Applications Of Hybrid Quantum Circuits In Quantum Computation

Hybrid quantum circuits combine two or more physical systems, with the goal of harnessing the advantages and strengths of the different systems in order to better ex-plore new phenomena and potentially bring about novel quantum technologies. Each system has its own advantages and disadvantages. Atomic and spin systems are well-studied systems. They have long coherence times, but operate slowly and have limited scalability. On the other hand, solid-state devices, such as superconducting circuits, offer flexibility, tunability and scalability, but have relatively short coherence times and in general are not identically reproducible. A promising idea pursued by various groups at the moment is to combine them together and build a new hybrid quantum structure that inherits only their advantages.This PhD thesis presents a brief overview of the progress achieved so far in the field of hybrid circuits involving atoms, spins, and solid-state devices (including super-conducting and nanomechanical systems). How these circuits combine elements from atomic physics, quantum optics, condensed matter physics, and nanoscience is dis-cussed, and different possible approaches for integrating various systems into a single circuit are presented.Furthermore, this thesis studies two different hybrid quantum circuits:We propose to achieve an the effective strong coupling between spins and the transmission-line resonator via a superconducting flux qubit used as a data bus, which is coupled to both of the spin ensemble and the resonator. The resulting coupling can be used to transfer quantum information between the spin ensemble and the resonator. In particular, in contrast to the direct coupling without a data bus, our approach requires far less spins to achieve a strong coupling between the spin ensemble and the resonator. Thus, this circuit could enable a long-time quantum memory when storing information in the spin ensemble.Also, we propose to implement the high-fidelity quantum storage using a hybrid quantum circuit consisting of two coupled superconducting flux qubits and a nitrogen-vacancy center ensemble. One of the flux qubits is considered as the quantum processor and the other acts as a quantum memory. By separating the processor and memory units, the influence of the quantum computing process on the quantum memory can be effectively eliminated, and hence the quantum storage of an arbitrary quantum state of the processor qubit could be achieved with a high fidelity.In addition, we propose to use atom-cavity lattices as a quantum simulator to em-ulate physical models in condensed-matter physics. By using the lattice of coupled cavities with embedded A-type three-level atoms, different spin-spin interactions can simultaneously be simulated. In particular, we show how to simulate Heisenberg mod-el and Kitaev model with atom-cavity lattices. It provides a feasible way of realizing these condensed-matter models, especially the topological quantum model, in control-lable quantum systems.