Fig. 5. Simulations of susceptibility fluctuations.

(a-d) Diamagnetic susceptibility (χ)-maps derived from Quantum Monte Carlo simulations of the quantum XY model at different temperatures for g = 0.5, 3, 3.6 and 3.7 respectively. When χ is large in magnitude (white) and uniform, the system has an overall diamagnetic response, indicating a superconducting regime. As temperature increases, fluctuations destroy the superconducting order with the introduction of dark regions that have reduced diamagnetic response. In these maps a small degree of disorder is introduced in order to pin fluctuations (see Methods). As the system approaches the SIT, the range of temperatures over which granularity is observed grows. (e) Standard deviation (STD) of susceptibility fluctuations vs. reduced temperature T/T_c for various values of g. The fluctuation region around T_c is enhanced as the QCP is approached with increasing g. (f) The long time-scale dynamics of currents along imaginary time τ is revealed by calculating the frequency-dependent connected current-current correlation function ${\Lambda }_{\tau \tau }({\omega }_n)=ℱ\mathcal{T}\left[\langle \delta J_{\tau }(\tau )\delta J_{\tau }(\tau \prime )\rangle \right]$ at zero frequency, plotted as a function of g. The correlator shows a peak around g_c ≈ 4, as it picks up the switching between zero and non-zero current states near the QCP due to increased quantum fluctuations.