Entanglement generated by dissipation and steady state entanglement of two macroscopic objects

Entanglement is a striking feature of quantum mechanics and an essential ingredient in most applications in quantum information. Typically, coupling of a system to an environment inhibits entanglement, particularly in macroscopic systems. Here we report on an experiment where dissipation continuously generates entanglement between two macroscopic objects. This is achieved by engineering the dissipation using laser and magnetic fields, and leads to robust event-ready entanglement maintained for 0.04 s at room temperature. Our system consists of two ensembles containing about 10(12) atoms and separated by 0.5 m coupled to the environment composed of the vacuum modes of the electromagnetic field. By combining the dissipative mechanism with a continuous measurement, steady state entanglement is continuously generated and observed for up to 1 h.     

Geometric entanglement in valance-bond-solid state

Multipartite entanglement, measured by the geometric entanglement (GE), is discussed for integer spin Valance-Bond-Solid (VBS) state respectively with periodic boundary condition (PBC) and open boundary condition (OBC) in this paper. The optimization in the definition of geometric entanglement can be reduced greatly by exploring the symmetry of VBS state, and then the fully separable state can be determined explicitly. Numerical evaluation for GE by the random simulation is also implemented in order to demonstrate the validity of the reductions. Our calculations show that GE is saturated by a finite value with the increment of particle number, that means that the total entanglement for VBS state would be divergent under the thermodynamic limit. Moreover it is found that the scaling behavior of GE with spin number s is fitted as α log(s + β/s + γ)+δ, in which the values of the parameters α, β, γ, σ are only dependent on the parity of spin s. A comparison with entanglement entropy of VBS state is also made, in order to demonstrate the essential differences between multipartite and bipartite entanglement in this model.     

Extracting excitations from model state entanglement

We extend the concept of an entanglement spectrum from the geometrical to the particle bipartite partition. We apply this to several fractional quantum Hall wave functions on both sphere and torus geometries to show that this new type of entanglement spectra completely reveals the physics of bulk quasihole excitations. While this is easily understood when a local Hamiltonian for the model state exists, we show that the quasihole wave functions are encoded within the model state even when such a Hamiltonian is not known. As a nontrivial example, we look at Jain's composite fermion states and obtain their quasiholes directly from the model state wave function. We reach similar conclusions for wave functions described by Jack polynomials.     

Diffusion near nonequilibrium steady state

The diffusion equation describing the response of a system in a steady nonequilibrium state to a nonuniform external perturbation is derived from the Boltzmann equation. For transient processes, the diffusion equation governs the slow (final) stage of the process, irrespective to the form of the initial distribution. The initial velocity distribution becomes the local steady-state distribution during the first fast stage of the process. This fast initial relaxation affects only the initial condition for the diffusion equation. Some principal features of the nonequilibrium diffusion constant are discussed. As an illustration, the diffusion constant of hot electrons is calculated in the electron temperature approximation.     

A steady state quantum classifier

We report that under some specific conditions a single qubit model weakly interacting with information environments can be referred to as a quantum classifier. We exploit the additivity and the divisibility properties of the completely positive (CP) quantum dynamical maps in order to obtain an open quantum classifier. The steady state response of the system with respect to the input parameters was numerically investigated and it's found that the response of the open quantum dynamics at steady state acts non-linearly with respect to the input data parameters. We also demonstrate the linear separation of the quantum data instances that reflects the success of the functionality of the proposed model both for ideal and experimental conditions. Superconducting circuits were pointed out as the physical model to implement the theoretical model with possible imperfections.     

Bi-layer steady state current cloak

We report that bi-layer copper and polystyrene cylinders can cloak steady current. We fabricated two hollow cylinders, the one made of copper, and the other made of polystyrene. Two hollow copper and polystyrene cylinders nested concentric bi-layer hollow copper and polystyrene cylinders. The background media are made of aluminum. Theory and experiment demonstrated that the electric potential gradients are parallel and equal outside the outer circle, the iso-potential lines are parallel outside the outer circle, and the electric potential is zero in the inner circle.     

Measurement-based entanglement purification for hybrid entangled state

Hybrid entangled states(HESs), which involve different particles with various degrees of freedom, have garnered significant attention and been applied in a wide range of quantum technologies. However, similar to other categories of entanglement, maximally HESs inevitably degrade to mixed states due to the environmental noise and operational imperfections. To address the degradation problem, measurement-based entanglement purification offers a feasible and robust solution alternative to conventional gate-based purification methods. In this paper, we propose a measurement-based hybrid entanglement purification protocol(MB-HEPP) for a certain kind of HES which consists of polarization photons and coherent states. We extend our methodology to several conditions, such as the multi-copy and multi-party scenarios, and the photon-loss condition. Compared with previous HEPPs, this protocol has several advantages. First, it does not depend on post-selection and the purified HESs can be retained for further application. Second, it does not require the Bell state measurement, but only uses the parity check with conventional linear optical elements, which makes it have the higher success probability and more feasible. Our MB-HEPP has potential applications in future heterogeneous quantum networks.     

