Posazhennikova, Anna
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Manipulating the Mott lobes : optical lattice bosons coupled to an array of atomic quantum dots
We analyze quantum phase transitions in a system of optical lattice bosons coupled to an array of atomic quantum dots, or pseudospins-1/2. The system parallels the Bose-Hubbard model with a single difference of the direct tunneling between the lattice sites being replaced by an assisted tunneling via coupling to the atomic quantum dots. We calculate the phase diagram of the combined system, numerically within the Gutzwiller ansatz and analytically using the mean-field decoupling approximation. The result of the assisted Bose-Hubbard model is that the Mott-superfluid transition still takes place, however, the Mott lobes strongly depend on the system parameters such as the detuning. One can even reverse the usual hierarchy of the lobes with the first lobe becoming the smallest. The phase transition in the bosonic subsystem is accompanied by a magnetization rotation in the pseudospin subsystem with the tilting angle being an effective order parameter. When direct tunneling is taken into account, the Mott lobes can be made disappear and the bosonic subsystem becomes superfluid throughout.
Phonon resonances in atomic currents through Bose-Fermi mixtures in optical lattices
We present an analysis of Bose-Fermi mixtures in optical lattices for the case where the lattice potential of the fermions is tilted and the bosons (in the superfluid phase) are described by Bogoliubov phonons. It is shown that the Bogoliubov phonons enable hopping transitions between fermionic Wannier-Stark states; these transitions are accompanied by energy dissipation into the superfluid and result in a net atomic current along the lattice. We derive a general expression for the drift velocity of the fermions and find that the dependence of the atomic current on the lattice tilt exhibits negative differential conductance and phonon resonances. Numerical simulations of the full dynamics of the system based on the time-evolving block decimation algorithm reveal that the phonon resonances should be observable under the conditions of a realistic measuring procedure.
Entanglement generation in a system of two atomic quantum dots coupled to a pool of interacting bosons
We discuss entanglement generation in a closed system of one or two atomic quantum dots (qubits) coupled via Raman transitions to a pool of cold interacting bosons. The system exhibits rich entanglement dynamics, which we analyze in detail in an exact quantum mechanical treatment of the problem. The bipartite setup of only one atomic quantum dot coupled to a pool of bosons turns out to be equivalent to two qubits which easily get entangled from an initial product state. We show that both the number of bosons in the pool and the boson-boson interaction crucially affect the entanglement characteristics of the system. The tripartite system of two atomic quantum dots and a pool of bosons reduces to a qubit-qutrit-qubit realization. We consider entanglement possibilities of the pure system as well as of reduced ones by tracing out one of the constituents, and show how the entanglement can be controlled by varying system parameters. We demonstrate that the qutrit, as expected, plays a leading role in entangling the two qubits and the maximum entanglement depends in a nontrivial way on the pool characteristics.
Nonequilibrium Josephson oscillations in Bose-Einstein condensates without dissipation
We perform a detailed field theoretical study of nonequilibrium Josephson oscillations between interacting Bose-Einstein condensates confined in a finite-size double-well trap. We find that the Josephson junction can sustain multiple undamped Josephson oscillations up to a characteristic time scale tc without quasipartcles being excited in the system. This may explain recent related experiments. At larger times the dynamics of the junction is governed by fast Rabi oscillations between the descrete quasiparticle levels. We predict that a selftrapped BEC state will be destroyed by these Rabi oscillations.
Entanglement generation in a system of two atomic quantum dots coupled to a pool of interacting bosons
We discuss entanglement generation in a closed system of one or two atomic quantum dots (qubits) coupled via Raman transitions to a pool of cold interacting bosons. The system exhibits rich entanglement dynamics, which we analyze in detail in an exact quantum mechanical treatment of the problem. The bipartite setup of only one atomic quantum dot coupled to a pool of bosons turns out to be equivalent to two qubits which easily get entangled being initially in a product state. We show that both the number of bosons in the pool and the boson-boson interaction crucially affect the entanglement characteristics of the system. The tripartite system of two atomic quantum dots and a pool of bosons reduces to a qubit-qutrit-qubit realization. We consider entanglement possibilities of the pure system as well as of reduced ones by tracing out one of the constituents, and show how the entanglement can be controlled by varying system parameters. We demonstrate that the qutrit, as expected, plays a leading role in entangling of the two qubits and the maximum entanglement depends in a nontrivial way on the pool characteristics.
Two atomic quantum dots interacting via coupling to BECs
We consider a system of three weakly coupled Bose-Einstein condensates and two atomic quantum dots embedded in the barriers between the condensates. Each dot is coupled to two neighboring condensates by optical transitions and can be described as a two-state system, or a pseudospin 1/2. Although there is no direct coupling between the dots, an effective interaction between the pseudospins is induced due to their coupling to the condensate reservoirs. We investigate this effective interaction, depending on the strengths of the dot-condensate coupling T and the direct coupling J between the condensates. In particular, we show that an initially ferromagnetic arrangement of the two pseudospins stays intact even for large T/J. However, antiferromagnetically aligned spins undergo peculiar "breathing" modes for weak coupling T/J