[Show abstract][Hide abstract] ABSTRACT: We analyze the low energy excitations of spin lattice systems in two dimensions at zero temperature within the framework of projected entangled pair state models. Perturbations in the bulk give rise to physical excitations located at the edge. We identify the corresponding degrees of freedom, give a procedure to derive the edge Hamiltonian, and illustrate that it can exhibit a rich phase diagram. For topological models, the edge Hamiltonian is constrained by the topological order in the bulk, which gives rise to one-dimensional edge models with unconventional properties; for instance, a topologically ordered bulk can protect a ferromagnetic Ising chain at the edge against spontaneous symmetry breaking.
[Show abstract][Hide abstract] ABSTRACT: We propose a scheme for the deterministic generation of steady-state entanglement between the two nuclear spin ensembles in an electrically defined double quantum dot. Because of quantum interference in the collective coupling to the electronic degrees of freedom, the nuclear system is actively driven into a two-mode squeezedlike target state. The entanglement buildup is accompanied by a self-polarization of the nuclear spins towards large Overhauser field gradients. Moreover, the feedback between the electronic and nuclear dynamics leads to multistability and criticality in the steady-state solutions.
[Show abstract][Hide abstract] ABSTRACT: We show that Projected Entangled-Pair States (PEPS) in two spatial dimensions
can describe chiral topological states by explicitly constructing a family of
such states with a non-trivial Chern number. They are ground states of two
different kinds of free-fermion Hamiltonians: (i) local and gapless; (ii)
gapped, but with hopping amplitudes that decay according to a power law. We
also prove that they are necessarily non-injective, and cannot correspond to
exact ground states of gapped, local parent Hamiltonians. We provide numerical
evidence that they can nevertheless approximate well the physical properties of
topological insulators with local Hamiltonians at arbitrary temperatures.
[Show abstract][Hide abstract] ABSTRACT: We propose and analyze a nanoengineered vortex array in a thin-film type-II superconductor as a magnetic lattice for ultracold atoms. This proposal addresses several of the key questions in the development of atomic quantum simulators. By trapping atoms close to the surface, tools of nanofabrication and structuring of lattices on the scale of few tens of nanometers become available with a corresponding benefit in energy scales and temperature requirements. This can be combined with the possibility of magnetic single site addressing and manipulation together with a favorable scaling of superconducting surface-induced decoherence.
[Show abstract][Hide abstract] ABSTRACT: We analyze a criterion which guarantees that the ground states of certain
many body systems are stable under perturbations. Specifically, we consider
PEPS, which are believed to provide an efficient description, based on local
tensors, for the low energy physics arising from local interactions. In order
to assess stability in the framework of PEPS, one thus needs to understand how
physically allowed perturbations of the local tensor affect the properties of
the global state. In this paper, we show that a restricted version of the Local
Topological Quantum Order (LTQO) condition provides a checkable criterion which
allows to assess the stability of local properties of PEPS under physical
perturbations. We moreover show that LTQO itself is stable under perturbations
which preserve the spectral gap, leading to nontrivial examples of PEPS which
possess LTQO and are thus stable under arbitrary perturbations.
[Show abstract][Hide abstract] ABSTRACT: We show the feasibility of tensor network solutions for lattice gauge
theories in Hamiltonian formulation by applying matrix product states
algorithms to the Schwinger model with zero and non-vanishing fermion mass. We
introduce new techniques to compute excitations in a system with open boundary
conditions, and to identify the states corresponding to low momentum and
different quantum numbers in the continuum. For the ground state and both the
vector and scalar mass gaps in the massive case, the MPS technique attains
precisions comparable to the best results available from other techniques.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: We propose to use subwavelength confinement of light associated with the near field of plasmonic systems to create nanoscale optical lattices for ultracold atoms. Our approach combines the unique coherence properties of isolated atoms with the subwavelength manipulation and strong light-matter interaction associated with nanoplasmonic systems. It allows one to considerably increase the energy scales in the realization of Hubbard models and to engineer effective long-range interactions in coherent and dissipative many-body dynamics. Realistic imperfections and potential applications are discussed.
