We investigate two-photon Autler-Townes splitting and strong-field effects of ${}^{85}\mathrm{Rb}$ Rydberg atoms in a room-temperature vapor cell. To observe the level structure we employ electromagnetically induced transparency. We first study the two-photon $62S_{1/2}\text{−}63S_{1/2}$ microwave transition using an electric-field reference measurement obtained with the one-photon $62S_{1/2}\text{−}62P_{3/2}$ transition. We then study the $61D_{5/2}\text{−}62D_{5/2}$ transition where the microwave electric-field range is extended up to $\sim 40\mathrm{V}/\mathrm{m}$. A Floquet analysis is used to model field-induced level shifts and state-mixing effects present in the strongly driven quantum systems under consideration. Calculations are found to be in good agreement with experimental observations.

We derive an effective ring model in momentum space for trapped bosons with synthetic spin-orbit coupling. This effective model is characterized by a peculiar form of the interparticle interactions, which is crucially modified by the external confinement. The ring model allows for an intuitive understanding of the phase diagram of trapped condensates with isotropic spin-orbit coupling and, in particular, for the existence of skyrmion lattice phases. The model, which may be generally applied for spinor condensates of arbitrary spin and spin-dependent interactions, is illustrated for the particular cases of spin-$\frac{1}{2}$ and spin-1 condensates.

We compute second-order transport coefficients of the dilute Fermi gas at unitarity. The calculation is based on kinetic theory and the Boltzmann equation at second order in the Knudsen expansion. The second-order transport coefficients describe the shear stress relaxation time, nonlinear terms in the strain-stress relation, and nonlinear couplings between vorticity and strain. An exact calculation in the dilute limit gives ${\tau }_R=\eta /P$, where ${\tau }_R$ is the shear stress relaxation time, $\eta$ is the shear viscosity, and $P$ is pressure. This relation is identical to the result obtained using the Bhatnagar-Gross-Krook approximation to the collision term, but other transport coefficients are sensitive to the exact collision integral.

Within the context of Hartree-Fock-Bogoliubov theory, we study the behavior of superfluid Fermi systems when they pass from a small to a large container. Such systems can be now realized thanks to recent progress in experimental techniques. It will allow researchers to better understand pairing properties at overflow and in general in rapidly varying external potentials.

From an appropriate parameterization of the three-dimensional (3D) coherency matrix R that characterizes the second-order, classical states of polarization, the coherency matrices are classified and interpreted in terms of incoherent decompositions. The relevant physical quantities derived from R, such as the intensity, degree of polarimetric purity, the indices of polarimetric purity, angular momentum, degree of directionality, and degree of linear polarization, are identified and interpreted in light of the case study performed. The information provided by R about the direction of propagation is clarified and it is found that coherency matrices with $\mathrm{rank}\mathbf{R}=2$ do not always represent states with a well-defined direction of propagation. Moreover, the existence of 3D mixed states that cannot be decomposed into a superposition of a pure state, a two-dimensional (2D) unpolarized state, and a 3D unpolarized state is demonstrated. Appropriate representation and interpretation for all the different types of 3D coherency matrices are provided through physically consistent criteria. Under the approach proposed, the conventional 2D model arises naturally.

In our previous work [Phys. Rev. A 89, 022329 (2014)], we proposed a teleportation-based quantum communication scheme in a quantum wireless multihop network with arbitrary types of Bell pairs shared between adjacent nodes. Here we revised the scheme by introducing a result-mapping method for halving the classical communication cost, with which the source and intermediate nodes map the measurement outcomes to those corresponding to the target Bell pair type assigned by the destination node. A routing protocol is utilized to piggyback the target Bell pair type notification information which introduces no extra communication cost.

We report experimental results on the action of selected local environments on the fidelity of the quantum teleportation protocol, taking into account nonideal, realistic entangled resources. Different working conditions are theoretically identified, where a noisy protocol can be made almost insensitive to the further addition of noise. We put to test these conditions on a photonic implementation of the quantum teleportation algorithm, where two polarization entangled qubits act as the entangled resource and a path qubit on Alice encodes the state to be teleported. Bob's path qubit is used to implement a local environment, while the environment on Alice's qubit is simulated as a weighted average of different pure states. We obtain a good agreement with the theoretical predictions, experimentally recreate the conditions to obtain a noise-induced enhancement of the protocol fidelity, and identify parameter regions of increased insensibility to interactions with specific noisy environments.

