Effects of time reversal symmetry in dynamical decoupling
  Dynamical decoupling (DD) is a technique for preserving the coherence of quantum mechanical states in the presence of a noisy environment. It uses sequences of inversion pulses to suppress the environmental perturbations by periodically refocusing them. It has been shown that different sequences of inversion pulses show vastly different performance, in particular also concerning the correction of experimental pulse imperfections. Here, we investigate specifically the role of time-reversal symmetry in the building-blocks of the pulse sequence. We show that using time symmetric building blocks often improves the performance of the sequence compared to sequences formed by time asymmetric building blocks. Using quantum state tomography of the echoes generated by the sequences, we analyze the mechanisms that lead to loss of fidelity and show how they can be compensated by suitable concatenation of symmetry-related blocks of decoupling pulses. 
Alexandre M. Souza, Gonzalo A. \'Alvarez and Dieter Suter
quant-ph