  The modern search for extraterrestrial intelligence (SETI) began with the
seminal publications of Cocconi & Morrison (1959) and Schwartz & Townes (1961),
who proposed to search for narrow-band signals in the radio spectrum, and for
optical laser pulses. Over the last six decades, more than one hundred
dedicated search programs have targeted these wavelengths; all with null
results. All of these campaigns searched for classical communications, that is,
for a significant number of photons above a noise threshold; with the
assumption of a pattern encoded in time and/or frequency space. I argue that
future searches should also target quantum communications. They are preferred
over classical communications with regards to security and information
efficiency, and they would have escaped detection in all previous searches. The
measurement of Fock state photons or squeezed light would indicate the
artificiality of a signal. I show that quantum coherence is feasible over
interstellar distances, and explain for the first time how astronomers can
search for quantum transmissions sent by ETI to Earth, using commercially
available telescopes and receiver equipment.
