Entangled Quantum Clocks for Measuring Proper-Time Difference
  We report that entangled pairs of quantum clocks (non-degenerate quantum bits) can be used as a specialized detector for precisely measuring difference of proper-times that each constituent quantum clock experiences. We describe why the proposed scheme would be more precise in the measurement of proper-time difference than a scheme of two-separate-quantum-clocks. We consider possibilities that the proposed scheme can be used in precision test of the relativity theory. 
W.Y. Hwang, D. Ahn, S.W. Hwang, Y.D. Han