The spin-3/2 Affleck-Kennedy-Lieb-Tasaki (AKLT) valence-bond state on a hexagonal lattice was shown to be a universal resource state for measurement-based quantum computation (MBQC). Can AKLT states of higher spin magnitude support universal MBQC? We demonstrate that several hybrid two-dimensional AKLT states involving a mixture of spin-2 and other lower-spin entities, such as spin-3/2 and spin-1, are also universal for MBQC. This significantly expands universal resource states in the AKLT family. Even though frustration may be a hindrance to quantum computational universality, lattices can be modified to yield AKLT states that are universal. The family of AKLT states thus provides a versatile playground for quantum computation.

Measurement-device-independent quantum key distribution (MDI-QKD) can remove all detector loopholes. When it is combined with the decoy-state method, the final key is unconditionally secure, even if Alice and Bob do not have strict single-photon sources. However, active modulation of source intensity, which is used to generate the decoy state, may leave side channels and leak additional information to Eve. In this paper, we consider the MDI-QKD with a passive decoy state, in which both Alice and Bob send pulses to an untrusted third party, Charlie. Then, in order to estimate the key generation rate, we derive two tight formulas to estimate the lower bound of the yield and the upper bound of the error rate that both Alice and Bob send a single-photon pulse to Charlie. Furthermore, the statistical fluctuation due to the finite length of data is also taken into account based on the standard statistical analysis.

The presence of net circular dichroism in the photoionization of nonchiral homonuclear molecules has been put in evidence recently through the measurement of molecular-frame photoelectron angular distributions in dissociative photoionization of ${\text{H}}_2$ [Dowek et al., Phys. Rev. Lett. 104, 233003 (2010)]. In this work we present a detailed study of circular dichroism in the photoelectron angular distributions of ${\text{H}}_2$ and ${\text{D}}_2$ molecules, oriented perpendicularly to the propagation vector of the circularly polarized light, at different photon energies (20, 27, and 32.5 eV). Circular dichroism in the angular distributions at 20 and to a large extent 27 eV exhibits the usual pattern in which inversion symmetry is preserved. In contrast, at 32.5 eV, the inversion symmetry breaks down, which eventually leads to total circular dichroism after integration over the polar emission angle. Time-dependent ab initio calculations support and explain the observed results for ${\text{H}}_2$ in terms of quantum interferences between direct photoionization and delayed autoionization from the ${\mathcal{Q}}_1$ and ${\mathcal{Q}}_2$ doubly excited states into ionic states $(1s{\sigma }_g$ and $2p{\sigma }_u)$ of different inversion symmetry. Nevertheless, for ${\text{D}}_2$ at 32.5 eV, there is a particular case where theory and experiment disagree in the magnitude of the symmetry breaking: when ${\text{D}}^{+}$ ions are produced with an energy of around 5 eV. This reflects the subleties associated to such simple molecules when exposed to this fine scrutiny.

We consider a quasiperiodic Aubry-Andre (AA) model and add a weak time-space-periodic perturbation. The undriven AA model is chosen to be well in the localized regime. The driving term controls the effective number of propagation channels. For a spatial resonance which reduces the reciprocal space dynamics to an effective one-dimensional two-leg ladder, the ac perturbation resonantly couples certain groups of localized eigenstates of the undriven AA model and turns them into extended ones. Slight detuning of the spatial and temporal frequencies off resonance returns these states into localized ones. We analyze the details of the resonant extended eigenstates using Floquet representations. In particular, we find that their size grows linearly with the system size. Initial wave packets overlap with resonant extended eigenstates and lead to ballistic spreading.

Optical bistability of a single-mode cavity containing a two-level atom in the Purcell regime has been analyzed. Compared with the case of a symmetric cavity, we find that the bistable regime of input power in an asymmetric cavity can be controlled by adjusting the cavity-loss rates of the asymmetric walls. Such results provide an optimal window of input power to realize giant optical nonreciprocity, which has promising application as the smallest all-optical diode unit with low operation power and high transmitted contrast.

We address the problem of continuous-variable quantum phase estimation in the presence of linear disturbance at the Hamiltonian level by means of Gaussian probe states. In particular we discuss both unitary and random disturbance by considering the parameter which characterizes the unwanted linear term present in the Hamiltonian as fixed (unitary disturbance) or random with a given probability distribution (random disturbance). We derive the optimal input Gaussian states at fixed energy, maximizing the quantum Fisher information over the squeezing angle and the squeezing energy fraction, and we discuss the scaling of the quantum Fisher information in terms of the output number of photons, $n_{\mathsf{out}}$. We observe that, in the case of unitary disturbance, the optimal state is a squeezed vacuum state and the quadratic scaling is conserved. As regards the random disturbance, we observe that the optimal squeezing fraction may not be equal to one and, for any nonzero value of the noise parameter, the quantum Fisher information scales linearly with the average number of photons. Finally, we discuss the performance of homodyne measurement by comparing the achievable precision with the ultimate limit imposed by the quantum Cramér–Rao bound.

