Albert Einstein: Jim Al Khalili explains quantum physics

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Scientists engaged in the Search for Extraterrestrial Intelligence (SETI) normally look for signals coming to Earth via classical communication channels.

However, theoretical physicist Professor Arjun Berera told Express.co.uk, it is quite possible that alien civilizations may have progressed to more advanced forms of communication — and we should be looking for signs of a quantum approach.

He said: “If you think about how communication is evolving on Earth, in 50 years time probably classical communication will be completely gone and everything will be quantum.

“If you think about it sociologically, then, in this extraterrestrial world maybe they don’t even have any more classical communication — maybe they’ll just use the technology we have.”

The advantage of sending messages via quantum communication channels is that such can allow for significantly more information to be transmitted.

Rather than sending information as classical bits, with each having a value of either zero or one, information can be sent as a quantum bit, or “qubit”.

Qubits can represent one, or zero, or — crucially — as a quantum superposition of these two states.

In this way, more data can inherently be transmitted via quantum states than would be possible classically.

In their study, Prof. Berera and his colleague Jaime Calderón-Figueroa set out to determine if it would even be possible to use photon-based quantum communication over interstellar distances in the first place.

For such a signal to work over the vast distances involved, the photons used would need to remain “coherent” — that is, to remain in a quantum state.

However, both gravity and potential interactions with other particles have been proposed as capable of causing decoherence.

This would collapse the quantum state of the photons and ruin the signal being sent.

Regarding the influence of gravity, first, the team made the case that, at least for photons, “there would be no decoherence induced by gravitational fields”.

Gravity could affect communication by causing a loss of its fidelity, or quality, however — thanks to a relativistic effect dubbed “Wigner rotation” that shifts the signal’s phase.

The duo note that, up to a certain point, the receiver of such a distorted signal should be able to compensate for the phase shift if they knew where the message came from.

They calculated, for example, that a photon under the influence of the Sun’s gravitational field that came no closer to the star than the orbit of planet Mercury would be able to travel a whopping 127 light years before it became impossible to reconstruct the original signal.

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As the researchers explained: “Thus, the photon could travel a considerable portion of the Milky Way before the limit is violated.

“Moreover, such distance is comfortably larger than the distance to the closest system of exoplanets, Proxima Centauri, which is at 1.3 parsecs from us.”

Next, the researchers turned their attention to the potential for interactions between the photons being used to carry a quantum signal and stray electrons, atoms and photons out in the depths of space.

Physicists call the distance a particle can travel before changing direction, energy, or other properties — such as via a collision with another particle — the “mean free path”.

The team’s calculations indicated that the typical mean free path distance for a quantum signal photon was much longer than the observable universe, meaning that the risk of signal decoherence is likely minimal.

Furthermore, they noted, X-ray photons in particular have both longer mean free paths and are also less susceptible to interference from powerful magnetic fields.

This would make them best suited to transmit a quantum signal — and potentially something to consider in the ongoing search for extraterrestrial intelligence.

The full findings of the study were published in the journal Physical Review D.

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