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Physical Review A (Atomic, Molecular, and Optical Physics) 
Print Issue of November 2001 

Phys. Rev. A 64, 056101 (2001) (3 pages)

Comment on "Manipulating the frequency-entangled states by an acoustic-optical modulator"

     K. J. Resch, S. H. Myrskog, J. S. Lundeen, and A. M. Steinberg 
     Department of Physics, University of Toronto, 60 St. George Street, Toronto, Canada ON M5S 2K9 

(Received 24 October 2000; published 4 October 2001) 

A recent theoretical paper by Shi et al. [Phys. Rev. A 61, 064102 (2000)] 
proposes a scheme for entanglement swapping utilizing acousto-optic modulators without
requiring a Bell-state measurement. In this Comment, we show that the proposal is 
flawed and no entanglement swapping can occur without measurement. 
©2001 The American Physical Society 

III. CONCLUSION

The authors of [7] used a nonunitary transformation to
describe the action of an AOM on a pair of input photon
modes, and this appeared to lead to unconditional entanglement
swapping. We have shown that when a unitary transformation
is used instead, as required by quantum mechanics,
no entanglement between the photons from different
sources is achieved. Due to the same erroneous transformation,
the claim that an AOM could create a GHZ state using
a pair of two-photon entangled states is also incorrect. In
general, no unitary transformation on one pair of photons can
ever modify the reduced density matrix of a different pair.
This is why effects such as entanglement swapping [5] and
quantum teleportation [6] always require a nonunitary interaction
(measurement).

URL: http://link.aps.org/abstract/PRA/v64/e056101 
DOI: 10.1103/PhysRevA.64.056101 
PACS: 03.65.Ta        Additional Information 


Manipulating the frequency-entangled states by an acoustic-optical modulator

     Bao-Sen Shi, Yun-Kun Jiang, and Guang-Can Guo 
     Laboratory of Quantum Communication and Quantum Computation, Department of Physics, 
University of Science and Technology of China, Hefei 230026, People's Republic of China 

(Received 8 October 1999; revised 4 January 2000; published 17 May 2000) 

In this paper, we describe how to realize conditional frequency entanglement swapping 
and to produce probabilistically a three-photon frequency entangled state from two 
pairs of frequency entangled states by using an acoustic-optical modulator. Both schemes 
are very simple and may be implementable in practice. 
©2000 The American Physical Society 

URL: http://link.aps.org/abstract/PRA/v61/e064102 
DOI: 10.1103/PhysRevA.61.064102 
PACS: 03.65.Bz, 42.79.Sz        Additional Information 


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