We show that the spreading of the center-of-mass density of ultracold attractively interacting bosons can become superballistic in the presence of decoherence, via one-, two-, and/or three-body losses. In the limit of weak decoherence, we analytically solve the numerical model introduced in [Phys. Rev. A 91, 063616 (2015)]. The analytical predictions allow us to identify experimentally accessible parameter regimes for which we predict superballistic spreading of the center-of-mass density. Ultracold attractive Bose gases form weakly bound molecules, quantum matter-wave bright solitons. Our computer simulations combine ideas from classical field methods (“truncated Wigner”) and piecewise deterministic stochastic processes. While the truncated Wigner approach to use an average over classical paths as a substitute for a quantum superposition is often an uncontrolled approximation, here it predicts the exact root-mean-square width when modeling an expanding Gaussian wave packet. In the superballistic regime, the leading order of the spreading of the center-of-mass density can thus be modeled as a quantum superposition of classical Gaussian random walks in velocity space.

We consider dynamically controllable periodic structures (gratings), resulting from Raman interaction of a weak probe field with a standing-wave pump and a second control laser field in four-level atomic media of $N$ type. The gratings under study are induced due to periodic spatial modulation of the Raman gain in a standing pump field and fundamentally differ from the ones based on electromagnetically induced transparency. We show that spectral and transmission properties of these gratings can be controlled with the help of an additional weak field (control field) by varying its intensity or frequency. Small variations of the control field intensity can change the system from opaque to transparent and vice versa and this structure can operate as an all-optical transistor. Such a structure can also be used as a tunable nonlinear mirror with amplification.

The efficiency of laser cooling relies on the existence of an almost closed optical-transition cycle in the energy spectrum of the considered species. In this respect, rare-earth elements exhibit many transitions which are likely to induce noticeable leaks from the cooling cycle. In this work, to determine whether laser cooling of singly ionized erbium ${\mathrm{Er}}^{+}$ is feasible, we have performed accurate electronic-structure calculations of energies and spontaneous-emission Einstein coefficients of ${\mathrm{Er}}^{+}$, using a combination of ab initio and least-squares-fitting techniques. We identify five weak closed transitions suitable for laser cooling, the broadest of which is in the kilohertz range. For the strongest transitions, by simulating the cascade dynamics of spontaneous emission, we show that repumping is necessary, and we discuss possible repumping schemes. We expect our detailed study on ${\mathrm{Er}}^{+}$ to give good insight into the laser cooling of neighboring ions such as ${\mathrm{Dy}}^{+}$.

We report an experimental realization of adaptive Bayesian quantum state tomography for two-qubit states. Our implementation is based on the adaptive experimental design strategy proposed in the work by Huszár and Houlsby [F. Huszár and N. M. T. Houlsby, Phys. Rev. A 85, 052120 (2012).] and provides an optimal measurement approach in terms of the information gain. We address the practical questions which one faces in any experimental application: the influence of technical noise and the behavior of the tomographic algorithm for an easy-to-implement class of factorized measurements. In an experiment with polarization states of entangled photon pairs, we observe a lower instrumental noise floor and superior reconstruction accuracy for nearly pure states of the adaptive protocol compared to a nonadaptive protocol. At the same time, we show that for the mixed states, the restriction to factorized measurements results in no advantage for adaptive measurements, so general measurements have to be used.

The purport of quantum teleportation is to completely transfer information from one party to another distant partner. However, from the perspective of parameter estimation, it is the information carried by a particular parameter, not the information of total quantum state that needs to be teleported. Due to the inevitable noise in environments, we propose two schemes to enhance quantum Fisher information (QFI) teleportation under amplitude damping noise with the technique of partial measurements. We find that post-partial measurement can greatly enhance the teleported QFI, while the combination of prior partial measurement and post-partial measurement reversal could completely eliminate the effect of decoherence. We show that, somewhat consequentially, enhancing QFI teleportation is more economic than that of improving fidelity teleportation. Our work extends the ability of partial measurements as a quantum technique to battle decoherence in quantum information processing.

