Quantum computing: what it actually is, and how far it has really come

Why it matters

Quantum computing is often described as the next great leap in computing power, capable of cracking encryption, designing new drugs and revolutionising logistics almost overnight. The reality is more nuanced. The technology could eventually reshape fields such as chemistry, materials science, cryptography and optimisation, which is why governments, including the UK’s, have made it a strategic priority alongside AI and other deep technologies. But it remains an early-stage science with formidable engineering hurdles still to clear. Understanding what quantum computers actually do, and what they do not yet do, matters for anyone trying to judge the claims made about them.

Bits versus qubits

Ordinary computers store information as bits, which are either 0 or 1. Quantum computers use qubits, which can exist in a mixture of both states at once, a property known as superposition. Qubits can also be linked through entanglement, so that the state of one qubit depends on another even when they are physically separated. These properties allow a quantum computer to explore many possible solutions to a problem simultaneously, at least in principle, rather than working through them one at a time as a classical computer does.

This is not simply a faster version of existing computing. For most everyday tasks, such as browsing the internet or running a spreadsheet, classical computers remain far more practical and efficient. Quantum computers are expected to offer an advantage only for specific categories of problem, such as simulating molecules for drug and materials discovery, optimising complex systems like supply chains, or breaking certain types of cryptographic code. Even for these tasks, the advantage depends on building machines that are both large enough and reliable enough to outperform the best classical alternatives, which has not yet been convincingly demonstrated for problems of real practical value.

Where the technology actually stands

Qubits are extraordinarily delicate. They can be built from superconducting circuits cooled to near absolute zero, trapped ions held in electromagnetic fields, photons of light, or other exotic physical systems, and each approach has its own advocates and trade-offs. Whatever the method, qubits are prone to losing their quantum state through interactions with their environment, a problem known as decoherence. This introduces errors that accumulate quickly as more qubits are added, which is why today’s machines are often described as being in the “noisy intermediate-scale quantum” era, or NISQ. They have enough qubits to run experiments and demonstrate quantum effects, but not enough error correction to run large, reliable calculations that would clearly beat classical computers on useful problems.

Building effective error correction is widely seen as the central challenge. It requires combining many physical qubits into a smaller number of highly stable “logical” qubits, which in turn demands vastly more hardware than exists in current machines. Researchers across academia and industry are making steady progress on error rates, qubit connectivity and control systems, and there have been genuine milestones in demonstrating specific quantum effects at small scale. However, credible experts in the field are generally cautious about timelines, and are wary of claims that fault-tolerant, commercially transformative quantum computers are imminent.

A realistic outlook

The most credible near-term applications involve hybrid approaches, where quantum processors handle a small part of a calculation while classical computers do the rest, particularly in chemistry simulations and optimisation problems where even modest improvements could have value. Cryptographers are also taking the long-term threat seriously, since a sufficiently powerful quantum computer could eventually break widely used encryption methods, which is why work on “post-quantum” cryptography, designed to resist quantum attacks, is already under way regardless of when large quantum machines arrive.

In the UK, government-backed research programmes and national facilities support work on quantum hardware, software and applications, reflecting a view that the country should build capability now rather than wait for the technology to mature elsewhere. For the general reader, the sensible approach is to treat quantum computing as a serious long-term research effort with real scientific promise, rather than a technology on the verge of arriving. Genuine breakthroughs are likely to be incremental, technical and unevenly reported, so claims of sudden transformation are usually worth treating with caution.