Direct estimations of linear and nonlinear functionals of a quantum state.

  title={Direct estimations of linear and nonlinear functionals of a quantum state.},
  author={Artur K. Ekert and C. Moura Alves and Daniel K. L. Oi and Michal Horodecki and Paweł Horodecki and Leong Chuan Kwek},
  journal={Physical review letters},
  volume={88 21},
We present a simple quantum network, based on the controlled-SWAP gate, that can extract certain properties of quantum states without recourse to quantum tomography. It can be used as a basic building block for direct quantum estimations of both linear and nonlinear functionals of any density operator. The network has many potential applications ranging from purity tests and eigenvalue estimations to direct characterization of some properties of quantum channels. Experimental realizations of… Expand

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