Near-Quantum-Limited Operation of a Charge-Sensitive Electrical Amplifier

Here, we report the operation of a superconducting single-electron transistor (S-SET) as a fast and nearly quantum-limited charge-sensitive amplifier. By operating the S-SET such that Cooper pair tunneling is resonant across both of its tunnel junctions, we are able to achieve an excellent charge sensitivity of δq = 1.6 × 10^-6 e/√(Hz), while also minimizing back action. We find that near its optimal operating point, the S-SET can have both a very low noise temperature T_n ≈ 50 mK and a large power gain A_SET >;>; 1. These results indicate that our S-SET operates within a factor of 2 of the quantum limit.