A single Cooper-pair transistor (SCPT) can work as a fast supercurrent switch. However, a direct combination of an SCPT with other superconducting-electronics circuits is not easy because of the smallness of the switchable supercurrent with it. In this work, scalability of the supercurrent that can be modulated with an SCPT structure is studied using multiple SCPTs connected in parallel having a common gate electrode. It is found that the magnitude of the maximum supercurrent is scaled up almost linearly with the increase in the number of SCPTs, N, and that it is modulated with the gate voltage as in the case for a single SCPT. The modulability, however, decreases with the increase in N, which is demonstrated to be mostly because of the charge-state fluctuations of either islands of the constituent SCPTs caused by the capacitive coupling among them. A practically possible method of improving the modulability is also discussed.
We demonstrate that the supercurrent branch of a single Cooper-pair transistor (SCPT) can be used for sensitive phonon detection. We built a theoretical model of the phonon detection based on the quasiparticle poisoning and demonstrated its operation with a SCPT and a superconducting quantum interference device as a phonon source both composed of mesoscopic Al/AlO_x/Al junctions. The sensitivity of the SCPT detector turned out to be approximately 10^3 times as high as that of the traditional superconducting-tunnel-junction detector.
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