An advanced alien civilization could modify the light coming off of stars in order to communicate across enormous distances, according to a preprint paper published last week by Imperial College London quantum physicist Terry Rudolph.
The idea is that aliens may use entangled photons from various stars to send messages that appear to be random blinking to bystanders, and while this is pure speculation, it is technically conceivable in terms of physics.
Rudolph writes that they've shown that "free-space diffraction of photons distributes highly useful entanglement." Quantum entanglement occurs when two or more particles link up in a certain way: No matter how far apart they are, observations of one of the entangled particles can automatically reveal information about the other entangled particles, and any action taken on one of these particles will always have an impact on the others in the entangled system, per NASA.
Receivers of the propagating modes — how radio signals move from a sending antenna to a receiving antenna — may perform distributed quantum computations using just linear optics and photon counting. While distributed computing necessitates traditional communication between receivers, much like standard measurement-based computation, this communication is based on entirely random outcomes and hence might be confused with noise.
But how does this tie with the possibility of us actually spying on alien conversations whenever we look at the sky? To make things a bit clearer, Rudolf's theory is predicated on the idea that a cautious civilization could "hide their photonic entanglement dispersion by using the thermal light already being emitted by the different starts they visit."
This would necessitate knowing the number of photons in the modes they've chosen to employ, which would also require performing a quantum non-demolition measurement of the photon number. "Because the thermal light they are measuring is diagonal in the number basis even this process can be rendered in principle indiscernible to those of us excluded from the conversation," Rudolf wrote.
Aliens swiftly communicating across star systems does require some complex physics that can be difficult to grasp, but you learn more about Rudolf's theory by reading the study here. If Rudolf's theory were to be accurate, an advanced civilization colonizing the Milky Way while communicating via stars would explain why no evidence of life beyond Earth has ever been discovered.