Relying on weak cross-Kerr nonlinearities, we propose a nearly deterministic generation scheme of the three-photon polarization-entangled perfect $W$ state which can be applied to the perfect teleportation of an unknown single-photon state and has robust entanglement against the loss of one photon of them. Three photons entangle together by virtue of the bus function of the coherent state serving as the intermediate among them. In the scheme, three processes are executed successively and two kinds of modules are inserted into the circuit, where the homodyne measurement and the photon number measurement are aptly performed. By means of classical feedforward techniques, single-photon unitary transformation operations are performed on the corresponding photons based on the obtained measurement outcomes, by which the generation efficiency of the perfect $W$ state aims to nearly unity. Moreover, some currently available optical elements are applied in the generation process, which offer facilities for the practical implementation.

In this paper, we study the geometry of reduced density matrices for states with symmetry-protected topological (SPT) order. We observe ruled surface structures on the boundary of the convex set of low-dimensional projections of the reduced density matrices. In order to signal the SPT order using ruled surfaces, it is important that we add a symmetry-breaking term to the boundary of the system—no ruled surface emerges in systems without a boundary or when we add a symmetry-breaking term representing a thermodynamic quantity. Although the ruled surfaces only appear in the thermodynamic limit where the ground-state degeneracy is exact, we analyze the precision of our numerical algorithm and show that a finite-system calculation suffices to reveal the ruled surface structures.

Photon subtraction can enhance the performance of continuous-variable quantum key distribution (CV QKD). However, the enhancement effect will be reduced by the imperfections of practical devices, especially the limited efficiency of a single-photon detector. In this paper, we propose a non-Gaussian postselection method to emulate the photon substraction used in coherent-state CV QKD protocols. The virtual photon subtraction not only can avoid the complexity and imperfections of a practical photon-subtraction operation, which extends the secure transmission distance as the ideal case does, but also can be adjusted flexibly according to the channel parameters to optimize the performance. Furthermore, our preliminary tests on the information reconciliation suggest that in the low signal-to-noise ratio regime, the performance of reconciliating the postselected non-Gaussian data is better than that of the Gaussian data, which implies the feasibility of implementing this method practically.

We describe the dimensional crossover in a noninteracting Fermi gas in an anisotropic trap, obtained by populating various transverse modes of the trap. We study the dynamical structure factor and drag force. Starting from a dimension $d$, the $(d+1)$-dimensional case is obtained to a good approximation with relatively few modes. We show that the dynamical structure factor of a gas in a $d$-dimensional harmonic trap simulates an effective $2d$-dimensional box trap. We focus then on the experimentally relevant situation when only a portion of the gas in harmonic confinement is probed and give a condition to obtain the behavior of a $d$-dimensional gas in a box. Finally, we propose a generalized Tomonaga-Luttinger model for the multimode configuration and compare the dynamical structure factor in the two-dimensional limit with the exact result, finding that it is accurate in the backscattering region and at low energy.

We propose and study various realizations of a Hofstadter-Hubbard model on a cylinder geometry with fermionic cold atoms in optical lattices. The cylindrical optical lattice is created by copropagating Laguerre-Gauss beams, i.e., light beams carrying orbital angular momentum. By strong focusing of the light beams we create a real-space optical lattice in the form of rings, which are offset in energy. A second set of Laguerre-Gauss beams then induces a Raman-hopping between these rings, imprinting phases corresponding to a synthetic magnetic field (artificial gauge field). In addition, by rotating the lattice potential, we achieve a slowly varying flux through the hole of the cylinder, which allows us to probe the Hall response of the system as a realization of Laughlin's thought experiment. We study how in the presence of interactions fractional quantum Hall physics could be observed in this setup.

A single phonon in a nonlinear nanomechanical resonator (NAMR) can block the excitation of a second phonon [Phys. Rev. A 82, 032101 (2010)]. This intrinsically quantum effect is called phonon blockade, and is an analog of Coulomb blockade and photon blockade. Here we predict tunable multiphonon blockade in coupled nonlinear NAMRs, where nonlinearity is induced by two-level systems (TLSs) assuming dispersive (far off-resonance) interactions. Specifically, we derive an effective Kerr-type interaction in a hybrid system consisting of two nonlinearly interacting NAMRs coupled to two TLSs and driven by classical fields. The interaction between a given NAMR and a TLS is described by a Jaynes-Cummings-like model. We show that by properly tuning the frequency of the driving fields one can induce various types of phonon blockade, corresponding to the entangled phonon states of either two qubits, a qutrit and quartit, or two qudits. Thus, a $k$-phonon Fock state (with $k=1,2,3$) can impede the excitation of more phonons in a given NAMR, which we interpret as a $k$-phonon blockade (or, equivalently, phonon tunneling). Our results can be explained in terms of resonant transitions in the Fock space and via phase-space interference using the $s$-parametrized Cahill-Glauber quasiprobability distributions including the Wigner function. We study the nonclassicality, entanglement, and dimensionality of the blockaded phonon states during both dynamics and in the stationary limits.

