40,351 research outputs found
An Elementary Quantum Network of Single Atoms in Optical Cavities
Quantum networks are distributed quantum many-body systems with tailored
topology and controlled information exchange. They are the backbone of
distributed quantum computing architectures and quantum communication. Here we
present a prototype of such a quantum network based on single atoms embedded in
optical cavities. We show that atom-cavity systems form universal nodes capable
of sending, receiving, storing and releasing photonic quantum information.
Quantum connectivity between nodes is achieved in the conceptually most
fundamental way: by the coherent exchange of a single photon. We demonstrate
the faithful transfer of an atomic quantum state and the creation of
entanglement between two identical nodes in independent laboratories. The
created nonlocal state is manipulated by local qubit rotation. This efficient
cavity-based approach to quantum networking is particularly promising as it
offers a clear perspective for scalability, thus paving the way towards
large-scale quantum networks and their applications.Comment: 8 pages, 5 figure
Universal Interface of TAUOLA Technical and Physics Documentation
Because of their narrow width, tau decays can be well separated from their
production process. Only spin degrees of freedom connect these two parts of the
physics process of interest for high energy collision experiments. In the
following, we present a Monte Carlo algorithm which is based on that property.
The interface supplements events generated by other programs, with tau decays.
Effects of spin, genuine weak corrections or of new physics may be taken into
account at the time when a tau decay is generated and written into an event
record.Comment: 1+44 pages, 17 eps figure
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
Electromagnetic Duality and the Electric Memory Effect
We study large gauge transformations for soft photons in quantum
electrodynamics which, together with the helicity operator, form an ISO(2)
algebra. We show that the two non-compact generators of the ISO(2) algebra
correspond respectively to the residual gauge symmetry and its electromagnetic
dual gauge symmetry that emerge at null infinity. The former is helicity
universal (electric in nature) while the latter is helicity distinguishing
(magnetic in nature). Thus, the conventional large gauge transformation is
electric in nature, and is naturally associated with a scalar potential. We
suggest that the electric Aharonov-Bohm effect is a direct measure for the
electromagnetic memory arising from large gauge transformations.Comment: 16 pages, 1 figure, version to appear in JHE
Near-deterministic quantum teleportation and resource-efficient quantum computation using linear optics and hybrid qubits
We propose a scheme to realize deterministic quantum teleportation using
linear optics and hybrid qubits. It enables one to efficiently perform
teleportation and universal linear-optical gate operations in a simple and
near-deterministic manner using all-optical hybrid entanglement as off-line
resources. Our analysis shows that our new approach can outperforms major
previous ones when considering both the resource requirements and fault
tolerance limits.Comment: 10 pages, 5 figures; extended version, title, abstract and figures
changed, details added, to be published in Phys. Rev.
Spin Electronics and Spin Computation
We review several proposed spintronic devices that can provide new
functionality or improve available functions of electronic devices. In
particular, we discuss a high mobility field effect spin transistor, an
all-metal spin transistor, and our recent proposal of an all-semiconductor spin
transistor and a spin battery. We also address some key issues in
spin-polarized transport, which are relevant to the feasibility and operation
of hybrid semiconductor devices. Finally, we discuss a more radical aspect of
spintronic research--the spin-based quantum computation and quantum information
processing.Comment: 17 pages, 3 figure
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