141,302 research outputs found
The one-way quantum computer - a non-network model of quantum computation
A one-way quantum computer works by only performing a sequence of one-qubit
measurements on a particular entangled multi-qubit state, the cluster state. No
non-local operations are required in the process of computation. Any quantum
logic network can be simulated on the one-way quantum computer. On the other
hand, the network model of quantum computation cannot explain all ways of
processing quantum information possible with the one-way quantum computer. In
this paper, two examples of the non-network character of the one-way quantum
computer are given. First, circuits in the Clifford group can be performed in a
single time step. Second, the realisation of a particular circuit --the
bit-reversal gate-- on the one-way quantum computer has no network
interpretation. (Submitted to J. Mod. Opt, Gdansk ESF QIT conference issue.)Comment: 7 pages, 3 figure
Modularization of multi-qubit controlled phase gate and its NMR implementation
Quantum circuit network is a set of circuits that implements a certain
computation task. Being at the center of the quantum circuit network, the
multi-qubit controlled phase shift is one of the most important quantum gates.
In this paper, we apply the method of modular structuring in classical computer
architecture to quantum computer and give a recursive realization of the
multi-qubit phase gate. This realization of the controlled phase shift gate is
convenient in realizing certain quantum algorithms. We have experimentally
implemented this modularized multi-qubit controlled phase gate in a three qubit
nuclear magnetic resonance quantum system. The network is demonstrated
experimentally using line selective pulses in nuclear magnetic resonance
technique. The procedure has the advantage of being simple and easy to
implement.Comment: to appear in Journal of Optics B: Quantum and Semiclassical Optic
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
Distributed quantum sensing in a continuous variable entangled network
Networking plays a ubiquitous role in quantum technology. It is an integral
part of quantum communication and has significant potential for upscaling
quantum computer technologies that are otherwise not scalable. Recently, it was
realized that sensing of multiple spatially distributed parameters may also
benefit from an entangled quantum network. Here we experimentally demonstrate
how sensing of an averaged phase shift among four distributed nodes benefits
from an entangled quantum network. Using a four-mode entangled continuous
variable (CV) state, we demonstrate deterministic quantum phase sensing with a
precision beyond what is attainable with separable probes. The techniques
behind this result can have direct applications in a number of primitives
ranging from biological imaging to quantum networks of atomic clocks
Complex Networks from Classical to Quantum
Recent progress in applying complex network theory to problems in quantum
information has resulted in a beneficial crossover. Complex network methods
have successfully been applied to transport and entanglement models while
information physics is setting the stage for a theory of complex systems with
quantum information-inspired methods. Novel quantum induced effects have been
predicted in random graphs---where edges represent entangled links---and
quantum computer algorithms have been proposed to offer enhancement for several
network problems. Here we review the results at the cutting edge, pinpointing
the similarities and the differences found at the intersection of these two
fields.Comment: 12 pages, 4 figures, REVTeX 4-1, accepted versio
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