Quantum non-demolition measurement of an electron spin qubit

While single-shot spin detection of electron spin qubits in semiconductor quantum dots is now a laboratory routine, the need for quantum error correction in a large-scale system demands a quantum non-demolition (QND) implementation. The QND spin readout imposes minimal disturbance to the probed spin polarization and can therefore be repeated to extinguish measurement errors. Furthermore, it also works as a high-fidelity state preparation device. However, its realization is challenging as it involves exquisite exposure of the system to the external readout circuitry while maintaining the qubit coherence and integrity. In this talk, we show that an electron spin qubit in quantum dots can be measured in a highly non-demolition manner by probing a neighboring electron spin qubit Ising-coupled to the qubit spin. The high QND fidelity enables enhancement of the measurement fidelity of a single electron spin state by repeating the readout process several dozens of times. We describe the analysis method based on statistical inference, which is crucial to achieve the optimal performance in the presence of the qubit relaxation. We will further discuss how such QND measurements can improve the fidelity of measurement-based state preparation in addition to the measurement fidelity.