5 research outputs found
Toward Photon-Efficient Key Distribution over Optical Channels
This work considers the distribution of a secret key over an optical
(bosonic) channel in the regime of high photon efficiency, i.e., when the
number of secret key bits generated per detected photon is high. While in
principle the photon efficiency is unbounded, there is an inherent tradeoff
between this efficiency and the key generation rate (with respect to the
channel bandwidth). We derive asymptotic expressions for the optimal generation
rates in the photon-efficient limit, and propose schemes that approach these
limits up to certain approximations. The schemes are practical, in the sense
that they use coherent or temporally-entangled optical states and direct
photodetection, all of which are reasonably easy to realize in practice, in
conjunction with off-the-shelf classical codes.Comment: In IEEE Transactions on Information Theory; same version except that
labels are corrected for Schemes S-1, S-2, and S-3, which appear as S-3, S-4,
and S-5 in the Transaction
A refined analysis of the Poisson channel in the high-photon-efficiency regime
We study the discrete-time Poisson channel under the constraint that its
average input power (in photons per channel use) must not exceed some constant
E. We consider the wideband, high-photon-efficiency extreme where E approaches
zero, and where the channel's "dark current" approaches zero proportionally
with E. Improving over a previously obtained first-order capacity
approximation, we derive a refined approximation, which includes the exact
characterization of the second-order term, as well as an asymptotic
characterization of the third-order term with respect to the dark current. We
also show that pulse-position modulation is nearly optimal in this regime.Comment: Revised version to appear in IEEE Transactions on Information Theor