Dominic Walliman – Doctoral Research Film of the Year 2010

Duration

4.13 mins

Transcript

Modern day computers are pretty powerful things but it’s getting increasingly difficult to make them any more powerful because it’s harder and harder to miniaturise the basic components that make them up, the transistors.  So people have been looking at different models of computing to try and overcome this problem and the most promising of these is quantum computing.

Quantum computers work in a fundamentally different way to normal computers.  A normal computer is essentially just a series of switches or bits that are either off or on, 0 or 1.  In a quantum computer the fundamental unit is a qubit and in this case they can be in a state 0 or 1 or a mixed state where they’re both 0 and 1 at the same time.   This is a property unique to quantum mechanics and it’s what gives quantum computers the ability to solve certain problems a lot faster than normal computers.   Now to make this a bit more understandable I’ll give you an example – the travelling salesman problem.

Here we have a salesman who wants to visit a series of shops around town but he wants to find the route that will get him round them all in the shortest distance. The way to solve this using a normal computer is to just step by step try all the different possibilities until you’ve found the one with the shortest distance. The quantum computer would solve this in a completely different way. Because of the special mixed state, they’re able to try all of the different  possibilities at the same time and in one step find the shortest route.   This difference becomes even bigger as you add more shops. For a normal computer, adding one extra shop means you have to try way more combinations before you find the fastest route.  For a quantum computer it can still do it all in one step, which is pretty amazing. 

So if quantum computers are so good, why don’t we all have one?  Well, it turns out they’re pretty difficult to make and let me show you the kind of things you’d need to build one. Because quantum computers use quantum mechanics to do their computations, they’re incredibly sensitive to any sort of external noise. Quantum mechanics is basically the laws of Physics that dictates sort of the smallest things in our universe, so things like atoms, electrons, sub-atomic particles, that kind of thing. So if you’re doing measurements in the quantum world you need incredibly sensitive equipment and this is the sort of thing that you would need to use.

So the first thing we need to do is isolate our qubits from any sources of external electromagnetic radiation and this can come from external electronics or mobile phones and to do this what we do is we house our qubits in a sealed metal box and that stops any of the radiation from getting in and affecting the measurements.  Another source of noise is basically temperature, so to eliminate this we need to cool the qubits down to a few milli-degrees above absolute zero, like -270°C. To do this we place the probe into one of these which are basically like giant thermos flasks full of liquid helium and they cool the qubits down to very low temperatures. And finally even after you’ve done all of this, you still have the problem of how to measure the state of your qubits. This is the most difficult part because the leads that are used to make the measurements can bring their own noise to disturb the system. So what you need to do is use really sensitive electronics and also employ extensive filtering of the leads used to measure the qubits.

So that’s it, quantum computers are still in their infancy and the ones that have been built so far have only really started to do useful things. However, as experimental techniques get better and better, quantum computers may well revolutionise the way we do computing.

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