Exact calculations are performed on the two-dimensional strongly interacting unpolarized uniform Fermi gas with a zero-range attractive interaction. Two auxiliary-field approaches are employed which accelerate the sampling of imaginary-time paths using BCS trial wave functions and a force bias technique. Their combination enables calculations on large enough lattices to reliably compute ground-state properties in the thermodynamic limit. An equation of state is obtained with a parametrization provided, which can serve as a benchmark and allow accurate comparisons with experiments. The pressure, contact parameter, and condensate fraction are determined systematically vs $k_{F}a$. The momentum distribution, pairing correlation, and the structure of the pair wave function are computed. The use of force bias to accelerate the Metropolis sampling of auxiliary fields in determinantal approaches is discussed.

The interaction between two bright solitons in a one-dimensional dipolar Bose-Einstein condensate (BEC) is investigated, with the aim of finding the regimes where they form a stable bound state, known as the soliton molecule. To study soliton interactions in BECs we employed a method similar to that used in experimental investigation of the interaction between solitons in optical fibers. The idea consists in creating two solitons at some spatial separation from each other at initial time $t_0$ and then measuring the distance between them at a later time $t_1>t_0$. Depending on whether the distance between solitons has increased, decreased, or remained unchanged, compared to its initial value at $t_0$, we conclude that the soliton interaction was repulsive, attractive, or neutral, respectively. We propose an experimentally viable method for estimating the binding energy of a soliton molecule, based on its dissociation at critical soliton velocity. Our theoretical analysis is based on the variational approach, which appears to be quite accurate in describing the properties of soliton molecules in dipolar BECs, as reflected in the good agreement between the analytical and the numerical results.

The kicking sequence of the atom-optics kicked rotor at quantum resonance can be interpreted as a quantum random walk in momentum space. We show how such a walk can become the basis for nontrivial classical walks by applying a random sequence of intensities and phases of the kicking lattice chosen according to a probability distribution. This distribution converts on average into the final momentum distribution of the kicked atoms. In particular, it is shown that a power-law distribution for the kicking strengths results in a Lévy walk in momentum space and in a power law with the same exponent in the averaged momentum distribution. Furthermore, we investigate the stability of our predictions in the context of a realistic experiment with Bose-Einstein condensates.

We have studied optical activity in optical fibers that possess multihelical distribution of the refractive index profile. We have demonstrated through an analytical treatment that the effect of optical activity is caused by asymmetric spin-dependent influence of the spin-orbit interaction on the coupling between the fundamental mode and the higher-order modes. We have provided a simple analytical expression for the coefficient of optical activity.

Conditions for stable propagation of one-dimensional bright spatial solitons in media exhibiting optical nonlinearities up to the seventh order are investigated. The results show well-defined stability regions even when all the nonlinear terms are focusing. Conditions for onset of the supercritical collapse of the optical beam are identified too. A variational approximation (VA) is used to predict dependence of the soliton's propagation constant on the norm, and the respective stability regions are identified using the Vakhitov-Kolokolov criterion. Analytical results obtained by means of the VA are corroborated by numerical simulations of the cubic-quintic-septimal nonlinear Schrödinger equation.

In New J. Phys. 16, 053048 (2014), Wang et al. have experimentally demonstrated the formation of bright solitons with repulsive nonlinearity and compared their results numerically by making use of the modified nonlinear Schrödinger equation (NLSE) with constant coefficients. We have extended their study by making the coefficients of NLSE time dependent, as they represent more realistic scenarios and are helpful in understanding the physics of the system in a better way. We have then analytically presented the bright soliton solutions with repulsive nonlinearity for the variable coefficient NLSE. These results have been obtained for the small variation of the time-dependent localized cubic and quintic nonlinearities. Additionally, we have discussed other possibilities which may support the formation of bright solitons in yttrium iron garnet thin film strips.

