Speaker
Description
In this talk we report the observation of offset charge jumps induced by external radiation in Si/SiGe quantum dots that serve as spin qubits. Such charge jumps are important for quantum dot qubits, because they directly alter the operating point of the qubit in gate voltage space, and such uncontrolled shifts can induce errors in qubit initialization, readout, and manipulation. Using the linear accelerator at the Johns Hopkins University Applied Physics Lab (JHU-APL), we observe offset charge shifts in a Si/SiGe Tunnel Falls chip [1,2] with two 3-dot, 1-sensor devices inside a dilution refrigerator. The accelerator allows for the precise injection of few-to-single high energy electrons (approximately 16 MeV) into the device with energy deposition similar to that of cosmic-ray muons. We observe both correlated and uncorrelated jumps across three separate quantum dots whose offset charge is monitored simultaneously. In a separate experiment, we also observe offset charge jumps arising from e-h pairs generated optically at the backside of the handle wafer on which the SiGe heterostructure is grown. These photon bursts emulate the effects of environmental radiation when energy is deposited in the substrate, whose thickness far exceeds that of the SiGe heterostructure itself. We find that such absorption at the back of the wafer also results in discrete jumps in the offset charge, and we discuss possible locations for the trapping of such charge. An important difference between superconducting qubits and Si/SiGe qubits is that the heterostructure hosting the latter has a lattice constant larger than that of bulk silicon, necessitating the removal of atomic density and leading to layers with defects caused by this mismatch [3]. An important result from measurements of photon-induced e-h pair generation at the back of the chip is that carrier can and do cross these layers and become trapped very close to the quantum dots at the top of the heterostructure.
[1] Neyens, S. et al. Probing single electrons across 300-mm spin qubit wafers. Nature 629, 80–85 (2024).
[2] George, H. C. et al. 12-Spin-Qubit Arrays Fabricated on a 300 mm Semiconductor Manufacturing Line. Nano Letters 25, 793–799 (2025).
[3] F. Schäffler, High-mobility Si and Ge structures, Semiconductor Science and Technology 12, 1515 (1997).
