Measuring quantum entanglement without prior state reconstruction
Abstract
Description
It is shown that, despite strong nonlinearity, entanglement of formation of two-qubit state can be measured without prior state reconstruction. Collective measurements on small number of copies are provided that allow to determine quantum concurrence {\it via} estimation of only {\it four} parameters. It is also pointed out that another entanglement measure based on so called negativity can also be measured in similar way. The result is related to general problem: what kind of information can be extracted efficiently from unknown quantum state ?
Improved version. For two qubits number of estimated parameters reduced to four; title changed; list of references extended. RevTeX
Improved version. For two qubits number of estimated parameters reduced to four; title changed; list of references extended. RevTeX