Quantum computing explained simply: what it can and cannot do
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Quantum computing attracts more breathless headlines than almost any other technology. Some say it will break the internet's security, cure diseases and make today's supercomputers obsolete. The reality is more interesting and more limited. Here is what quantum computers are, what they might be good at, and why they will not replace your laptop.
Bits and qubits
An ordinary computer stores information as bits, each either 0 or 1. A quantum computer uses qubits, which follow the rules of quantum physics. Three ideas matter:
- Superposition. A qubit can be in a combination of 0 and 1 until it is measured, when it gives a definite answer.
- Entanglement. Qubits can be linked so that their measurement results are correlated in ways ordinary bits cannot be.
- Interference. Quantum algorithms are designed so that the paths leading to wrong answers cancel out and those leading to right answers reinforce each other.
A common misunderstanding is that a quantum computer simply tries every answer at once. It does not. It only helps when a clever algorithm can use interference to make the right answer likely when you measure.
What they could be good at
Quantum computers are expected to outperform ordinary machines only on certain kinds of problem:
- Simulating molecules and materials. Nature is quantum, so modelling chemical reactions, catalysts or new materials is a natural fit. This is widely seen as one of the most promising early uses.
- Factoring large numbers. Shor's algorithm, published in 1994, showed that a large enough, reliable quantum computer could break widely used public-key encryption such as RSA.
- Some search and optimisation problems. Grover's algorithm gives a more modest speed-up for unstructured search. Claims of dramatic gains in logistics or finance are much less certain.
What they cannot do
- They will not make everyday tasks such as browsing, gaming or word processing faster.
- They are not general replacements for classical computers; they would work alongside them, like a specialised accelerator.
- For many problems, there is no known quantum speed-up at all.
Why they are so hard to build
Qubits are extremely fragile. Heat, vibration and stray electromagnetic signals cause errors, a problem called decoherence. Many designs need to be cooled close to absolute zero. Leading approaches include superconducting circuits, trapped ions, neutral atoms and photons, each with different trade-offs.
The key challenge is error correction: combining many physical qubits into one more reliable logical qubit. A milestone came in late 2024, when Google reported in Nature that with its Willow chip, errors fell as it used larger groups of qubits for each logical qubit, a long-sought result known as operating below threshold. Researchers describe this as an important step rather than a finished, useful machine. Running algorithms that matter in practice is generally thought to require far larger numbers of high-quality qubits than exist today.
Quantum advantage claims
You will see announcements that a quantum computer has done something no classical computer could manage in a reasonable time. These are usually carefully chosen test problems with no practical use, and classical researchers have sometimes found faster methods afterwards. They show progress, but they are not the same as solving a real-world problem better.
What about encryption?
Even though large code-breaking machines do not exist yet, governments are preparing now. One concern is that attackers could store encrypted data today and decrypt it once quantum computers are capable. In 2024 the US National Institute of Standards and Technology (NIST) published its first post-quantum cryptography standards, FIPS 203, 204 and 205, designed to resist quantum attacks. Technology companies are gradually adding these methods to browsers, messaging apps and operating systems. For ordinary users, keeping software updated is the main thing to do.
How to read the news
- Ask whether a claim involves physical or error-corrected logical qubits; the numbers are not comparable.
- Treat qubit counts alone with caution; quality and error rates matter as much as quantity.
- Be sceptical of firm dates for world-changing uses.
What would you most like quantum computers to tackle if they work as hoped? Share your thoughts in the comments.