  It is commonly expected that quantum theory is universal, in that it
describes the world at all scales. Yet, quantum effects at the macroscopic
scale continue to elude our experimental observation. This fact is commonly
attributed to decoherence processes affecting systems of sufficiently large
number of constituent subsystems leading to an effective macro-scale beyond
which a quantum description of the whole system becomes superfluous. Here, we
show both theoretically and experimentally that the existence of such a scale
is unjustifiable from an information-theoretic perspective. We introduce a
variant of the Wigner's friend experiment in which a multiparticle quantum
system is observed by the friend. The friend undergoes rapid decoherence
through her interactions with the environment. In the usual version of this
thought experiment, decoherence removes the need for Wigner to treat her as a
quantum system. However, for our variant we prove theoretically and observe
experimentally that there exist partitions of the subsystems in which the
friend is entangled with one of the particles in assistance with the other
particle, as observed by Wigner. Importantly, we show that the friend is
indispensable for the entanglement to be observed. Hence Wigner is compelled to
treat the friend as part of a larger quantum system. By analyzing our scenario
in the context of a quantum key distribution protocol, we show that a
semi-classical description of the experiment is suboptimal for security
analysis, highlighting the significance of the quantum description of the
friend.
