Decoherence and multipartite entanglement

We study the dynamics of multipartite entanglement under the influence of decoherence. A suitable generalization of concurrence reveals distinct scaling of the entanglement decay rate of Greenberger-Horne-Zeilinger and W states, for various environments.     

Concurrence of mixed bipartite quantum states in arbitrary dimensions

We derive a lower bound for the concurrence of mixed bipartite quantum states, valid in arbitrary dimensions. As a corollary, a weaker, purely algebraic estimate is found, which detects mixed entangled states with a positive partial transpose.     

Solitonic eigenstates of the chaotic Bose–Hubbard Hamiltonian

We study the evolving energy spectrum of interacting ultra-cold atoms in an optical lattice as a function of an external parameter, the tilt of the lattice. In a regime where the quantum mechanical model, the Bose–Hubbard Hamiltonian, shows predominantly chaotic behavior, we identify regular structures in the parametric level evolution and characterize the eigenstates associated with these structures. The mechanism generating these structures is found to be different from Stark localization or energetic isolation and is induced by an interplay of driving and interaction.     

Entanglement measures as physical observables

We discuss why regular observables cannot be proper entanglement measures, and how observables in a generalized setting can be used to make an entanglement monotone a directly observable quantity for the case of pure states. For the case of mixed states, these generalized observables can be used to find valid lower bounds on entanglement monotones that can be measured in laboratory experiments in the same fashion as it can also be done for pure states. PACS 03.67.-a; 03.67.Mn; 89.70.+c     

Entanglement dynamics under decoherence: from qubits to qudits

We investigate the time evolution of entanglement for bipartite systems of arbitrary dimensions under the influence of decoherence. For qubits, we determine the precise entanglement decay rates under different system-environment couplings, including finite temperature effects. For qudits, we show how to obtain upper bounds for the decay rates and also present exact solutions for various classes of states.     

Quantum gates using electronic and nuclear spins of Yb+ in a magnetic field gradient

An efficient scheme is proposed to carry out gate operations on an array of trapped Yb⁺ ions, based on a previous proposal using both electronic and nuclear degrees of freedom in a magnetic field gradient. For this purpose we consider the Paschen-Back regime (strong magnetic field) and employ a high-field approximation in this treatment. We show the possibility to suppress the unwanted coupling between the electron spins by appropriately swapping states between electronic and nuclear spins. The feasibility of generating the required high magnetic field is discussed.     

Concurrence of mixed multipartite quantum States

We propose generalizations of concurrence for multipartite quantum systems that can distinguish qualitatively distinct quantum correlations. All introduced quantities can be evaluated efficiently for arbitrary mixed sates.     

Observable entanglement measure for mixed quantum States

We quantify an unknown mixed quantum state's entanglement by suitable, local parity measurements on its twofold copy. The associated observable qualifies as a generalized entanglement witness.     

Entanglement screening by nonlinear resonances

We show that nonlinear resonances in a classically mixed phase space allow us to define generic, strongly entangled multipartite quantum states. The robustness of their multipartite entanglement increases with the particle number, i.e., in the semiclassical limit, for those classes of diffusive noise which assist the quantum-classical transition.     

Measuring multipartite concurrence with a single factorizable observable

We show that, for any composite system with an arbitrary number of finite-dimensional subsystems, it is possible to directly measure the multipartite concurrence of pure states by detecting only one single factorizable observable, provided that two copies of the composite state are available. This result can be immediately put into practice in trapped-ion and entangled-photon experiments.