Physics Letters A 315 (2003) 352–357
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Two-photon interference without bunching two photons
Yoon-Ho Kim
Center for Engineering Science Advanced Research, Computer Science & Mathematics Division, Oak Ridge National Laboratory,
Oak Ridge, TN 37831, USA
Received 9 June 2003; received in revised form 7 July 2003; accepted 11 July 2003
Communicated by P.R. Holland
Abstract
We report an experiment which conclusively demonstrates that the two-photon entangled state interference cannot be pictured
as the overlap and ‘bunching’ of two individual photons on a beamsplitter. We also demonstrate that photon ‘bunching’ does not
occur if the two-photon Feynman amplitudes are distinguishable, even though individual photons do overlap on a beamsplitter.
2003 Elsevier B.V. All rights reserved.
PACS: 03.65.Bz; 42.50.Dv
Two-photon quantum interference effects in spon-
taneous parametricdown-conversion(SPDC) [1] fields
have been playing an important role from the study of
fundamental problems of quantum physics [2,3] to re-
cent advances in quantum cryptography [4] due to the
entanglement between the two down-converted pho-
tons.
Among many different quantum interference ef-
fects in SPDC, the observation of null (experimen-
tally, close to zero) coincidence counts between the
detectors placed at the two output ports of a beamsplit-
ter, when two SPDC pair photons are brought back
together on the beamsplitter from the different input
ports at the same time, has attracted a lot of atten-
tion over the years. It was first observed by Shih and
Alley [5,6] and later by Hong, Ou, and Mandel [7,
8]. This effect, which we refer to as SA/HOM effect,
has the following formal interpretation: because the
E-mail address: yokim@umbc.edu (Y.-H. Kim).
two two-photon (or biphoton) amplitudes leading to
a coincidence count (both photons are reflected at the
beamsplitter, r–r, or both photons are transmitted at the
beamsplitter, t–t) become indistinguishable, even in
principle, when the photons arrive at the beamsplitter
simultaneously from the different input ports, fourth-
order two-photon quantum interference occurs. Due to
the destructive nature of the interference (each photon
accumulates i phase shift upon reflection at the beam-
splitter) between r–r and t–t amplitudes, zero coinci-
dence counts are expected [7–9].
This formal interpretation is, however, always ac-
companied by a physical picture that two individual
photons somehow become bunched together at the
beamsplitter when they arrive at the same time. Since
now bunched two photons leave the beamsplitter from
the same output port, null coincidence is expected.
Due to this picture, it is indeed quite common for re-
searchers to think that two photons must overlap at
the beamsplitter for these types of two-photon inter-
ference effects to occur [10]. Such a picture, however,
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doi:10.1016/S0375-9601(03)01114-9