Just found this that also may be used for quantum computers:
"Seeing a photon without absorbing it
...A single photon can be detected repeatedly by combining several nondestructive devices. This also provides new possibilities for using single photons in quantum communication and quantum information processing. The successful transfer of a photon in a quantum network could be detected without destroying the fragile quantum information encoded in it. Based on the mechanism used for single-photon detection, it should also be possible to realize a deterministic, universal quantum gate between a reflected single photon and the single atom and even between two photons. Because quantum gates are the functional building blocks of a quantum computer, this is a long-standing dream in optical quantum computing"
What? Can detect a photon without absorbing it!? I thought that was physically impossible! Well, I learn something new every day.