In the circuit QED architecture, the large degree of control over the designed system parameters and their tunability during experiments are extremely attractive for quantum computation (A. Blais et al.). This system provides an ideal setting to reduce the coupling of the quantum two-level system, i.e. the qubit, to the environment in order to reduce decoherence, while at the same time allowing for the strong and controlled coupling to a single degree of freedom, i.e. a single mode of a quantum electromagnetic field, to both control and read-out the qubit state. At Yale, we have recently demonstrated (D. Schuster et al.) that the off-resonant strong coupling between the qubit and the resonator can be used to perform a quantum non-demolition (QND) dispersive read-out (A. Blais et al.) of the qubit state. The effective isolation of the qubit from its environment allowed us to measure one of the longest coherence times of any charge qubit investigated so far (D. Schuster et al.). These properties allow for time-resolved control and readout of the qubit state, which we have demonstrated on the single qubit level (A. Wallraff et al.). In our work at ETH, the coherence of charge qubits will be investigated using advanced microwave pulse sequences for performing Ramsey interferometry and spin-echo measurements. Such experiments can be performed efficiently using the state of the art microwave techniques and large bandwidth data acquisition systems that we have developed at Yale for the circuit QED experiments. In the quantum device lab lab we will identify sources of decoherence with the aim to eliminate them to a level that error correction becomes feasible. Full control of the qubit state will be demonstrated. We will realize coupling of multiple qubits using the resonator as a coupling bus (A. Blais et al.) which allows for the implementation of gates similar to the Cirac-Zoller gates as demonstrated in ion-traps. Different implementations of two-qubit logic and simple gate operations will be explored. This will allow for the first simple quantum algorithms to be implemented in a solid state architecture.

The circuit QED qubit read-out is predicted to be able to reach high fidelity and single-shot capability (A. Blais et al.), which will allow us to investigate correlations in multi-qubit systems, e.g. to determine the fidelity of an entangled state or to perform a measurement of Bell's inequality. The entanglement of photons, with qubits, the transfer of entanglement between different entities, and the use of microwave photons as flying qubits mediating couplings over large distances, is going to be studied in my lab. These attractive features make this architecture a prominent candidate for addressing quantum computation problems beyond the single qubit level using techniques that are also applicable to other approaches for solid state quantum computation.