Searching for Interstellar Quantum Communications

Abstract

The modern search for extraterrestrial intelligence began with the seminal publications of Cocconi & Morrison and Schwartz & Townes, who proposed searching for narrowband signals in the radio spectrum and optical laser pulses. Over the last six decades, more than 100 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 regard 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.

Viability of quantum communication across interstellar distances

“…In that sense, X-rays may be the best alternative for interstellar quantum communication, although the optical band is also worth exploring in spite of the smaller mean free paths. In [105], following on the observation in [17] for intelligent quantum communication signals to be possible over interstellar distances, a series of tests using available technology were proposed to look for intelligent quantum signals. As such, these could mainly exploit optical wavelenghts for which, as already said, there is limited distance in the interstellar medium for quantum coherence of the photons to be maintained.…”

Viability of quantum communication across interstellar distances

“…In that sense, X-rays may be the best alternative for interstellar quantum communication, although the optical band is also worth exploring in spite of the smaller mean free paths. In [105], following on the observation in [17] for intelligent quantum communication signals to be possible over interstellar distances, a series of tests using available technology were proposed to look for intelligent quantum signals. As such, these could mainly exploit optical wavelenghts for which, as already said, there is limited distance in the interstellar medium for quantum coherence of the photons to be maintained.…”

Artificial Broadcasts as Galactic Populations. II. Comparing Individualist and Collective Bounds on Broadcast Populations in Single Galaxies

“…Thermal noise in the receiver usually overwhelms the wave noise from sources, with a greater noise temperature, but it, too, is the chaotic sum of many microscopic fluctuations. Artificial broadcasts may be coherent (Hippke 2021; see also Appendix B of Cordes et al 1997), but wave noise is still present in the background and in the mutual interference of many broadcasts.…”

“…A photon can be localized to a single mode, but no further. In terms of temporal properties, a mode includes only one polarization and has a bandwidth-duration product T μ B μ = 1 (Yamamoto & Haus 1986;Caves & Drummond 1994;Nityananda 1994;Hippke 2021). Amplitudes add linearly: the amplitude measured in mode μ is…”