[Show abstract][Hide abstract] ABSTRACT: Atoms coupled to nanophotonic interfaces represent an exciting frontier for
the investigation of quantum light-matter interactions. While most work has
considered the interaction between statically positioned atoms and light, here
we demonstrate that a wealth of phenomena can arise from the self-consistent
interaction between atomic internal states, optical scattering, and atomic
forces. We consider in detail the case of atoms coupled to a one-dimensional
nanophotonic waveguide, and show that this interplay gives rise to
self-organization of atomic positions along the waveguide, which can be probed
experimentally through distinct characteristics of the reflection and
[Show abstract][Hide abstract] ABSTRACT: We propose and analyze a scheme to observe topological phenomena with ions in
microtraps. We consider a set of trapped ions forming a regular structure in
two spatial dimensions and interacting with lasers. We find phonon bands with
non-trivial topological properties, which are caused by the breaking of time
reversal symmetry induced by the lasers. We investigate the appearance of edge
modes, as well as their robustness against perturbations. Long-range hopping of
phonons caused by the Coulomb interaction gives rise to flat bands which,
together with induced phonon-phonon interactions, can be used to produce and
explore strongly correlated states. Furthermore, some of these ideas can also
be implemented with cold atoms in optical lattices.
Physical Review A 10/2012; 87(1). · 3.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We show that by magnetically trapping a superconducting microsphere close to a quantum circuit, it is possible to perform ground-state cooling and prepare quantum superpositions of the center-of-mass motion of the microsphere. Due to the absence of clamping losses and time-dependent electromagnetic fields, the mechanical motion of micrometer-sized metallic spheres in the Meissner state is predicted to be very well isolated from the environment. Hence, we propose to combine the technology of magnetic microtraps and superconducting qubits to bring relatively large objects to the quantum regime.
[Show abstract][Hide abstract] ABSTRACT: The realization of devices which harness the laws of quantum mechanics represents an excit-ing challenge at the interface of modern tech-nology and fundamental science[1, 2]. An ex-emplary paragon of the power of such quantum primitives is the concept of "quantum money" . A dishonest holder of a quantum bank-note will invariably fail in any forging attempts; in-deed, under assumptions of ideal measurements and decoherence-free memories such security is guaranteed by the no-cloning theorem . In any practical situation, however, noise, decoherence and operational imperfections abound. Thus, the development of secure "quantum money"-type primitives capable of tolerating realistic infideli-ties is of both practical and fundamental impor-tance. Here, we propose a novel class of such pro-tocols and demonstrate their tolerance to noise; moreover, we prove their rigorous security by determining tight fidelity thresholds. Our pro-posed protocols require only the ability to pre-pare, store and measure single qubit quantum memories, making their experimental realization accessible with current technologies [5–7]. Recent extensions to Wiesner's original "quantum money" protocol  have garnered significant interest [8– 11]. One particular extension enables the authentication of quantum tokens via classical public communication with a trusted verifier . However, to tolerate noise, the verification process must condone a certain finite frac-tion of qubit failures; naturally, such a relaxation of the verification process enhances the ability for a dishonest user to forge quantum tokens. It is exactly this interplay which we, here, seek to address, by focusing on a class of "quantum token"-protocols which involve either direct physical or classical communication verification of qubit memories.
Proceedings of the National Academy of Sciences 10/2012; 109(40). · 9.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We theoretically show that intriguing features of coherent many-body physics can be observed in electron transport through a quantum dot (QD). We first derive a master-equation-based framework for electron transport in the Coulomb-blockade regime which includes hyperfine (HF) interaction with the nuclear spin ensemble in the QD. This general tool is then used to study the leakage current through a single QD in a transport setting. We find that, for an initially polarized nuclear system, the proposed setup leads to a strong current peak, in close analogy with superradiant emission of photons from atomic ensembles. This effect could be observed with realistic experimental parameters and would provide clear evidence of coherent HF dynamics of nuclear spin ensembles in QDs.
