Singlet-Triplet Readout for Donor-Based Qubits in Silicon

Abstract

In the pursuit of realising a full-scale universal quantum computer, the phosphorus donor in silicon platform provides a simple, low magnetic and low charge noise environment. In this thesis we consider the singlet-triplet (T0) qubit encoding in this system which allows for fast all electrical control. In a first scalable, double quantum dot design, we optimised the readout circuit to achieve single-shot single-gate RF dispersive readout of the singlet and triplet (T-) states with a fidelity of 90% at 5 kHz bandwidth. By atomic engineering of the donor number and positions, we optimised the tunnel coupling between the two dots to 3 GHz, ideal to observe coherent interaction of the qubit states. However surprisingly this was not observed due to the fast relaxation rate (>1 MHz) of the triplet T0 to the singlet ground state. This fast rate is a result of the large difference in Zeeman energy between the two electrons (ΔEZ ~ 200 MHz) present which induces mixing between the triplet T0 and singlet states, exceeding the readout bandwidth of the dispersive sensor. Motivated by this discovery, we designed 2 different charges sensors to map the short-lived triplet (1,1)T0 state to a longer lived (2,1) charge state in a process called latched readout. In the first device, we designed a novel single lead quantum dot (SLQD) sensor. Despite realising an operational sensor, the charge noise in this device was found to be too high. In the second device, we used an single electron transistor (SET) charge sensor where we were able to demonstrate latched readout for the first time in the Si:P platform with a fidelity of 99.7%. We showed that the latched method is robust at higher temperatures, with a 97.1% fidelity measured at 3.7 K. Using this sensor we were able to observe coherent oscillations around the Z-axis of an all donor singlet-triplet qubit with a coherence time of T2*~ 23 ns. Finally the role of phosphorus donor nuclear spins on X-gate operations are discussed.