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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Instantaneous nonlocal measurements

Lev Vaidman A

Published Jul 23, 2003• 0 citations
Publisher

Abstract

It is shown, under the assumption of possibility to perform an arbitrary local operation, that all nonlocal variables related to two or more separate sites can be measured instantaneously. The measurement is based on teleportation method. It is a verification measurement: it yields reliably the eigenvalues of the nonlocal variables, but it does not prepare the eigenstates of the system.

DOI
10.1117/12.517851

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It is shown, under the assumption of possibility to perform an arbitrary local operation, that all nonlocal variables related to two or more separate sites can be measured instantaneously. The measurement is based on teleportation method. It is a verification measurement: it yields reliably the eigenvalues of the nonlocal variables, but it does not prepare the eigenstates of the system.
With reference to a system of n spin‐1/2 particles which are distributed between two space‐like separated parties, we present two procedures to perform an instantaneous and local measurement of any nondegenerate observable having as its eigenstates the orthonormal basis of n‐partite entangled states introduced in [S. Bose, V. Vedral, and P. L. Knight, Phys. Rev. A 57, 822 (1998)], whose vectors reduce to the Bell states for n = 2 and to the GHZ states for n = 3. The first procedure implements a genuine nondemolition measurement by resorting to two n‐particle entangled states as an additional resource. The second one distinguishes between those eigenstates, though without preserving them (i.e., it is a demolition measurement), but it requires only a single couple of additional particles in a (maximally entangled) Bell state. Both methods are instantaneous, in the sense that the outcome of the measurement process is completely determined as soon as the local operations, whose duration can be made as short as one wishes, have been independently performed in the two distant regions where the particles are located.