- F. Cinti, P. Jain, M. Boninsegni, A. Micheli, P. Zoller, G. Pupillo, A superfluid-droplet crystal and a free-space supersolid in a dipole-blockaded gas, (2010-08-24), (ID: 717214) Toggle Abstract
A novel supersolid phase is predicted for an ensemble of Rydberg atoms in the dipole-blockade regime, interacting via a repulsive dipolar potential \"softened\" at short distances. Using exact numerical techniques, we study the low temperature phase diagram of this system, and observe an intriguing phase consisting of a crystal of mesoscopic superfluid droplets. At low temperature, phase coherence throughout the whole system, and the ensuing bulk superfluidity, are established through tunnelling of identical particles between neighbouring droplets.
Alternative URL (local restricted copy) - A. Micheli, Z. Idziaszek, G. Pupillo, M. Baranov, P. Zoller, P. S. Julienne, Universal rates for reactive ultracold polar molecules in reduced dimensions, Phys. Rev. Lett. 105, 073202, (2010-08-13), URL doi:10.1103/PhysRevLett.105.073202 (ID: 717207) Toggle Abstract
Analytic expressions describe universal elastic and reactive rates of quasi-two-dimensional and quasi-one-dimensional collisions of highly reactive ultracold molecules interacting by a van der Waals potential. Exact and approximate calculations for the example species of KRb show that stability and evaporative cooling can be realized for spin-polarized fermions at moderate dipole and trapping strength, whereas bosons or unlike fermions require significantly higher dipole or trapping strengths.
Alternative URL (local restricted copy) - S. Diehl, A. Tomadin, A. Micheli, R. Fazio, P. Zoller, Dynamical Phase Transitions and Instabilities in Open Atomic Many-Body Systems, Phys. Rev. Lett. 105, 015702, (2010-07-01), URL doi:10.1103/PhysRevLett.105.015702 (ID: 717159) Toggle Abstract
We discuss an open driven-dissipative many-body system, in which the competition of unitary Hamiltonian and dissipative Liouvillian dynamics leads to a nonequilibrium phase transition. It shares features of a quantum phase transition in that it is interaction driven, and of a classical phase transition, in that the ordered phase is continuously connected to a thermal state. Within a generalized Gutzwiller approach which includes the description of mixed state density matrices, we characterize the complete phase diagram and the critical behavior at the phase transition approached as a function of time. We find a novel fluctuation induced dynamical instability, which occurs at long wavelength as a consequence of a subtle dissipative renormalization effect on the speed of sound.
Alternative URL (local restricted copy) - G. Pupillo, A. Micheli, M. Boninsegni, I. Lesanovsky, P. Zoller, Strongly correlated gases of Rydberg-dressed atoms: quantum and classical dynamics, Phys. Rev. Lett. 104, 223002, (2010-06-01), URL (ID: 716985) Toggle Abstract
We discuss techniques to generate long-range interactions in a gas of ground state alkali atoms, by weakly admixing excited Rydberg states with laser light. This provides a tool to engineer strongly correlated phases with reduced decoherence from inelastic collisions and spontaneous emission. As an illustration, we discuss the quantum phases of dressed atoms with dipole-dipole interactions confined in a harmonic potential, as relevant to experiments. We show that residual spontaneous emission from the Rydberg state acts as a heating mechanism, leading to a quantum-classical crossover.
Alternative URL (local restricted copy)
- G. Pupillo, A. Griessner, A. Micheli, M. Ortner, Wang, Daw-Wei, P. Zoller, Cold Atoms and Molecules in Self-Assembled Dipolar Lattices, Phys. Rev. Lett. 100, 050402, (2008-02-05), URL doi:10.1103/PhysRevLett.100.050402 (ID: 519056) Toggle Abstract
We study the realization of lattice models, where cold atoms and molecules
move as extra particles in a dipolar crystal of trapped polar molecules. The
crystal is a self-assembled floating mesoscopic lattice structure with quantum
dynamics given by phonons. We show that within an experimentally accessible
parameter regime extended Hubbard models with tunable long-range
phonon-mediated interactions describe the effective dynamics of dressed
particles.
Alternative URL (local restricted copy) - S. Diehl, A. Micheli, A. Kantian, B. Kraus, H. Büchler, P. Zoller, Quantum States and Phases in Driven Open Quantum Systems with Cold Atoms, Nature Physics 4, 878, (2008-09-07), URL doi:10.1038/nphys1073 (ID: 576233) Toggle Abstract
An open quantum system, whose time evolution is governed by a master equation, can be driven into a given pure quantum state by an appropriate design of the system-reservoir coupling. This points out a route towards preparing many body states and non-equilibrium quantum phases by quantum reservoir engineering. Here we discuss in detail the example of a \emph{driven dissipative Bose Einstein Condensate} of bosons and of paired fermions, where atoms in an optical lattice are coupled to a bath of Bogoliubov excitations via the atomic current representing \emph{local dissipation}. In the absence of interactions the lattice gas is driven into a pure state with long range order. Weak interactions lead to a weakly mixed state, which in 3D can be understood as a depletion of the condensate, and in 1D and 2D exhibits properties reminiscent of a Luttinger liquid or a Kosterlitz-Thouless critical phase at finite temperature, with the role of the ``finite temperature'' played by the interactions.
