Readout of Superconducting Qubits Based on a Power Sensor

Several important applications are currently emerging from circuit quantum electrodynamics such as a quantum computer that is superior to classical supercomputers for certain tasks. Thermal sensors hold potential for enhancing such devices because they do not add quantum noise and they are smaller, simpler and consume up to six orders of magnitude less power than the frequently used travelling-wave parametric amplifiers. However, despite great progress in the speed and noise levels of thermal sensors, no bolometer has previously met the threshold for circuit quantum electrodynamics, which lies at a time constant of a few hundred nanoseconds and a simultaneous energy resolution of the order of h x 10 GHz (where h is the Planck constant). Here we experimentally demonstrate a bolometer that operates at this threshold, with a noise-equivalent power of 30 zW/Hz^0.5, comparable to the lowest value reported so far, at a thermal time constant two orders of magnitude shorter, at 500 ns. Both of these values are measured directly on the same device, giving an accurate estimation of h x 10 GHz for the calorimetric energy resolution. The minimum observed time constant of 200 ns is well below the dephasing times of roughly 100 microseconds reported for superconducting qubits and matches the timescales of currently used readout schemes. Finally, we report on our latest efforts on the experimental implementation of qubit readout using a bolometer, i.e., a power sensor, which yields a fundamentally different way to measure the quantum properties of microwaves in comparison to voltage amplification.