  The deconfined quantum critical point (DQCP) represents a paradigm shift in
quantum matter studies, presenting a "beyond Landau" scenario for order--order
transitions. Its experimental realization, however, has remained elusive. Using
high-pressure $^{11}$B NMR measurements on the quantum magnet
SrCu$_2$(BO$_3$)$_2$, we here demonstrate a magnetic-field induced
plaquette-singlet to antiferromagnetic transition above 1.8 GPa at a remarkably
low temperature, $T_{\rm c}\simeq 0.07$ K. First-order signatures of the
transition weaken with increasing pressure, and we observe quantum critical
scaling at the highest pressure, 2.4 GPa. Supported by model calculations, we
suggest that these observations can be explained by a proximate DQCP inducing
critical quantum fluctuations and emergent O(3) symmetry of the order
parameters. Our findings take the DQCP from a theoretical concept to a concrete
experimental platform.
