We study the decoherence caused by particle interaction for a conceptually simple model, a quantum walk on a bipartite one-dimensional lattice with decay from every second site. The corresponding noninteracting (linear) system has been shown to have a topological transition described by the average displacement before decay. Here, we use this topological quantity to distinguish coherent quantum dynamics from incoherent classical dynamics caused by a breaking of the translational symmetry. Furthermore, we analyze the behavior by means of a rate equation providing a quantitative description of the incoherent nonlinear dynamics.

We consider the problem of establishing a two-qubit entangled state of optimal fidelity across a noisy quantum channel when only a single use of the channel and local postprocessing by trace-preserving operations are allowed. We show that the optimal fidelity is obtained only when part of an appropriate nonmaximally entangled state is transmitted through the channel. The entanglement of this state can be vanishingly small when the channel becomes very noisy. Moreover, in the optimal case no further local processing is required to enhance the fidelity. We further show that local postprocessing can enhance fidelity if and only if the amount of noise is larger than a critical value and entanglement of the transmitted state is bounded from below. A notable consequence of these results is that the ordering of states under an entanglement monotone can be reversed even when the states undergo the same local interaction via a trace-preserving, completely positive map.

We investigate the dissipative dynamics of a hybrid system consisting of two inductively coupled superconducting flux qubits (FQs), each of which couples magnetically with a nitrogen-vacancy-center spin ensemble (NVE). The system displays a series of damped oscillations under various experimental situations, where we obtain analytically the time-dependent populations associated with arbitrary values of the FQ-FQ inductively coupling strength $g$ and the FQ-NVE magnetically coupling strength $G$ in the one-excitation case. Our results show that a reliable high-fidelity quantum state transfer between the two distant NVEs can be realized by modulating the parameters $g$ and $G.$ Furthermore, we also present a potentially practical idea to entangle the two separate NVEs, which is within reach due to the currently available technology.

There are only two known kinds of zero-capacity channels. The first kind produces entangled states that have positive partial transpose; and the second, states that are cloneable. We consider the family of “hybrid” quantum channels, which lies in the intersection of the above classes of channels and we investigate its properties. This study gives rise to explicit examples of the channels which create bound entangled states that have the property of being cloneable to the arbitrary finite number of parties. Hybrid channels provide an example of highly cloneable binding entanglement channels for which known superactivation protocols must fail; superactivation is the effect where two channels, each with zero quantum capacity, have positive capacity when used together. We give two methods to construct a hybrid channel from any binding entanglement channel. We also find the low-dimensional counterparts of hybrid states: bipartite qubit states which are extendible and possess two-way keys.

We propose an efficient method to generate a Greenberger-Horne-Zeilinger entangled state of $n$ photons in $n$ microwave cavities (or resonators) via resonant interaction to a single superconducting qutrit. The deployment of a qutrit, instead of a qubit, as the coupler enables us to use resonant interactions exclusively for all qutrit-cavity and qutrit-pulse operations. This unique approach significantly shortens the time of operation, which is advantageous for reducing the adverse effects of qutrit decoherence and cavity decay on the fidelity of the protocol. Furthermore, the protocol involves no measurement on either the state of the qutrit or cavity photons. We also show that the protocol can be generalized to other systems by replacing the superconducting qutrit coupler with different types of physical qutrits, such as an atom in the case of cavity QED, to accomplish the same task.

We investigate the quality of quantum state transfer through a uniformly coupled antiferromagnetic spin chain in a multiexcitation subspace. The fidelity of state transfer using multiexcitation channels is found to compare well with communication protocols based on the ground state of a spin chain with ferromagnetic interactions. Our numerical results support the conjecture that the fidelity of state transfer through a multiexcitation subspace only depends on the number of initial excitations present in the chain and is independent of the excitation ordering. Based on these results, we describe a communication scheme which requires little effort for preparation.