We propose a method to decouple the nanomechanical resonator in optomechanical systems from the environmental noise by introducing a chaotic coherent feedback loop. We find that the chaotic controller in the feedback loop can modulate the dynamics of the controlled optomechanical system and induce a broadband response of the mechanical mode. This broadband response of the mechanical mode will cut off the coupling between the mechanical mode and the environment and thus suppress the environmental noise of the mechanical modes. As an application, we use the protected optomechanical system to act as a quantum memory. It is shown that the noise-decoupled optomechanical quantum memory is efficient for storing information transferred from coherent or squeezed light.

Decoy-state quantum key distribution (QKD) is a standard technique in current quantum cryptographic implementations. Unfortunately, existing experiments have two important drawbacks: the state preparation is assumed to be perfect without errors and the employed security proofs do not fully consider the finite-key effects for general attacks. These two drawbacks mean that existing experiments are not guaranteed to be proven to be secure in practice. Here, we perform an experiment that shows secure QKD with imperfect state preparations over long distances and achieves rigorous finite-key security bounds for decoy-state QKD against coherent attacks in the universally composable framework. We quantify the source flaws experimentally and demonstrate a QKD implementation that is tolerant to channel loss despite the source flaws. Our implementation considers more real-world problems than most previous experiments, and our theory can be applied to general discrete-variable QKD systems. These features constitute a step towards secure QKD with imperfect devices.

Quantum computation with a complete graph of superconducting qubits has been recently proposed, and applications to amplitude amplification, phase estimation, and the simulation of realistic atomic collisions given [Phys. Rev. A 91, 062309 (2015)]. This single-excitation-subspace (SES) approach does not require error correction and is practical now. Previously it was shown how to implement symmetric $n\times n$ unitaries in a single step, but not general unitaries. Here we show that any element in the unitary group $\mathrm{U}\left(n\right)$ can be executed in no more than three steps, for any $n$. This enables the implementation of highly complex operations in constant time and, in some cases, even allows for the compilation of an entire algorithm down to only three operations. By using this protocol we show how to prepare any pure state of an SES chip in three steps and also how to compute, for a given SES state $\rho$, the expectation value $\mathrm{Tr}\left(\rho O\right)$ of any $n\times n$ Hermitian observable $O$ in a constant number of steps.

We propose a scheme for preparing a coherent ground-state superposition for an atom through external drivings assisted by spontaneous emission. In the scheme, the dynamics induced by the competition between the spontaneous emission and the external drives contributes to the superposition of the ground states. Compared with schemes based on the stimulated Raman adiabatic passage, such a scheme is more easily implemented because the preparation of special initial states is no longer needed, which simplifies the operation process. Moreover, since spontaneous emission is involved to act as a positive factor, a higher fidelity superposition state is achieved.

We examine coherent memory manipulation in a $\mathrm{\Lambda }$-type medium, using the second-order solution presented by Groves, Clader, and Eberly [J. Phys. B: At. Mol. Opt. Phys. 46, 224005 (2013)] as a guide. The analytical solution obtained using the Darboux transformation and a nonlinear superposition principle describes complicated soliton-pulse dynamics which, by an appropriate choice of parameters, can be simplified to a well-defined sequence of pulses interacting with the medium. In this report, this solution is reviewed and put to test by means of a series of numerical simulations, encompassing all the parameter space and adding the effects of homogeneous broadening due to spontaneous emission. We find that even though the decohered results deviate from the analytical prediction they do follow a similar trend that could be used as a guide for future experiments.

We propose a setup to generate giant Kerr nonlinearity and polaritonic solitons via matter-wave superradiant scattering. The system we consider is a long cigar-shaped Bose-Einstein condensate (BEC), pumped by a red-detuned laser field with a space-dependent intensity distribution in transverse directions. The pump and the scattered fields propagate along the longitudinal direction. We show that by means of the atom-photon and atom-atom interactions in the system it is possible to produce a giant nonlinear optical effect. We further show that a backward scattering of the laser field from the BEC is favorable for the formation and stable propagation of polaritonic solitons, which are collective nonlinear excitations of the BEC coupled with the scattered laser field. In the case of backward Stokes (anti-Stokes) scattering the system may support robust bright (dark) polaritonic solitons propagating with superluminal (subluminal) velocity.