We apply the variational method to obtain the universal and analytical lower bounds for parameter precision in some noisy systems. We first derive a lower bound for phase precision in lossy optical interferometry at nonzero temperature. Then we consider the effect of both amplitude damping and phase diffusion on phase-shift precision. Finally, we extend the constant phase estimation to the case of continuously fluctuating phase estimation and find that due to photon losses the corresponding mean square error transits from the stochastic Heisenberg limit to the stochastic standard quantum limit as the total photon flux increases.

We investigate the remarkable role of position-dependent damping in determining the parametric regions of symmetry breaking in nonlinear $\mathcal{PT}$-symmetric systems. We illustrate the nature of $\mathcal{PT}$-symmetry preservation and breaking with reference to a remarkable integrable scalar nonlinear system. In the two-dimensional cases of such position-dependent damped systems, we unveil the existence of a class of twofold-$\mathcal{PT}$-symmetric systems which have twofold $\mathcal{PT}$ symmetries. We analyze the dynamics of these systems and show how symmetry breaking occurs, that is, whether the symmetry breaking of the two $\mathcal{PT}$ symmetries occurs in pair or occurs one by one. The addition of linear damping in these nonlinearly damped systems induces competition between the two types of damping. This competition results in a $\mathcal{PT}$ phase transition in which the $\mathcal{PT}$ symmetry is broken for lower loss or gain strength and is restored by increasing the loss or gain strength. We also show that by properly designing the form of the position-dependent damping, we can tailor the $\mathcal{PT}$-symmetric regions of the system.

The geometric measure of entanglement for pure states has attracted much attention. On the other hand, the spectral theory of non-negative tensors (hypermatrices) has been developed rapidly. In this paper, we show how the spectral theory of non-negative tensors can be applied to the study of the geometric measure of entanglement for pure states. For symmetric pure multipartite qubit or qutrit states an elimination method is given. For symmetric pure multipartite qudit states, a numerical algorithm with randomization is presented. We also show that a nonsymmetric pure state can be augmented to a symmetric one whose amplitudes can be encoded in a non-negative symmetric tensor, so the geometric measure of entanglement can be calculated. Several examples, such as $m\mathrm{GHZ}$ states, $W$ states, inverted $W$ states, qudits, and nonsymmetric states, are used to demonstrate the power of the proposed methods. Given a pure state, one can always find a change of basis (a unitary transformation) so that all the probability amplitudes of the pure state are non-negative under the new basis. Therefore, the methods proposed here can be applied to a very wide class of multipartite pure states.

We classify entanglement distribution protocols based on whether or not entanglement gain is observed with respect to communicated and initial entanglement. We call a protocol nonexcessive if the gain of entanglement is bounded by the communicated entanglement and excessive if it violates this bound. We present examples of excessive protocols that achieve significant gain, independently of the presence of the initial and (or) communicated entanglement. We show that, for certain entanglement measures, excessive entanglement distribution is possible even with pure states, which sheds light on the possibility of formulating a unifying approach to quantifiers of quantum correlations. We point out a “catalytic” effect, where a protocol is turned into an excessive one by sending an intermediate particle (which does not change the initial entanglement) in advance of the designated carrier. Finally, we analyze the protocols in noisy scenarios and show that, under suitable conditions, excessive distribution may be the only way to achieve entanglement gain.

We propose several schemes for implementing a two-qubit quantum phase gate between two Rydberg atoms. The schemes could be realized in one step without adiabatic passage which depends on the specifical shapes and tailored pulse sequences of the laser fields. When the Rydberg-Rydberg-interaction (RRI) strength and the parameters of the driving fields satisfy some certain conditions, the effective Rabi oscillation between the two-excitation Rydberg state and the ground state would be generated, which is out of the Rydberg blockade regime and essential for our scheme. In addition, the individual addressing of the atoms is not required. And the schemes can work under strong or weak RRI strength. The imperfections induced by the variation of RRI strength and spontaneous emission are discussed through solving the master equation numerically.

