WP 3. Analysis and control of decoherence and errors
In a quantum computer, the information has an analogue nature and the continuity of the manifold of states of a quantum bit prevents the use of only stable states. The challenge is thus to represent quantum information in a way which minimises the influence of both small and large perturbations. It has been shown that error correction schemes can be invented to take care of both type of perturbations. An important condition though for these schemes to work is that the individual bit registers need to be of sufficiently good quality to start with (the ratio of decoherence rate to the operation frequency must approximately be one part in 10 000). Although in theory the decoherence rate of JJ qubits should satisfy this requirement, the best experiments are still at least an order of magnitude away. We plan to investigate the physical mechanisms that determine the decoherence rate of single and coupled qubits in actual superconducting devices made with present day technology to understand to what extent the importance of these mechanisms could be reduced. Another related issue will be the investigation of how the imperfections of circuits will produce errors in the operations of the logic gates. These errors should also occur with a frequency below the 100 ppm level. The experimenal challenge will be to minimise the influence of fluctuations and noise from a number of sources: control systems, electromagnetic environment, other qubits, two-level charge fluctuators in the device material producing 1/f noise. A problem of fundamental importance may be decoherence due to the intrisic noise from the multi-qubit system itself We will develop theory for the dynamics of multi-qubit systems to interpret measurements and improve circuit designs. We will devise measurement protocols in the form of simple quantum algorithms to study the fidelity of different logic operations with various JJ qubit gates and circuits.