240 research outputs found
Proper magnetic fields for nonadiabatic geometric quantum gates in NMR
In a scheme of nonadiabatic purely geometric quantum gates in nuclear
magnetic resonance(NMR) systems we propose proper magnitudes of magnetic fields
that are suitable for an experiment. We impose a natural condition and reduce
the degree of freedom of the magnetic fields to the extent. By varying the
magnetic fields with essentially one-dimensional degree of freedom, any spin
state can acquire arbitrary purely geometric phase
\phi_{g}=-2\pi(1-cos{theta}), 0 < cos{\theta} < 1. This is an essential
ingredient for constructing universal geometric quantum gates.Comment: LaTeX, 4page
Networking quantum networks with minimum cost aggregation
A quantum internet holds promise for accomplishing distributed quantum
sensing and large-scale quantum computer networks, as well as quantum
communication among arbitrary clients all over the globe. The main building
block is efficient distribution of entanglement, entangled bits (ebits), across
quantum networks. This could be achieved by aggregating quantum repeater
protocols. However, the existing protocol is not practical as it requires
point-to-point entanglement generation, the first step of the protocol, not
only to suppress the error, depending on the whole size of the networks, but
also to be run more than necessary. Here we present a practical recipe on how
to aggregate quantum networks in order to present ebits to clients with minimum
cost. This is combined with a conception of concatenation to enable arbitrary
clients to have arbitrary long-distance communication with fixed error across
quantum networks, regardless of the overall size. Our recipe forms the basis of
designing a quantum internet protocol to control a self-organizing large-scale
quantum network.Comment: 7 pages, 3 figure
Tight bound on coherent-state-based entanglement generation over lossy channels
The first stage of the hybrid quantum repeaters is entanglement generation
based on transmission of pulses in coherent states over a lossy channel.
Protocols to make entanglement with only one type of error are favorable for
rendering subsequent entanglement distillation efficient. Here we provide the
tight upper bound on performances of these protocols that is determined only by
the channel loss. In addition, we show that this bound is achievable by
utilizing a proposed protocol [arXiv:0811.3100] composed of a simple
combination of linear optical elements and photon-number-resolving detectors.Comment: 12 pages, 3 figure
Quantum repeaters and computation by a single module
We present a protocol of remote nondestructive parity measurement (RNPM) on a
pair of quantum memories. The protocol works as a single module for key
operations such as entanglement generation, Bell measurement, parity check
measurement, and an elementary gate for extending one-dimensional cluster
states. The RNPM protocol is achieved by a simple combination of devices such
as lasers, optical fibers, beam splitters, and photon detectors. Despite its
simplicity, a quantum repeater composed of RNPM protocols is shown to have a
communication time that scales sub-exponentially with the channel length, and
it can be further equipped with entanglement distillation. With a reduction in
the internal losses, the RNPM protocol can also be used for generating cluster
states toward measurement-based quantum communication.Comment: 7 pages, 4 figure
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