Loss unlimited quantum communications

Presenting Author: Dawei Ding, Stanford University
Contributing Author(s): Saikat Guha

The maximum rate of quantum-secured communication over an optical channel with two-way authenticated public communication, under an all-powerful quantum adversary, is -log(1-eta) secure bits/mode, where eta is the channel's transmissivity, no matter how high the transmit power is. Since eta = e^{-alpha L} in a length L fiber, and -log(1-eta) ~ 1.44 eta for eta << 1, the rate decays exponentially with L. We propose a reverse-reconciliation-based protocol but with the assumption of a slightly weakened Eve. We assume that Eve's copy of the first round of the reverse public communication is corrupted by a tiny amount of additive noise. However, Eve is still assumed to hear the remainder of the public communication noiselessly, wiretap the transmitted-but-lost photons perfectly, and can do arbitrary collective measurements. We show that with this seemingly inconsequential weakening of the conventional eavesdropping model, Alice and Bob can achieve a private communication rate of (1/2)log(1+ 4 eta N_S) bits/mode on the pure-loss channel using a simple laser-light modulation and homodyne detection, N_S being the mean transmit photon number per mode. The rate of our protocol has no upper limit, regardless of how lossy the channel is, for a high enough transmit power. Our protocol also works with arbitrary i.i.d. noise (not necessarily Gaussian) injected by Eve in every use, and the error probability decays super-exponentially with the block length n.

Read this article online: https://drive.google.com/file/d/1Li4k523S9HeNWEOVLl0VwqBl9ZLYkKfM/view?usp=sharing

(Session 5 : Thursday from 5:00pm - 7:00 pm)


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