  Ergodic quantum many-body systems undergoing unitary dynamics evolve towards
increasingly entangled states characterized by an extensive scaling of
entanglement entropy with system volume. At the other extreme, quantum systems
repeatedly measured may be stabilized in a measurement eigenstate, a phenomenon
known as the quantum Zeno effect. Recently, the intermediate regime in which
unitary evolution is interspersed with quantum measurements has become of
interest. Numerical studies have reported the existence of distinct phases
characterized by volume- and area-law entanglement entropy scaling for
infrequent and frequent measurement rates, respectively, separated by a
critical measurement rate. The experimental investigation of these dynamic
quantum phases of matter on near-term quantum hardware is challenging due to
the need for repeated high-fidelity mid-circuit measurements and fine control
over the evolving unitaries. Here, we report the realization of a
measurement-induced entanglement transition on superconducting quantum
processors with mid-circuit readout capability. We directly observe extensive
and sub-extensive scaling of entanglement entropy in the volume- and area-law
phases, respectively, by varying the rate of projective measurements. We
further demonstrate phenomenological critical behavior of the transition by
performing a data collapse for different system sizes. Our work paves the way
for the use of mid-circuit measurement as an effective resource for quantum
simulation on near-term quantum computers, for instance by facilitating the
study of dynamic and long-range entangled quantum phases.
