What is quantum computing?
What quantum computing is, why people are building quantum computers, what they might be good for, and how to tell the real promise from the hype.
What you’ll learn
- Explain how a quantum computer differs from an ordinary one.
- Say why "it tries every answer at once" is wrong.
- Name the problem types where quantum computers might help, and how strong the evidence is.
Prerequisites: None
On this page
- Ordinary computers, in one paragraph
- The problems that grow too fast
- Feynman's idea: use nature to simulate nature
- What makes a quantum computer different
- Where the comparison with waves breaks down
- What quantum computers might be good for
- Where things stand today
- Not to be confused with
- Common questions
- Sources and further reading
Quantum computing is a different way of processing information, built on the rules that govern atoms, electrons and light. It isn't a faster version of the computer you're using now, and it won't replace it. It's a specialist tool that may one day solve a small set of problems that ordinary computers can't handle in any reasonable time. This lesson explains what that means, why people are building these machines, and how to tell the real promise from the hype. There's no maths in it.
Ordinary computers, in one paragraph
Every computer you've used, from a phone to a supercomputer, stores information as bits. A bit is a tiny switch that is either off or on, written 0 or 1. Your photos, messages and this page are all long strings of them. A computer works by following instructions that flip and combine bits, billions of times a second. It's an extraordinarily good design, and for almost everything people do with computers it's the right one.
The problems that grow too fast
Some problems get harder much faster than computers get better. A good example is working out exactly how the electrons in a molecule behave. Electrons follow the rules of quantum physics, and an exact description has to keep track of every way they can be arranged together. Each extra place the electrons can be found roughly doubles the work, so the effort doubles again and again as the molecule grows.
Chemists get around this with clever approximations, and those work well much of the time. But for some important molecules, such as new battery materials or the enzyme that some bacteria use to make fertiliser, the approximations break down exactly where the interesting chemistry happens. No ordinary computer, however large, can simply calculate the exact answer.
Feynman's idea: use nature to simulate nature
In 1981 the physicist Richard Feynman gave a lecture with a simple suggestion. If nature runs on quantum rules, and quantum rules are so hard for ordinary computers to imitate, why not build a computer that follows quantum rules itself? Such a machine wouldn't have to imitate the quantum world step by painful step. It would behave like it.
That idea, published in 1982, is where quantum computing began. In 1994 the mathematician Peter Shor showed that the same kind of machine could also solve a famous mathematical problem, factoring very large numbers, far faster than any known ordinary method. The field has been growing ever since.
What makes a quantum computer different
A quantum computer replaces bits with qubits. You'll meet them properly in the next lesson, but here is the short version, in three ideas.
- A qubit can hold a blend of 0 and 1 until you look at it. This is called superposition. The blend has a weighting that the computer can steer.
- Looking gives you one answer. When you measure a qubit you get a plain 0 or 1, and the blend is gone. You never get to read the whole blend directly.
- The blends can be made to cancel or reinforce, a bit like waves on water. This is called interference. Qubits can also be linked so they behave as one system, which is called entanglement.
A quantum program is a carefully designed sequence of steps that steers all those blends so that, by the end, the wrong answers have cancelled out and the right answer has built up. Then you measure, and the right answer is the one you're likely to get.
A quantum computer doesn't try every answer at once and pick the best. It arranges for wrong answers to cancel, so the right one is likely when you look.
That last point is worth stressing, because the opposite is the most common myth about quantum computing. If a quantum computer simply held every possible answer and handed you one at random, it would be no better than guessing. Interference is one important way quantum algorithms make useful results more likely. Another promising use is modelling quantum systems using hardware that follows quantum rules. Neither offers a speed-up for every problem.
Where the comparison with waves breaks down
Waves are a helpful picture for interference, and a coin spinning in the air is a helpful picture for a blend that settles into one result when it lands. Both pictures break down if you push them. A spinning coin is secretly heads or tails at every moment; a qubit isn't. And no ordinary wave can be linked across a room the way entangled qubits can. Use the pictures to get started, then let the next lessons replace them with the real thing.
What quantum computers might be good for
Researchers expect quantum computers to help with a short list of problem types. The evidence for each is very different.
| Problem type | What a quantum computer might do | How strong is the case today? |
|---|---|---|
| Simulating molecules and materials | Model chemistry that ordinary computers can only approximate | The original motivation and widely seen as the most promising, but it needs larger, more reliable machines than exist now |
| Factoring very large numbers | Run Shor's algorithm, which would break some of today's encryption | Shown in theory in 1994; needs a large error-corrected machine that doesn't exist yet |
| Searching unstructured possibilities | A modest speed-up (Grover's algorithm) | Proven in theory, but the gain often shrinks once real-world costs are counted |
| Optimisation and machine learning | Possibly better answers to some hard planning and pattern problems | Unproven; an active research question, often overstated in marketing |
The factoring row explains why you may have heard about quantum computing in the news about security. If a large enough machine is ever built, it could unlock some of the encryption that protects today's internet. New forms of encryption designed to resist quantum computers already exist and are being rolled out now. The security lessons, starting with Harvest now, decrypt later, explain this step by step.