We study photon blockade and antibunching in the cavity of an optomechanical system in which the mechanical resonator is coupled to a two-level system (TLS). In particular, we analyze the effects of the coupling strength (to the mechanical mode), transition frequency, and decay rate of TLS on the photon blockade. The statistical properties of the cavity field are affected by the TLS, because the TLS changes the energy-level structure of the optomechanical system via dressed states formed by the TLS and the mechanical resonator. We find that the photon blockade and tunneling can be significantly changed by the transition frequency of the TLS and the coupling strength between the TLS and the mechanical resonator. Therefore, our study provides a method to tune the photon blockade and tunneling using a controllable TLS.

Polarization beating quasi-phase-matching (PB QPM) in high-harmonic generation (HHG) is currently understood as a purely-intensity-modulation quasi-phase-matching (QPM) technique. In PB QPM, the driver ellipticity oscillates during propagation, modulating the HHG conversion efficiency since the single-atom HHG yield decreases rapidly with increasing ellipticity of the driving laser. We show that PB QPM is in fact based on both intensity and phase modulations. Unfortunately, in PB QPM the intensity and phase contributions oppose each other, resulting in significant reduction in the QPM efficiency.

We consider the quantum-witness test of macroscopic realism and derive an upper bound for possible violations of this equality due to quantum mechanics. The bound depends only on the number of possible outcomes for the blind measurement at the heart of the witness protocol. Mirroring recent results for the related Leggett-Garg inequality, we show that quantum mechanics can saturate the algebraic bound for large systems. We also establish a connection between the quantum witness and the trace distance between density matrices and discuss how the quantum witness can be used to obtain a bound on the Hilbert-space dimension of the system under study.

We utilize a discrete (sequential) measurement protocol to investigate quantum process tomography of a single two-level quantum system, with an unknown initial state, undergoing Rabi oscillations. The ignorance of the dynamical parameters is encoded into a continuous-variable classical system which is coupled to the two-level quantum system via a generalized Hamiltonian. This combined estimate of the quantum state and dynamical parameters is updated by using the information obtained from sequential measurements on the quantum system and, after a sufficient waiting period, faithful state monitoring and parameter determination is obtained. Numerical evidence is used to demonstrate the convergence of the state estimate to the true state of the hybrid system.

Dynamical decoupling has been actively investigated since Viola first suggested using a pulse sequence to protect a qubit from decoherence. Since then, many schemes of dynamical decoupling have been proposed to achieve high-order suppression, both analytically and numerically. However, hitherto, there has not been a systematic framework to understand all existing uniform $\pi$-pulse dynamical decoupling schemes. In this report, we use the projection pulse sequences as basic building blocks and concatenation as a way to combine them. We derived a concatenated-projection dynamical decoupling, a framework in which we can systematically construct pulse sequences to achieve arbitrary high suppression order. All previously known uniform dynamical decoupling sequences using $\pi$ pulse can be fit into this framework. Understanding uniform dynamical decoupling as successive projections on the Hamiltonian will also give insights on how to invent new ways to construct better pulse sequences.

Plasma environment effects on atomic structure are analyzed using various atomic structure codes. To monitor the effect of high free-electron density or low temperatures, Fermi-Dirac and Maxwell-Boltzmann statistics are compared. After a discussion of the implementation of the Fermi-Dirac approach within the ion-sphere model, several applications are considered. In order to check the consistency of the modifications brought here to extant codes, calculations have been performed using the Los Alamos Cowan Atomic Structure (cats) code in its Hartree-Fock or Hartree-Fock-Slater form and the parametric potential Flexible Atomic Code (fac). The ground-state energy shifts due to the plasma effects for the six most ionized aluminum ions have been calculated using the fac and cats codes and fairly agree. For the intercombination resonance line in ${\mathrm{Fe}}^{22+}$, the plasma effect within the uniform electron gas model results in a positive shift that agrees with the multiconfiguration Dirac-Fock value of B. Saha and S. Fritzsche [J. Phys. B 40, 259 (2007)]. Last, the present model is compared to experimental data in titanium measured on the terawatt Astra facility and provides values for electron temperature and density in agreement with the maria code.