We demonstrate here a controllable variation in the Casimir force. Changes in the force of up to $20\%$ at separations of $~$$100$ nm between Au and Ag-In-Sb-Te (AIST) surfaces were achieved on crystallization of an amorphous sample of AIST. This material is well known for its structural transformation, which produces a significant change in the optical properties and is exploited in optical data storage systems. The finding paves the way to the control of forces in nanosystems, such as micro- or nanoswitches, by stimulating the phase-change transition via localized heat sources.

We investigate the onset of thermalization and quantum chaos in finite one-dimensional gapped systems of hard-core bosons. Integrability in these systems is broken by next-nearest-neighbor repulsive interactions, which also generate a superfluid to insulator transition. By employing full exact diagonalization, we study chaos indicators and few-body observables. We show that with increasing system size, chaotic behavior is seen over a broader range of parameters and, in particular, deeper into the insulating phase. Concomitantly, we observe that, as the system size increases, the eigenstate thermalization hypothesis extends its range of validity inside the insulating phase and is accompanied by the thermalization of the system.

We study the performance of common quantum stabilizer codes in the presence of asymmetric and correlated errors. Specifically, we consider the depolarizing noisy quantum memory channel and perform quantum error correction via the five- and seven-qubit stabilizer codes. We characterize these codes by means of the entanglement fidelity as a function of the error probability and the degree of memory. We show that their performances are lowered by the presence of correlations, and we compute the error probability threshold values for code effectiveness. Furthermore, we uncover that the asymmetry in the error probabilities does not affect the performance of the five-qubit code, while it does affect the performance of the seven-qubit code, which results in being less effective when considering correlated and symmetric depolarizing errors but more effective for correlated and asymmetric errors.

The coherent evolution of two qubits mediated by a set of bosonic field modes is investigated. By assuming a specific asymmetric encoding of the quantum states in the internal levels of the qubits, we show that entangling quantum gates can be realized, with high fidelity, even when a large number of mediating modes is involved. The effect of losses and imperfections on the gates’ operation is also considered in detail.

This article presents a detailed analysis of the $K\alpha$ x-ray spectra of Si induced by $11.4$ MeV/u Ca projectiles penetrating a low-density SiO${}_2$ aerogel target measured with high spectral and spatial resolution at the UNILAC accelerator at GSI-Darmstadt. The low-density material used in the experiment was crucial for the space-resolved studies of the Si x-ray radiation (for different energies of stopping Ca ions). The stopping length of the $11$ MeV/u Ca ions reaches up to $10$ mm in the low-density SiO${}_2$ aerogel, whereas in regular quartz solid targets it is about $100$ times shorter. The analysis of the x-ray spectra emitted by the stopping medium has shown a high level of the $L$-shell ionization, especially in the later considered phase ($E_p~5$ MeV/u) of the stopping process. It has been further demonstrated that the population of the highly ionized states produced in the ion-atom collisions can be substantially reduced in the time between the collision and the x-ray emission due to the very intense rearrangement processes occurring in Si situated in the chemical environment of oxygen atoms. Moreover, comparison of the experimental values of the $K\alpha$ $L$-shell satellite energy shifts with the results of the multiconfiguration Dirac-Fock calculations allows us to find that Si valence electron configuration is enriched due to electron transfer from valence-electron-rich oxygen atoms into highly ionized silicon atoms. Our results indicate that the Coulomb explosion in a highly ionized track core is prevented by rapid neutralization in the femtosecond time scale.

