Direct estimations of linear and nonlinear functionals of a quantum state

A K Ekert, C M Alves, D K L Oi, M Horodecki, P Horodecki, L C Kwek

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215 Citations (Scopus)


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 the proposed network are within the reach of quantum technology that is currently being developed.

Original languageEnglish
Article number217901
Pages (from-to)-
Number of pages4
JournalPhysical Review Letters
Issue number21
Publication statusPublished - 27 May 2002


  • quantum states
  • quantum tomography
  • quantum estimations
  • linear
  • nonlinear


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