Relativistic effects in one-particle densities of hydrogenic systems are quantified by means of global and local density functionals: the Jensen-Shannon and the Jensen-Fisher divergences, respectively. The Schrödinger and Dirac radial densities are compared, providing complementary results in position and momentum spaces. While the electron cloud gets compressed towards the origin in the Dirac case, the momentum density spreads out over its domain, and the raising of minima in position space does not occur in the momentum space. Regarding the dependence on the nuclear charge and the state quantum numbers for all divergences here considered, as well as their mutual interconnection, accurate powerlike laws $y\approx C{x}^a$ are found systematically. The parameters $\{C,a\}$ defining the respective dependences are extremely sensitive to the closeness of the system to the ground and/or the circular state. Particularly interesting are the analyses of (i) the plane subtended by the Jensen-Shannon and Jensen-Fisher divergences, in a given space (position or momentum), and (ii) either of the above two divergences in the position-momentum plane. These kinds of results show the complementary role of global and local divergences and that of both conjugate spaces.

Existence, stability, and dynamics of $\mathcal{PT}$-symmetric fundamental bright solitons supported by localized super-Gaussian potentials in a focusing Kerr medium are investigated theoretically. We address how the shape and the magnitude of the transverse profile of the loss-gain distribution affect soliton stability. We find the stability region for nonlinear wave packets via a linear stability analysis, interpreting the insurgence of instability as an unbalanced flow of energy on the transverse plane. We confirm our results via numerical simulations, showing that an unstable soliton first undergoes longitudinal oscillations in propagation due to the interference between the soliton and the exponentially growing perturbation modes, eventually forming a highly localized single peak in the gain region.

The NOON state, which was introduced as a resource for quantum-enhanced metrology, is in fact a special case of a superposition of two SU(2) coherent states. Here we show explicitly the derivation of the NOON state from the superposition state. This derivation makes clear the connection between these seemingly disparate states and shows how the NOON state can be generalized to a superposition of SU(2) coherent states.

Quantum state embezzlement is the transformation $|\mu \rangle \mapsto |\mu \rangle |\phi \rangle$ using only local operations, where $|\phi \rangle$ and $|\mu \rangle$ are multipartite quantum states. Exact embezzlement is an impossible task since it implies the increase of entanglement without communication. Surprisingly, van Dam and Hayden [Phys. Rev. A 67, 060302 (2003)] find a universal embezzling family of states $|\mu \rangle$ that enables embezzlement in the bipartite setting with arbitrary precision as the dimension of $|\mu \rangle$ increases. Furthermore, the family is independent of the state $|\phi \rangle$ to be embezzled. We study embezzlement in the bipartite setting. We present various requirements and consequences, and infinitely many universal embezzling families inequivalent to that proposed by van Dam and Hayden. We include numerical studies of up to 33-qubit large local systems.

We present calculations for the dependence of the two-photon double ionization (DI) of ${\mathrm{H}}_2$ on the relative orientation of the linear laser polarization to the internuclear axis and the length of the pulse. We use the fixed-nuclei approximation at the equilibrium distance of 1.4 $a_0$, where $a_0=0.529\times {10}^{-10}\mathrm{m}$ is the Bohr radius. Central photon energies cover the entire direct DI domain from 26.5 to 34.0 eV. In contrast to the parallel geometry studied earlier [X. Guan, K. Bartschat, B. I. Schneider, and L. Koesterke, Phys. Rev. A 83, 043403 (2011)], the effect of the pulse duration is almost negligible for the case when the two axes are perpendicular to each other. This is a consequence of the symmetry rules for dipole excitation in the two cases. In the parallel geometry, doubly excited states of ${}^1{\Sigma }_u^{+}$ symmetry affect the cross section, while in the perpendicular geometry only much longer-lived ${}^1{\Pi }_u$ states are present. This accounts for the different convergence patterns observed in the calculated cross sections as a function of the pulse length. When the photon energy approaches the threshold of sequential DI, a sharp increase of the generalized total cross section (GTCS) with increasing pulse duration is also observed in the perpendicular geometry, very similar to the case of the molecular axis being oriented along the laser polarization direction. Our results differ from those of Colgan et al. [J. Colgan, M. S. Pindzola, and F. Robicheaux, J. Phys. B 41, 121002 (2008)] and Morales et al. [F. Morales, F. Martín, D. A. Horner, T. N. Rescigno, and C. W. McCurdy, J. Phys. B 42, 134013 (2009)], but are in excellent agreement with the GTCSs of Simonsen et al. [A. S. Simonsen, S. A. Sørngård, R. Nepstad, and M. Førre, Phys. Rev. A 85, 063404 (2012)] over the entire domain of direct DI.