Coherent Manipulation In Multiple Charge Qubits

A qubit is a basic unit for quantum computation. Compared with other quantum computing systems like superconducting qubits or ion trap system, qubits based on quantum dots in solid state materials owns the advantages of easy integration, march for industrial production, and so on. In this thesis, we studied the strong capacitive coupling among multiple qubits and the quantum controlled-logic operations in GaAs/AIGaAs quantum dots, both theoretically and experimentally. We discussed the dependence of the fidelity of controlled-logic gates and the inter-qubit coupling energy by theoretical simulations. Specificically, we demonstrated Controlled-NOT gate in double charge qubits and the static Toffoli gate in trible charge qubits.The followings are the main contents in this thesis:1. We introduced the constant interaction model to help understand the basic phenomena in single and double quantum dots, like electrical potential, charging energy, coloumb blockade, coloumb diamond, honeycomb diagram, and so on.2. We described the processing processdures and needed faciities in fabircating nano-scale quantum dots, such as MJB3 lithography, electron beam lithography and electron beam evaporation. We also showed our custom-designed and home-made measurement systems for GaAs wafer characterization.3. We showed various ways of manipulating a single charge qubit, such as photon assisted tunneling, Larmor oscillation by square pulse, LZS interference, Rabi oscillation and Ramsey fringes by microwave.4. By designing a novel structure of double charge qubits, we achieved strong and tunable capacitive coupling between two charge qubits. Based on this, we realised CNOT gate operation and measured the operation fidelity.5. We achieved strong coupling in a triple charge qubit syatem, and demonstrated a three-qubit logical gate.The main innovations of this thesis are as following:1. By introducing a pair of open slot horizontal gates in double charge qubits, we achieved strong and tunable capacitive coupling, which showed universal controbility for different electron charge numbers.2. We theoretically simulated the effects of coupling energy J on two-qubit operations, such as CNOT gate fidelity and degree of entanglement.3. We experimentally demonstrated CNOT gate operations based on strong coupling betweem two charge qubits. We showed CNOT logic for both amplitude and phase of thestates of qubits. We got a CNOT fidelity of 68%, which is comparable to the best result in this field.4. In a triple charge qubit system, we showed that strong inter-qubit couplings could be achieved, and demonstrated a static controlled-controlled-not gate (or Toffoli gate). There has been no report on three-qubit gates in semiconductor qubuts yet. Our experiment would also be helpful for developing multi-qubit systems.