We study the dynamics of a harmonically trapped quasi-one-dimensional Bose-Einstein condensate subjected to a moving disorder potential of finite extent. We show that, due to the inhomogeneity of the sample, only a percentage of the atoms is localized at supersonic velocities of a random potential. We find that this percentage can be sensitively increased by introducing suitable correlations in the disorder potential such as those provided by random dimers.

We study the collective excitations of polarized single-component quasi-two-dimensional dipolar fermions in an isotropic harmonic trap by solving the collisional Boltzmann-Vlasov (CBV) equation via the method of moments. We study the response to monopole and quadrupole perturbations of the trap potential and investigate the dynamical character of excitations in each case. Simple analytic formulas are found using the linearized scaling ansatz approximation and accurate numerical results are obtained by satisfying the first eight moments of the CBV equation. Except for the lowest-lying monopole mode that is weakly affected by collisions, the quadrupole and the higher-order monopole modes undergo a transition from the collisionless regime to a dissipative crossover regime and finally approach the hydrodynamic regime upon increasing the dipolar interaction strength. For strong transverse confinement (2D limit), we predict the existence of a temperature window within which the characteristics of the collective modes become temperature independent. This plateau, which is a direct consequence of dipole-dipole scatterings, persists as long as the scattering energies remain in the near-threshold regime. The predictions of this work are expected to be observable in the current experiments.

We consider spinor Bose gas with the isotropic Rashba spin-orbit coupling in two dimensions. We argue that at low density, its ground state is a composite fermion state with a Chern-Simons gauge field and filling factor of one. The chemical potential of such a state scales with the density as $\mu \propto n^{3/2}$. This is a lower energy per particle than $\mu \propto n$ for the earlier suggested ground-state candidates: a condensate with broken time-reversal symmetry and a spin-density wave state.

We experimentally demonstrate an all-optical switching (AOS) scheme based on double slow light (DSL) pulses, in which one pulse is switched by another due to the cross-Kerr nonlinearity. The interaction time is prolonged by optically dense atomic media and matched group velocities. The interaction strength is maintained at a high level by keeping both fields at their electromagnetically-induced-transparency resonances to minimize the linear loss. In the AOS without the DSL scheme, the group velocity mismatch sets an upper limit on the switching efficiency of two photons per atomic cross section as discussed by Harris and Hau [Phys. Rev. Lett. 82, 4611 (1999)]. Compared to that limit, we have obtained an enhanced switching efficiency by a factor of 3 with our DSL scheme. The nonlinear efficiency can be further improved by increasing the optical depth of the medium. Our work advances low-light-level nonlinear optics and provides essential ingredients for quantum many-body physics using strongly interacting photons.

We study laser-assisted photoionization by attosecond pulses using a time-independent formalism based on diagrammatic many-body perturbation theory. Our aim is to provide an ab initio route to the “delays” for this above-threshold ionization process, which is essential for a quantitative understanding of attosecond metrology. We present correction curves for characterization schemes of attosecond pulses, such as “streaking,” that account for the delayed atomic response in ionization from neon and argon. We also verify that photoelectron delays from many-electron atoms can be measured using similar schemes if, instead, the so-called continuum-continuum delay is subtracted. Our method is general and it can be extended also to more complex systems and additional correlation effects can be introduced systematically.

We present an analytical study of the quantum phase transition between the topologically ordered toric-code-model ground state and the disordered spin-polarized state. The phase transition is induced by applying an external magnetic field, and the variation in topological order is detected via two nonlocal quantities: the Wilson loop and the topological Rényi entropy of order 2. By exploiting an equivalence with the transverse-field Ising model and considering two different variants of the problem, we investigate the field dependence of these quantities by means of an exact treatment in the exactly solvable variant and complementary perturbation theories around the limits of zero and infinite fields in both variants. We find strong evidence that the phase transition point between topological order and disorder is marked by a discontinuity in the topological Rényi entropy and that the two phases around the phase transition point are characterized by its different constant values. Our results therefore indicate that the topological Rényi entropy is a proper topological invariant: its allowed values are discrete and can be used to distinguish between different phases of matter.

