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I’ve just put an order in for a new piece of lab equipment – an experimental set-up that will allow students to carry out the two-slit, one-photon experiment. This is one of the ‘classic’ experiments of quantum mechanics – showing just how strangly small things behave.

Interference patterns are commonly observable when you have two sources of waves close to each other. You can see them with water waves (e.g. the first link below) – you can also observe them with light. If you look at the colours on thin film (e.g. a soap bubble) you are observing an interference pattern – in this case interference between light that bounces off the outer surface of the bubble and that which bounces off the inner surface. You see the colours because the interference depends upon the wavelength (colour) of the light. So the white light is separated out into regions where red predominates, regions where blue predominates, etc.

If you shine light (e.g. just daylight coming in through the window) onto a CD or DVD you’ll also see a similar thing – a pattern of colours ‘reflected’ from the disc. The CD contains tiny pits in its surface, and the reflections from each pit interfere with each other.

We can do this is a more controlled manner in the lab – for example we can make two small, very closely separated slits in an opaque medium and shine a laser at them. We’ll observe a pattern of repeating bright and dark bands on the other side, caused by the interference of the light waves leaving the two slits.

Now, here’s the wierd stuff. We can also refer to light as being made up of particles, called photons. The intensity of a light source depends on how many photons are being released per second. It’s possible to use a very low intensity beam in an interference experiment – so low that there can only possibly be one photon travelling through the slits at once. What do we see?  The same intereference pattern as before. (It takes time for the pattern to build up, since there’s such a low intensity, but all we need do is wait and watch.) That’s odd. The conclusion is that the particle must travel through both slits at once. Can it do that?

Here’s where it gets really odd. We can try to measure which slit the particle actually goes through, by putting a detector at each slit. When we do that, the interference pattern goes away – we are just left with what we’d expect for a single slit. The very act of observing what is happening has changed what is happening. If we don’t look, the one photon goes through both slits at once – but if we look to see which it goes through then it obliges and just goes through one or the other for us.

The fact that observing something changes it is an inescapable conclusion from looking at the quantum world and is very puzzling.

So, later on in the year, my students should be able to do this experiment for themselves.

The video below gives a great analysis of what’s happening (though with electrons not photons) – also the second link gives you an opportunity to be a bit interactive.