Example of observing without absorbing needed

[Ulysees]

I saw somewhere that the act of observing can affect whether light will show its wave nature or its particle nature ie whether the wavefunction will collapse.

But then, it seems to me, it's not real observing as in the macroscopic world, we don't just observe, we absorb and therefore destroy photons in order to observe them.

Is there any example where a photon remains intact after the act of observing?

 

[Cthugha]

Well, that depends on what you define as intact. There are so called QND (quantum non demolition) measurements.

If you have access to peer reviewed journals, you might want to have a look at the following paper:

Progressive field-state collapse and quantum non-demolition photon counting, Nature 448, 889-893 (23 August 2007)

In this paper the photon number inside a cavity is measured by sending rubidium atoms in a well defined state through this cavity and using these atoms as "clocks". The presence of the photons alters the clock rate and therefore the state of the atom after it has left the cavity is a measure for the photon number inside the cavity, which is left unaltered.

However, this is non destructive in terms of photon number. Phase information is of course altered during this kind of measurement.

 

[Ulysees]

Alright. Any thoughts how it can be, that the clock rate of those atoms can be affected by the number of photons circulating in that cavity?