Delayed choice

A more sophisticated experiment was proposed by John A. Wheeler (1911-) of the University of Princeton. He suggested that you can determine what kind of condition you want to set up at a time when the photon is already 'on its way' through the apparatus. If you decide to offer it one path only, it will behave as a particle, showing no interference. If you decide that it could have gone 'along both paths at once', it will behave as a wave, and will obey the rules of interference.

In the delayed choice extension of the experiment, the route is randomly closed or opened when the photon has almost completed its journey towards mirror 4. The photon 'does not know' whether the 'lower route' is closed or open when it comes to mirror 1.

Obviously, this must be done faster than by moving a brick, but it can be done electronically. The experiment was carried out successfully by Carroll Alley and co-workers at the University of Maryland in 1986.

Instead of a 'brick', Alley used an optical switch which could open or close a route within 5 x 10-9 s (5 ns). The length of time for a photon to go through the apparatus was about 30 ns. The result was as predicted by Wheeler. It does not matter when the route is closed. If the photon could have taken either route, interference occurs, and detector B remains dark, whereas if one route had been closed, even in the middle of its journey, there is no interference and detector B registers 'clicks'.

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