Controlling and measuring quantum dot qubits in Si/SiGe heterostructures

In this talk I describe two recent advances in the control and measurement of Si/SiGe quantum dot qubits. In the first part of the talk, I describe how relatively small changes in gate voltage can lead to dramatic tunability of 2-electron energy eigenstates, which are important for readout of Loss-Divincenzo, singlet-triplet, and quantum dot hybrid qubits. For the latter qubit, the ability to choose, on demand, singlet-triplet splittings in neighboring quantum dots is particularly useful, because this splitting determines (in the first dot) the qubit zero-to-one energy splitting and (in the second dot) the energy gap for qubit readout. I will show data from the same device demonstrating singlet-triplet splittings varying from below 5 GHz up to 15 GHz. In the low frequency end of this range we demonstrate the key role of electron-electron correlations and Wigner molecule physics. I also present data on the use of three-dimensional integration in dispersive readout measurement schemes for Si/SiGe quantum dot qubits, and I discuss the implications for scaling of qubit numbers.