We consider the Casimir interaction between perfectly conducting and infinitely permeable corrugated parallel plates. We first study smooth plates which face each other at the distance $H$ with mixed boundary conditions (BCs) and show that the Casimir force is repulsive between them. Then we consider the effect of corrugation on the Casimir interaction. We investigate two geometries with mixed BCs, one with a corrugated plate and a flat plate (cf) and another with two corrugated surfaces. For the geometry of cf, at small distances the correction to the Casimir energy is $E_{\mathrm{cf}}^{\mathrm{N},\mathrm{D}}\sim \frac{+\hslash c{a}^2}{H^5}$, while at large distances $E_{\mathrm{cf}}^{\mathrm{N},\mathrm{D}}\sim \frac{+\hslash c{a}^2}{H^4\lambda }$, where $a$ and $\lambda$ are amplitude and wavelength of the sinusoidal corrugated surface respectively. On the other hand, when both surfaces are sinusoidally corrugated with a wavelength of $\lambda$, they experience a periodic sinusoidal lateral casimir force $F_l\sin (2\pi l/\lambda )$, where $l$ is the lateral displacement of the plates with respect to each other. In the limit of $\frac{H}{\lambda }\ll 1$, $F_l\sim -\frac{\hslash c{a}^2}{H^5\lambda }$ while at the region of $\frac{H}{\lambda }\gg 1$, $F_l$ decays exponentially as $F_l\sim -\frac{\hslash c{a}^2}{H{\lambda }^5}{e}^{-2\pi H/\lambda }$. For fixed $H$ and $\lambda$, stable and unstable mechanical equilibrium states of the system occur when $l=n\lambda$ and $l=(n+\frac{1}{2})\lambda$ respectively, where $n$ is an integer number.

We investigate heteronuclear collisions in isotope mixtures of laser-cooled metastable (${}^3{P}_2$) neon. Experiments are performed with spin-polarized atoms in a magnetic trap for all two-isotope combinations of the stable neon isotopes ${}^{20}$Ne, ${}^{21}$Ne, and ${}^{22}$Ne. We determine the rate coefficients for heteronuclear ionizing collisions to ${\beta }_{21,20}=(3.9\pm 2.7)\times {10}^{-11}$ cm${}^3$/s, ${\beta }_{22,20}=(2.6\pm 0.7)\times {10}^{-11}$ cm${}^3$/s, and ${\beta }_{21,22}=(3.9\pm 1.9)\times {10}^{-11}$ cm${}^3$/s. We also study heteronuclear elastic collision processes and give upper bounds for heteronuclear thermal relaxation cross sections. This work significantly extends the limited available experimental data on heteronuclear ionizing collisions for laser-cooled atoms involving one or more rare gas atoms in a metastable state.

Reentrance is an unusual feature where the phase boundaries of a system exhibit a succession of transitions between two phases $A$ and $B$, such as $A$-$B$-$A$-$B$, when just one parameter is varied monotonically. This type of reentrance is displayed by the Bose-Hubbard model in one spatial dimension between its Mott insulator (MI) and superfluid phase as the hopping amplitude is increased from zero. Here we analyze this counterintuitive phenomenon directly in the thermodynamic limit by utilizing the infinite time-evolving block decimation algorithm to variationally minimize an infinite matrix product state (MPS) parameterized by a matrix size $\chi$. Exploiting the direct restriction on the half-chain entanglement imposed by fixing $\chi$, we determined that reentrance in the MI lobes only emerges in this approximation when $\chi \ge 8$. This entanglement threshold is found to be coincident with the ability of an infinite MPS to be simultaneously particle-number symmetric and capture the kinetic energy carried by particle-hole excitations above the MI. Focusing on the tip of the MI lobe, we then applied a general finite-entanglement scaling analysis of the infinite-order Kosterlitz-Thouless (KT) critical point located there. By analyzing values of $\chi$ up to a very moderate $\chi =70$, we obtained an estimate of the KT transition as $t_{\mathrm{KT}}=0.30\pm 0.01$, demonstrating how a finite-entanglement approach can provide not only qualitative insight but also quantitatively accurate predictions.

