Could interstellar quantum communications involve Earth or solve the Fermi paradox?

Thus far, the search for extraterrestrial intelligence (SETI) has used strategies based on classical science—listening for radio waves, telescopes watching for optical signals, telescopes in orbit scouring light from the atmospheres of exoplanets, scanning for laser light that might come from aliens. Could a quantum mechanical approach do better?

Latham Boyle says maybe. “It’s interesting that our galaxy (and the sea of cosmic background radiation in which it’s embedded) ‘does’ permit interstellar quantum communication in certain frequency bands,” he says.

A researcher at the Higgs Center for Theoretical Physics at the University of Edinburgh in Scotland, Boyle has investigated the possibility and says, “But whereas our current telescopes are big enough to allow interstellar ‘classical’ communication, interstellar ‘quantum’ communication requires huge telescopes—much bigger than anything we’ve built so far.”

Further, his analysis leads to another potential solution to the Fermi paradox.

For interstellar communication, Boyle wrote “it is natural to ask whether it is also possible to send or receive interstellar quantum communications.” His preprint was released on the arXiv preprint server and has been submitted to a peer-reviewed journal.

The idea is to use entangled qubit pairs, one kept by the sender and the other sent to Earth. A few years ago it was discovered that two quantum particles could retain a quantum coherence over interstellar and even galactic distances, even entangled with one another—somehow linked so that determining a property of one entangled qubit immediately determines that of the other.

This strange connection has already been demonstrated between photons over a thousand kilometers apart, with one on Earth’s surface and the other in a spacecraft orbiting the planet.

A qubit is a unit of quantum information. Quantum mechanics allows, via quantum superposition, for a particle like a photon to be in two states at once, for example, spin up and spin down. Whereas in classical communication, a photon is in a single state, a bit, that is, either spin up or spin down, but not both at the same time. The qubit’s difference makes them more powerful for many applications.

Boyle concentrated on the physical requirements and limitations of sending and detecting such a qubit signal, beginning with the “quantum capacity” of a transmission—the maximum rate at which a quantum communications channel can transmit quantum information.

Much is already known about quantum communications channels from studies and experiments of quantum teleportation, quantum cryptography, quantum entanglement and other quantum phenomena. Protocols based on quantum communication are exponentially faster than those based on classical communication—channels passing one bit at a time from transmitter to receiver—for some tasks.

Using known constraints on the quantum capacity for so-called quantum erasure channels, and properties of the interstellar medium, Boyle was able to obtain two important results: a quantum capacity greater than zero requires the exchanged photons lie within certain allowed frequency bands, and that the effective diameter of both the sending and receiving telescopes must be greater than a value which is proportional to the square root of the photon’s wavelength multiplied by the distance between the telescopes.

According to Boyle’s analysis, a quantum capacity that doesn’t vanish requires the exchanged photons to have a wavelength less than 26.5 cm, mostly to avoid complications with the cosmic microwave background.

Moreover, while classical communications can happen if the receiver receives only a tiny percentage of the photons transmitted (as with radio signals), quantum communications requires that a majority of the photons sent be detected in the receiver’s telescope.

For a ground-based telescope, that diameter would be enormous. The photon’s wavelength must be at least 320 nm to get through Earth’s atmosphere, and given that the distance to our nearest star, Proxima Centauri, is 4.25 light-years, Boyle finds a ground-based telescope would need to be at least 100 kilometers in diameter.

Needless to say, that’s a vast difference from the largest ground-based telescope now under construction, the European Extremely Large Telescope under construction in Chile, which will have a diameter of 0.04 km (40 meters).

“In fact,” Boyle said, “the required telescopes are so large that if the extraterrestrial sender has a big enough transmitting telescope, they can necessarily also see that we have not yet built a sufficiently large receiving telescope, so they would know that it doesn’t yet make sense to communicate with us.”

And that’s maybe we haven’t heard from them, he notes. “In other words, the assumption that extraterrestrials communicate quantum mechanically seems sufficient to explain the Fermi paradox.”

Above the atmosphere, shorter wavelengths could be utilized that would require a smaller telescope, perhaps on the moon or at Earth’s L2 Lagrange Point, but even gamma rays with wavelengths of order 0.001 nm would still require telescope diameters of about 200 meters.

The telescope need not be a single dish—it could be many small dishes packed close together (either on earth or in space), but they would have to close together, “like the cells in a honeycomb,” Boyle said.

A series of relays or quantum repeaters could also be placed on the line between the sender and the target, but for diameters less than 100 meters the repeater telescopes would need to be placed every tenth of an astronomical unit, which includes inside our own solar system. Keeping them in alignment might be a problem (for them at first, not us).

A missing piece is how the receiver would know that an arriving signal is quantum mechanical instead of classical, “viz.” part of an entangled pair, if aliens and humans start off with no prior communication. “I think that answer is at least one additional paper in its own right,” Boyle said.

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