61 research outputs found
Continuous-variable entanglement distillation over a pure loss channel with multiple quantum scissors
Entanglement distillation is a key primitive for distributing high-quality
entanglement between remote locations. Probabilistic noiseless linear
amplification based on the quantum scissors is a candidate for entanglement
distillation from noisy continuous-variable (CV) entangled states. Being a
non-Gaussian operation, quantum scissors is challenging to analyze. We present
a derivation of the non-Gaussian state heralded by multiple quantum scissors in
a pure loss channel with two-mode squeezed vacuum input. We choose the reverse
coherent information (RCI)---a proven lower bound on the distillable
entanglement of a quantum state under one-way local operations and classical
communication (LOCC), as our figure of merit. We evaluate a Gaussian lower
bound on the RCI of the heralded state. We show that it can exceed the
unlimited two-way LOCCassisted direct transmission entanglement distillation
capacity of the pure loss channel. The optimal heralded Gaussian RCI with two
quantum scissors is found to be significantly more than that with a single
quantum scissors, albeit at the cost of decreased success probability. Our
results fortify the possibility of a quantum repeater scheme for CV quantum
states using the quantum scissors.Comment: accepted for publication in Physical Review
R\'enyi generalizations of quantum information measures
Quantum information measures such as the entropy and the mutual information
find applications in physics, e.g., as correlation measures. Generalizing such
measures based on the R\'enyi entropies is expected to enhance their scope in
applications. We prescribe R\'enyi generalizations for any quantum information
measure which consists of a linear combination of von Neumann entropies with
coefficients chosen from the set {-1,0,1}. As examples, we describe R\'enyi
generalizations of the conditional quantum mutual information, some quantum
multipartite information measures, and the topological entanglement entropy.
Among these, we discuss the various properties of the R\'enyi conditional
quantum mutual information and sketch some potential applications. We
conjecture that the proposed R\'enyi conditional quantum mutual informations
are monotone increasing in the R\'enyi parameter, and we have proofs of this
conjecture for some special cases.Comment: 9 pages, related to and extends the results from arXiv:1403.610
Unconstrained distillation capacities of a pure-loss bosonic broadcast channel
Bosonic channels are important in practice as they form a simple model for
free-space or fiber-optic communication. Here we consider a single-sender
two-receiver pure-loss bosonic broadcast channel and determine the
unconstrained capacity region for the distillation of bipartite entanglement
and secret key between the sender and each receiver, whenever they are allowed
arbitrary public classical communication. We show how the state merging
protocol leads to achievable rates in this setting, giving an inner bound on
the capacity region. We also evaluate an outer bound on the region by using the
relative entropy of entanglement and a `reduction by teleportation' technique.
The outer bounds match the inner bounds in the infinite-energy limit, thereby
establishing the unconstrained capacity region for such channels. Our result
could provide a useful benchmark for implementing a broadcasting of
entanglement and secret key through such channels. An important open question
relevant to practice is to determine the capacity region in both this setting
and the single-sender single-receiver case when there is an energy constraint
on the transmitter.Comment: v2: 6 pages, 3 figures, introduction revised, appendix added where
the result is extended to the 1-to-m pure-loss bosonic broadcast channel. v3:
minor revision, typo error correcte
Bounds on entanglement distillation and secret key agreement for quantum broadcast channels
The squashed entanglement of a quantum channel is an additive function of
quantum channels, which finds application as an upper bound on the rate at
which secret key and entanglement can be generated when using a quantum channel
a large number of times in addition to unlimited classical communication. This
quantity has led to an upper bound of on the capacity
of a pure-loss bosonic channel for such a task, where is the average
fraction of photons that make it from the input to the output of the channel.
The purpose of the present paper is to extend these results beyond the
single-sender single-receiver setting to the more general case of a single
sender and multiple receivers (a quantum broadcast channel). We employ
multipartite generalizations of the squashed entanglement to constrain the
rates at which secret key and entanglement can be generated between any subset
of the users of such a channel, along the way developing several new properties
of these measures. We apply our results to the case of a pure-loss broadcast
channel with one sender and two receivers.Comment: 35 pages, 1 figure, accepted for publication in IEEE Transactions on
Information Theor
Unconstrained Capacities of Quantum Key Distribution and Entanglement Distillation for Pure-Loss Bosonic Broadcast Channels
We consider quantum key distribution (QKD) and entanglement distribution
using a single-sender multiple-receiver pure-loss bosonic broadcast channel. We
determine the unconstrained capacity region for the distillation of bipartite
entanglement and secret key between the sender and each receiver, whenever they
are allowed arbitrary public classical communication. A practical implication
of our result is that the capacity region demonstrated drastically improves
upon rates achievable using a naive time-sharing strategy, which has been
employed in previously demonstrated network QKD systems. We show a simple
example of the broadcast QKD protocol overcoming the limit of the
point-to-point strategy. Our result is thus an important step toward opening a
new framework of network channel-based quantum communication technology.Comment: 9 pages, 5 figure
Persistence of nonlocality for bipartite quantum spin systems
Generic forms of the entangled states of two spin-1 (and spin-3/2) particles,
along with the set of appropriate spin observables that together exhibit
maximum nonlocality under the Hardy's nonlocality test are given; the maximum
nonlocality is shown to be 0.09017. It is conjectured that this result holds
good for a system of two spin- particles for all values of . It is also
shown that no maximally entangled state of two spin- particles responds to
the Hardy's nonlocality test.Comment: Revised version in Revtex format, 20 pages, single column; few small
corrections made, two small subsections added, two .eps figures adde
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