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arXiv:2408.02445

On Interstellar Quantum Communication and the Fermi Paradox

Latham A. Boyle

Published Aug 5, 2024• 0 citations
arXivHTMLPDF

Abstract

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 $λ< 26.5\,cm$ (to avoid depolarization by the cosmic microwave background), and \emph{enormous} telescopes of effective diameter $D>0.78\sqrt{λL}$ (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.

DOI
10.48550/arxiv.2408.02445

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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} QQQ of an interstellar channel. We point out that, to have non-zero quantum capacity Q>0Q>0Q>0, interstellar communication over a distance LLL must use wavelengths λ<26.5 cmλ< 26.5\,cmλ<26.5cm (to avoid depolarization by the cosmic microwave background), and \emph{enormous} telescopes of effective diameter D>0.78λLD>0.78\sqrt{λL}D>0.78λL​ (to satisfy quantum erasure constraints). For example, for two telescopes of diameter DDD on Earth and Proxima Centauri, this implies D>100 kmD>100\,kmD>100km! 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.
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.
The possibility of achieving quantum communication using photons across interstellar distances is examined. For this, different factors are considered that could induce decoherence of photons, including the gravitational field of astrophysical bodies, the particle content in the interstellar medium, and the more local environment of the Solar System. The x-ray region of the spectrum is identified as the prime candidate to establish a quantum communication channel, although the optical and microwave bands could also enable communication across large distances. Finally, we discuss what could be expected from a quantum signal emitted by an extraterrestrial civilization, as well as the challenges for the receiver end of the channel to identify and interpret such signals.
Quantum coherence could be sustained up to interstellar distances. It is shown that the photon mean free path in certain regions of the electromagnetic spectrum, such as within the radio or x-ray ranges, could allow sustaining of the quantum state of a photon up to galactic distances. Therefore processes involving quantum entanglement, such as quantum teleportation, could be realized over very long distances in the Milky Way or other galaxies. This is of fundamental interest and offers a new direction in the role of quantum mechanics. Some limited applications of this observation are discussed.
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Since his famous discussions with Niels Bohr, Albert Einstein considered quantum entanglement (QE) as a spooky action at a distance, due to the violation of locality necessary so that two entangled particles can share this effect in an instantaneous way despite of being at a great distance from each other, i.e., not being local. In other words, a notification about the change of state in one of them could only cover the space that separates them at a speed superior to that of light, which we know is impossible according to the Theory of Relativity (TR). Besides, QE faces directly the two main pillars of Physics: TR and Quantum Theory (QT); becoming the bone of contention between both theories. Quite the contrary, in this work we will see that QE is the meeting point of both theories, so much so, that QE could be considered as the cornerstone of the Theory of Everything (TOE). Consistent with this, the entangled particles retain certain autonomy unknown to date, and in addition, they will have relativistically entangled alter-egos, which will hold the effect even when the original entangled particles are extremely separated from each other. These alter-egos can be considered as black holes (with their corresponding temperature and entropy) giving rise to a wormhole. This is possible since the locality dilates according to the Lorentz factor, which is accompanied by a contraction in the effective channel and in the temporal delay to cross that channel. All this takes place while space-time is curved (hyperspace) to generate the wormhole between both black holes. In other words, QE is a local effect of infinite range so it does not outpace the speed of light, and therefore QT is a complete theory that does not clash with TR. Finally, everything we have said has direct consequences on the link between entangled particles from the point of view of quantum communications in terms of the channel and its bandwidth, latency, capacity, robustness and security.
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