arXiv subjects
Publications and source records attributed to Lin Tian.
Decoherence of a solid state based qubit can be caused by coupling to microscopic degrees of freedom in the solid. We lay out a simple theory and use it to estimate decoherence for a recently proposed superconducting persistent current design. All considered sources of decoherence are found to be quite weak, leading to a high quality factor for this qubit.
Off-resonant effects are a significant source of error in quantum computation. This paper presents a group theoretic proof that off-resonant transitions to the higher levels of a multilevel qubit can be completely prevented in principle. This result can be generalized to prevent unwanted transitions due to qubit-qubit interactions. A simple scheme exploiting dynamic pulse control techniques is presented that can cancel transitions to higher states to arbitrary accuracy.
We present the design of a superconducting qubit that has circulating currents of opposite sign as its two states. The circuit consists of three nano-scale aluminum Josephson junctions connected in a superconducting loop and controlled by magnetic fields. The advantages of this qubit are that it can be made insensitive to background charges in the substrate, the flux in the two states can be detected with a SQUID, and the states can be manipulated with magnetic fields. Coupled systems of qubits are also discussed as well as sources of decoherence.
The electron-phonon interaction in $C_{70}$ anions is studied by making use of a lattice relaxation approach. We find there exists a Jahn-Teller effect in $C_{70}^{3-}$ system, due to an extra electron being doped to the double degenerate $E_{1}^{''}$ state. As a result of this effect, the original $D_{5h}$ symmetry of the ground state becomes unstable, which causes distortion of the lattice configuration. The only symmetry maintained in the final state of the relaxation is the $x-y$ plane reflection symmetry. We further find that besides the Jahn-Teller active $A_{1}^{'}, A_{2}^{'}, E_{2}^{'}$ modes, the non-Jahn-Teller active $E_{1}^{'}$ vibrations also contribute to the relaxation process, which come from the nonlinear effect and are two or three orders smaller than those of the Jahn-Teller active modes. We suggest that the $C_{70}^{3-}$ molecule is a promising Berry Phase candidate in this effective $E \otimes e$ Jahn-Teller system.