Quantum communication might be the best way to send messages between star systems – it would even have the bonus of making sure no hostile species are eavesdropping. The scientists who have calculated the viability of this interstellar telephone system have even suggested some ways we can look out for signs aliens are already using it. However, they reject the idea it would make faster than light communication possible.
As its name suggests, quantum communication utilizes the quantum features of light to transmit messages. The dots and dashes of binary code can be indicated through the quantum state of photons, rather than amplitude or frequency. Such communication has been demonstrated over distances of hundreds of kilometers, and even between a communicator on the ground and in a satellite.
Of course, that's just peanuts compared to sending messages to Mars, let alone a distant star – but Professor Arjun Berera and Jaime Calderón-Figueroa of the University of Edinburgh claim that these are possible too, writing in Physical Review D. The obstacles to more distant communications are engineering problems, not fundamental laws of physics, they show.
The main advantage of quantum communication is the amount of information that can be carried efficiently, without needing exceptionally high-powered transmitters. Given the years of delay between sending and receiving signals to other star systems, messages that don't carry a lot of information may be judged not worth the effort, the authors suggest. Ease of encryption is a secondary benefit.
Quantum communication relies on coherence, a consistency in the frequency and waveforms of the transmitted photons. The major challenge it faces is induced decoherence, seen when we try to transmit signals too far through Earth's atmosphere, for example. In space, gravitational fields or interstellar gas could cause decoherence in the photons carrying a message.
While this is a challenge, it can be minimized, Berera and Calderón-Figueroa point out. In particular, they conclude decoherence will be much less of a threat to interstellar quantum messages using the X-ray part of the spectrum, although both sender and receiver might need to be outside an atmosphere. If we want the transmission to occur from a planet itself, the authors show the optical and microwave bands should also work.
In the (very, very) long run, colonies in other star systems may use this to communicate with the home planet. However, the authors are interested in exploring more immediate applications. If extraterrestrial civilizations have decided on the merits of quantum communication, the galaxy could be abuzz with their messages. Perhaps we should be trying to listen in? The paper suggests a potentially attractive wavelength to try.
Distinguishing alien signals from natural sources can be tricky, as shown by surges of excitement when we discover new classes of objects such as pulsars and fast radio bursts. However, quantum teleportation – the form of quantum communication the authors consider most attractive – requires two correlated signals, which would distinguish any alien quantum message from anything natural.
“There would be some amount of guesswork and trial and error in understanding the quantum communication signal that has been received, but all the same one would at least be aware the signal appears to be from an intelligent source,” the authors write.
Although quantum entanglement allows quantum states to be changed instantaneously at a distance (an idea that horrified Einstein), the transmission of information through quantum teleportation is restricted to light speed. If we want an ansible, therefore, we'll have to look elsewhere.