Zeptojoule Microwave Calorimetry with Nanosecond Recovery in Graphene
Graphene calorimeters are predicted to combine extreme energy sensitivity with rapid thermal recovery, yet experiments remain far from ideal estimates. Here, we directly calibrate a graphene Josephson-junction microwave calorimeter using nanosecond pulses and independently determine the thermal parameters of the same operating device. At 110 mK, calibrated pulse measurements yield a characteristic switching-energy scale of 18.1 meV (2.90 zJ), while time-resolved two-pulse thermometry gives an intrinsic relaxation time of 19.5 ns. Pulse-width-dependent calorimetry and bolometric thermometry show that the electronic heat capacity is 12.4 times and the electron-phonon coupling is approximately 50 times larger than ideal estimates. A thermal model using these measured parameters predicts 21.2 meV, quantitatively reproducing the observed switching response. These results identify the microscopic origin of the sensitivity gap in graphene calorimeters and establish a self-consistent calibration framework for ultrafast nanoscale thermal detectors.