Measuring quantum entanglement without prior state reconstruction

It is shown that, despite strong nonlinearity, entanglement of formation of a two-qubit state can be measured without prior state reconstruction. Collective measurements on a small number of copies are provided that allow one to determine quantum concurrence via estimation of only four parameters. It is also pointed out that another entanglement measure based on so-called "negativity" can also be measured in a similar way. The result is related to the general problem: What kind of information can be extracted efficiently from an unknown quantum state?     

Experimental bound entanglement in a four-photon state

Bound entanglement is central to many exciting theoretical results in quantum information processing, but has thus far not been experimentally realized. In this work, we consider a one-parameter family of four-qubit Smolin states. We experimentally produce these states in the polarization of four optical photons produced from parametric down-conversion. Within a range of the parameter, we show that our states are entangled and undistillable, and thus bound entangled. Using these bound-entangled states we demonstrate entanglement unlocking.     

On thermodynamics near a steady state

The statistical deduction of a stability criterion for steady states (a generalization of the Glansdorff-Prigogine criterion) given in a previous paper is extended in some respects. Dynamical coefficients for the motion of small deviations from a steady state are expressed by correlation functions. The stability criterion is applied to some models of chemical reactions and to a model for second break-down in semiconductors.     

Pure state estimation and the characterization of entanglement

A connection between the state estimation problem and the separability problem is noticed and exploited to find efficient numerical algorithms to solve the first one. Based on these ideas, we also derive a systematic method to obtain upper bounds on the maximum local fidelity when the states are distributed among several distant parties.     

Analysis of mixed state entanglement with intrinsic decoherence

We analyze different entanglement measures for a mixed state two-level system in the presence of intrinsic decoherence. The information about entanglement is obtained by comparing the results for the atomic Wehrl entropy and negativity with the analytical results for a simple case. For the strong decoherence case we find that a similar and long-lived maximum Wehrl entropy and negativity between atom and field are shown. The results highlight the important roles played by both the decoherence parameter and the initial state setting in determining the evolution of the atomic Wehrl entropy and negativity.     

Small fluctuations in a steady non-equilibrium state

An expression for the probability of fluctuations in a given probability distribution deduced in an earlier paper is used to derive a relation between autocorrelations of velocities and the dynamics of small fluctuations in a stable steady state. The steady state may lie in the region of nonlinear thermodynamics. The considerations are restricted to systems with a distinctly separated behaviour on short and long time scale and the corresponding use of Mori distributions.     

Generation of multiparticle three-dimensional entanglement state via adiabatic passage

A scheme is proposed for generating a multiparticle three-dimensional entangled state by appropriately adiabatic evolutions, where atoms are respectively trapped in separated cavities so that individual addressing is needless. In the ideal case, losses due to the spontaneous transition of an atom and the excitation of photons are efficiently suppressed since atoms are all in ground states and the fields remain in a vacuum state. Compared with the previous proposals, the present scheme reduces its required operation time via simultaneously controlling four classical fields. This advantage would become even more obvious as the number of atoms increases. The experimental feasibility is also discussed. The successful preparation of a high-dimensional multiparticle entangled state among distant atoms provides better prospects for quantum communication and distributed quantum computation.     

Equilibrium state and non-equilibrium steady state in an isolated human system

The principle of increasing entropy （PIE） is commonly considered as a universal physical law tbr natural systems. It also means that a non-equilibrium steady state （NESS） must not appear in any isolated natural systems. Here we experimentally investigate an isolated human social system with a clustering effect. We report that the PIE cannot always hold, and that NESSs can come to appear. Our study highlights the role of human adaptability in the PIE, and makes it possible to study human social systems by using some laws originating from traditional physics.     

A further visit to the steady state universe

It is shown that within the framework of stochastic electrodynamics observations of the apparent magnitudes and redshifts of galaxies as well as number counts cannot be considered as arguments against the steady state cosmology. A bright (a black hole viewed from inside) universe cosmology is briefly considered.     

Optical fibre: Spatial transient and steady state

The concept of spatial transient is introduced to better appreciate the role of leaky modes on optical waveguides of circular cross section. Refracting leaky modes represent a relatively fast transient while tunnelling leaky modes represent a relatively slow transient. Bound modes provide the spatial steady-state solution. The greater the waveguide parameter V, the greater the duration of the transient. The relevance of leaky modes on fibres with loss and irregularities is discussed.