  The Yang-Lee edge singularity was originally studied from the standpoint of
mathematical foundations of phase transitions, and its physical demonstration
has been of active interest both theoretically and experimentally. However, the
presence of an imaginary magnetic field in the Yang-Lee edge singularity has
made it challenging to develop a direct observation of the anomalous scaling
with negative scaling dimension associated with this critical phenomenon. We
experimentally implement an imaginary magnetic field and demonstrate the
Yang-Lee edge singularity through a nonunitary evolution governed by a
non-Hermitian Hamiltonian in an open quantum system, where a classical system
is mapped to a quantum system via the equivalent canonical partition function.
In particular, we directly observe the partition function in our experiment
using heralded single photons. The nonunitary quantum criticality is identified
with the singularity at an exceptional point. We also demonstrate
unconventional scaling laws for the finite-temperature dynamics unique to
quantum systems.
