We usually think of light as a wave, but it's actually made of millions of tiny particles called photons – photons are the quantum particle of light – and are being harnessed to help create new technologies like quantum computers. In this animation we look at research being carried out at the University of Oxford to generate photons one at a time - no easy feat - and where this technology could take us in the future.

Why quantum?

Quantum mechanics is a theory proposed around 100 years ago, so it’s not all that new. Still, despite being around for such a long time, it still provokes debate because it is a strange theory. In quantum mechanics, many quantities we think should be definite don't always have definite values; for example, an electron in an atom typically does not exist at any specific location. Whilst this sounds weird, this is not so unfamiliar: what note is the sound of a snare drum? It doesn’t have a definite note. Or, what colour is white light? No definite colour: we know from rainbows that white light contains all colours.

In the 1980s, physicists were led to the interesting concept of “quantum information”. In a normal computer, 1s and 0s are represented by electronic charges. But in quantum theory, electron charges can be non-definite. So what if we had a computer where the bits were “quantum”, with non-definite values? This is permitted by the theory. But could such a machine be useful? It could do everything a normal computer could do, because ordinary bits are just a special case of quantum bits. There were strong hints it could also do more than a normal computer, simply because simulating how a quantum computer behaves turned out to be very difficult for our best classical computers; In fact, we now have strong theoretical evidence to suggest that a quantum computer could be vastly more powerful than a classical computer, at least for a select, but growing number, of tasks, such as weather forecasting and decryption. This has motivated decades of research into trying to build a quantum computer, something which has just recently become feasible technologically.

Why Photons?

We’ve been discussing electrons, and in theory, we could make a quantum computer based on electronics. However, quantum effects are difficult to see in standard electronic components – this explains why our computers are not already quantum computers! It is much easier to see quantum effects with light. In fact, most of us already think of light as a kind of fluid, without a definite position, so the question “what is the position of this light?” reads as a grammatical error. Actually, just as electric currents are carried by electrons, light is made up of particles called photons. True to form, they obey quantum rules, so photons do not always have definite positions. This makes them ideal as quantum bits. Moreover, they are much easier to control than electrons, since photons are not disturbed by stray electric and magnetic fields.

The one and only

As discussed in the animation, pulses of light, containing billions of photons, are already used in modern fibre-optics to send broadband data over the internet. But quantum bits must be encoded on individual photons. In fact it is an on-going research challenge that we’re working on here at the University of Oxford to produce a light source that delivers one and only one photon on demand. A single atom can be used, but it is hard to reliably hold a single atom in place. And each photon is emitted in a random direction, so these photons are hard to catch. The video explains two parallel lines of research that seek to solve this problem.

Read more about this video on the Oxford Sparks website:
http://www.oxfordsparks.ox.ac.uk/run-for-your-light