16,253 research outputs found
Quantum Entanglement and Communication Complexity
We consider a variation of the multi-party communication complexity scenario
where the parties are supplied with an extra resource: particles in an
entangled quantum state. We show that, although a prior quantum entanglement
cannot be used to simulate a communication channel, it can reduce the
communication complexity of functions in some cases. Specifically, we show
that, for a particular function among three parties (each of which possesses
part of the function's input), a prior quantum entanglement enables them to
learn the value of the function with only three bits of communication occurring
among the parties, whereas, without quantum entanglement, four bits of
communication are necessary. We also show that, for a particular two-party
probabilistic communication complexity problem, quantum entanglement results in
less communication than is required with only classical random correlations
(instead of quantum entanglement). These results are a noteworthy contrast to
the well-known fact that quantum entanglement cannot be used to actually
simulate communication among remote parties.Comment: 10 pages, latex, no figure
Communication Complexity Lower Bounds by Polynomials
The quantum version of communication complexity allows the two communicating
parties to exchange qubits and/or to make use of prior entanglement (shared
EPR-pairs). Some lower bound techniques are available for qubit communication
complexity, but except for the inner product function, no bounds are known for
the model with unlimited prior entanglement. We show that the log-rank lower
bound extends to the strongest model (qubit communication + unlimited prior
entanglement). By relating the rank of the communication matrix to properties
of polynomials, we are able to derive some strong bounds for exact protocols.
In particular, we prove both the "log-rank conjecture" and the polynomial
equivalence of quantum and classical communication complexity for various
classes of functions. We also derive some weaker bounds for bounded-error
quantum protocols.Comment: 16 pages LaTeX, no figures. 2nd version: rewritten and some results
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Gaussian Entanglement Distribution via Satellite
In this work we analyse three quantum communication schemes for the
generation of Gaussian entanglement between two ground stations. Communication
occurs via a satellite over two independent atmospheric fading channels
dominated by turbulence-induced beam wander. In our first scheme the
engineering complexity remains largely on the ground transceivers, with the
satellite acting simply as a reflector. Although the channel state information
of the two atmospheric channels remains unknown in this scheme, the Gaussian
entanglement generation between the ground stations can still be determined. On
the ground, distillation and Gaussification procedures can be applied, leading
to a refined Gaussian entanglement generation rate between the ground stations.
We compare the rates produced by this first scheme with two competing schemes
in which quantum complexity is added to the satellite, thereby illustrating the
trade-off between space-based engineering complexity and the rate of
ground-station entanglement generation.Comment: Closer to published version (to appear in Phys. Rev. A) 13 pages, 6
figure
From communication complexity to an entanglement spread area law in the ground state of gapped local Hamiltonians
In this work, we make a connection between two seemingly different problems.
The first problem involves characterizing the properties of entanglement in the
ground state of gapped local Hamiltonians, which is a central topic in quantum
many-body physics. The second problem is on the quantum communication
complexity of testing bipartite states with EPR assistance, a well-known
question in quantum information theory. We construct a communication protocol
for testing (or measuring) the ground state and use its communication
complexity to reveal a new structural property for the ground state
entanglement. This property, known as the entanglement spread, roughly measures
the ratio between the largest and the smallest Schmidt coefficients across a
cut in the ground state. Our main result shows that gapped ground states
possess limited entanglement spread across any cut, exhibiting an "area law"
behavior. Our result quite generally applies to any interaction graph with an
improved bound for the special case of lattices. This entanglement spread area
law includes interaction graphs constructed in [Aharonov et al., FOCS'14] that
violate a generalized area law for the entanglement entropy. Our construction
also provides evidence for a conjecture in physics by Li and Haldane on the
entanglement spectrum of lattice Hamiltonians [Li and Haldane, PRL'08]. On the
technical side, we use recent advances in Hamiltonian simulation algorithms
along with quantum phase estimation to give a new construction for an
approximate ground space projector (AGSP) over arbitrary interaction graphs.Comment: 29 pages, 1 figur
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