Perhaps They [aka Aliens] are Everywhere? Undetectable Distributed Quantum Computation and Communication for Alien Civilizations can be Established Using Thermal Light from Stars - 2021

Astrogeology, Asteroids, Robots, Space Flight & Extraterrestrial Matters / Extraterrestrial Matters

Rudolph reported in early 2021 that free-space diffraction of photons distributes highly useful entanglement: the receivers of the propagated modes can do a distributed quantum computation using only linear optics and photon counting. The distributed computation requires classical communication between receivers, however, similar to standard measurement-based computation, that communication is of purely random outcomes and so can be indistinguishable from noise.

The speculation in the above title arises from the further observation that the natural way for a circumspect civilization to hide their photonic entanglement distribution is to use the thermal light already being emitted from the various stars they visit. This requires them knowing the number of photons in the modes they have chosen to use, and as such they would need to perform a quantum non-demolition measurement of 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.

The author concluded that in matter-based systems creation, the of useful entanglement involves interaction. For photons it does not - free evolution generates considerable amounts of entanglement deterministically. Photons can generate high dimensional entanglement both by evolving into more modes, and by inhabiting higher occupation numbers within a single mode. (The latter possibility has not been considered here.) Yet photonic quantum computing typically follows routes originally conceived for matter-based systems. These are unnatural and inefficient.

It takes considerable effort to free photons from the qubit tyranny - the obstacle is simply that fault tolerance is very much better understood for stabilizer states, and the qubit versions thereof in particular. It is generally believed (by the physicists on this planet) that to claim a true understanding of quantum theory one of our tasks is an explanation of why useful entanglement is so fragile that the world we experience is classical.

Entanglement, in our experience, only manifests itself when the cleverest of our species capture and protect it appropriately in controlled and delicate experiments. However, once one comprehends how incredibly robust, pervasive and useful photonic entanglement is, the question is flipped - why is it that we did not evolve to make use of it? Why is it that this entanglement did not help us in finding mates or bananas?