We use multi-pulse dynamical decoupling to increase the coherence lifetime (T2) of large numbers of nitrogen-vacancy (NV) electronic spins in room temperature diamond, thus enabling scalable applications of multi-spin quantum information processing and metrology. We realize an order-of-magnitude extension of the NV multi-spin T2 for diamond samples with widely differing spin environments. For samples with nitrogen impurity concentration <~1 ppm, we find T2 > 2 ms, comparable to the longest coherence time reported for single NV centers, and demonstrate a ten-fold enhancement in NV multi-spin sensing of AC magnetic fields.
@article{arxiv.1201.5686,
title = {Enhanced solid-state multi-spin metrology using dynamical decoupling},
author = {L. M. Pham and N. Bar-Gill and C. Belthangady and D. Le Sage and P. Cappellaro and M. D. Lukin and A. Yacoby and R. L. Walsworth},
journal= {arXiv preprint arXiv:1201.5686},
year = {2015}
}