In this paper, the method of adaptive measurement for qubit-state preparation developed by Jacobs is reconsidered, and an alternative scheme that works even under unknown unitary evolution of the state is shown. The key idea is that the measurement is adaptively changed so that one of the eigenstates of the measured observable is always set between the current and the target states while that eigenstate converges to the target. The most significant feature of this scheme is that the measurement strength can be taken constant, unlike in Jacobs’ scheme, which eventually provides fine robustness of the controlled state against the uncertainty of the unitary evolution.

We show that the length of a qubit-qutrit separable state is equal to $\max (r,s)$, where $r$ is the rank of the state and $s$ the rank of its partial transpose. We refer to the ordered pair $(r,s)$ as the birank of this state. We also construct examples of qubit-qutrit separable states of any feasible birank $(r,s)$. We determine the closure of the set of normalized two-qutrit entangled states having positive partial transpose (PPT) of rank 4. The boundary of this set consists of all separable states of length at most 4. We prove that the length of any qubit-qudit separable state of birank $(d+1,d+1)$ is equal to $d+1$. We also show that all qubit-qudit PPT entangled states of birank $(d+1,d+1)$ can be built in a simple way from edge states. If $V$ is a subspace of dimension $k<d$ in a $2\otimes d$ space such that $V$ contains no product vectors, we show that the set of all product vectors in $V^{\perp }$ is a vector bundle of rank $d-k$ over the projective line. Finally, we explicitly construct examples of qubit-qudit PPT states (both separable and entangled) of any feasible birank.

The task of privacy amplification, in which Alice holds some partially secret information with respect to an adversary Eve and wishes to distill it until it is completely secret, is known to be solvable almost optimally in both the classical and quantum worlds. Unfortunately, when considering an adversary who is limited only by nonsignaling constraints such a statement cannot be made in general. We here consider systems which violate the chained Bell inequality and prove that under the natural assumptions of a time-ordered nonsignaling system, which allow past subsystems to signal future subsystems (using the device's memory for example), superpolynomial privacy amplification by any hashing is impossible. This is of great relevance when considering practical device-independent key-distribution protocols which assume a superquantum adversary.

The scaling of nuclear-spin-dependent parity-violating effects with increasing nuclear charge $Z$ is discussed in two series of isoelectronic open-shell diatomic molecules. The parameter $W_{\mathrm{a}}$ characterizing the strength of parity violation in diatomic molecules is calculated in the framework of the zeroth-order regular approximation and found to be in good agreement with the $R\left(Z\right)Z^k$ scaling law derived for atoms, in which $R\left(Z\right)$ represents a relativistic enhancement factor. The influence of electron correlation is studied on the molecular level, with spin-polarization effects being conveniently accounted for by a previously established approximate relation between the hyperfine coupling tensor and $W_{\mathrm{a}}$. For high-accuracy predictions of parity-violating effects in radium fluoride, the necessity for systematically improvable correlation calculations is emphasized.

In the present study, we investigate the effect of Debye-Hückel potentials on the frequency-dependent dipole polarizability of the helium atom using highly correlated wave functions within the framework of the pseudostate summation technique. The dynamic dipole polarizability of He (1$s^2$ ${}^{1}S$) as a function of the scaled number density of the plasma electrons for arbitrary plasma temperature is presented. Screening effects on the resonance frequencies are also presented. In a free-atomic system, our calculated results are in agreement with the available theoretical and experimental predictions.

We apply the concept of photoemission time delay to the process of single-photon one-electron ionization of the H${}_2$ molecule. We demonstrate that, by resolving the photoelectron detection in time on the attosecond scale, one can extract differential photoionization cross sections for particular field and molecule orientations from the measurement on a randomly oriented molecule

Recently, Tang and Cohen [Y. Tang and A. E. Cohen, Science 332, 333 (2011)] proposed and demonstrated the use of “superchiral” electromagnetic fields to enhance optical enantioselectivity. Their work generated much excitement as enantioselective signals are typically quite small, and it appeared that the enhancement factor could be extremely large. In this paper we explicitly show the limitations of such fields by including the magnetic susceptibility term. This term is small and is ignored in most cases compared to the electric polarizability term. However, for the fields used, the enhancement was obtained at the electric field energy node. Due to conservation of field energy, the magnetic field energy is then maximum, and the magnetic susceptibility contribution can no longer be ignored. This then is what limits the enhancement of the optical enantioselectivity. For a counterpropagating left- and right-circularly polarized light field, as used in the aforementioned experiment, we show that this fundamentally limits the enhancement to one or two orders of magnitude in general, determined by the ratio of the magnetic susceptibility to the electric polarizability of the material used. We also generalize the dissymmetry factor to include optical rotation effects present in chiral media, as opposed to fields being in vacuum. In the process, we generalize Lipkin's “$Z^{000}$ zilch” (or “optical chirality”) to that for a linear medium. This generalization shows that chirality of the material cannot be completely separated from chirality of the field and that opposite enantiomers are symmetric in terms of the dissymmetry factor enhancement. Finally, an analogy between ellipsometric chiroptical signal enhancement and enhanced optical enantioselectivity using a standing wave chiral field is discussed. Our analysis and generalization can be used as a guide for future searches of locally enhanced chiral fields.

