Abstract

The Cooper-pair transistor (CPT) is a three-terminal superconducting device composed of a mesoscale superconducting island connected to drain and source leads via ultrasmall (~100 nm) Josephson junctions (see Figure 16.1). Due to its small size, the energy to add a single Cooper pair to the island can be large compared with the typical temperatures at which similar quantum circuits are operated, T < 100 mK. Because of this, the transport properties of the device can be strongly dependent on the polarization charge presented by a capacitively coupled gate electrode and operated as a transistor with high input impedance. As an electrometer, it is useful as it can be operated with very little dissipation and should minimally influence the systems that it is measuring. As we will see, its similarity to another quantum circuit, the Cooper-pair box (CPB), marks it as a useful device to study decoherence in superconducting quantum bits (qubits) due to unpaired electrons.