. Deterministic high-quality conversion of dual-rail qubits to single-rail qubits is possible with an array of homodyne detectors where the phases of the measured quadratures are controlled by a feedforward scheme, which represents an alternative to conversion via homodyne detection with a real-time continuous feedback [T. C. Ralph, A. P. Lund, and H. M. Wiseman, J. Opt. B 7, S245 (2005)].

Maximal violation of the CHSH-Bell inequality is usually said to be a feature of anticommuting observables. In this work we show that even random observables exhibit near-maximal violations of the CHSH-Bell inequality. To do this, we use the tools of free probability theory to analyze the commutators of large random matrices. Along the way, we introduce the notion of “free observables,” which can be thought of as infinite-dimensional operators that reproduce the statistics of random matrices as their dimension tends towards infinity. We also study the fine-grained uncertainty of a sequence of free or random observables and use this to construct a steering inequality with a large violation.

We consider the quantum search problem with a continuous-time quantum walk for networks characterized by a finite spectral dimension $d_s$ of the network Laplacian. For general networks of fractal (integer or noninteger) dimension $d_f$, for which in general $d_f\ne d_s$, it suggests that it is $d_s$ that determines the computational complexity of the quantum search. Our results continue those of A. M. Childs and J. Goldstone [Phys. Rev. A 70, 022314 (2004)] for lattices of integer dimension, where $d=d_f=d_s$. Thus, we find for general fractals that the Grover limit of quantum search can be obtained whenever $d_s>4$. This complements the recent discussion of mean-field (i.e., $d_s\to \infty$) networks by S. Chakraborty et al. [Phys. Rev. Lett. 116, 100501 (2016)] showing that for all those networks, spatial search by quantum walk is optimal.

At low temperatures bosons typically condense to minimize their single-particle kinetic energy while interactions stabilize superfluidity. Optical lattices with artificial spin-orbit coupling challenge this paradigm, because here kinetic energy can be quenched in an extreme regime where the single-particle band flattens. To probe the fate of superfluidity in the absence of kinetics we construct and numerically solve interaction-only tight-binding models in flatbands. We find that superfluid states arise entirely from interactions operating in quenched kinetic energy bands, thus revealing a distinct and unexpected condensation mechanism. Our results have important implications for the identification of quantum condensed phases of ultracold bosons beyond conventional paradigms.

We numerically study an analog black hole with two horizons with parameters similar to a recent experiment. We find that the Hawking radiation exists on a background which contains a density oscillation, a zero-frequency ripple. The Hawking radiation evolves from spontaneous to self-amplifying, while the background ripple grows steadily with no qualitative change. It is seen that the self-amplifying Hawking radiation has a nonzero frequency. This frequency is independent of the reference frame since it is the magnitude of the wave which oscillates, as in a standing wave. The background ripple appears even before the inner horizon is created, in contrast to predictions. Furthermore, we find that technical noise and shot-to-shot variations in the number of atoms are not sufficient to cause the observed correlation function. This work is in agreement with the recent observation of self-amplifying Hawking radiation, and explains some of the features seen. In contrast to some recent works, our study differentiates between the Hawking radiation observed, and the evolution of the background.

We experimentally investigate the atom optics kicked particle at quantum resonance using finite duration kicks. Even though the underlying process is quantum interference, it can be well described by an $\epsilon$-pseudoclassical model. The $\epsilon$-pseudoclassical model agrees well with our experiments for a wide range of parameters. We investigate the parameters yielding maximal momentum transfer to the atoms and find that this occurs in the regime where neither the short pulse approximation nor the Bragg condition is valid. Nonetheless, the momentum transferred to the atoms can be predicted using a simple scaling law, which provides a powerful tool for choosing optimal experimental parameters. We demonstrate this in a measurement of the Talbot time (from which $h/M$ can be deduced), in which we coherently split atomic wave functions into superpositions of momentum states that differ by 200 photon recoils. Our work may provide a convenient way to implement large momentum difference beam splitters in atom interferometers.

Motivated by advances in the manipulation and detection of ultracold atoms with multiple internal degrees of freedom, we present a finite-temperature lattice Monte Carlo calculation of the density and pressure equations of state, as well as Tan's contact, of attractively interacting SU(4)- and SU(6)-symmetric fermion systems in one spatial dimension. We also furnish a nonperturbative proof of a universal relation whereby quantities computable in the SU(2) case completely determine the virial coefficients of the SU($N_f$) case. These one-dimensional systems are appealing because they can be experimentally realized in highly constrained traps and because of the dominant role played by correlations. The latter are typically nonperturbative and are crucial for understanding ground states and quantum phase transitions. While quantum fluctuations are typically overpowered by thermal ones in one and two dimensions at any finite temperature, we find that quantum effects do leave their imprint in thermodynamic quantities. Our calculations show that the additional degrees of freedom, relative to the SU(2) case, provide a dramatic enhancement of the density and pressure (in units of their noninteracting counterparts) in a wide region around vanishing $\beta \mu$, where $\beta$ is the inverse temperature and $\mu$ the chemical potential. As shown recently in experiments, the thermodynamics we explore here can be measured in a controlled and precise fashion in highly constrained traps and optical lattices. Our results are a prediction for such experiments in one dimension with atoms of high nuclear spin.