Abstract
The optical excitation of graphene electrons is known to obey a simple selection rule in regard to the binary states of photonic circular polarization and electronic valley pseudospin, with the rule giving a close link - an approximate one-to-one correspondence - between the corresponding binary states. Thus, a highly faithful quantum state transfer (QST) can be implemented, at least in principle, between the polarization and the pseudospin, which could have important implications for quantum repeater protocol-based quantum communications. In this thesis, we perform a proof-of-principle study and investigate the QST between a photon qubit and a valley pair qubit - the system of two entangled valley pseudospins separately confined in coupled graphene quantum dots. For the demonstration, we consider the specific configuration where the valley qubit is placed inside both a micro-cavity and a photonic crystal cavity. The two cavities together provide i) enhancement of the electron-photon interaction and ii) differentiation between the incoming and outgoing light paths in the QST. The QST in the configuration proceeds in a series of steps. It starts with a) the arrival of a signal photon with quantum information already encoded in the polarization, followed by b) its entry into the micro-cavity and c) subsequent interaction with the valley qubit to form an entangled photon-valley state by the valley-polarization correspondence, and last, d) detection and measurement of the leaking photon from the photonic crystal cavity about its linear polarization. The last step d) projects out the photon component from entangled photon-valley state, thus leaving behind the quantum information in valley sector alone and completing the final transfer of quantum information between the two qubits. We set up a quantum mechanical description of the forgoing QST process and evaluate the corresponding yield and fidelity, which are defined, respectively, by (no. of detected photons) / (no. of signal photons) and |overlap between the transferred valley state and the signal state|2. Numerical work in our study shows promising results for the two forgoing figures of merits. Dependences of the yield and fidelity on various cavity and qubit parameters are also examined.