We solve the problem of ultrashort pulse propagation in a two-level medium beyond the rotating-wave (RWA) and slowly varying-envelope approximations. The method of solution is based on the Maxwell-Bloch equations represented in the form that allows one to switch between RWA and general (non-RWA) cases in the framework of a single numerical algorithm. Using this method, the effect of a subcycle pulse (containing less than a single period of field oscillations) on the two-level medium was analyzed. It is shown that for such short pulses, the clear breakdown of the area theorem occurs for the pulses of large enough area. Moreover, deviations from the area theorem appear to be strongly dependent on the pulse shape that cannot be observed for longer few-cycle pulses.

We address a two-level system in an environment interacting with an electromagnetic field in the dipole approximation. The resonant optical bistability induced by local-field effects is studied by considering the relationship between the population difference and the excitation field. The diversity of various systems is included by accounting for system self-action via the surface part of Green's dyadic in the general form. The bistability condition and the exact solution of the steady-state optical Bloch equations at the absolute bistability threshold are derived analytically.

Noiseless amplification or attenuation are two heralded filtering operations that enable amplifying or de-amplifying a quantum state of light with no added noise, at the cost of a small success probability. We show that inserting such noiseless operations in a transmission line improves the performances of continuous-variable quantum key distribution over this line. Remarkably, these noiseless operations do not need to be physically implemented but can simply be simulated in the classical data postprocessing stage. Hence, virtual noiseless amplification or attenuation amounts to performing a Gaussian postselection, which enhances the secure range or tolerable excess noise while keeping the benefits of Gaussian security proofs.

We excite the 717-nm electric quadrupole 6$D_{3/2}$ $\leftrightarrow$ 7$S_{1/2}$ transition in a laser-cooled ${}^{232}$Th${}^{3+}$ ion crystal. The transition frequency and the lifetime of the metastable 7$S_{1/2}$ level are measured to be 417 845 964(30) MHz and 0.60(7) s, respectively. We subsequently employ the 7$S_{1/2}$ level to drive the ions with nanosecond-long 269-nm laser pulses into the 7$P_{1/2}$ level. The latter is connected to the 7$S_{1/2}$ electronic level within the ${}^{229}$Th nuclear isomer manifold by the strongest available electron-bridge transition, forming a basis for its laser excitation.

The strong system-bath correlation is a typical initial condition in many condensed matter and some quantum optical systems. Here, the dynamics of a spin interacting with a spin bath through an intermediate spin are studied. Initial correlations between the spin and the intermediate spin are taken into account. The exact analytical expression for the evolution operator of the spin is found. Furthermore, correlated projection superoperator techniques are applied to the model and a time-convolutionless master equation to second order is derived. It is shown that the time-convolutionless master equation to second order reproduces the exact dynamics for time scales of the order $1/\gamma ,$ where $\gamma$ is the coupling of the central spin to the intermediate spin. It is found that there is a strong dependence on the initial system-bath correlations in the dynamics of the reduced system, which cannot be neglected.

We extend the time-dependent variational principle to the setting of dissipative dynamics. This provides a locally optimal (in time) approximation to the dynamics of any Lindblad equation within a given variational manifold of mixed states. In contrast to the pure-state setting, there is no canonical information geometry for mixed states, and this leads to a family of possible trajectories—one for each information metric. We focus on the case of the operationally motivated family of monotone Riemannian metrics and show further that, in the particular case where the variational manifold is given by the set of fermionic Gaussian states, all of these possible trajectories coincide. We illustrate our results in the case of the Hubbard model subject to spin decoherence.