Recently, the fundamental limits of covert, ie, reliable-yet-undetectable, communication have
been established for general memoryless channels and for lossy-noisy bosonic (quantum)
channels with a quantum-limited adversary. The key import of these results was the square-
root law (SRL) for covert communication, which states that O (√ n) covert bits, but no more,
can be reliably transmitted over n channel uses with O (√ n) bits of secret pre-shared
between communicating parties. Here we prove the achievability of the SRL for a general …

We investigate covert communication over general memoryless classical-quantum channels
with fixed finite-size input alphabets. We show that the square root law (SRL) governs covert
communication in this setting when product of $ n $ input states is used: $ L_ {\rm SRL}\sqrt
{n}+ o (\sqrt {n}) $ covert bits (but no more) can be reliably transmitted in $ n $ uses of
classical-quantum channel, where $ L_ {\rm SRL}> 0$ is a channel-dependent constant that
we call covert capacity. We also show that ensuring covertness requires $ J_ {\rm SRL}\sqrt …