@article{PhysRevFocus.19.12,
  title = {Four States are Better than Two},
  author = {Sherwood, Jonathan},
  journal = {Phys. Rev. Focus},
  volume = {19},
  issue = {},
  pages = {12},
  numpages = {1},
  year = {2007},
  month = {Apr},
  doi = {10.1103/PhysRevFocus.19.12},
  url = {http://link.aps.org/doi/10.1103/PhysRevFocus.19.12},
  publisher = {American Physical Society}
}