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In June, an IBM computing executive said quantum computers were entering the “utility” phase, in which high-tech experimental devices become useful. In September, Australia’s chief scientist, Cathy Foley, went so far as to declare “the dawn of the quantum age.”

This week, Australian physicist Michelle Simmons won the country’s top science award for her work developing silicon-based quantum computers.

Obviously, quantum computers are having their moment. But, backing up a bit, what exactly are they?

## What is a quantum computer?

One way to think about computers is in terms of the types of numbers they work with.

The digital computers we use every day are based on integers (or integers), which represent information as strings of zeros and ones that they rearrange according to complicated rules. There are also analog computers, which represent information as continuously varying numbers (or real numbers), manipulated by electrical circuits or rotating rotors or moving fluids.

In the 16th century, Italian mathematician Girolamo Cardano invented another type of number called complex numbers to solve seemingly impossible tasks, such as finding the square root of a negative number. In the 20th century, with the advent of quantum physics, it turned out that complex numbers also naturally describe the fine details of light and matter.

In the 1990s, physics and computer science collided when it was discovered that some problems could be solved much faster with algorithms that worked directly with complex numbers encoded in quantum physics.

The next logical step was to build devices that run on light and matter to do those calculations automatically. This was the birth of quantum computing.

## Why is quantum computing important?

We usually think of the things our computers do in terms that mean something to us: balancing my spreadsheet, streaming my live video, finding my transportation to the airport. However, these are all ultimately computational problems, expressed in mathematical language.

Since quantum computing is still a nascent field, most of the problems we know quantum computers will solve are expressed in abstract mathematics. Some of them will have “real world” applications that we cannot yet foresee, but others will find a more immediate impact.

One of the first applications will be cryptography. Quantum computers will be able to crack current Internet encryption algorithms, so we will need quantum-resistant cryptographic technology. Provably secure cryptography and a fully quantum Internet would use quantum computing technology.

In materials science, quantum computers will be able to simulate molecular structures at the atomic scale, making it faster and easier to discover new and interesting materials. This can have important applications in batteries, pharmaceuticals, fertilizers and other chemistry-based domains.

Quantum computers will also speed up many difficult optimization problems, where we want to find the “best” way to do something. This will allow us to address larger scale problems in areas such as logistics, finance and weather forecasting.

Machine learning is another area where quantum computers can accelerate progress. This could happen indirectly, by speeding up subroutines in digital computers, or directly if quantum computers can be reinvented as learning machines.

## What is the current panorama?

In 2023, quantum computing will move out of the underground laboratories of university physics departments and into industrial research and development facilities. The move is backed by the checkbooks of multinational corporations and venture capitalists.

Contemporary quantum computing prototypes (built by IBM, Google, IonQ, Rigetti, and others) are still far from perfection.

Current machines are modest in size and susceptible to errors, in what has been called the “intermediate-scale noisy quantum” phase of development. The delicate nature of small quantum systems means that they are prone to many sources of error, and correcting these errors is a major technical hurdle.

The holy grail is a large-scale quantum computer that can correct its own errors. An entire ecosystem of research factions and commercial companies pursue this goal through various technological approaches.

## Superconductors, ions, silicon, photons.

The current approach uses loops of electrical current within superconducting circuits to store and manipulate information. This is the technology adopted by Google, IBM, Rigetti and others.

Another method, “trapped ion” technology, works with clusters of electrically charged atomic particles, using the particles’ inherent stability to reduce errors. This approach has been spearheaded by IonQ and Honeywell.

A third avenue of exploration is to confine electrons within small particles of semiconductor material, which could then be fused into the well-established silicon technology of classical computing. Silicon Quantum Computing is following this angle.

Yet another direction is to use individual particles of light (photons), which can be manipulated with high fidelity. A company called PsiQuantum is designing intricate “guided light” circuits to perform quantum calculations.

There is no clear winner yet among these technologies, and it may well be a hybrid approach that ultimately prevails.

## Where will the quantum future take us?

Trying to forecast the future of quantum computing today is akin to predicting flying cars and ending up with cameras on our phones. However, there are some milestones that many researchers would agree will likely be reached in the next decade.

Better bug fixing is one of the most important. We hope to see a transition from the era of noisy devices to small devices that can sustain computing by actively correcting errors.

Another is the advent of post-quantum cryptography. This means establishing and adopting cryptographic standards that quantum computers cannot easily break.

Commercial benefits from technologies such as quantum sensing are also on the horizon.

A true “quantum advantage” will also likely be demonstrated. This means a compelling application where a quantum device is indisputably superior to the digital alternative.

And an ambitious goal for the next decade is the creation of an error-free (with active error correction) large-scale quantum computer.

When this has been achieved, we can be sure that the 21st century will be the “quantum age.”