- B. Kraus, H. Büchler, S. Diehl, A. Kantian, A. Micheli, P. Zoller, Preparation of Entangled States by Quantum Markov Processes, Phys. Rev. A 78, 042307, (2008-05-02), URL doi:10.1103/PhysRevA.78.042307 (ID: 576234) Toggle Abstract
We investigate the possibility of using a dissipative process to prepare a quantum system in a desired state. We derive for any multipartite pure state a dissipative process for which this state is the unique stationary state and solve the corresponding master equation analytically. For certain states, such as the cluster states, we use this process to show that the jump operators can be chosen quasilocally, i.e. they act nontrivially only on a few, neighboring qubits. Furthermore, the relaxation time of this dissipative process is independent of the number of subsystems. We demonstrate the general formalism by considering arbitrary matrix-product states or projected entangled pair states. In particular, we show that the ground state of the Affleck-Kennedy-Lieb-Tasaki model can be prepared employing a quasi-local dissipative process.
- A. V. Gorshkov, P. Rabl, G. Pupillo, A. Micheli, P. Zoller, M. Lukin, H. Büchler, Suppression of Inelastic Collisions Between Polar Molecules With a Repulsive Shield, Phys. Rev. Lett. 101, 073201, (2008-08-14), URL doi:10.1103/PhysRevLett.101.073201 (ID: 582039) Toggle Abstract
We propose and analyze a technique that allows to suppress inelastic collisions and simultaneously enhance elastic interactions between cold polar molecules. The main idea is to cancel the leading dipole-dipole interaction with a suitable combination of static electric and microwave fields in such a way that the remaining van-der-Waals-type potential forms a three-dimensional repulsive shield. We analyze the elastic and inelastic scattering cross sections relevant for evaporative cooling of polar molecules and discuss the prospect for the creation of crystalline structures
Alternative URL (local restricted copy) - X. Lu, C. Wu, A. Micheli, G. Pupillo, Structure and melting behavior of classical bilayer crystals of dipoles, Phys. Rev. B 78, 024108, (2008-07-11), URL doi:10.1103/PhysRevB.78.024108 (ID: 596533) Toggle Abstract
We study the structure and melting of a classical bilayer system of dipoles, in a setup where the dipoles are oriented perpendicular to the planes of the layers and the density of dipoles is the same in each layer. Due to the anisotropic character of the dipole-dipole interactions, we find that the ground-state configuration is given by two hexagonal crystals positioned on top of each other, independent of the interlayer spacing and dipolar density. For large interlayer distances these crystals are independent, while in the opposite limit of small interlayer distances the system behaves as a two-dimensional crystal of paired dipoles. Within the harmonic approximation for the phonon excitations, the melting temperature of these crystalline configurations displays a non-monotonic dependence on the interlayer distance, which is associated with a re-entrant melting behavior in the form of solid-liquid-solid-liquid transitions at fixed temperature.
- G. Brennen, A. Micheli, P. Zoller, Designing spin-1 lattice models using polar molecules, New Journal of Physics 9, 138, (2007-05-18), URL doi:10.1088/1367-2630/9/5/138 (ID: 430363) Toggle Abstract
We describe how to design a large class of always on spin-1 interactions between polar molecules trapped in an optical lattice. The spin degrees of freedom correspond to the hyperfine levels of a ro-vibrational ground state molecule. Interactions are induced using a microwave field to mix ground states in one hyperfine manifold with the spin entangled dipole-dipole coupled excited states. Using multiple fields anistropic models in one, two, or three dimensions, can be built with tunable spatial range. An illustrative example in one dimension is the generalized Haldane model, which at a specific parameter has a gapped valence bond solid ground state. The interaction strengths are large compared to decoherence rates and should allow for probing the rich phase structure of strongly correlated systems, including dimerized and gapped phases.
Alternative URL (local restricted copy) - A. Micheli, G. Pupillo, H. Büchler, P. Zoller, Cold polar molecules in 2D traps: Tailoring interactions with external fields for novel quantum phases, Phys. Rev. A 76, 043604, (2007-10-03), URL doi:10.1103/PhysRevA.76.043604 (ID: 460560) Toggle Abstract
We discuss techniques to engineer effective long-range interactions between polar molecules using external static electric and microwave fields. We consider a setup where molecules are trapped in a two-dimensional pancake geometry by a far-off-resonance optical trap, which ensures the stability of the dipolar collisions. We detail how to modify the shape and the strength of the long-range part of interaction potentials, which can be utilized to realize interesting quantum phases in the context of cold molecular gases.
Alternative URL (local restricted copy) - H. Büchler, E. Demler, M. Lukin, A. Micheli, N. Prokofev, G. Pupillo, P. Zoller, Strongly correlated 2D quantum phases with cold polar molecules: controlling the shape of the interaction potential, Phys. Rev. Lett. 98, 060404, (2007-02-08), URL doi:10.1103/PhysRevLett.98.060404 (ID: 376275) Toggle Abstract
We discuss techniques to tune and shape the long-range part of the interaction potentials in quantum gases of bosonic polar molecules by dressing rotational excitations with static and microwave fields. This provides a novel tool towards engineering strongly correlated quantum phases in combination with low-dimensional trapping geometries. As an illustration, we discuss the 2D superfluid-crystal quantum phase transition for polar molecules interacting via an electric-field-induced dipole-dipole potential.
Alternative URL (local restricted copy) - H. Büchler, A. Micheli, P. Zoller, Three-body interactions with cold polar molecules, Nature Physics 3, 726, (2007-07-22), URL doi:10.1038/nphys678 (ID: 462909) Toggle Abstract
We show that polar molecules driven by microwave fields give naturally rise to strong three-body interactions, while the two-particle interaction can be independently controlled and even switched off. The derivation of these effective interaction potentials is based on a microscopic understanding of the underlying molecular physics, and follows from a well controlled and systematic expansion into many-body interaction terms. For molecules trapped in an optical lattice, we show that these interaction potentials give rise to Hubbard models with strong nearest-neighbor two-body and three-body interaction. As an illustration, we study the one-dimensional Bose-Hubbard model with dominant three-body interaction and derive its phase diagram.
Alternative URL (local restricted copy)