[Show abstract][Hide abstract] ABSTRACT: We investigate dissipative phase transitions in an open central spin system. In our model the central spin interacts coherently with the surrounding many-particle spin environment and is subject to coherent driving and dissipation. We develop analytical tools based on a self-consistent Holstein-Primakoff approximation that enable us to determine the complete phase diagram associated with the steady states of this system. It includes first- and second-order phase transitions, as well as regions of bistability, spin squeezing, and altered spin-pumping dynamics. Prospects of observing these phenomena in systems such as electron spins in quantum dots or nitrogen-vacancy centers coupled to lattice nuclear spins are briefly discussed.
Physical Review A 07/2012; 86:012116. · 3.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stable quantum bits, capable both of storing quantum information for macroscopic time scales and of integration inside small portable devices, are an essential building block for an array of potential applications. We demonstrate high-fidelity control of a solid-state qubit, which preserves its polarization for several minutes and features coherence lifetimes exceeding 1 second at room temperature. The qubit consists of a single (13)C nuclear spin in the vicinity of a nitrogen-vacancy color center within an isotopically purified diamond crystal. The long qubit memory time was achieved via a technique involving dissipative decoupling of the single nuclear spin from its local environment. The versatility, robustness, and potential scalability of this system may allow for new applications in quantum information science.
[Show abstract][Hide abstract] ABSTRACT: The realization of a scalable quantum information processor has emerged over the past decade as one of the central challenges at the interface of fundamental science and engineering. Here we propose and analyse an architecture for a scalable, solid-state quantum information processor capable of operating at room temperature. Our approach is based on recent experimental advances involving nitrogen-vacancy colour centres in diamond. In particular, we demonstrate that the multiple challenges associated with operation at ambient temperature, individual addressing at the nanoscale, strong qubit coupling, robustness against disorder and low decoherence rates can be simultaneously achieved under realistic, experimentally relevant conditions. The architecture uses a novel approach to quantum information transfer and includes a hierarchy of control at successive length scales. Moreover, it alleviates the stringent constraints currently limiting the realization of scalable quantum processors and will provide fundamental insights into the physics of non-equilibrium many-body quantum systems.
[Show abstract][Hide abstract] ABSTRACT: A new approach for the realization of a quantum interface between single photons and single ions in an ion crystal is proposed and analyzed. In our approach the coupling between a single photon and a single ion is enhanced via the collective degrees of freedom of the ion crystal. Applications including single-photon generation, a memory for a quantum repeater, and a deterministic photon-photon, photon-phonon, or photon-ion entangler are discussed.
[Show abstract][Hide abstract] ABSTRACT: We propose a method to prepare and verify spatial quantum superpositions of a nanometer-sized object separated by distances of the order of its size. This method provides unprecedented bounds for objective collapse models of the wave function by merging techniques and insights from cavity quantum optomechanics and matter-wave interferometry. An analysis and simulation of the experiment is performed taking into account standard sources of decoherence. We provide an operational parameter regime using present-day and planned technology.
[Show abstract][Hide abstract] ABSTRACT: Room temperature atomic ensembles in a spin-protected environment are useful systems both for quantum information science and metrology. Here we utilize a setup consisting of two atomic ensembles as a memory for quantum information initially encoded in the polarization state of two entangled light modes. We also use the ensembles as a radio frequency entanglement-assisted magnetometer with projection noise limited sensitivity below femtoTesla/. The performance of the quantum memory as well as the magnetometer was improved by spin-squeezed or entangled atomic states generated by quantum non demolition measurements. Finally, we present preliminary results of long lived entangled atomic states generated by dissipation. With the method presented, one should be able to generate an entangled steady state.
Journal of Physics Conference Series 01/2011; 264(1):012022.