Noisy Feedback and Loss Unlimited Private Communication
Dawei Ding, Saikat Guha

TL;DR
This paper demonstrates that introducing a small amount of noise into Eve's feedback channel dramatically increases the private communication capacity over bosonic channels, potentially making it unbounded with respect to input photon number.
Contribution
The authors show that a slight noise in Eve's feedback can transform the private capacity from a limited to an unbounded rate, highlighting the importance of physical assumptions in quantum communication models.
Findings
Capacity becomes unbounded with input photon number when Eve's feedback is slightly noisy.
The protocol remains effective under arbitrary i.i.d. noise injected by Eve.
Error probability decays super-exponentially with blocklength.
Abstract
Alice is transmitting a private message to Bob across a bosonic wiretap channel with the help of a public feedback channel to which all parties, including the fully-quantum equipped Eve, have completely noiseless access. We find that by altering the model such that Eve's copy of the initial round of feedback is corrupted by an iota of noise, one step towards physical relevance, the capacity can be increased dramatically. It is known that the private capacity with respect to the original model for a pure-loss bosonic channel is at most bits per mode, where is the transmissivity, in the limit of infinite input photon number. This is a very pessimistic result as there is a finite rate limit even with an arbitrarily large number of input photons. We refer to this as a loss limited rate. However, in our altered model we find that we can achieve a rate of $(1/2) \log(1…
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