High-Speed Low-Crosstalk Detection of a $^{171}$Yb$^+$ Qubit using Superconducting Nanowire Single Photon Detectors.

Qubits used in quantum computing tend to suffer from errors, either from the qubit interacting with the environment, or from imperfect control when quantum logic gates are applied. Fault-tolerant construction based on quantum error correcting codes (QECC) can be used to recover from such errors. Effective implementation of QECC requires a high fidelity readout of the ancilla qubits from which the error syndrome can be determined, without affecting the data qubits in which relevant quantum information is stored for processing. Here, we present a detection scheme for \yb trapped ion qubits, where we use superconducting nanowire single photon detectors and utilize photon time-of-arrival statistics to improve the fidelity and speed. Qubit shuttling allows for creating a separate detection region where an ancilla qubit can be measured without disrupting a data qubit. We achieve an average qubit state detection time of 11$\mu$s with a fidelity of $99.931(6)\%$. The error due to the detection crosstalk, defined as the probability that the coherence of the data qubit is lost due to the process of detecting an ancilla qubit, is reduced to $\sim2\times10^{-5}$ by creating a separation of 370$\mu$m between them.