Optical Phase Information Due to the Vacuum in Two-Photon Down-Conversion
According to QED single photons carry no phase information about the optical field, yet it is possible for a superposition of states, including the vacuum, to carry phase memory. We have demonstrated this principle by showing experimentally that the photon pairs generated in the process of parametric down-conversion,1 through their entanglement with the vacuum state, contain information about the phase of the pump field, as was recently pointed out by Grangier et al.2 However, our experimental technique is different from the one they proposed.
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