618 research outputs found
Quantum repeaters with imperfect memories: cost and scalability
Memory dephasing and its impact on the rate of entanglement generation in
quantum repeaters is addressed. For systems that rely on probabilistic schemes
for entanglement distribution and connection, we estimate the maximum
achievable rate per employed memory for our optimized partial nesting protocol.
We show that, for any given distance , the polynomial scaling of rate with
distance can only be achieved if quantum memories with coherence times on the
order of or longer, with being the speed of light in the channel, are
available. The above rate degrades as a power of
with distance when the coherence time .Comment: Extended version with 5 figure
On quantum non-signalling boxes
A classical non-signalling (or causal) box is an operation on classical
bipartite input with classical bipartite output such that no signal can be sent
from a party to the other through the use of the box. The quantum counterpart
of such boxes, i.e. completely positive trace-preserving maps on bipartite
states, though studied in literature, have been investigated less intensively
than classical boxes. We present here some results and remarks about such maps.
In particular, we analyze: the relations among properties as causality,
non-locality and entanglement; the connection between causal and entanglement
breaking maps; the characterization of causal maps in terms of the
classification of states with fixed reductions. We also provide new proofs of
the fact that every non-product unitary transformation is not causal, as well
as for the equivalence of the so-called semicausality and semilocalizability
properties.Comment: 18 pages, 7 figures, revtex
Quantum discord bounds the amount of distributed entanglement
The ability to distribute quantum entanglement is a prerequisite for many
fundamental tests of quantum theory and numerous quantum information protocols.
Two distant parties can increase the amount of entanglement between them by
means of quantum communication encoded in a carrier that is sent from one party
to the other. Intriguingly, entanglement can be increased even when the
exchanged carrier is not entangled with the parties. However, in light of the
defining property of entanglement stating that it cannot increase under
classical communication, the carrier must be quantum. Here we show that, in
general, the increase of relative entropy of entanglement between two remote
parties is bounded by the amount of non-classical correlations of the carrier
with the parties as quantified by the relative entropy of discord. We study
implications of this bound, provide new examples of entanglement distribution
via unentangled states and put further limits on this phenomenon.Comment: 8 pages, 1 figure, RevTeX4; Accepted for publication in Phys. Rev.
Let
Experimental bound entanglement in a four-photon state
Entanglement [1, 2] enables powerful new quantum technologies [3-8], but in
real-world implementations, entangled states are often subject to decoherence
and preparation errors. Entanglement distillation [9, 10] can often counteract
these effects by converting imperfectly entangled states into a smaller number
of maximally entangled states. States that are entangled but cannot be
distilled are called bound entangled [11]. Bound entanglement is central to
many exciting theoretical results in quantum information processing [12-14],
but has thus far not been experimentally realized. A recent claim for
experimental bound entanglement is not supported by their data [15]. Here, we
consider a family of four-qubit Smolin states [16], focusing on a regime where
the bound entanglement is experimentally robust. We encode the state into the
polarization of four photons and show that our state exhibits both entanglement
and undistillability, the two defining properties of bound entanglement. We
then use our state to implement entanglement unlocking, a key feature of Smolin
states [16].Comment: 10 pages, 6 figures. For a simultaneously submitted related work see
arXiv:1005.196
Universal resources for approximate and stochastic measurement-based quantum computation
We investigate which quantum states can serve as universal resources for
approximate and stochastic measurement-based quantum computation, in the sense
that any quantum state can be generated from a given resource by means of
single-qubit (local) operations assisted by classical communication. More
precisely, we consider the approximate and stochastic generation of states,
resulting e.g. from a restriction to finite measurement settings or from
possible imperfections in the resources or local operations. We show that
entanglement-based criteria for universality obtained for the exact,
deterministic case can be lifted to the much more general approximate,
stochastic case, moving from the idealized situation considered in previous
works, to the practically relevant context of non-perfect state preparation. We
find that any entanglement measure fulfilling some basic requirements needs to
reach its maximum value on some element of an approximate, stochastic universal
family of resource states, as the resource size grows. This allows us to rule
out various families of states as being approximate, stochastic universal. We
provide examples of resources that are efficient approximate universal, but not
exact deterministic universal. We also study the robustness of universal
resources for measurement-based quantum computation under realistic assumptions
about the (imperfect) generation and manipulation of entangled states, giving
an explicit expression for the impact that errors made in the preparation of
the resource have on the possibility to use it for universal approximate and
stochastic state preparation. Finally, we discuss the relation between our
entanglement-based criteria and recent results regarding the uselessness of
states with a high degree of geometric entanglement as universal resources.Comment: 17 pages; abstract shortened with respect to the published version to
respect the arXiv limit of 1,920 character
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