  The Yang-Lee edge singularity is a quintessential nonunitary critical
phenomenon accompanied by anomalous scaling laws. However, an imaginary
magnetic field involved in this critical phenomenon makes its physical
implementation difficult. By invoking the quantum-classical correspondence to
embed the Yang-Lee edge singularity in a quantum system with an ancilla qubit,
we demonstrate a physical realization of the nonunitary quantum criticality in
an open quantum system. Here the nonunitary criticality is identified with the
singularity at an exceptional point caused by postselection of quantum
measurement.
