Perhaps they are everywhere? Undetectable distributed quantum computation and communication for alien civilizations can be established using thermal light from stars.
We show 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 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.

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