Circuit complexity near critical points

Abstract: We consider the Bose-Hubbard model in two and three spatial dimensions and numerically compute the quantum circuit complexity of the ground state in the Mott insulator and superfluid phases using a mean field approximation with additional quadratic fluctuations. After mapping to a qubit system, the result is given by the complexity associated with a Bogoliubov transformation applied to the reference state taken to be the mean field ground state. In particular, the complexity has peaks at the $O(2)$ critical points where the system can be described by a relativistic quantum field theory. Given that we use a gaussian approximation, near criticality the numerical results agree with a free field theory calculation. To go beyond the gaussian approximation we use general scaling arguments that imply that, as we approach the critical point $t\rightarrow t_c$, there is a non-analytic behavior in the complexity $c_2(t)$ of the form $|c_2(t) - c_2(t_c)| \sim |t-t_c|^{\nu d}$, up to possible logarithmic corrections. Here $d$ is the number of spatial dimensions and $\nu$ is the usual critical exponent for the correlation length $\xi\sim|t-t_c|^{-\nu}$. As a check, for $d=2$ this agrees with the numerical computation if we use the gaussian critical exponent $\nu=\frac{1}{2}$. Finally, using AdS/CFT methods, we study higher dimensional examples and confirm this scaling argument with non-gaussian exponent $\nu$ for strongly interacting theories that have a gravity dual.