Where things stand today
Today's quantum computers are real, and anyone can run small programs on one over the internet. But they are small and noisy. Their qubits are so delicate that heat, vibration or a stray signal can scramble a calculation, and even the best of them make an error roughly once every few hundred to a few thousand operations. The physicist John Preskill named this stage of the technology NISQ, for "noisy intermediate-scale quantum": big enough to be scientifically interesting, too noisy for the famous algorithms.
The fix is called error correction: combining many fragile qubits so they behave as one reliable qubit. It has been shown to work in small experiments, but it multiplies the hardware needed. That's why useful machines are still some way off, and why nobody can honestly give a firm date. Experiments have shown quantum machines outpacing ordinary computers, but mostly on tasks chosen to suit the hardware rather than problems anyone needed solved.
Not to be confused with
Quantum computing is one of several quantum technologies. They are often mentioned together, but they do different jobs.
- Quantum communication uses the quantum behaviour of light to send information in ways that reveal eavesdropping. Quantum key distribution is the best-known example.
- Quantum sensing uses the quantum behaviour of atoms or light to measure things extremely precisely, such as magnetic fields.
- Post-quantum cryptography isn't quantum technology at all. It's ordinary encryption, maths running on ordinary computers and chips, designed to stay safe even against future quantum computers.
Common questions
Will I have a quantum computer at home? Almost certainly not, and you wouldn't need one. Many quantum computers need extreme conditions, such as cooling to near absolute zero or a near-perfect vacuum, and they're used over the cloud alongside ordinary computers.
Is quantum computing just hype? No, but there is a lot of hype around it. The underlying science is solid and decades old, and the machines exist. What's uncertain is how soon they'll be large and reliable enough to be useful, and for which problems.
Do I need physics or maths to understand it? No. The ideas can be explained in plain words, which is what these lessons do. A few later lessons use small numbers, and say so first.
What you’ve learned
- A quantum computer is a specialist machine that follows quantum rules; it isn't a faster ordinary computer.
- Its power comes from steering blends so wrong answers cancel, not from trying every answer at once.
- Today's machines are small and noisy; the strongest expected uses are simulating nature and, eventually, factoring.
Check your understanding optional
A friend says a quantum computer is powerful because it tries every possible answer at the same time. What's wrong with that?
Show the answer
Measuring a quantum computer gives you just one answer, so holding every answer and handing one back at random would be no better than guessing. The power comes from steering the calculation so wrong answers cancel and the right one becomes likely.
Written by Muhammad Atif · Reviewed 19 October 2026 · How sources are chosen and checked · Report a correction
Sources and further reading
Tags show what kind of source each one is. A standard or government guidance is an official document; a peer-reviewed paper has been checked by other experts; a preprint has not been peer-reviewed yet; an experiment reports a real-world demonstration; a company announcement is the company's own account; vendor documentation is a supplier's own technical guide; expert commentary is a specialist's published analysis or opinion; a news report is a journalist's account. Dates and figures were checked against these sources on 19 October 2026. Spotted an error? Email hello@plainquantum.com and it will be corrected, with a note.
- Peer-reviewed paperSimulating physics with computersRichard Feynman, International Journal of Theoretical Physics, 1982
- Peer-reviewed paperPolynomial-time algorithms for prime factorization and discrete logarithms on a quantum computerPeter Shor, SIAM Journal on Computing, 1997
- Peer-reviewed paperA fast quantum mechanical algorithm for database searchLov Grover, STOC, 1996
- Peer-reviewed paperElucidating reaction mechanisms on quantum computers (nitrogen fixation)Reiher et al., PNAS, 2017
- Peer-reviewed paperQuantum Computing in the NISQ era and beyondJohn Preskill, Quantum, 2018
- ExperimentQuantum error correction below the surface code thresholdGoogle Quantum AI and collaborators, Nature, 2024
- ExperimentQuantum supremacy using a programmable superconducting processorArute et al. (Google), Nature, 2019
- Government guidanceQuantum Key Distribution (QKD) and Quantum Cryptography (QC)US National Security Agency
- StandardNIST releases first 3 finalized post-quantum encryption standardsNIST, 2024
- BookQuantum Computation and Quantum Information (10th anniversary edition)Michael Nielsen and Isaac Chuang, Cambridge University Press, 2010
- CourseBasics of quantum information (free course)IBM Quantum Learning
- Government guidanceMagnetic and Electric Fields (atom-based sensors)NIST