It has been shown that control over the design of single- or multimode-transparency windows can be achieved by control-probe quantum interferometry using two delayed phase-locked ultrashort pulses. We have investigated how single- or multimode-transparency windows can be designed by eliminating the absorption to a set of odd or even vibrational levels of a molecule covered by the spectral bandwidth of the pulses. Suppression of absorption can be achieved by controlling the quantum interference of two vibrational wave packets excited by these two pulses, leading to destructive interference of transition amplitudes for one set of vibrational levels even or odd. Simuntaneously the interference becomes constructive for the other set of odd or even vibrational levels, leading to absorption to these levels. Thus, a set of transparency windows is designed in between two absorption peaks at two consecutive odd (even) vibrational levels by eliminating absorption at even (odd) vibrational levels between two absorption peaks. If the spectral bandwidth of the pulse [full width at half maximum (FWHM)] is less than the energy difference between two consecutive absorption peaks, the pulse will be transmitted without absorption through the transparency window. However, for pulses with a larger spectral bandwidth (FWHM), a set of broad transparency windows will be generated and different bands of pulse frequencies will be transmitted through these windows. We have shown that coherent control of the design of transparency windows can be achieved by controlling pulse duration, carrier frequency, delay, and the phase difference of two ultrashort pulses.

We investigate the level population statistics and degree of coherence encoded in the single-particle density matrix of harmonically trapped low-dimensional [quasi-one-dimensional (quasi-1D) or quasi-two-dimensional (quasi-2D)] Bose gases with repulsive contact interactions. Using a variational analysis, we derive fragmentation of the condensate in the weakly confining directions into two (quasi-1D) and three (quasi-2D) mutually incoherent macroscopic pieces, upon increasing a dimensionless interaction measure beyond a critical value. Fragmented condensate many-body states in low-dimensional systems therefore occur well before the thermodynamic limit of infinite extension is reached, in which phase fluctuations of the matter wave field create an infinite number of nonmacroscopic fragments.

We present a general analysis of two-dimensional optical lattice models that give rise to topologically nontrivial insulating states. We identify the main ingredients of the lattice models that are responsible for the nontrivial topological character and argue that such states can be realized within a large family of realistic optical lattice Hamiltonians with cold atoms. We focus our quantitative analysis on the properties of topological states with broken time-reversal symmetry specific to cold-atom settings. In particular, we analyze finite-size effects, multiorbital phenomena that give rise to a variety of distinct topological states and transitions between them, the dependence on the trap geometry, and, most importantly, the behavior of the edge states for different types of soft and hard boundaries. Furthermore, we demonstrate the possibility of experimentally detecting the topological states through light Bragg scattering of the edge and bulk states.

We address spectral tunneling of walking spatial solitons in photorefractive media with nonlocal diffusion component of the nonlinear response and an imprinted shallow optical lattice. In contrast to materials with local nonlinearities, where solitons traveling across the lattice close to the Bragg angle suffer large radiative losses, in photorefractive media with diffusion nonlinearity resulting in self-bending, solitons survive when their propagation angle approaches and even exceeds the Bragg angle. In the spatial frequency domain this effect can be considered as tunneling through the band of spatial frequencies centered around the Bragg frequency where the spatial group velocity dispersion is positive.

The Casimir-Polder (CP) potential and transition rates of a Rydberg atom above a plane metal surface at finite temperature are discussed. As an example, the CP potential and transition rates of a rubidium atom above a copper surface at 300 K are computed. Close to the surface we show that the quadrupole correction to the force is significant and increases with increasing principal quantum number $n$. For both the CP potential and decay rates one finds that the dominant contribution comes from the longest wavelength transition and the potential is independent of temperature. We provide explicit scaling laws for potential and decay rates as functions of atom-surface distance and principal quantum number of the initial Rydberg state.

We describe a phase-amplification technique using a Sagnac interferometer. We monitor the relative phase between two paths of a precisely misaligned interferometer by measuring the average position of a split-Gaussian mode in the dark port. Although we monitor only the dark port, we show that the signal varies linearly with phase and that we can obtain similar sensitivity to balanced homodyne detection. We derive the source of the amplification using classical wave optics.

We consider a hidden-variable theoretic description of successive measurements of noncommuting spin observables on an input spin-$s$ state. In this scenario, the hidden-variable theory leads to a Hardy-type argument that quantum predictions violate it. We show that the maximum probability of success of Hardy’s argument in quantum theory is $(\frac{1}{2}){}^{4s}$, which is more than in the spatial case.

