Probing Spin Qubits with Radiofrequency Reflectometry

Spin qubits in semiconductors are a promising platform for scalable quantum computing due to their small footprint, VLSI compatibility and demonstrated operation fidelities above fault-tolerant thresholds. In the path to scaling, a key aspect is developing compact and fast high-fidelity readout techniques to enable highly connected qubit architectures as well as mid-circuit measurements, a necessary feature for many dynamic algorithms and quantum error correction. In this talk, I will introduce the concept of radio-frequency reflectometry for fast readout of spin qubits as a method to achieve the goal. By embedding the devices in radio- or microwave resonant circuits, high-bandwidth and novel compact methods for readout can be implemented. I will review the state-of-the-art of the field and present a series of results that highlight improvements at all stages of the readout chain that enable achieving readout fidelities above fault-tolerant thresholds in timescales comparable to the gate operation times. First, at the device level, I will describe optimal device fabrication recipes and introduce methods to abstract spin qubit devices into classical circuit elements for efficient simulation. Then, at the resonator level, I will present optimized designs tailored to the specific measurement method. At the amplifier level, I will discuss quantum-limited techniques specific for spin qubit devices. Finally, I will touch upon scaling aspect of the methodology such as frequency- and time-domain multiplexing, VLSI integration, and footprint reduction.