Since the first modern computer in the 1960s, the power of computers has grown exponentially, allowing them to get smaller and more powerful simultaneously. But this process is about to meet its physical limits. Computer parts are approaching the size of an atom, and this is a problem. However, a paradigm shift in computing technology is on the horizon, a development that could not only solve existing problems but also increase the computing power available to humanity by factors of possibly millions. Quantum computing has the power to transform our fundamental perception of reality, and it is quickly becoming a reality.
 
Classical computers are simply very sophisticated calculators – they are made up of many transistors, or switches, that can represent bits (1s or 0s), depending on whether they are on or off. Transistors can allow (1) or block the flow of electrons (0), and the more transistors we have, the more information we can store. However, as we try to cram more transistors into successive generations of chips, the transistors are getting so small that if they got any smaller, they would be unable to block the electrons.
 
Therefore the next era of computing will be quantum computing. In quantum computing, a qubit, or quantum bit, can be both 0 and 1 in a probabilistic, unknown state until measured – this is known as quantum superposition. A qubit contains a coefficient for the likelihood of being 0 and a coefficient for the likelihood of being 1. Only when the qubit is measured, or observed, does it drop out of superposition and take one position or the other. For example, if you hold a coin in your hand, it can either be heads or tails – like normal bits. But if you toss it, it has a chance of landing on heads, and a chance of landing on tails. Until you measure it, by catching the coin, it can be either. If we have 2 normal bits, they can take the values 00, 01, 10 or 11 – two qubits take all these values, or states, at once. Therefore, n qubits can represent 2ⁿ bits. A qubit allows for uncertainty and that’s what makes quantum computers so powerful.
 
Furthermore, particles can become coupled in probabilistic outcomes in a phenomenon called quantum entanglement. Normally, if you flip two coins, the result of one coin toss does not affect the other – they are independent. With entanglement, two particles are linked together, even if they are physically separate. If one comes up heads, the other one will also be heads. This sounds silly, and in fact, even physicists don’t fully understand how or why it works. But in the realm of quantum computing, it means that you can move information around, even if it contains uncertainty. And if you can string together multiple qubits, you can tackle problems that would take our best computers millions of years to solve. But how is this new technology useful in the real world?
 
Quantum computing holds much promise in the future, as superposition and entanglement help enormously with simulations and optimisation problems. The physical world behaves according to quantum mechanics. Therefore, if we want to simulate physical phenomena on a computer, we should use quantum mechanical principles as well. Quantum computers are far more efficient than supercomputers as they use the principles of quantum mechanics to better approach problems. This allows them to perform some computations at blistering speeds. In 2019, Google claimed to have designed a machine that took 200 seconds to solve a problem that would take the world's fastest supercomputer 10,000 years, an achievement they called quantum supremacy.
 
Despite this, it is important to get a common misconception out of the way. Most people, upon hearing that quantum computers can solve some problems trillions of times faster, mistakenly assume that quantum computers are universally computationally faster than classic computers. And that is simply wrong. Currently, quantum computers are only useful for a narrow set of uses, and most everyday processing will be better handled by conventional computers. So don’t expect to plug a quantum processor into your MacBook and do everything millions of times quicker; we’ll likely always have both classical and quantum in the future.
 
The first major application of quantum computing is for optimisation problems. An optimisation problem is any problem where your goal is to find the optimal solution with respect to certain variables and constraints, such as escaping a maze. If you ask a normal computer to navigate a maze, it will explore each path one by one, until it finds the right path. This is a very time-consuming process. Now consider a quantum computer navigating the same maze. Remember that a quantum particle has the unique property of being able to represent many states at the same time, due to the principle of quantum superposition. Therefore, a quantum computer can go down every path of the maze at once, allowing it to navigate the maze in exponentially less time than the classical computer. Therefore, quantum computers could optimise the supply-chain network for efficient manufacturing and distribution, the production of fertilisers to reduce world hunger, and much more.
 
The second major application of quantum computers is in quantum simulations. By mimicking the complex chemical and physical processes of nature at the atomic level, they can create, simulate, and design molecular structures down to the atomic level. In medicine, for example, scientists can simulate how a new drug will work in a human being — without first testing on humans or animals. They could be used to design more energy-efficient materials. In particular, researchers hope that it will provide more insight into superconducting materials, which can transport electricity without losing energy.
 
Cryptography will be another key application. Currently, many encryption systems force computers into factoring large numbers to decrypt an encryption (a thing called Rivest Shamir Adleman encryption), because, for classical computers, it’s slow, expensive, and impractical. But quantum computers can do it easily, putting our data at risk. The solution is quantum encryption, which exploits the exciting properties of quantum mechanics. The best-known quantum encryption so far is the quantum key distribution (QKD), which uses the method of quantum communication to determine a shared key between two end-nodes. The magic of QKD comes because the mere act of listening in on this communication will cause changes in the connection, which provides absolute security to the communication channel. This relies on the uncertainty principle – the idea that you can’t measure something without influencing the result. To break quantum encryption, you would have to break quantum physics itself. Now, this all sounds fantastic, but building a quantum computer is incredibly complex.
 
Quantum computers are inexplicably fragile. Almost anything can knock a qubit out of the delicate state of superposition; even a molecule of air or a light particle can render them ineffective. This is why the quantum chip in the laboratories of both IBM and Google is located at the bottom of a freezer in a large cabinet with components made of gold and copper, which cool the chip to absolute zero. This construction, called a cryostat, is the only thing that allows researchers to perform calculations on a quantum chip at all.
 
Our current technology is not advanced enough to operate a completely reliable quantum computer. Most big breakthroughs so far have been in controlled settings or using problems that we already know the answer to. Researchers have made great progress in developing the algorithms that quantum computers will use. But the devices themselves still need a lot more work. Quantum computing could change the world - but we still have a while to wait before quantum computers can do all the things they promise.