3,242 research outputs found
Applications of atomic ensembles in distributed quantum computing
Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes - stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium - an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single step Q-qubit GHZ states with success probability p(success) similar to eta(Q/2), where eta is the combined detection and source efficiency. This is signifcantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer
Deterministic atom-light quantum interface
The notion of an atom-light quantum interface has been developed in the past
decade, to a large extent due to demands within the new field of quantum
information processing and communication. A promising type of such interface
using large atomic ensembles has emerged in the past several years. In this
article we review this area of research with a special emphasis on
deterministic high fidelity quantum information protocols. Two recent
experiments, entanglement of distant atomic objects and quantum memory for
light are described in detail.Comment: 50 pages (bookstyle) 15 graphs, to be published in "Advances in
Atomic, Molecular, and Optical Physics" Vol. 54. (2006)(Some of the graphs
here have lower resolution than in the version to be published
Enhancing quantum transduction via long-range waveguide mediated interactions between quantum emitters
Efficient transduction of electromagnetic signals between different frequency
scales is an essential ingredient for modern communication technologies as well
as for the emergent field of quantum information processing. Recent advances in
waveguide photonics have enabled a breakthrough in light-matter coupling, where
individual two-level emitters are strongly coupled to individual photons. Here
we propose a scheme which exploits this coupling to boost the performance of
transducers between low-frequency signals and optical fields operating at the
level of individual photons. Specifically, we demonstrate how to engineer the
interaction between quantum dots in waveguides to enable efficient transduction
of electric fields coupled to quantum dots. Owing to the scalability and
integrability of the solid-state platform, our transducer can potentially
become a key building block of a quantum internet node. To demonstrate this, we
show how it can be used as a coherent quantum interface between optical photons
and a two-level system like a superconducting qubit.Comment: The maintext has 6 pages, two column and 4 figure
Single-Photon Pulse Induced Transient Entanglement Force
We show that a single photon pulse (SPP) incident on two interacting
two-level atoms induces a transient entanglement force between them. After
absorption of a multi-mode Fock state pulse, the time-dependent atomic
interaction mediated by the vacuum fluctuations changes from the van der Waals
interaction to the resonant dipole-dipole interaction (RDDI). We explicitly
show that the RDDI force induced by the SPP fundamentally arises from the
two-body transient entanglement between the atoms. This SPP induced
entanglement force can be continuously tuned from being repulsive to attractive
by varying the polarization of the pulse. We further demonstrate that the
entanglement force can be enhanced by more than three orders of magnitude if
the atomic interactions are mediated by graphene plasmons. These results
demonstrate the potential of shaped SPPs as a powerful tool to manipulate this
entanglement force and also provides a new approach to witness transient
atom-atom entanglement.Comment: 5 pages, 5 figures and a supplementary materia
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