Pochung Chen

Induced decoherence and entanglement by interacting quantum spin baths

The reduced dynamics of a single qubit or two qubits coupled to an interacting quantum spin bath modeled by an XXZ spin chain is investigated. By using the method of a time-dependent density matrix renormalization group t-DMRG, we go beyond the uniform coupling central spin model and nonperturbatively evaluate the induced decoherence and entanglement. It is shown that both the decoherence and the entanglement strongly depend on the phase of the underlying spin bath. We show that in general, spin baths can induce entanglement for an initially disentangled pair of qubits. Furthermore, when the spin bath is in the ferromagnetic phase because the qubits directly couple to the order parameter, the reduced dynamics shows an oscillatory type behavior. On the other hand, only for the paramagnetic and the antiferromagnetic phases do the initially entangled states suffer from an entanglement sudden death. By calculating the concurrence, the finite disentanglement time is mapped out for all of the phases in the phase diagram of the spin bath.

Field-induced spin supersolidity in frustrated S = 1 2 spin-dimer models

By means of the recently developed algorithm based on the tensor product states, the magnetization process of frustrated spin-1/2 spin-dimer models on a square lattice is investigated. Clear evidence of a supersolid phase over a finite regime of magnetic field is observed. Besides, critical fields at various field-induced transitions are determined accurately. Our work hence sheds light on the search of the supersolid phase in real frustrated spin-dimer compounds.

Edge states, entanglement entropy spectra and critical hopping couplings of anisotropic honeycomb lattices

For bipartite honeycomb lattices, we show that the Berry phase depends not only on the shape of the system but also on the hopping couplings. Using the entanglement entropy spectra obtained by diagonalizing the block Greens function matrices, the maximally entangled states with the eigenvalue λm=1/2 of the reduced density matrix are shown to have one-to-one correspondences to the zero-energy states of the lattice with open boundaries. The existence of these states depends on the Berry phase. For systems with finite bearded edges along the x-direction, we show that new maximally entangled states (zero-energy states) appear pair-by-pair when one increases the hopping coupling h over the critical values hcs.

Quench dynamics of topological maximally entangled states

We investigate the quench dynamics of the one-particle entanglement spectra (OPES) for systems with topologically nontrivial phases. By using dimerized chains as an example, it is demonstrated that the evolution of OPES for the quenched bipartite systems is governed by an effective Hamiltonian which is characterized by a pseudospin in a time-dependent pseudomagnetic field S(k,t). The existence and evolution of the topological maximally entangled states (tMESs) are determined by the winding number of S(k,t) in the k-space. In particular, the tMESs survive only if nontrivial Berry phases are induced by the winding of S(k,t). In the infinite-time limit the equilibrium OPES can be determined by an effective time-independent pseudomagnetic field Seff(k). Furthermore, when tMESs are unstable, they are destroyed by quasiparticles within a characteristic timescale in proportion to the system size.

Quantum phase transitions in a two-species hard-core boson Hubbard model in two dimensions.

We investigate various quantum phase transitions of attractive two-species bosons in a square lattice. Using the algorithm based on the tensor product states, the phase boundaries of the pair superfluid states with nonzero pair condensate density and vanishing atomic condensate density are determined. Various quantum phase transitions across the phase boundaries are characterized. Our work thus provides guides to the experimental search of the pair superfluid phase in lattice boson systems.

Quantum shape effects on Zeeman splittings in semiconductor nanostructures

We develop a general method to calculate Zeeman splittings of electrons and holes in semiconductor nanostructures within the tight-binding framework. The calculation is carried out in the electron-hole picture and is extensible to the excitonic calculation by including the electron-hole Coulomb interaction. The method is suitable for the investigation of quantum shape effects and the anisotropy of the

A Memory of Majorana Modes through Quantum Quench

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Universal dynamics of quantum spin decoherence in a nuclear spin bath

factors. Numerical results for CdSe and CdTe nanostructures are presented.

Relaxation of the entanglement spectrum in quench dynamics of topological systems

We study the sudden quench of a one-dimensional p-wave superconductor through its topological signature in the entanglement spectrum. We show that the long-time evolution of the system and its topological characterization depend on a pseudomagnetic field Reff(k). Furthermore, Reff(k) connects both the initial and the final Hamiltonians, hence exhibiting a memory effect. In particular, we explore the robustness of the Majorana zero-mode and identify the parameter space in which the Majorana zero-mode can revive in the infinite-time limit.

Two-photon Interference and Polarization Entanglement of Photon Pair Beam by Path Overlap Scheme

Received 11 January 2008; revised manuscript received 21 March 2008; published 30 May 2008We systematically investigate the universal spin decoherence dynamics of a localized electron in an arbitrarynuclear spin bath, which can even be far away from equilibrium due to the weak nuclear-lattice interaction. Weshow that the electron spin relaxation dynamics as well as spin pure dephasing and Hahn echo decay can