I just overheard an engineer saying "There are no two clocks in the world that tell identical time". She was describing a time syncing mechanism to another engineer, but it made me think... In theory, can something large enough to be used as a clock become fully entangled with a copy of itself? For example, could a pair of unstable atomic isotopes be formed that are fully entangled with each other? Or perhaps something based on Neutrino oscillation? Or entangle mesons? If so, it opens up another line of experimentation - since you don't need to query each member of the pair at the same time.