We study continuous-time quantum walks mimicking the quantum search based on Grover’s procedure. This allows us to consider structures, that is, databases, with arbitrary topological arrangements of their entries. We show that the topological structure of the database plays a crucial role by analyzing, both analytically and numerically, the transition from the ground to the first excited state of the Hamiltonian associated with different (fractal) structures. Additionally, we use the probability of successfully finding a specific target as another indicator of the importance of the topological structure.

We derive an approximate local density functional for the exchange-correlation energy to be used in density-functional calculations of two-dimensional systems. In the derivation we employ the Colle-Salvetti wave function within the scheme of Salvetti and Montagnani [Phys. Rev. A 63, 052109 (2001)] to satisfy the sum rule for the exchange-correlation hole. We apply the functional to the two-dimensional homogeneous electron gas as well as to a set of quantum dots and find a very good agreement with exact reference data.

Measurements of elastic-scattering cross sections are presented for positron collisions with helium, neon, argon, krypton, and xenon around the threshold energy for positronium (Ps) formation. The elastic cross section falls slowly with increasing energy above the Ps formation threshold in helium and neon, whereas in argon, krypton, and xenon it exhibits an increase, which appears both more prominent and more sustained as the atomic number of the gas increases. It is proposed that this coupling is a result of an intermediate virtual Ps state that enhances branching into the (atom plus positron) final state.

Polarization bremsstrahlung (PBS) on scattering of $0.7$-keV electrons on Xe clusters of various size is studied experimentally. The PBS spectrum is found to change upon passage from Xe gas to clusters. There takes place a shift of the PBS maximum to lower energies and a narrowing of its spectral profile. Calculations of the PBS cross section are made for Xe clusters in two models: one takes into account dielectric permeability of the environment of an emitting dipole and the other takes into consideration the frequency dependence of the dipole damping. The calculated curves fit the experimental PBS spectra well and account for the spectral changes occurring upon transition from the gas to the clusters. It is found that cooperative effects in a solid strongly influence the PBS spectrum.

We investigate arrays of three traps with two fermionic or bosonic atoms. The tunneling interaction between neighboring sites is used to prepare multisite dark states for the empty site (i.e., the hole) which allows for the coherent manipulation of its external degrees of freedom. By means of an ab initio integration of the Schrödinger equation, we investigate the adiabatic transport of a hole between the two extreme traps of a triple-well potential. Furthermore, a quantum-trajectory approach based on the de Broglie–Bohm formulation of quantum mechanics is used to get physical insight into the transport process. Finally, we discuss the use of the hole for the construction of a coherent single hole diode and a coherent single hole transistor.

We study the effect of the induced interaction on the superfluid transition temperature of a spin-polarized Fermi gas. In the BCS limit, the polarization is very small in the superfluid state, and the effect of the induced interaction is almost the same as in the spin-balanced case. The temperature $T_t$ and the polarization $P_t$ of the tricritical point are both reduced from mean-field results by a factor about $2.22$. This reduction is also significant beyond the BCS limit. In the unitary limit, we find $(P_t,T_t/T_F)=(0.42,0.16)$, in comparison with mean-field and experimental results.

The coherent matter waves of a dipolar condensate in deep two-dimensional (2D) tilted and nontilted optical lattices are studied both analytically and numerically. It is shown that, in tilted lattices, by properly designing the sign and the magnitude of the contact interaction and the dipolar interaction, it is possible to control the decoherence of Bloch oscillations. Contrary to the usual short-range interacting Bose system, long-lived Bloch oscillations of the dipolar condensate are achieved when the dipolar interaction, the contact interaction, and the lattice dimension satisfy an analytical condition. Furthermore, we predict that, in untilted lattices, stable coherent 2D moving soliton and breather states of the dipolar condensate exist. This fact is very different from the purely short-range interacting Bose system (where the moving soliton cannot be stabilized in high-dimensional lattices). The dipolar interaction can lead to some novel phenomena that can not appear in short-range interacting BEC system.