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Why Two‐Photon but Not Two Photons?
Author(s) -
Shih Y.H.,
Strekalov D.V.,
Pittman T.D.,
Rubin M.H.
Publication year - 1998
Publication title -
fortschritte der physik
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.469
H-Index - 71
eISSN - 1521-3978
pISSN - 0015-8208
DOI - 10.1002/(sici)1521-3978(199811)46:6/8<627::aid-prop627>3.0.co;2-1
Subject(s) - photon , physics , quantum mechanics
In his famous book, The Principles of Quantum Mechanics , Dirac stated that “. . . photon . . . interferes only with itself. Interference between two different photons never occurs.” However, two‐photon interference has been demonstrated experimentally. Was Dirac wrong? The experiment we reported here shows that two‐photon interference can not be considered the interference of two photons. “Two‐photon” is not two photons. Two‐photon is a single entity, which is just like a photon. We may call it biphoton . Dirac was correct. Biphoton interferes only with itself. The consistent description of an entangled two‐particle system is to consider it as a nonlocal single object: an entangled Biphoton system with zero entropy. However, it is an experimental choice to study only one subsystem and to ignore the other. The results of this kind measurement look peculiar. Our experiment demonstrated a unusual wavepacket of a single photon from a two‐photon entangled system, which depends on its unmeasured twin. The experiment also concluded that even though the two‐particle system is in a pure state with zero entropy, the subsystems are both in mixed states with entropy greater than zero. Does it mean negative entropy is present somewhere in the two‐particle system?