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The beginnings of quantum computing -

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The beginnings of quantum computing

A quantum algorithm has been encoded on a chip for the first time. The approach is known as
waveguide on chip. It was first proposed in 2001. Single photons of light are controlled and store
information. Compared to an electron spin or a neutron spin, a photon doesn't react with the
environment and thus has very low noise. When used in computing, it increases the speed and power
of the computer many times. The challenge has been in preserving the information encoded.

Transcript

Robyn Williams: We begin with some news from the British Science Festival, just finished in
Guildford, about quantum computers, using a single photon. Yes, take your solitary particle of
light and insert. Sounds completely impossible, doesn't it. And the leader of the team that
achieved this incredible result is an Australian, professor of physics Jeremy O'Brien.

Jeremy O'Brien: So it shows, for the first time, a very small-scale quantum algorithm on a photonic
chip using single photons of light to encode quantum information, and then implementing a very
small-scale algorithm, and when I say 'small-scale' I mean it factors the number 15 into its prime
factors and any schoolkid can tell you what that is probably quicker than our chip, but it's an
important proof of principle.

Robyn Williams: Put this in context. Quantum computing is on a scale so much faster and bigger than
any computers we can conceive. How much more powerful would they be once they're established?

Jeremy O'Brien: We say they're exponentially faster in the sense that as the number that you're
trying to factor gets bigger and bigger, a conventional computer...the time it takes to solve that
problem grows exponentially, whereas for a quantum computer it's exponentially faster than that. So
if you make the number big enough those two things diverge increasingly, so you can't really say
it's a million times faster or 100 million times faster but...

Robyn Williams: It can be. The big problem has been trying to get those quantum elements under
control. Isn't that the real challenge with quantum computers?

Jeremy O'Brien: That's exactly right. So these quantum systems that we encode the information in
are inherently fragile, so we typically need to get very small or very cold or both to in order to
observe these unique quantum properties in a system. One of the big challenges is preserving the
information that we encode in them and then, as you say, being able to control those systems. And
we think this wave-guide on chip approach is a really promising way to control a single photon,
single particles of light for this application.

Robyn Williams: It seems boggling to think of controlling single particles of light in any way, let
alone in computing.

Jeremy O'Brien: Yes, indeed, and in fact when this scheme was first proposed back in 2001 by Knill,
Laflamme and Gerard Milburn who's at the University of Queensland. Many people regarded it as,
okay, well, that's just mathematically proven to be possible in principle but who really believes
that you can operate a computer with single photons whizzing around at the speed of light?

And over the time since, so the last six or seven years, I think it's become apparent that actually
there's a lot of advantages to using single photons and the key one is that they're very low noise,
so they tend not to interact with their environment in contrast to other systems like electron spin
or a nuclear spin or an artificial atom or something like that which tends to interest with
anything else in its environment. So that's a real positive.

The other great thing is that as well as quantum computing the other main quantum technology that
people are interested in is quantum communication and it seems pretty clear that photons are the
only logical choice for doing that, and in fact there are four or five companies around the world
who will today sell you a quantum communication system. So it's just becoming a commercial
enterprise, these quantum technologies.

Robyn Williams: So using photons...you actually started your work at the University of NSW. How
many different approaches are there around the world of different teams trying to crack this
quantum computing puzzle?

Jeremy O'Brien: I guess there's probably dozens of different ways that people are trying to do
this, but it's widely regarded that five or six of them are sort of the leading approaches, and
that's really quantified by the US government roadmap to quantum computing and the funding that
follows that roadmap. So there are sort of five or six leading approaches, and this photonics
approach is one of them. And as I said earlier, photons are sort of already in the game in terms of
the broader field of quantum technologies in that they're essential for quantum communication. And
so even if we don't ultimately realise a full-scale photonic quantum computer, it seems clear that
we'll so some sort of small-scale processing at least in these future quantum communication
networks.

Robyn Williams: When you do have quantum computers, how will the world be different?

Jeremy O'Brien: I guess this big application of factoring is also the most difficult one and the
application of that is in cracking the encryption schemes that we use currently, so it's not just a
mathematical abstraction factoring these large numbers...

Robyn Williams: You could crack any code in the world, couldn't you?

Jeremy O'Brien: At least the ones that are based on these large so-called semi-primes, so the
product of two prime numbers. Now, that might not seem like a good thing, but obviously there are
people out there who are interested in that, but the good news is that these quantum communication
systems that we already have are not subject to that insecurity, so they have security that's based
on the laws of physics itself. But that's really the very long-term goal because you'd need a
quantum computer of millions of qubits to do that sort of task, and I think that's fair to say that
that's at least 20 years into the future. In fact it's so far into the future that we can't even
really make sensible predictions of when it might happen.

But in the nearer term we can extend these current quantum communication systems which are
currently limited to point-to-point communication to something more like the internet we have today
where you have a network of different parties that can communicate securely. And there you'd need
some nodes in that network that were able to relay signals and repeat signals and so on, and that
would be a small-scale process.

And then going further into the future there's prospects for simulating quantum systems with a
quantum computer, and that's initially of somewhat esoteric interest to condensed matter physicists
who have models of systems that they can't exactly solve and so on. But it may be that we're able
to use that approach to model important materials, maybe even pharmaceuticals and so on, to provide
an exact quantum simulation of them.

Robyn Williams: So put in the quantum description of anything and out could come that material or
an object. The basis for 'beaming me up Scotty' perhaps. That paper by Jeremy O'Brien and his team
from the University of Bristol was published in the journal Science.