Nonlocal effects of partial measurements and quantum erasure
Abstract
Partial measurement turns the initial superposition not into a definite outcome but into a greater probability for it. The probability can approach 100%, yet the measurement can undergo complete quantum erasure. In the EinsteinPodolskyRosen (EPR) setting, we prove that (i) every partial measurement nonlocally creates the same partial change in the distant particle, and (ii) every erasure inflicts the same erasure on the distant particle's state. This enables an EPR experiment where the nonlocal effect does not vanish after a single measurement but keeps ``traveling'' back and forth between particles. We study an experiment in which two distant particles are subjected to interferometry with a partial ``which path'' measurement. Such a measurement causes a variable amount of correlation between the particles. A new inequality is formulated for sameangle polarizations, extending Bell's inequality for different angles. The resulting nonlocality proof is highly visualizable, as it rests entirely on the interference effect. Partial measurement also gives rise to a new form of entanglement, where the particles manifest correlations of multiple polarization directions. Another novelty in that the measurement to be erased is fully observable, in contrast to prevailing erasure techniques in which it can never be observed. Some profound conceptual implications of our experiment are briefly pointed out.
 Publication:

Physical Review A
 Pub Date:
 June 2001
 DOI:
 10.1103/PhysRevA.63.062109
 arXiv:
 arXiv:quantph/0012091
 Bibcode:
 2001PhRvA..63f2109E
 Keywords:

 03.65.Ta;
 03.67.a;
 Foundations of quantum mechanics;
 measurement theory;
 Quantum information;
 Quantum Physics
 EPrint:
 To be published in Phys. Rev. A 63 (2001). 19 pages, 12 figures, RevTeX 3.1