Single charge devices for fundamental metrology

Metallic single electron tunnelling circuits with line widths in the range of 100 nm allow very sensitive electrometers and, in principle, quantum standards for electrical current and capacitance is to be constructed. Superconducting single Cooper pair devices offer the chance to increase the operation speed up to the GHz range. This would improve the precision of the quantum standards by three orders of magnitude. Moreover, single Cooper pair devices are at present the only solid state qubit systems with de-coherence times long enough to analyse the behaviour of quantum states, the ultimate challenge to metrology. PTB has designed a superconducting charge qubit circuit consisting of a Bloch transistor integrated in a superconducting loop which is connected to a resonance RF readout system. Realizing such circuits is a technological challenge. It is experimentally difficult to fulfil the condition that the Josephson coupling energy and the charging energy must be of the same order of magnitude and both much larger than the fluctuation energies. Furthermore the presently used Al/ Al₂O₃ technology for the superconducting circuit preparation is not optimum. It is advantageous to use Nb instead of Al due to its higher energy gap. Few steps to develop integrated circuit technology of Nb are described.