4,048 research outputs found
Remote preparation of quantum states
Remote state preparation is the variant of quantum state teleportation in
which the sender knows the quantum state to be communicated. The original paper
introducing teleportation established minimal requirements for classical
communication and entanglement but the corresponding limits for remote state
preparation have remained unknown until now: previous work has shown, however,
that it not only requires less classical communication but also gives rise to a
trade-off between these two resources in the appropriate setting. We discuss
this problem from first principles, including the various choices one may
follow in the definitions of the actual resources. Our main result is a general
method of remote state preparation for arbitrary states of many qubits, at a
cost of 1 bit of classical communication and 1 bit of entanglement per qubit
sent. In this "universal" formulation, these ebit and cbit requirements are
shown to be simultaneously optimal by exhibiting a dichotomy. Our protocol then
yields the exact trade-off curve for arbitrary ensembles of pure states and
pure entangled states (including the case of incomplete knowledge of the
ensemble probabilities), based on the recently established quantum-classical
trade-off for quantum data compression. The paper includes an extensive
discussion of our results, including the impact of the choice of model on the
resources, the topic of obliviousness, and an application to private quantum
channels and quantum data hiding.Comment: 21 pages plus 2 figures (eps), revtex4. v2 corrects some errors and
adds obliviousness discussion. v3 has section VI C deleted and various minor
oversights correcte
On Classical Teleportation and Classical Nonlocality
An interesting protocol for classical teleportation of an unknown classical
state was recently suggested by Cohen, and by Gour and Meyer. In that protocol,
Bob can sample from a probability distribution P that is given to Alice, even
if Alice has absolutely no knowledge about P. Pursuing a similar line of
thought, we suggest here a limited form of nonlocality - "classical
nonlocality". Our nonlocality is the (somewhat limited) classical analogue of
the Hughston-Jozsa-Wootters (HJW) quantum nonlocality. The HJW nonlocality
tells us how, for a given density matrix rho, Alice can generate any
rho-ensemble on the North Star. This is done using surprisingly few resources -
one shared entangled state (prepared in advance), one generalized quantum
measurement, and no communication. Similarly, our classical nonlocality
presents how, for a given probability distribution P, Alice can generate any
P-ensemble on the North Star, using only one correlated state (prepared in
advance), one (generalized) classical measurement, and no communication.
It is important to clarify that while the classical teleportation and the
classical non-locality protocols are probably rather insignificant from a
classical information processing point of view, they significantly contribute
to our understanding of what exactly is quantum in their well established and
highly famous quantum analogues.Comment: 8 pages, Version 2 is using the term "quantum remote steering" to
describe HJW idea, and "classical remote steering" is the main new result of
this current paper. Version 2 also has an additional citation (to Gisin's 89
paper
On the capacities of bipartite Hamiltonians and unitary gates
We consider interactions as bidirectional channels. We investigate the
capacities for interaction Hamiltonians and nonlocal unitary gates to generate
entanglement and transmit classical information. We give analytic expressions
for the entanglement generating capacity and entanglement-assisted one-way
classical communication capacity of interactions, and show that these
quantities are additive, so that the asymptotic capacities equal the
corresponding 1-shot capacities. We give general bounds on other capacities,
discuss some examples, and conclude with some open questions.Comment: V3: extensively rewritten. V4: a mistaken reference to a conjecture
by Kraus and Cirac [quant-ph/0011050] removed and a mistake in the order of
authors in Ref. [53] correcte
Optimal Gaussian Entanglement Swapping
We consider entanglement swapping with general mixed two-mode Gaussian states
and calculate the optimal gains for a broad class of such states including
those states most relevant in communication scenarios. We show that for this
class of states, entanglement swapping adds no additional mixedness, that is
the ensemble average output state has the same purity as the input states. This
implies that, by using intermediate entanglement swapping steps, it is, in
principle, possible to distribute entangled two-mode Gaussian states of higher
purity as compared to direct transmission. We then apply the general results on
optimal Gaussian swapping to the problem of quantum communication over a lossy
fiber and demonstrate that, contrary to negative conclusions in the literature,
swapping-based schemes in fact often perform better than direct transmission
for high input squeezing. However, an effective transmission analysis reveals
that the hope for improved performance based on optimal Gaussian entanglement
swapping is spurious since the swapping does not lead to an enhancement of the
effective transmission. This implies that the same or better results can always
be obtained using direct transmission in combination with, in general, less
squeezing.Comment: 10 pages, 2 figures, minor corrections in version 2 with one
reference added (ref.9
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