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ABSTRACT: Topology plays a central role in ensuring the robustness of a wide variety of physical phenomena. Notable examples range from the current-carrying edge states associated with the quantum Hall and the quantum spin Hall effects to topologically protected quantum memory and quantum logic operations. Here we propose and analyse a topologically protected channel for the transfer of quantum states between remote quantum nodes. In our approach, state transfer is mediated by the edge mode of a chiral spin liquid. We demonstrate that the proposed method is intrinsically robust to realistic imperfections associated with disorder and decoherence. Possible experimental implementations and applications to the detection and characterization of spin liquid phases are discussed.
Nature Communications 03/2013; 4:1585. · 7.40 Impact Factor
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ABSTRACT: Topological states of fermionic matter can be induced by means of a suitably
engineered dissipative dynamics. Dissipation then does not occur as a
perturbation, but rather as the main resource for many-body dynamics, providing
a targeted cooling into a topological phase starting from an arbitrary initial
state. We explore the concept of topological order in this setting, developing
and applying a general theoretical framework based on the system density matrix
which replaces the wave function appropriate for the discussion of Hamiltonian
ground-state physics. We identify key analogies and differences to the more
conventional Hamiltonian scenario. Differences mainly arise from the fact that
the properties of the spectrum and of the state of the system are not as
tightly related as in a Hamiltonian context. We provide a symmetry-based
topological classification of bulk steady states and identify the classes that
are achievable by means of quasi-local dissipative processes driving into
superfluid paired states. We also explore the fate of the bulk-edge
correspondence in the dissipative setting, and demonstrate the emergence of
Majorana edge modes. We illustrate our findings in one- and two-dimensional
models that are experimentally realistic in the context of cold atoms.
02/2013;
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ABSTRACT: We discuss a hybrid quantum system where a dielectric membrane situated
inside an optical cavity is coupled to a distant atomic ensemble trapped in an
optical lattice. The coupling is mediated by the exchange of sideband photons
of the lattice laser, and is enhanced by the cavity finesse as well as the
square root of the number of atoms. In addition to observing coherent dynamics
between the two systems, one can also switch on a tailored dissipation by laser
cooling the atoms, thereby allowing for sympathetic cooling of the membrane.
The resulting cooling scheme does not require resolved sideband conditions for
the cavity, which relaxes a constraint present in standard optomechanical
cavity cooling. We present a quantum mechanical treatment of this modular open
system which takes into account the dominant imperfections, and identify
optimal operation points for both coherent dynamics and sympathetic cooling. In
particular, we find that ground state cooling of a cryogenically pre-cooled
membrane is possible for realistic parameters.
01/2013;
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ABSTRACT: We propose and analyze a physical system that naturally admits two-dimensional topological nearly flat bands. Our approach utilizes an array of three-level dipoles (effective S=1 spins) driven by inhomogeneous electromagnetic fields. The dipolar interactions produce arbitrary uniform background gauge fields for an effective collection of conserved hard-core bosons, namely, the dressed spin flips. These gauge fields result in topological band structures, whose band gap can be larger than the corresponding bandwidth. Exact diagonalization of the full interacting Hamiltonian at half-filling reveals the existence of superfluid, crystalline, and supersolid phases. An experimental realization using either ultracold polar molecules or spins in the solid state is considered.
Physical Review Letters 12/2012; 109(26):266804. · 7.37 Impact Factor
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ABSTRACT: We investigate dissipation-induced p-wave paired states of fermions in two dimensions and show the existence of spatially separated Majorana zero modes in a phase with vanishing Chern number. We construct an explicit and natural model of a dissipative vortex that traps a single of these modes, and establish its topological origin by mapping the problem to a chiral one-dimensional wire where we observe a nonequilibrium topological phase transition characterized by an abrupt change of a topological invariant (winding number). We show that the existence of a single Majorana zero mode in the vortex core is intimately tied to the dissipative nature of our model. Engineered dissipation opens up possibilities for experimentally realizing such states with no Hamiltonian counterpart.
Physical Review Letters 09/2012; 109(13):130402. · 7.37 Impact Factor
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Chemical Reviews 08/2012; 112(9):5012-61. · 40.20 Impact Factor
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ABSTRACT: We study the non-equilibrium many-body dynamics of a cold gas of ground state
alkali atoms weakly admixed by Rydberg states with laser light. On a timescale
shorter than the lifetime of the dressed states, effective dipole-dipole or van
der Waals interactions between atoms can lead to the formation of strongly
correlated phases, such as atomic crystals. Using a semiclassical approach, we
study the long-time dynamics where decoherence and dissipative processes due to
spontaneous emission and blackbody radiation dominate, leading to heating and
melting of atomic crystals as well as particle losses. These effects can be
substantially mitigated by performing active laser cooling in the presence of
atomic dressing.
