Abstract
Bénoit Doucot (LPTHE, CNRS, Jussieu, France)
Physical implementation of protected qubits.
One of the most serious problems raised by attempts to build devices based on qubits is the decoherence induced by the residual couplings between these small systems and their environment. Instead of trying to eliminate this quantum noise, an alternative strategy has been proposed some years ago by Kitaev, namely to build quantum systems which would be to a large extent insensitive to external perturbations. This is achieved by a deeply non-local coding of the information in terms of many-body wave-functions. I'll present a recent physical implementation of such protected qubits with small Josephson junction arrays. The experimental results obtained at Rutgers by M. Gershenson and his group show clearly that the protection mechanism works as expected for static perturbations. This system provides a direct simulation of a $Z_2$ lattice gauge theory in a strongly deconfined regime. I will also discuss an alternative way to realize a protected qubit from a quantum Ising-like model with a set of anticommuting non-local symmetries attached to rows and columns of the lattice. This could be implemented in a system of trapped ions. Finally, I will discuss theoretically the effect of dynamical noise on such systems. The main conclusion is that the protection is still effective provided the spectral width of the noise is smaller than the energy gap separating the ground-state doublet with higher excited states.