Joint Bachelor's Defense

Rune Kutchinsky & Emil Mathias K. Rødbro

Title: Manipulating and detecting the charge state of a quantum dot in real time


New technology yields new potential for physics experiments. A new generation of quantum control electronics, capable of measurement, data analysis and conditional generation of control signals, all on sub-millisecond timescales, opens up new types of quantum physics experiments, where measurement and quantum system manipulation can be done on timescales shorter than the typical coherence time of the system. In this project, we manipulated the potential of a quantum dot and detected the resulting changes of quantum dot occupation (single-electron tunneling on/off the dot) in real-time using FPGA-based control electronics. The dot potential was manipulated using baseband voltage pulses applied to metallic gate electrodes, and its charge state was detected via a proximal sensor dot using radio-frequency reflectometry. Unlike previous experiments that employed homodyne detection via analog mixers, the reflectometry carrier received from the cryostat was demodulated fully digitally on the control electronics. When detecting single-electron tunneling events, we were able to achieve a signal-to-noise ratio above 3 with a temporal resolution of 4 microseconds. Importantly, the demodulated signal can be analyzed on the fly, allowing subsequent gate-voltage changes based on the detected charge state of the quantum dot. By applying a large external magnetic field to a singly-occupied GaAs quantum dot, we were able to demonstrate most of the control techniques necessary for real-time spin-to-charge conversion, although clear signatures of spin-dependent tunneling remained elusive. We propose that the pre-distortion of pulse shapes needs to be improved in order to adequately compensate high-pass pulse distortions that occur inside the cryostat. Our techniques should be transferable to silicon quantum dot devices in which spin coherence times are expected to be much longer than the sub-microsecond latency associated with our control electronics.