Enhanced solid-state multi-spin metrology using dynamical decoupling
  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. 
L. M. Pham, N. Bar-Gill, C. Belthangady, D. Le Sage, P. Cappellaro, M.   D. Lukin, A. Yacoby, R. L. Walsworth
cond-mat.mes-hall quant-ph