We study the effect of losses on the phase sensitivity of the SU(1,1) interferometer for different configurations. We find that this type of interferometer is robust against losses that result from an inefficient detection system. This type of loss only introduces an overall prefactor to the sensitivity but does not change the $1/n$ scaling, where $n$ is the average number of particles inside the interferometer, characteristic of the Heisenberg limit. In addition, we show that under some conditions the SU(1,1) interferometer with coherent state inputs is also robust against internal losses. These results show that the SU(1,1) interferometer is a viable candidate for experimentally reaching the Heisenberg limit with current technology. Possible implementations of this interferometer using four-wave mixing in atomic vapors or an atom interferometer in a spinor Bose-Einstein condensate are compared.

We characterize the two-photon excitation of an ultracold gas of rubidium atoms to Rydberg states analyzing the induced atomic losses from an optical dipole trap. Extending the duration of the Rydberg excitation to several milliseconds, the ground-state atoms are continuously coupled to the formed positively charged plasma. In this regime we measure the $n$ dependence of the plasma-induced blockade effect and we characterize the interaction of the excited states and the ground state with the plasma. We also investigate the influence of the quasielectrostatic trapping potential on the system, confirming the validity of the ponderomotive model for states with $20\le n\le 120$.

We describe a scheme that allows for the transfer of a quantum state between a trapped atomic Bose condensate and an optomechanical end mirror mediated by a cavity field. Coupling between the mirror and the cold gas arises from the fact that the cavity field can produce density oscillations in the gas which in turn acts as an internal Bragg mirror for the field. After adiabatic elimination of the cavity field we find that the coherent dynamics of the atomic condensate–mirror hybrid system is described by an effective state transfer beam-splitter Hamiltonian. The state transfer fidelity is limited principally by the quantum noise associated with the intracavity field.

We present a simple hidden-variable model for the singlet state of a pair of qubits, characterized by two kinds of hierarchically ordered hidden variables. The model is such that, averaging over both types of variables, one reproduces all the quantum mechanical correlations of the singlet state. In contrast, averaging only over the hidden variables of the lower level, one obtains a general formal theoretical scheme exhibiting correlations stronger than the quantum ones, but with faster-than-light communication forbidden. By varying the parameters of the model, it is possible to obtain an arbitrary violation of the Clauser-Horne-Shimony-Holt and Tsirelson bounds. This result is interesting by itself since it shows that a violation of the quantum bound for nonlocal correlations can be implemented in a precise physical manner and not only mathematically and it suggests that resorting to two levels of nonlocal hidden variables might lead to a deeper understanding of the physical principles at the basis of quantum nonlocality. It is useful to mention that a model having features similar to ours has already been considered in the literature. However, from the point of view of its nonlocal aspects, it is physically much less appealing since averaging over the lower-level hidden variables yields correlations that can take only a discrete set of values for any choice of the upper-level variables and not the continuous set of values characterizing our approach. We discuss this problem in the concluding section of our work.

We study bifurcation measurement of a multilevel superconducting qubit using a nonlinear resonator biased in the straddling regime, where the resonator frequency sits between two qubit transition frequencies. We find that high-fidelity bifurcation measurements are possible because of the enhanced qubit-state-dependent pull of the resonator frequency, the behavior of qubit-induced nonlinearities, and the reduced Purcell decay rate of the qubit that can be realized in this regime. Numerical simulations find up to a threefold improvement in qubit readout fidelity when operating in, rather than outside of, the straddling regime. High-fidelity measurements can be obtained at much smaller qubit-resonator couplings than current typical experimental realizations, reducing spectral crowding and potentially simplifying the implementation of multiqubit devices.

The two-photon transition of ${}^{87}$Rb ($5S_{1/2}-4D_{3/2}$, $5S_{1/2}-4D_{5/2}$) in a vapor cell is investigated with direct frequency-comb spectroscopy. When strong chirp is imposed on a frequency comb, it is found that the Doppler-free signal amplitude barely degrades despite a significant reduction in peak intensity. In addition, it is found that the Doppler-broadened signal is strongly dependent on the applied chirp. We present a simple model that explains this counterintuitive behavior based on the elongated spatial extent of chirped pulses. Keeping Doppler-free signals almost constant, Doppler-broadened background signals can be strongly suppressed with chirped-pulse excitations. Consequently, in the case of uniformly distributed target atoms, chirped-pulse direct frequency-comb spectroscopy can be a useful method to improve the signal-to-noise ratio in two-photon direct frequency-comb spectroscopy.

