@article{JMLR:v16:koltchinskii15a,
  author  = {Vladimir Koltchinskii and Dong Xia},
  title   = {Optimal Estimation of Low Rank Density Matrices},
  journal = {Journal of Machine Learning Research},
  year    = {2015},
  volume  = {16},
  number  = {53},
  pages   = {1757--1792},
  url     = {http://jmlr.org/papers/v16/koltchinskii15a.html}
}