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WP 3. Analysis and control of decoherence and errors

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**Lead contractor: UNIKARL**

**Participants: All teams **

WP 3 deals with controlling decoherence times and qubit couplings in various JJ circuits,
laying the foundation for controlled qubit operations. Also here an important part concens
development of theory for interpreting the experiments, as well as designing new or
improved JJ qubit circuits.
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.