# Un-measuring a Photon

Young’s double slit experiment does a great job of demonstrating the wave nature of light. But as we also know, a couple of centuries later, that light can also travel as a stream of particles called photons. Why, then, is the particle nature of light not exhibited in the experiment?

To check this, we’re going to be sneaky and we’re going to send in one photon at a time. We send the first photon through, and it’s appeared as a spot on the screen, as expected. Then we send more and more in one at a time, and we see a very peculiar result. An interference pattern gradually builds up, the same one shown as before. It’s almost as if each photon interferes with itself, and then recombines to form a spot on the screen!

This our first very weird observation of quantum mechanics the day. You’ll soon realise that weirdness because the norm when you deal with physics at this level.

This result illustrates that the interference pattern not only represents the superposition of two waves, but also a quantum superposition. The interference pattern is representative of the probability distribution of a photon arriving at a particular location. Where there is high intensity, there is a high probability of finding a photon there. This probability distribution can be calculated by modelling the beam of light as a wavefunction.

We’re now going to be even more sneaky, and we’re going to try and check which slit each photon has gone through, by placing a detector in front of each slit. Then we send the photons one by one as before.

We find that the result dramatically changes. Instead of an interference pattern, we just see two bands of light, as if the photons have just gone through a slit and ended up on the screen without interfering with one another.

This result has shown that the mere act of measuring which slit the photon went through destroyed the interference pattern. In the Schrodinger’s cat analogy, opening the box (the measurement) destroys the superposition of the cat being both dead and alive .

This is our second very weird observation of the day, that photons can ‘know’ if conscious observer is measuring them. There is no definite explanation as to why this happens, but there are multiple interpretations, or theories that try to describe the process of what happens during when a superposition becomes a well-defined state (i.e. both dead and alive to either just dead or just alive when we open the box).

Now we’re going to be extremely sneaky. We’re going to place the detectors next to the screen – this checks which slits the photons gone through after they’ve gone through the slits. This way, the photons must decide to act as a wave or a particle before they get detected.

And here comes our third very weird observation of the day. No interference pattern shows again – somehow the photons know that they will be measured, and choose to act as a stream of particles, and this is another very unsettling consequence of quantum mechanics. And if you were to repeat the experiment but remove the detectors, you would find an interference pattern again – by not measuring the photons they become a superposition again.

This experiment may be simple, but it is powerful in demonstrating the existence of quantum superpositions. And this is how we know that Schrodinger’s cat is indeed both dead and alive – before you open the box, of course.

Yanhao