Quantum non-demolition spin measurement in quantum dots

Single-shot measurement of spin qubits in quantum dots is now common practice. However, a quantum non-demolition (QND) implementation will be necessary in a large-scale quantum computing device e.g. for quantum error correction. In the QND spin readout, only minimal disturbance is imposed to the probed spin polarization. Therefore, unlike conventional destructive counterparts, measurements can be repeated to extinguish errors and the readout process serves as a qubit preparation device. Unfortunately, it is challenging to maintain the qubit coherence and integrity when the system is inevitably exposed to the external circuitry for readout. This stringent condition for QND measurement has been achieved in several recent experiments [1-3] for the first time in quantum dots for a different type of spin measured in each case: a single electron spin in GaAs [1] and Si [3] and a single Si-29 nuclear spin [2]. We will discuss how the high non-demolition fidelity is realized to allow for readout repetitions of a single spin state and how consecutive QND readout outcomes can be collectively used to maximize the measurement and preparation fidelities, with the main focus on the Si electron spin qubit case [3]. These results will be important for measurement-based quantum information protocols in quantum dots.

References
[1] T. Nakajima et al., Nature Nanotechnology 14, 555 (2019).
[2] B. Hensen et al., Nature Nanotechnology 15, 13 (2020).
[3] J. Yoneda et al., Nature Communications 11, 1144 (2020).