Light-induced offset charge in Si/SiGe quantum dots as a proxy for radiation impacts

Recently, it has been discovered that environmental radiation when absorbed in the silicon substrate of superconducting qubits causes correlated errors [1,2]. It is of great interest to know if Si/SiGe quantum dot qubits, which are also fabricated on silicon substrates, are similarly sensitive to radiation. One key difference between superconducting and Si/SiGe qubits is that SiGe heterostructures have additional interfaces between the bulk silicon substrate and the qubit region. These interfaces are of interest for two reasons: first, they are locations at which charge can be trapped, which can induce qubit errors. Second, in principle, they can block charge migration, which (if true) could be beneficial for qubits. It is therefore interesting to monitor radiation impacts and track the location of the resulting trapped charge.



Here, we imitate such radiation impacts in Si/SiGe quantum dot devices using a fiber optic connection in a 3K cryogenic refrigerator to deposit energy from multiple 1.6 eV photon-impacts on the back side of the host silicon substrate. We show that such photon impacts shift the Coulomb blockade behavior of gate-defined quantum dots at the surface of the chip. We track multiple offset charge shifts from a series of photon bursts by using active feedback to sit on the side of a Coulomb blockade peak [3]. Using this technique, we find that small photon bursts result in a repeatable offset charge shift, indicating that charge is trapped at a consistent interface. A Poisson solver is used to find a vertical location for this charge to be trapped that is consistent with experimentally observed shifts.

[1] Vepsäläinen, A. P. et al. Impact of ionizing radiation on superconducting qubit coherence. Nature 584, 551–556 (2020).

[2] Wilen, C.D., Abdullah, S., Kurinsky, N.A. et al. Correlated charge noise and relaxation errors in superconducting qubits. Nature 594, 369–373 (2021).

[3] Nakajima, Takashi, Kojima, Yohei et al. Real-Time Feedback Control of Charge Sensing for Quantum Dot Qubits, Phys. Rev. Appl., 15, L031003 (2021)