Imaging Quantum Fluctuations Near Criticality*

A quantum phase transition (QPT) is a fascinating physical phenomenon in which a system exhibits a dramatic change of behavior as a function of a non-thermal tuning parameter at zero temperature. The quantum fluctuations associated with the quantum critical point (QCP) leave an imprint over an extended temperature range. Here we explore the nature of these fluctuations by performing quantum Monte Carlo calculations of the local diamagnetic susceptibility of a 2D Josephson junction array model in the vicinity of the QPT from a superconductor to an insulator. Our work is supported by scanning SQUID measurements of diamagnetic currents in NbTiN thin film superconductors. We find fluctuations of the diamagnetic response in both space and time, and identify the origin of these fluctuations as Cooper pair tunneling events. As we approach the QCP the lateral dimension of these fluctuations grows and they survive well below the transition temperature, thus demonstrating the contribution from quantum fluctuations in addition to thermal fluctuations. This research provides a unique tool for studying QPTs and has implications for current efforts in quantum information and communication.