Search

We define single electron spin qubits in a silicon metal-oxide-semiconductor double quantum dot system. By mapping the qubit resonance frequency as a function of a gate-induced electric field, the spectrum reveals an anticrossing that is consistent with an intervalley spin-orbit coupling. We fit the data from which we extract an intervalley coupling strength of 43 MHz. In addition, we observe a narrow resonance near the primary qubit resonance when we operate the device in the (1,1) charge configuration. The experimental data are consistent with a simulation involving two weakly exchanged-coupled spins with a Zeeman energy difference of 1 MHz, of the same order as the Rabi frequency. We conclude that the narrow resonance is the result of driven transitions between the T- and T+ triplet states, using an electron spin resonance signal of frequency located halfway between the resonance frequencies of the two individual spins. The findings presented here offer an alternative method of implementing two-qubit gates, of relevance to the operation of larger-scale spin qubit systems.

We study Andreev reflection and Josephson currents in topological bilayer exciton condensates (TECs). These systems can create 100% spin-entangled nonlocal currents with high amplitudes due to perfect nonlocal Andreev reflection. This Andreev reflection process can be gate tuned from a regime of purely retro reflection to purely specular reflection. We have studied the bound states in TEC–topological-insulator–TEC Josephson junctions and find a gapless dispersion for perpendicular incidence. The presence of a sharp transition in the supercurrent-phase relationship when the system is in equilibrium is a signature of fractional charge, which can be further revealed in ac measurements faster than relaxation processes via Landau-Zener processes.

We have modeled SQUIDs with topologically non-trivial superconducting junctions and performed an optimization study on the Majorana fermion detection. We find that the SQUID parameters beta_L, and beta_C can be used to increase the ratio of Majorana tunneling to standard Cooper pair tunneling by more than two orders of magnitude. Most importantly, we show that dc SQUIDs including topologically trivial components can still host strong signatures of the Majorana fermion. This paves the way towards the experimental verification of the theoretically predicted Majorana fermion., Comment: accepted by Physical Review B