  We study disorder operator, defined as a symmetry transformation applied to a
finite region, across a continuous quantum phase transition in $(2+1)d$. We
show analytically that at a conformally-invariant critical point with U(1)
symmetry, the disorder operator with a small U(1) rotation angle defined on a
rectangle region exhibits power-law scaling with the perimeter of the
rectangle. The exponent is proportional to the current central charge of the
critical theory. Such a universal scaling behavior is due to the sharp corners
of the region and we further obtain a general formula for the exponent when the
corner is nearly smooth. To probe the full parameter regime, we carry out
systematic computation of the U(1) disorder parameter in the square lattice
Bose-Hubbard model across the superfluid-insulator transition with large-scale
quantum Monte Carlo simulations, and confirm the presence of the universal
corner correction. The exponent of the corner term determined from numerical
simulations agrees well with the analytical predictions.
