# Understanding quantum computers

# How quantum computers work and when they will become a reality

Mike Mullane

It is a common fallacy that quantum computers are faster than classical computers and it is easy to see where the idea comes from. After all, as processors have shrunk in size, computing has become quicker, which has led some to assume that quantum computers must be even faster because they work with subatomic particles. The truth is that quantum computers are not really faster, but rather do things differently, thanks to the quantum

properties of superposition, entanglement and interference.

The computers we have today store data using bits, which have two states — either on or off — represented as a 1 or a 0. They perform a Boolean function: a sort of binary logic, commonly seen in advanced search engines, that works with modifiers such as ‘AND’ or ‘NOT’. The transistor receives two incoming signals and depending on what it encounters, sends out a new electrical signal.

Computer scientists describe classical bits as “discrete”, which is the opposite of continuous. To compute more efficiently, they use a method called parallel processing, which involves splitting a computation into parts that can be executed simultaneously on different processors attached to the same machine.

Quantum computing replaces binary bits with qubits that have more states that are changing continuously. Qubits can take infinitely different values, which means they can be on, off or somewhere in between all at the same time. Superposition enables qubit-based computers to carry out far more calculations at the same time, effectively taking parallel processing to

the nth degree.

When qubits become entangled, they share all the possible combinations of the quantum states of the individual qubits, substantially boosting computational power in the process.

A by-product of superposition, quantum interference determines the computer’s function by exploiting the probabilistic behaviour of particles and waves. Notoriously difficult to explain, interference is usually demonstrated using the double-slit experiment, in which single photons are beamed through two slits on a screen. Furthermore, in the experiment, the act of

observing the photons appears to affect their behaviour. The implication is that quantum

computers may be virtually unhackable, as any attempt to eavesdrop would

probably corrupt the data.

## What is a quantum computer?

There is no single way to build a quantum computer. Among the technologies used are trapped ions, silicon quantum dots, topological qubits, diamond vacancies and photonics. They all have different strengths and weaknesses. At present, the most prevalent are gate-based computers using superconducting loops. They work in a similar way to classical computers and build on the existing semiconductor industry.

The main challenge is increasing the small number of qubits possible today to an industrial scale, which is difficult because it is a struggle to keep qubits in their quantum state. The qubits only function “coherently” when they are cooled down to mere thousandths of a degree above absolute zero, which also protects them from the destabilizing effects of radiation, light, sound, vibrations and magnetic fields. They are also prone to errors.

Only when it is possible to increase the number of qubits will we have computers powerful enough to run quantum algorithms such as Shor’s “decryption” algorithm. Until then, the focus is on developing meaningful algorithms for today’s Noisy intermediate-scale quantum (NISQ) technology.

Computers based on quantum annealing take a radically different approach. Quantum annealers run adiabatic quantum computing algorithms. Instead of allowing the entanglement of all qubits, they create an environment where only restricted, local connections are possible. When they attain superposition, they can be used to mediate and control longer-range coherences. This makes them suitable for a much narrower range of tasks, such as solving optimization problems — i.e. choosing the best solution from all feasible solutions.

Quantum annealers have already been used to solve such problems in the domains of finance and the aerospace industry, among others, with potential users limited only by the upwards of 10 million dollars cost of a quantum annealer device. As with gate-based quantum computing, decoherence is a major challenge for quantum annealers and they too require massive refrigeration units. The limited number of tasks that quantum annealers can

perform means, for example, that they are also unable to run Shor’s algorithm.

It may take another 10 to 15 years before fully functional quantum computers become a reality. There is even talk of a quantum internet, with photons carried through fibre optic cables. Scientists believe that quantum computing will bring massive benefits, such as accelerating medical research, making advances in artificial intelligence and perhaps even finding answers to climate change.

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Mike Mullane

Mike is an advocate for international standards with a background in broadcasting and communications. He writes about AI, cybersecurity and digital transformation.

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