Effects of postcollision interaction (PCI) observable in low-energy 3$d$ photoelectron spectra of Kr, which are associated with double Auger decay of the created inner-shell vacancy, are investigated by a combined experimental and theoretical approach. Measurements are based on an efficient multielectron coincidence method. Calculations have been carried out in the framework of a semiclassical approach. Our investigation reveals strong PCI distortion of the photoelectron line shapes, which depends on the kinematics of the process and the characteristics of the double Auger decay.

We investigate the emission times of high-order harmonics from the asymmetric molecule HeH${}^{2+}$ exposed to intense laser fields numerically and analytically. We show that for each laser cycle, the emission times of harmonics follow two characteristic electron trajectories. The first one matches the prediction of the general ground-state-based quantum orbital theory (GSB-QOT), as the second one differs significantly from the GSB-QOT. We show that the excited state plays an important role in this difference. As a result, the emission times of harmonics can be used as a tool to monitor the dynamics of excited states of the asymmetry molecule in strong laser fields.

Pairing in a population-imbalanced Fermi system in a two-dimensional optical lattice is studied using determinant quantum Monte Carlo simulations and mean-field calculations. The approximation-free numerical results show a wide range of stability of the Fulde-Ferrell-Larkin-Ovchinnikov phase. Contrary to claims of fragility with increased dimensionality, we find that this phase is stable across a wide range of values for the polarization, temperature, and interaction strength. Both homogeneous and harmonically trapped systems display pairing with finite center-of-mass momentum, with clear signatures either in momentum space or real space, which could be observed in cold-atomic gases loaded in an optical lattice. We also use the harmonic level basis in the confined system and find that pairs can form between particles occupying different levels, which can be seen as the analog of the finite center-of-mass momentum pairing in the translationally invariant case. Finally, we perform mean-field calculations for the uniform and confined systems and show the results to be in good agreement with quantum Monte Carlo. This leads to a simple picture of the different pairing mechanisms, depending on the filling and confining potential.

In this paper, we derive and study two versions of the short pulse equation (SPE) in $(2+1)$ dimensions. Using Maxwell's equations as a starting point, and suitable Kramers-Kronig formulas for the permittivity and permeability of the medium, which are relevant, e.g., to left-handed metamaterials and dielectric slab wave guides, we employ a multiple scales technique to obtain the relevant models. General properties of the resulting $(2+1)$-dimensional SPEs, including fundamental conservation laws, as well as the Lagrangian and Hamiltonian structure and numerical simulations for one- and two-dimensional initial data, are presented. Ultrashort one-dimensional breathers appear to be fairly robust, while rather general two-dimensional localized initial conditions are transformed into quasi-one-dimensional dispersing wave forms.

We present numerical results for the particle (photon) creation rate of dynamical Casimir effect (DCE) radiation in a resonant cylindrical microwave cavity. Based on recent experimental proposals, we model an irradiated semiconducting diaphragm (SCD) using a time-dependent “plasma sheet”' where we show that the number of photons created for the transverse magnetic mode TM${}_{011}$ is considerably enhanced even for low laser powers (of $\mu$J order). Conversely to the moving mirror case, we also show that the fundamental TM mode (TM${}_{010}$) is not excited for an irradiated plasma sheet. We show that polarization (arising due to the back reaction of pair created photons with the plasma SCD) implies losses for TM, but not transverse elelctric (TE) modes. However, we argue that these losses can be reduced by lowering the laser power and shortening the relaxation time. The results presented here lead support to the idea that TE and, in particular, TM modes are well suited to the detection of DCE radiation in a cylindrical cavity.