Circuit QED for Quantum Information Processing

During the last decades enormous progress in the abilities to manipulate atomic, molecular and solid-state quantum systems has been made attesting to the beginning of a new era in science and technology. In particular, information technology is shaped by the research and development of circuits on constantly smaller scales where quantum effects already play a role. Eventually, having coherent control over individual two-level quantum objects (quantum bits) the vision of quantum computation allowing for an efficient solution of specific algorithmic problems comes into reach. In particular, solid-state implementations of quantum bits based on superconducting electronics seem promising due to the availability of standard fabrication processes borrowed from existing integrated circuit technology. This implies also potential for scalability, i.e. the smooth migration from single to multi-qubit architecture, as an important prerequisite for doing larger-scale quantum computation. Recently, further progress has been made by coupling superconducting qubits to 1D microwave transmission line resonators. This architecture -- known as Circuit Quantum Electrodynamics - provides the possibility to implement fast manipulations of the qubit by the coherent interaction between qubit and single microwave photons while effectively shielding the qubits from the electromagnetic environment. Typical coherence times of several microseconds are contrasted by characteristic operation times in the nanosecond range. Moreover, experimental demonstrations of the coupling of distant qubits are a first step towards two-qubit manipulations and, ultimately, scalable quantum computation and further enhance the outstanding significance of this rapidly evolving field. The goal of this project is to implement a first quantum algorithm comprising multiple qubits in circuit QED. Since even the most basic algorithm consists of several sequential single and two-qubit operations (quantum gates), reliable coherence-preserving manipulation techniques have to be developed in order to obtain gate fidelities high enough for concatenated gate sequences. Neither the particular choice of a basic set of quantum gates needed for the implementation of arbitrary algorithms, nor the physical implementation of a particular gate is unique. Consequent-ly, different versions of qubit gates, e.g. geometric quantum gates, will be evaluated for both speed and fidelity by a quantitative analysis resulting in an optimized multi-qubit architecture for the generation of arbitrary entangled quantum states. Full state tomography will be implemented to characterize the entanglement properties of both multi-qubit states and quantum gates. Eventually, the read-out scheme will be improved to achieve high signal-to-noise ratio. While solid-state qubit implementations provide fast quantum gates other systems like Rydberg atoms are ideally suited as quantum memory due to their long coherence times, though slower operation times. Thus, investigations in hybrid quantum system will be commenced to possibly extend available coherence times for more complex operations. The high level of expertise in quantum mechanics with superconducting devices of the host -- apparent from his pioneering contributions to this field -- and the experience of the fellow in both theoretical and experimental quantum mechanics are expected to make this project a success. The gained insights and expertise on physics with superconducting quantum devices will consequently reinforce the physics research in Austria with its strong focus on quantum optics with various other systems.