45 research outputs found
Maximum privacy without coherence, zero-error
We study the possible difference between the quantum and the private capacities of a quantum channel in the zero-error setting. For a family of channels introduced by Leung et al. [Phys. Rev. Lett. 113, 030512 (2014)], we demonstrate an extreme difference: the zero-error quantum capacity is zero, whereas the zero-error private capacity is maximum given the quantum output dimension
Zero-error channel capacity and simulation assisted by non-local correlations
Shannon's theory of zero-error communication is re-examined in the broader
setting of using one classical channel to simulate another exactly, and in the
presence of various resources that are all classes of non-signalling
correlations: Shared randomness, shared entanglement and arbitrary
non-signalling correlations. Specifically, when the channel being simulated is
noiseless, this reduces to the zero-error capacity of the channel, assisted by
the various classes of non-signalling correlations. When the resource channel
is noiseless, it results in the "reverse" problem of simulating a noisy channel
exactly by a noiseless one, assisted by correlations. In both cases, 'one-shot'
separations between the power of the different assisting correlations are
exhibited. The most striking result of this kind is that entanglement can
assist in zero-error communication, in stark contrast to the standard setting
of communicaton with asymptotically vanishing error in which entanglement does
not help at all. In the asymptotic case, shared randomness is shown to be just
as powerful as arbitrary non-signalling correlations for noisy channel
simulation, which is not true for the asymptotic zero-error capacities. For
assistance by arbitrary non-signalling correlations, linear programming
formulas for capacity and simulation are derived, the former being equal (for
channels with non-zero unassisted capacity) to the feedback-assisted zero-error
capacity originally derived by Shannon to upper bound the unassisted zero-error
capacity. Finally, a kind of reversibility between non-signalling-assisted
capacity and simulation is observed, mirroring the famous "reverse Shannon
theorem".Comment: 18 pages, 1 figure. Small changes to text in v2. Removed an
unnecessarily strong requirement in the premise of Theorem 1