A detailed theoretical study is carried out for electron interactions with magnesium oxide (MgO) with incident energies ranging from 0.01 to 5000 eV. This wide range of energy has allowed us to investigate a variety of processes and report data on resonances through eigenphase study, vertical electronic excitation energies, differential, momentum transfer, and total cross sections (TCS), as well as scattering rate coefficients. MgO has a large number of low-lying $\pi$ excited states and the present study finds overall a good agreement with earlier reported data. In order to compute total cross sections, we have employed the ab initio R-matrix method (0.01 to $\sim 20$ eV) and the spherical complex optical potential method ($\sim 20$ to 5000 eV). The R-matrix calculations are performed using a close-coupling method with the aid of 34 target states, 1436 configuration state functions, and 213 channels employing a static exchange plus polarization model. The present study reports evidence for electron scattering resonances through analysis of eigenphase diagrams at low energies below the ionization threshold. In the absence of any theoretical or experimental data for resonances, we have done double differentiation of TCS to confirm the resonances reported here. The present study is a maiden effort to report excitation cross sections, differential cross sections, momentum transfer cross sections, and scattering rate coefficients at low energies below the ionization threshold of the target. Additionally, in the absence of any experimental data and sparse theoretical data for a total elastic cross section, the present comprehensive study will provide a reference data set over such an extensive impact energy range.

Recently introduced time-dependent renormalized-natural-orbital theory (TDRNOT) is based on the equations of motion for the so-called natural orbitals, i.e., the eigenfunctions of the one-body reduced density matrix. Exact TDRNOT can be formulated for any time-dependent two-electron system in either spin configuration. In this paper, the method is tested against high-order-harmonic generation (HHG) and Fano profiles in absorption spectra with the help of a numerically exactly solvable one-dimensional-model He atom, starting from the spin-singlet ground state. Such benchmarks are challenging because Fano profiles originate from transitions involving autoionizing states, and HHG is a strong-field phenomenon well beyond the linear response. TDRNOT with just one natural orbital per spin in the helium spin-singlet case is equivalent to time-dependent Hartree-Fock or time-dependent density functional theory (TDDFT) in the exact exchange-only approximation. It is not unexpected that TDDFT fails in reproducing Fano profiles due to the lack of doubly excited, autoionizing states. HHG spectra, on the other hand, are widely believed to be well captured by TDDFT. However, HHG spectra of helium may display a second plateau that originates from simultaneous HHG in $\mathrm{He}^{+}$ and neutral He. It is found that TDRNOT with two natural orbitals per spin is already sufficient to capture this effect as well as the Fano profiles on a qualitative level. With more natural orbitals (6–8 per spin), quantitative agreement can be reached. Errors due to the truncation to a finite number of orbitals are identified.

Differential measurements using simultaneous atom interferometers provide unprecedented precision and stability for explorations on the scientific frontiers. Phase extraction between two atom interferometers, however, imposes additional limitations on the overall instrument performance due to nonlinear multiparameter fit and associated reduced data rate and sensitivity. We propose an active differential phase extraction method, which is self-calibratable and yields the theoretical performance of differential measurement for uncorrelated errors, and demonstrate the scheme on a transportable gravity gradiometer. The gravity gradient sensitivity of the instrument is improved by a factor of 3 with the implementation of the technique, which is in consistent with independently measured detection noise. We also demonstrate the accuracy and applicability of the scheme with 33-kg test masses, and achieve $1E$ uncertainty after 4000 s.

The concept of potential-energy landscapes is applied in many areas of science. We experimentally realize a random potential-energy landscape (RPEL) to which colloids are exposed. This is achieved by exploiting the interaction of matter with light. The optical setup is based on a special diffuser, which creates a top-hat beam containing a speckle pattern. This is imposed on colloids. The effect of the speckle pattern on the colloids can be described by a RPEL. The speckle pattern and the RPEL are quantitatively characterized. The distributions of both intensity and potential-energy values can be approximated by $\mathrm{\Gamma }$ distributions. They can be tuned from exponential to approximately Gaussian with variable standard deviation, which determines the contrast of the speckles and the roughness of the RPEL. Moreover, the characteristic length scales, e.g., the speckle size, can be controlled. By rotating the diffuser, furthermore, a flat potential can be created and hence only radiation pressure can be exerted on the particles.