5,994 research outputs found
Random bipartite entanglement from W and W-like states
We describe a protocol for distilling maximally entangled bipartite states
between random pairs of parties from those sharing a tripartite W state, and
show that, rather surprisingly, the total distillation rate (the total number
of EPR pairs distilled per W, irrespective of who shares them) may be done at a
higher rate than distillation of bipartite entanglement between specified pairs
of parties. Specifically, the optimal distillation rate for specified
entanglement for the W has been previously shown to be the asymptotic
entanglement of assistance of 0.92 EPR pairs per W, while our protocol can
asymptotically distill 1 EPR pair per W between random pairs of parties, which
we conjecture to be optimal. We thus demonstrate a tradeoff between the overall
asymptotic rate of EPR distillation and the distribution of final EPR pairs
between parties. We further show that by increasing the number of parties in
the protocol that there exist states with fixed lower-bounded distillable
entanglement for random parties but arbitrarily small distillable entanglement
for specified parties.Comment: 5 pages, 1 figure, RevTeX. v2 - upper bound on random distillation is
expressed more generally and corollaries to the bound added. Minor notation
changes. v3 - further notation changes (Ernd now designated Et), discussion
of finite distillation rounds and single-copy bound on Et added. Theorem
added - relative entropy is shown to be an upper bound to Et for all pure
states. Discussion of W formation from EPRs (previously shown in others'
work) removed. Some addition, removal and reordering of reference
Distillation protocols: Output entanglement and local mutual information
A complementary behavior between local mutual information and average output
entanglement is derived for arbitrary bipartite ensembles. This leads to bounds
on the yield of entanglement in distillation protocols that involve
disinguishing. This bound is saturated in the hashing protocol for
distillation, for Bell-diagonal states.Comment: 4 pages, RevTeX, no figures; v2: presentation improved, results
unchanged; v3: published versio
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
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
Assisted Entanglement Distillation
Motivated by the problem of designing quantum repeaters, we study
entanglement distillation between two parties, Alice and Bob, starting from a
mixed state and with the help of "repeater" stations. To treat the case of a
single repeater, we extend the notion of entanglement of assistance to
arbitrary mixed tripartite states and exhibit a protocol, based on a random
coding strategy, for extracting pure entanglement. The rates achievable by this
protocol formally resemble those achievable if the repeater station could merge
its state to one of Alice and Bob even when such merging is impossible. This
rate is provably better than the hashing bound for sufficiently pure tripartite
states. We also compare our assisted distillation protocol to a hierarchical
strategy consisting of entanglement distillation followed by entanglement
swapping. We demonstrate by the use of a simple example that our random
measurement strategy outperforms hierarchical distillation strategies when the
individual helper stations' states fail to individually factorize into portions
associated specifically with Alice and Bob. Finally, we use these results to
find achievable rates for the more general scenario, where many spatially
separated repeaters help two recipients distill entanglement.Comment: 25 pages, 4 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
On the efficiency of nonlocal gates generation
We propose and study a method for using non-maximally entangled states to
implement probabilistically non-local gates. Unlike distillation-based
protocols, this method does not generate a maximally entangled state at
intermediate stages of the process. As a consequences, the method becomes more
efficient at a certain range of parameters. Gates of the form
with , can be implemented with
nearly unit probability and with vanishingly small entanglement, while for the
distillation-based method the gate is produced with a vanishing success
probability. We also derive an upper bound to the optimal success probability
and show that in the small entanglement limit, the bound is tight.Comment: 6 pages, 3 figure
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