Quantum Bits

With support from extensive theoretical calculations, it was suggested by a German group that superconducting Coulomb blockade devices would be very well suited as qubits; the basic building block for quantum computers. The Coulomb blockade devices have the very important advantage that they can be easily integrated to larger systems, since they are based on the microelectronics fabrication technology, which is very well advanced. These systems are macroscopic and can offer a macroscopically coherent quantum state from superconductivity. Until 1999 this concept was only theoretical, since quantum coherence had never been observed in this kind of macroscopic system. In a ground-breaking experiment, a Japanese group from NEC demonstrated macroscopic quantum coherence in a so-called single Cooper-pair box. This result was published in Nature in late April of that year. The combination of these two very impor-tant results shows that it would be possible to implement the basic qubit operations in a Coulomb-blockade-based device. The decoherence time of a single qubit has been measured to be greater than nanoseconds, but the theory predicts a lifetime of microseconds. With gate switching rates of picoseconds, this gives us the potential for tens of thousands of operations per coherence time.

During the last few years an effort to make Coulomb blockade devices has been successfully pursued at the Micro-Device Lab (MDL) of JPL. These transistors consist of extremely small tunnel junctions with areas of the order of 70x70 nm^2. The MDL lab has excellent nanofabrication facilities, which are well suited for this type of research. Several single electron transistors have been fabricated and tested. A charge sensitivity of one ten-thousandth electronic charges per root-Hertz has been demonstrated. At JPL, we have fabricated single Cooper-pair boxes in close proximity to a single electron transistor (SET); see above figure. We are in the process of testing the properties of the qubit with a radio frequency (RF) technique. Prof. Per Delsing, who is leading the group at Chalmers and has done research in the field of Coulomb blockade for 15 years, has taken part in this JPL effort and is involved on a consulting basis also in the proposed project.