07/2012;
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ABSTRACT: We study the driven-dissipative dynamics of photons interacting with an array
of micromechanical membranes in an optical cavity. Periodic membrane driving
and phonon creation result in an effective photon-number conserving non-unitary
dynamics, which features a steady state with long-range photonic coherence. If
the leakage of photons out of the cavity is counteracted by incoherent driving
of the photonic modes, we show that the system undergoes a dynamical phase
transition to the state with long-range coherence. A minimal system, composed
of two micromechanical membranes in a cavity, is studied in detail, and it is
shown to be a realistic setup where the key processes of the driven-dissipative
dynamics can be seen.
06/2012;
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ABSTRACT: The enormous experimental progress in atomic, molecular and optical (AMO)
physics during the last decades allows us nowadays to isolate single, a few or
even many-body ensembles of microscopic particles, and to manipulate their
quantum properties at a level of precision, which still seemed unthinkable some
years ago. This versatile set of tools has enabled the development of the
well-established concept of engineering of many-body Hamiltonians in various
physical platforms. These available tools, however, can also be harnessed to
extend the scenario of Hamiltonian engineering to a more general Liouvillian
setting, which in addition to coherent dynamics also includes controlled
dissipation in many-body quantum systems. Here, we review recent theoretical
and experimental progress in different directions along these lines, with a
particular focus on physical realizations with systems of atoms and ions. This
comprises digital quantum simulations in a general open system setting, as well
as engineering and understanding new classes of systems far away from
thermodynamic equilibrium.
03/2012;
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ABSTRACT: We study the dissipative dynamics and the formation of entangled states in
driven cascaded quantum networks, where multiple systems are coupled to a
common unidirectional bath. Specifically, we identify the conditions under
which emission and coherent reabsorption of radiation drives the whole network
into a pure stationary state with non-trivial quantum correlations between the
individual nodes. We illustrate this effect in more detail for the example of
cascaded two-level systems, where we present an explicit preparation scheme
that allows one to tune the whole network through "bright" and "dark" states
associated with different multi-partite entanglement patterns. In a
complementary setting consisting of cascaded non-linear cavities, we find that
two cavity modes can be driven into a non-Gaussian entangled dark state.
Potential realizations of such cascaded networks with optical and microwave
photons are discussed.
12/2011;
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ABSTRACT: We discuss the preparation of many-body states of cold fermionic atoms in an
optical lattice via controlled dissipative processes induced by coupling the
system to a reservoir. Based on a mechanism combining Pauli blocking and phase
locking between adjacent sites, we construct complete sets of jump operators
describing coupling to a reservoir that leads to dissipative preparation of
pairing states for fermions with various symmetries in the absence of direct
inter-particle interactions. We discuss the uniqueness of these states, and
demonstrate it with small-scale numerical simulations. In the late time
dissipative dynamics, we identify a "dissipative gap" that persists in the
thermodynamic limit. This gap implies exponential convergence of all many-body
observables to their steady state values. We then investigate how these pairing
states can be used as a starting point for the preparation of the ground state
of Fermi-Hubbard Hamiltonian via an adiabatic state preparation process also
involving the parent Hamiltonian of the pairing state. We also provide a
proof-of-principle example for implementing these dissipative processes and the
parent Hamiltonians of the pairing states, based on Yb171 atoms in optical
lattice potentials.
11/2011;
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ABSTRACT: We investigate pairing and crystalline instabilities of bosonic and fermionic polar molecules confined to a ladder geometry. Combining analytical and numerical techniques, we show that gases of composite molecular dimers as well as trimers can be stabilized as a function of the density difference between the wires. A shallow optical lattice can pin both liquids, realizing crystals of composite bosons and fermions. We show that these exotic quantum phases are robust against conditions of confinement of the molecular gas to harmonic finite-size potentials.
Physical Review Letters 10/2011; 107(16):163202. · 7.37 Impact Factor
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ABSTRACT: We discuss the implementation of optical quantum networks where the interface
between stationary and photonic qubits is realized by optomechanical
transducers [K. Stannigel et al., PRL 105, 220501 (2010)]. This approach does
not rely on the optical properties of the qubit and thereby enables optical
quantum communication applications for a wide range of solid-state spin- and
charge-based systems. We present an effective description of such networks for
many qubits and give a derivation of a state transfer protocol for
long-distance quantum communication. We also describe how to mediate local
on-chip interactions by means of the optomechanical transducers that can be
used for entangling gates. We finally discuss experimental systems for the
realization of our proposal.
