## What is Quantum Computing

There is an international race to build a quantum computer that transcends the capacity of conventional computers and to build ultra-secure communication networks – a race that has been called the space race of the 21st century.

These technologies have the potential to transform the information economy and create the industries of the future, solving in hours or minutes problems that would take conventional computers – even supercomputers – centuries, and tackling otherwise intractable problems that even supercomputers could not solve in a useful timeframe.

Present-day computers are really fast, and they are getting very powerful, however they aren’t fast enough to perform all of the calculations that we need them to in a useful time frame.

Quantum computers use quantum mechanics to perform certain complex calculations in a smaller number of steps than an ordinary computer. However, not all algorithms run faster on quantum hardware – only certain ones with particular features. Identifying exactly which problems can benefit from quantum computing is an active area of research today.

Potential applications include machine learning, scheduling and logistical planning, financial analysis, stock market modelling, software and hardware verification, rapid drug design and testing, and early disease detection and prevention.

A 2020 report from CSIRO revealed that quantum computing in Australia has the potential to create 10,000 jobs and A$2.5 billion in annual revenue by 2040, while spurring breakthroughs in drug development, industrial processes, and machine learning.

A quantum computer is a machine that performs its calculations by harnessing the unique features of quantum mechanics.

In ordinary computing, information is stored in bits, and each bit stores either a 0 or a 1. Many bits together can represent all sorts of information using binary code, which computers can process.

Quantum computers process quantum information, which is stored in quantum bits, called qubits (pronounced “KYU-bits”). A qubit can be any quantum object with two states – for example, a single electron (spin up or spin down) or a single photon (polarised horizontally or vertically).

In everyday life, we usually have a good intuition regarding how the physical world will behave. Drop a glass and it will smash on the floor. Punch a concrete wall and your fist won’t go through it. But in the world of the ultra-small – atoms and electrons – none of the normal rules apply. Instead particles follow quantum rules that are quite baffling.

Like a bit, a qubit can be in one of its two states, labelled 0 or 1, but unlike a bit, a qubit can also be in a superposition of 0 and 1. Superposition is a subtle concept. Measuring a qubit always gives either 0 or 1, but superpositions can be manipulated beforehand so that one of the two outcomes is more likely.

Multiple qubits together can be put into more complicated superpositions. Measuring the qubits always gives a binary string of 0s and 1s, but the likelihood of what string appears can be controlled beforehand, and this is what a quantum computer does.

In fact, quantum computers work by first creating a superposition of lots of different possible solutions to a problem – encoded in qubits – and then manipulating that superposition so that wrong solutions cancel out and right ones are strengthened. This is because the alternatives in a superposition can interfere like waves do. This makes the right answer much more likely to appear when you measure the qubits. For certain types of problems, these two steps can be completed very quickly – outperforming any ordinary computer in solving the original problem.

Building the quantum computer hardware that will work reliably, and is large enough, to process quantum information without errors is a big challenge. Worldwide there is a huge experimental effort to do just that. There are many different designs being explored to build a universal quantum computer – some of these include superconducting circuits, ion traps, optics, and silicon.

In Australia, the Centre for Quantum Computation and Communication Technology (CQC²T) is a world leader in two of the most promising types of hardware for a quantum computer: optical qubits (made of light) and silicon qubits (made of either nuclear or electron spins).

A large-scale universal quantum processor capable of outperforming today’s computers for a wide-range of useful applications needs to have millions of qubits and very few errors.

Small-scale quantum computers called noisy intermediate-scale quantum (NISQ) processors already exist and can be accessed through the internet – i.e., through “cloud quantum computing.” These devices are currently relatively small in qubit number and error prone but are very important in pointing the way forward. To achieve commercial success, we require larger-scale quantum computers with error correction, and that is likely to take at least another 5 years and will continue to improve over the next decade.

Quantum Communication technology has the potential to send messages securely against any sort of hacker, no matter how powerful their computer is – even a quantum computer! The basic idea is simple. Heisenberg’s Uncertainty Principle implies that if you find out one property of a particle you necessarily create uncertainty in other properties. That is, quantum particles are disturbed by measurements. Because of this, an eavesdropper trying to read a secret message encoded in photons will leave unmistakable traces of this transgression on the message itself. These traces clearly reveal the attempt to eavesdrop, ensuring detection before any of your valuable information is compromised.

Quantum communication protocols were first developed in the 1980s. There are short-range systems in commercial operation in many countries, including an Australian one developed by CQC²T researchers. Recently ground-satellite quantum encryption links have also been demonstrated by scientists in USTC, China [Sheng-Kai Liao et al., ‘Satellite-to-ground quantum key distribution’, Nature, 2017, 549:43; Ji-Gang Ren et al., ‘Ground-to-satellite quantum teleportation, Nature, 2017, 549:70.) and MPL, Germany [K Günthner et al., ‘Quantum-limited measurements of optical signals from a geostationary satellite’, Optica, 2017, 4:611].

A grand challenge, which is being tackled worldwide, including at CQC²T, is to extend the range of secure communications into a global network. Because quantum messages cannot be copied, this requires using quantum repeaters to realise a large-scale quantum network. Analogous to the fibre repeater links in global fibre optics networks, quantum repeaters are special-purpose quantum devices that bridge a connection between a distant quantum source and receiver are critical infrastructure for a globally connected quantum network. Designing and making them – and showing their viability – is an active area of research.