Coulomb blockade electrometer based on single Cooper pair tunneling

We have studied the electrometric characteristics of the Bloch transistor, i.e. the structure comprising a double small-capacitance superconductor junction (where both the Josephson and the charging energies, E_J1,2∽E_C1,2≫k_BT) and an adjacent gate electrode polarizing its island. The transistor bias is realized through small-sized high-ohmic resistors (≫R_Q=h/4e²≈6.5 kΩ) to ensure a high electromagnetic impedance of the transistor environment. At low bias current, a coherent flow of single Cooper pairs occurs and the average voltage across the transistor shows a 2e-periodic dependence on the polarization charge Q₀. The sensitivity of such an electrometer has been evaluated and found to be comparable to that of the single-electron counterpart. The device has been fabricated with Al/AlO_x/Al junctions and two miniature on-chip Cr resistors (each of 80 kΩ and 10 μm long) which were located very close to the junctions. We used this device to measure 1/f noise of the background charge and found it to be about 9×10⁻⁴ e/Hz^1/2 at 10 Hz.