[intro music]
This optical bench apparatus here is a quantum
eraser experiment, which is a demonstration
of the nature of quantum information.
A photon emitted by a laser passes around
an optical bench, is split along two paths
by a beam splitter, and recombined at a second
beam splitter.
Because our photon is a wave, we get constructive
and destructive interference, and a circular
interference pattern can be seen on the screen.
In the language of quantum mechanics, the
path the photon has taken is a superposition
of the two paths.
This is unintuitive, surely a single particle
cannot take both paths at once – so we try
to overcome this - we can ‘mark’ the photon’s
path by passing it through polarising screens.
However, by doing so, we lose our interference
pattern.
By `measuring’ the path the photon has taken,
we have changed the photon from exhibiting
wave behaviour in the form of interference,
to exhibiting particle-like behaviour.
This phenomenon is a key feature of quantum
mechanics and is part of the conceptual underpinnings
of quantum computation.
[outro music]
