@misc{W1975677983,
  author = {Heinz‐Peter Breuer and Elsi-Mari Laine and Jyrki Piilo},
  title = {Measure for the Degree of Non-Markovian Behavior of Quantum Processes in Open Systems},
  year = {2009},
  howpublished = {Physical Review Letters},
  doi = {https://doi.org/10.1103/physrevlett.103.210401}
}