On Interstellar Quantum Communication and the Fermi Paradox

Since it began \cite{CocconiMorrison}, the search for extraterrestrial intelligence (SETI) has focused on interstellar \emph{classical} communication.

Recently, Berera \cite{Berera:2020rpl} pointed out that, at certain frequencies, photon qubits can retain their quantum coherence over interstellar (and even intergalactic) distances, raising the prospect of interstellar \emph{quantum} communication.

This is an intriguing possibility, since quantum communication permits certain tasks that would be impossible with classical communication, and allow exponential speed-ups for others.

(We suggest some motivations in the interstellar context.)

But quantum coherence alone is not sufficient for quantum communication: here, for the first time, we analyze the \emph{quantum capacity} $Q$ of an interstellar channel.

We point out that, to have non-zero quantum capacity $Q>0$, interstellar communication over a distance $L$ must use wavelengths $\lambda < 26.5\,cm$ (to avoid depolarization by the cosmic microwave background), and \emph{enormous} telescopes of effective diameter $D>0.78\sqrt{\lambda L}$ (to satisfy quantum erasure constraints).

For example, for two telescopes of diameter $D$ on Earth and Proxima Centauri, this implies $D>100\,km$!

This is a technological threshold that remains to be crossed in order for reliable one-way quantum communication to become possible, and suggests a fundamental new resolution of the Fermi paradox.

;Comment: 4+3 pages, 2 figures