Scientists say that communication based on the effect of quantum entanglement is not only possible, but also suitable for conversations with alien civilizations. And this explains the Fermi Paradox — the absence of signals from aliens.
Possibility of interstellar quantum communication
So far, the search for extraterrestrial intelligence (SETI) has used strategies based on classical science: listening to radio waves, observing optical signals with telescopes, searching for light in the atmospheres of exoplanets with orbiting telescopes, and scanning for laser light that may originate from extraterrestrials. Can a quantum mechanical approach do it better?
Latham Boyle says it does. “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 this 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.”
His analysis then leads to another potential solution to the Fermi paradox.
Using entangled qubit pairs for quantum communication
The idea is to use entangled qubit pairs, one stored at the sender and the other sent to Earth. A few years ago, it was discovered that two quantum particles could maintain quantum coherence over interstellar and even galactic distances, even when entangled with each other — so that determining the property of one entangled qubit immediately determines the property of the other.
This amazing connection has already been demonstrated between photons over a thousand kilometers apart, one on the Earth’s surface and the other in a spacecraft orbiting the planet.
A qubit is a unit of quantum information. Quantum mechanics allows particles such as a photon, due to quantum superposition, to be in two states at the same time, such as spinning up and spinning down. Whereas in classical communication, a photon is in one state, a bit, that is either spin up or spin down, but not both at the same time. This difference in qubits makes them more powerful for many applications.
Limitation of the theory for quantum communication
There is already much known about quantum communication channels from research and experiments on quantum teleportation, quantum cryptography, quantum entanglement, and other quantum phenomena. Protocols based on quantum communication are exponentially faster than protocols based on classical communication channels transmitting one bit at a time from transmitter to receiver-for some tasks.
Using known constraints on quantum bandwidth for so-called quantum erasure channels and properties of the interstellar medium, Boyle was able to obtain two important results: quantum bandwidth greater than zero requires that the photons exchanged lie within certain allowed frequency bands, and that the effective diameter of both the transmitting and receiving telescopes be greater than a value proportional to the square root of the photon’s wavelength multiplied by the distance between the telescopes.
According to Boyle’s analysis, quantum ability that doesn’t disappear requires that the photons exchanged have a wavelength of less than 26.5 centimeters, mainly to avoid complications from the cosmic microwave background.
Moreover, while classical communication can occur if the receiver receives only a tiny percentage of the transmitted photons (as in the case of radio signals), quantum communication requires that the majority of the photons sent be detected by the receiver’s telescope.
Requirements for ultra-large telescopes in quantum communications
For a ground-based telescope, that diameter was huge. The wavelength of a photon 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 believes that a ground-based telescope would need to be at least 100 kilometers in diameter.
It is clear that this is very different from the largest ground-based telescope currently under construction in Chile, the European Extremely Large Telescope, which will be 0.04 km (40 m) in diameter.
“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.”
Technological complexities of quantum communication realization
Outside the atmosphere, shorter wavelengths could be used, which would require a smaller telescope, perhaps on the Moon or at the L2 Lagrange Point, but even gamma rays with wavelengths on the order of 0.001 nm would still require a telescope diameter of about 200 m.
The telescope doesn’t have to be one dish — it can be many small dishes close together (on Earth or in space), but they need to be close together like cells in a honeycomb, Boyle says.
A series of relays or quantum repeaters could also be placed on the line between the sender and the target, but for diameters smaller than 100 meters, repeater telescopes would need to be placed every tenth of an astronomical unit, which includes our Solar System. Retaining them on the same line could be a problem (for them first, not us).
The missing part is how the receiver knows that the incoming signal is quantum mechanical and not classical, i.e. part of an entangled pair, if aliens and humans have no prior communication. “I think that answer is at least one additional paper in its own right,” Boyle said.
According to phys.org