06/2011;
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ABSTRACT: We discuss the implementation of quantum gate operations in a self-assembled
dipolar crystal of polar molecules. Here qubits are encoded in long-lived spin
states of the molecular ground state and stabilized against collisions by
repulsive dipole-dipole interactions. To overcome the single site
addressability problem in this high density crystalline phase, we describe a
new approach for implementing controlled single and two-qubit operations based
on resonantly enhanced spin-spin interactions mediated by a localized phonon
mode. This local mode is created at a specified lattice position with the help
of an additional marker molecule such that individual qubits can be manipulated
by using otherwise global static and microwave fields only. We present a
general strategy for generating state and time dependent dipole moments to
implement a universal set of gate operations for molecular qubits and we
analyze the resulting gate fidelities under realistic conditions. Our analysis
demonstrates the experimental feasibility of this approach for scalable quantum
computing or digital quantum simulation schemes with polar molecules.
06/2011;
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ABSTRACT: Robust edge states and non-Abelian excitations are the trademark of
topological states of matter, with promising applications such as
"topologically protected" quantum memory and computing. While so far
topological phases have been exclusively discussed in a Hamiltonian context, we
show that such phases and the associated topological protection and phenomena
also emerge in open quantum systems with engineered dissipation. The specific
system studied here is a quantum wire of spinless atomic fermions in an optical
lattice coupled to a bath. The key feature of the dissipative dynamics
described by a Lindblad master equation is the existence of Majorana edge
modes, representing a non-local decoherence free subspace. The isolation of the
edge states is enforced by a dissipative gap in the p-wave paired bulk of the
wire. We describe dissipative non-Abelian braiding operations within the
Majorana subspace, and we illustrate the insensitivity to imperfections.
Topological protection is granted by a nontrivial winding number of the system
density matrix.
05/2011;
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ABSTRACT: We study the BCS superfluid transition in a single-component fermionic gas of dipolar particles loaded in a tight bilayer trap, with the electric dipole moments polarized perpendicular to the layers. Based on the detailed analysis of the interlayer scattering, we calculate the critical temperature of the interlayer superfluid pairing transition when the layer separation is both smaller (dilute regime) and of the order or larger (dense regime) than the mean interparticle separation in each layer. Our calculations go beyond the standard BCS approach and include the many-body contributions resulting in the mass renormalization, as well as additional contributions to the pairing interaction. We find that the many-body effects have a pronounced effect on the critical temperature, and can either decrease (in the very dilute limit) or increase (in the dense and moderately dilute limits) the transition temperature as compared to the BCS approach. Comment: 23 pages, 10 figures, final approval from S. Ronen was not received due to his no-response
12/2010;
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ABSTRACT: We describe a new scheme to interconvert stationary and photonic qubits which is based on indirect qubit-light interactions mediated by a mechanical resonator. This approach does not rely on the specific optical response of the qubit and thereby enables optical quantum interfaces for a wide range of solid state spin and charge based systems. We discuss the implementation of state transfer protocols between distant nodes of a quantum network and show that high transfer fidelities can be achieved under realistic experimental conditions.
Physical Review Letters 11/2010; 105(22):220501. · 7.37 Impact Factor
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ABSTRACT: We show how dissipative dynamics can give rise to pairing for two-component fermions on a lattice. In particular, we construct a parent Liouvillian operator so that a BCS-type state of a given symmetry, e.g., a d-wave state, is reached for arbitrary initial states in the absence of conservative forces. The system-bath couplings describe single-particle, number-conserving and quasilocal processes. The pairing mechanism crucially relies on Fermi statistics. We show how such Liouvillians can be realized via reservoir engineering with cold atoms representing a driven dissipative dynamics.
Physical Review Letters 11/2010; 105(22):227001. · 7.37 Impact Factor
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ABSTRACT: We study one-dimensional fermionic and bosonic gases with repulsive power-law interactions 1/|x|(β), with β>1, in the framework of Tomonaga-Luttinger liquid (TLL) theory. We obtain an accurate analytical expression linking the TLL parameter to the microscopic Hamiltonian, for arbitrary β and strength of the interactions. In the presence of a small periodic potential, power-law interactions make the TLL unstable towards the formation of a cascade of lattice solids with fractional filling, a "Luttinger staircase." Several of these quantum phases and phase transitions are realized with ground state polar molecules and weakly bound magnetic Feshbach molecules.
Physical Review Letters 10/2010; 105(14):140401. · 7.37 Impact Factor
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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 neighboring droplets.
Physical Review Letters 09/2010; 105(13):135301. · 7.37 Impact Factor
