Quantum Physics

Dr Winfried Hensinger is Senior Lecturer in Atomic Molecular and Optical Physics at Sussex University. He and his team at Sussex are some of the world leaders in research in quantum computing. We thought he would be the man to tell us a little bit about quantum mechanics and why it’s so important.

Why should non-scientists care about quantum physics?

Quantum physics governs everything in this world. With any system – quantum physics can calculate how it works with other things.

It has some weird predictions – things that typically don’t happen in real life… or they do, but we don’t see them. For example, quantum mechanics predicts that it is entirely possible for you to walk through a brick wall, it’s just very unlikely. On the quantum level, though, it is entirely possible. Indeed, if you were to spend long enough walking against a brick wall, at some point you would walk straight through.

What is the double slit experiment and why is it important to the way we understand the world?

The experiment demonstrates one of the key features of quantum mechanics. We fire electrons at a wall with two slits in it, so that they have to go through one of the two slits in order to hit the wall behind. But what we find from the pattern on the back wall is that the electrons actually go through both slits at once! It seems they can be in two places at the same time.

So when you sit in your car, you have a choice to drive forward or backwards. But in the world of quantum mechanics, you can drive forward and backwards at the same time.

However, when we set up a device to look closely at the slits themselves to witness this, we then find that the electron ‘chooses’ and goes through one slit only. So as soon as we try to measure something in the quantum world (i.e. which path the electron has taken), we collapse the quantum state.

This has huge implications because quantum mechanics happens in every aspect – every system. In theory the earth could be in a super-position of summer and winter. The reason why it’s not is only measurement.

So humans are collapsing the quantum state of the world (and stopping it from being summer and winter at the same time) just by living in it?

Yes, but not just humans – everything; birds, trees, molecules. Many people still don’t realise, but Newtonian physics is actually wrong. It is only because everything is constantly being measured that we see this classical world around us.

Albert Einstein famously gave birth to quantum physics and then disowned it. Why?

Einstein didn't like the idea that "God throws dice". He thought this was the implication of quantum physics because the measurement process is probabilistic. For example, the outcome of the measurement which determines through which slit the electron passes is inherently random and cannot be predicted.

Einstein didn’t like the randomness of it all. He felt there must be an underlying property of a particle, or object, which would make it what it is. He felt there must be an underlying reality, or foundation to the world.

What are the practical ways in which quantum physics will effect the world?

More recently people started to use QP to create new technology. So now it is not just important philosophically, but practically as well. The most famous quantum technology is the quantum computer. It can perform calculations much faster.

With normal computers – every few years you can buy a new one and it will have roughly doubled its speed. That sounds very impressive. But even in a hundred years – there are many problems that we will still not be able to solve.

With quantum computers it’s very different. Quantum computers will be able solve problems that modern day computers couldn’t in 100 years.

They are different because they use quantum effects – things can be in two different places at a time. Normal computers operate on bits. They have definite values. But with quantum computers each bit can have more than one value at the same time. So it can operate on many more levels.

Quantum computers that use up to eight bits have already been made – so we have proof of principle. Here at Sussex we are trying to scale this up.

 

Below are some pictures that Winfried sent us. He asked us if we wanted captions for them, but we realised that this would have been utterly pointless as there is no way you could ever understand what it is happening in these images. Just have a look and leave the complicated stuff up to Winfried.


A machine that you will never understand

Chip thing that you will never understand

Something else that you will never understand