Quantum emitters coupled to surface plasmons of a nano-wire: A Green function approach

We investigate a system consisting of a single, as well as two emitters strongly coupled to surface plasmon modes of a nano-wire using a Green function approach. Explicit expressions are derived for the spontaneous decay rate into the plasmon modes and for the atom-plasmon coupling as well as a plasmon-mediated atom-atom coupling. Phenomena due to the presence of losses in the metal are discussed. In case of two atoms, we observe Dicke sub- and superradiance resulting from their plasmon-mediated interaction. Based on this phenomenon, we propose a scheme for a deterministic two-qubit quantum gate. We also discuss a possible realization of interesting many-body Hamiltonians, such as the spin-boson model, using strong emitter-plasmon coupling.Comment: 12 pages, 16 figure

Fault-tolerant Quantum Communication with Minimal Physical Requirements

We describe a novel protocol for a quantum repeater which enables long distance quantum communication through realistic, lossy photonic channels. Contrary to previous proposals, our protocol incorporates active purification of arbitrary errors at each step of the protocol using only two qubits at each repeater station. Because of these minimal physical requirements, the present protocol can be realized in simple physical systems such as solid-state single photon emitters. As an example, we show how nitrogen vacancy color centers in diamond can be used to implement the protocol, using the nuclear and electronic spin to form the two qubits.Comment: 4 pages, 3 figures. V2: Minor modifications. V3: Major changes in the presentation and new titl

Strong coupling of single emitters to surface plasmons

We propose a method that enables strong, coherent coupling between individual optical emitters and electromagnetic excitations in conducting nano-structures. The excitations are optical plasmons that can be localized to sub-wavelength dimensions. Under realistic conditions, the tight confinement causes optical emission to be almost entirely directed into the propagating plasmon modes via a mechanism analogous to cavity quantum electrodynamics. We first illustrate this result for the case of a nanowire, before considering the optimized geometry of a nanotip. We describe an application of this technique involving efficient single-photon generation on demand, in which the plasmons are efficiently out-coupled to a dielectric waveguide. Finally we analyze the effects of increased scattering due to surface roughness on these nano-structures.Comment: 34 pages, 7 figure

Coupling Nitrogen Vacancy Centers in Diamond to Superconducting Flux Qubits

We propose a method to achieve coherent coupling between Nitrogen-vacancy (NV) centers in diamond and superconducting (SC) flux qubits. The resulting coupling can be used to create a coherent interaction between the spin states of distant NV centers mediated by the flux qubit. Furthermore, the magnetic coupling can be used to achieve a coherent transfer of quantum information between the flux qubit and an ensemble of NV centers. This enables a long-term memory for a SC quantum processor and possibly an interface between SC qubits and light.Comment: Accepted in Phys. Rev. Lett. Updated text and Supplementary Material adde

Low temperature spin diffusion in the one-dimensional quantum O(3)O(3) nonlinear σ\sigma-model

An effective, low temperature, classical model for spin transport in the one-dimensional, gapped, quantum $O(3)$ non-linear $\sigma$-model is developed. Its correlators are obtained by a mapping to a model solved earlier by Jepsen. We obtain universal functions for the ballistic-to-diffusive crossover and the value of the spin diffusion constant, and these are claimed to be exact at low temperatures. Implications for experiments on one-dimensional insulators with a spin gap are noted.Comment: 4 pages including 3 eps-figures, Revte

Herding cats: observing live coding in the wild

After a momentous decade of live coding activities, this paper seeks to explore the practice with the aim of situating it in the history of contemporary arts and music. The article introduces several key points of investigation in live coding research and discusses some examples of how live coding practitioners engage with these points in their system design and performances. In the light of the extremely diverse manifestations of live coding activities, the problem of defining the practice is discussed, and the question raised whether live coding will actually be necessary as an independent category

Entangling many atomic ensembles through laser manipulation

We propose an experimentally feasible scheme to generate Greenberger-Horne-Zeilinger (GHZ) type of maximal entanglement between many atomic ensembles based on laser manipulation and single-photon detection. The scheme, with inherent fault tolerance to the dominant noise and efficient scaling of the efficiency with the number of ensembles, allows to maximally entangle many atomic ensemble within the reach of current technology. Such a maximum entanglement of many ensembles has wide applications in demonstration of quantum nonlocality, high-precision spectroscopy, and quantum information processing.Comment: 4 pages, 1 figur

Coherent control of trapped ions using off-resonant lasers

In this paper we develop a unified framework to study the coherent control of trapped ions subject to state-dependent forces. Taking different limits in our theory, we can reproduce two different designs of a two-qubit quantum gate --the pushing gate [1] and the fast gates based on laser pulses from Ref. [2]--, and propose a new design based on continuous laser beams. We demonstrate how to simulate Ising Hamiltonians in a many ions setup, and how to create highly entangled states and induce squeezing. Finally, in a detailed analysis we identify the physical limits of this technique and study the dependence of errors on the temperature. [1] J.I. Cirac, P. Zoller, Nature, 404, 579, 2000. [2] J.J. Garcia-Ripoll, P. Zoller, J.I. Cirac, PRL 67, 062318, 200

Speed of ion trap quantum information processors

We investigate theoretically the speed limit of quantum gate operations for ion trap quantum information processors. The proposed methods use laser pulses for quantum gates which entangle the electronic and vibrational degrees of freedom of the trapped ions. Two of these methods are studied in detail and for both of them the speed is limited by a combination of the recoil frequency of the relevant electronic transition, and the vibrational frequency in the trap. We have experimentally studied the gate operations below and above this speed limit. In the latter case, the fidelity is reduced, in agreement with our theoretical findings. // Changes: a) error in equ. 24 and table III repaired b) reference Jonathan et al, quant-ph/ 0002092, added (proposes fast quantum gates using the AC-Stark effect)Comment: 10 pages, 4 figure