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

So far, the search for extraterrestrial intelligence (SETI) has used strategies based on classical science: listening for radio waves, telescopes looking for optical signals, orbiting telescopes to analyze light from the atmospheres of exoplanets, scanning for laser light that might come from aliens. Could a quantum mechanical approach do better?

Latham Boyle responds that this may be the case. “It is interesting to note that our galaxy (and the sea of ​​cosmic background radiation in which it is embedded) allows interstellar quantum communication in certain frequency bands,” he says.

Boyle, a researcher at the Higgs Centre for Theoretical Physics at the University of Edinburgh in Scotland, has studied this possibility and says: “But while our current telescopes are large enough to enable ‘classical’ interstellar communication, ‘quantum’ interstellar communication requires enormous telescopes, far larger than anything we have built so far.”

Moreover, 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 posted on the website arXiv preprint server and has been submitted to a peer-reviewed journal.

The idea is to use pairs of entangled qubits, one held by the transmitter and the other sent to Earth. A few years ago, it was discovered that two quantum particles can maintain quantum coherence over interstellar and even galactic distances, even when entangled with each other – linked in some way so that determining a property of one entangled qubit immediately determines that of the other.

This strange connection has already been demonstrated between photons more than a thousand kilometers apart, one on the surface of the Earth and the other in a spacecraft orbiting the planet.

A qubit is a unit of quantum information. Quantum mechanics allows, through quantum superposition, 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 only one state, a bit, that is, either spin up or spin down, but not both at the same time. The difference in qubits makes them more powerful for many applications.

Boyle focused on the physical requirements and limitations of sending and detecting such a qubit signal, starting with the “quantum capacity” of a transmission—that is, the maximum rate at which a quantum communication channel can transmit quantum information.

Quantum communication channels are already well known from studies and experiments on 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 transmitting one bit at a time from the transmitter to the receiver) for some tasks.

Using known constraints on the quantum capacity of quantum erasure channels and the properties of the interstellar medium, Boyle was able to obtain two important results: a quantum capacity greater than zero requires that the exchanged photons lie within certain allowed frequency bands, and that the effective diameter of the transmitting and receiving telescopes must be greater than a value proportional to the square root of the photon wavelength times the distance between the telescopes.

According to Boyle’s analysis, a non-vanishing quantum capacity requires that the exchanged photons have a wavelength less than 26.5 cm, mainly to avoid complications with the cosmic microwave background.

Furthermore, while classical communications can occur if the receiver receives only a tiny percentage of the transmitted photons (as in the case of radio signals), quantum communications require that a majority of the photons sent are 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 pass through Earth’s atmosphere, and given that the distance to our nearest star, Proxima Centauri, is 4.25 light-years, Boyle estimates that a ground-based telescope would have to be at least 100 kilometers in diameter.

Needless to say, this is a big difference from the largest ground-based telescope currently 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 telescopes needed are so large that if the alien sender has a large enough transmitting telescope, he can necessarily also see that we have not yet built a large enough receiving telescope, so he would know that it does not yet make sense to communicate with us.”

And that may be what we haven’t heard about, he notes. “In other words, the hypothesis that aliens communicate through quantum mechanics seems sufficient to explain the Fermi paradox.”

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

The telescope doesn’t need to be a single dish: it could be made up of several smaller dishes placed close together (on Earth or in space), but they should be close together, “like the cells of a honeycomb,” Boyle explained.

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

There’s one missing piece: How would the receiver know that an incoming signal is quantum mechanical rather than classical, that is, part of an entangled pair, if the aliens and humans start out without prior communication? “I think that answer would deserve at least a separate paper of its own,” Boyle said.

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