*Interest in quantum computers is taking off as governments and big business look for ways to gain a competitive edge through cutting edge technology.*

*In this three-part series, Information Age looks at how quantum computers work, their applications, and the continued role Australian scientists and engineers play in the technology's development.*

In a 1981 lecture, Nobel Prize winning physicist Richard Feynman described a probabilistic computer that physicists could use to more accurately simulate the quantum world.

The problem, as presented by Feynman, was that computers as we still understand them today are unsuitable for accurately simulating quantum mechanical systems, in large part because these systems are inherently probabilistic.

Accurately simulating quantum systems using a deterministic computer is an intractable problem; the cost of computational resources (time, memory, etc.) grows exponentially with the size of the system it is trying to simulate.

“Nature isn't classical, damn it,” Feynman concludes his lecture. “And if you want to make a simulation of nature, you'd better make it quantum mechanical.

“And by golly it's a wonderful problem, because it doesn't look so easy.”

Fast forward to 1999.

Sydney was preparing for the 2000 Olympic Games, a golden age for the city as it took the international spotlight to show Australia off as a vibrant, culturally rich, modern nation; the envy of the world.

This was the backdrop in which a young physicist named Michelle Simmons had arrived from the UK to work on that wonderful problem of building a quantum computer.

Professor Simmons’s pursuit would see her become one of the founders the Australian Research Council’s Centre of Excellence for Quantum Computer Technology as she helped build in Sydney an ecosystem of quantum expertise that would output some of the field’s foremost minds for decades.

“Australia was the first group of people to really get into quantum computing in a big way,” Professor Simmons told *Information Age*.

“In the '90s there were small research teams scattered in the US or in Europe.

“But the [Centre of Excellence for Quantum Computer Technology], around 100 people getting together, was the first coordinated effort to try and build it.”

It was an unheard of attempt to build what was, at the time, still a largely theoretical device.

“The rest of the world was like us ‘We don’t even know if this is real yet, what are you doing?’” Professor Simmons said.

“They thought it was hilarious that [these Australians] were going into it at full tilt.”

Australia has since become a feeding ground for big tech companies and universities in the race to build quantum computers.

It’s a source both of pride for the local quantum community, and a source of concern that Australia could throw away its advantage after decades of leading the research in this emerging technology.

We’ll come back to Australia’s role in quantum computing later in the series, but first let’s understand what a quantum computer is, or importantly, what it isn’t.

**Not your average computer**

A classical computer – like those in the device you are reading this article on right now – manipulates information in the form of bits (0s and 1s) through logic gates (AND, NOT, OR, XOR).

A quantum computer holds information in qubits which are, when measured, also a collection of 1s and 0s.

But there is no standardisation of what exactly a qubit is. Many research teams take different approaches to developing qubits, including the polarisation or timing of a photon, or the charge of an electron.

Professor Simmons and her team at Silicon Quantum Computing use the spin of an electron for their qubits.

“If you put a magnetic field across an atom with one electron on it, and put it in a perpendicular magnetic field, the electron will either align with the field or against the field – that would be a one and zero,” she explained to *Information Age*.

“So if you imagine a sphere, we can call it the north pole and the south pole.

“In the quantum world we can pulse that electron so that it points to somewhere else on the surface of the sphere.

“If you put it halfway between the poles, then it’s in a superposition, essentially, of the up and down states.”

**Superposition**

Qubits are often represented using Bloch spheres which can describe these superpositions and the evolution of a qubit after it has been manipulated using different gates.

‘Superposition’ is a concept that has somewhat muddled the popular understanding of what a qubit is and how quantum computers operate.

Often you will see the superpositional state of a qubit as being ‘both 1 and 0’, giving qubits a kind of ontological mystique by virtue of this apparent physical contradiction.

But underneath all of quantum mechanics is a wave function, the Schrödinger equation, and wave functions – far from being relegated to the quantum realm – also describe effects in the macroscale world.

Professor Philip Moriarty from the University of Nottingham, UK, has suggested we think about a quantum state as being analogous to a guitar string where waveforms produced by a guitar string are also a superposition of resonances.

The point being that a ‘superposition’ is all maths – it’s waves, it’s linear algebra, it’s vectors – and as such the mathematical representations of qubits can be combined to create interesting effects.

When measured, a qubit in a superposition will collapse to be either 1 or 0.

And it does this based on chance.

**Playing dice with the universe**

There is an element of randomness in quantum mechanics that marks a radical departure from the physics that preceded it, and it is this randomness that makes the search for quantum computers so valuable.

Whether a single qubit will collapse into being 1 or 0 depends on its superposition. An electron halfway between the poles on a sphere, as mentioned earlier, might have a 50-50 chance of being 1 or 0.

But qubits can also interact with one other, become entangled, and move around that theoretical sphere in a way that affects how the total system will collapse into a given state.

Imagine a classical computer with two bits. At any time, it can be in one of four states (00, 01, 10, or 11) which depends on the state it was in previously and the way its information (bits) were manipulated.

Now suppose instead the computer has qubits where each potential state has an arbitrary probability totalling 100 per cent.

Say, for the sake of illustration, that there is a 25 per cent chance of each state occurring. Or that the state 00 has a 90 per cent chance, 01 a 10 per cent chance, and the final two no chance. Or that 00 and 11 each have a 50 per cent chance of occurring. Or some other combination of four percentages that total 100.

Now add another qubit; the number of possible states has doubled. Add another one; it doubles again. With each extra qubit the amount of potential probabilities for each possible state greatly increases.

**Quantum advantage**

Here is the where classical computers start to run up against a hard limitation.

The exponential growth of the amount of computational resources required to simulate more and more qubits means there is a point at which a fully realised quantum computer will outperform the most powerful computers in certain tasks.

Google claimed to have hit this point – which it called ‘quantum supremacy’ – in 2019.

Those researchers created a specific problem related to sampling its quantum processor’s output, a task they estimated would take the world’s fastest supercomputer “10,000 years”.

It was a bespoke challenge that alone doesn’t have any real-world applications.

But the result is a step toward what’s known as ‘quantum advantage’, a point when businesses, governments, scientists decide that using a quantum computer produces better outcomes than a classical computer.

What kinds of problems do we expect to see this new form of computing solve?

*Find out in Part Two of this series next Thursday.*