Single-Shot Readout of a Nuclear Spin Weakly Coupled to a   Nitrogen-Vacancy Center
  Single-shot readout of qubits is required for scalable quantum computing. Nuclear spins are superb quantum memories due to their long coherence times but are difficult to be read out in single shot due to their weak interaction with probes. Here we demonstrate single-shot readout of a weakly coupled $^{13}$C nuclear spin, which is unresolvable in traditional protocols. We use dynamical decoupling pulse sequences to selectively enhance the entanglement between the nuclear spin and a nitrogen-vacancy center electron spin, tuning the weak measurement of the nuclear spin to a strong, projective one. A nuclear spin coupled to the NV center with strength 330 kHz is read out in 200 ms with fidelity 95.5\%. This work provides a general protocol for single-shot readout of weakly coupled qubits and therefore largely extends the range of physical systems for scalable quantum computing. 
Gang-Qin Liu, Jian Xing, Wen-Long Ma, Chang-Hao Li, Ping Wang, Hoi   Chun Po, Ren-Bao Liu and Xin-Yu Pan
quant-ph