What is a quantum computer?
A quantum computer calculates with qubits instead of ordinary bits. A qubit can be in a superposition of 0 and 1, and several qubits can be entangled with each other. Cleverly built quantum programs use this to solve certain problems in far fewer steps than an ordinary computer. It is not an all-rounder: for most everyday tasks the ordinary computer stays faster and cheaper.
Bit and qubit: the difference
An ordinary computer stores everything as bits. A bit is like a light switch: on or off, 1 or 0. Photos, music and this page all end up as a very large number of such switches.
A qubit behaves differently. Before measurement it is not simply 0 or 1 but in a state that contains both possibilities with certain weights. Only measurement gives a definite result, at random, with a probability that depends on those weights. A good picture is a coin spinning in the air: only when it lands is it clear whether heads or tails is up.
What is special shows up with many qubits. To describe the state of n qubits completely you need 2 to the power of n numbers. For 10 qubits that is 1,024 numbers, for 35 qubits more than 34 billion. That is exactly why quantum computers are so hard to reproduce on ordinary machines.
Try it: the qubit as a coin Superposition, entanglement, interference: the three tricks
Superposition means that a qubit carries several possibilities at once until it is measured. A quantum program changes these possibilities step by step with so-called gates.
Entanglement links several qubits so that their measurement results are related. Measuring one immediately tells you something about the other, even though each individual result looks random on its own. This creates joint states that can no longer be split into single qubits.
Interference is the actual computing trick. The weights of the possibilities behave like waves: they can reinforce or cancel each other. A good quantum program is built so that wrong answers cancel out and the right one is reinforced. In the end you measure the result you are looking for with high probability.
Try it: two entangled coins What can a quantum computer do, and what not?
A quantum computer is not a faster general-purpose machine. It only pays off for problems where quantum programs with a real advantage exist. These include simulating molecules and materials, certain search and optimisation problems and factoring very large numbers.
A well-known example is Grover’s search: for a list of one million entries an ordinary computer needs up to one million tries in the worst case, a quantum computer only about a thousand steps. That is a large but not unlimited advantage.
For e-mail, spreadsheets, videos or games a quantum computer brings nothing. It will therefore not replace the ordinary computer but complement it as a specialist tool, much as a graphics card complements the main processor.
Can a quantum computer break encryption?
Not today. Many encryption schemes on the internet, such as RSA, rely on ordinary computers being practically unable to factor very large numbers. A sufficiently large, error-corrected quantum computer could do this much faster with Shor’s algorithm.
How large “sufficiently large” is keeps being estimated more precisely. A paper by Craig Gidney from May 2025 arrives at fewer than one million noisy qubits and a run time of under a week for 2048-bit RSA. The largest devices today have around a thousand qubits, and these are considerably noisier than assumed there. The gap is still large, but it is shrinking.
That is why the switch is already under way: the US standards agency NIST published the first three standards for quantum-safe encryption on 13 August 2024. Important data that must stay secret for a long time should be protected with them in the medium term.
You can try the principle safely with us: our combination lock experiment searches for your code with a simulated quantum search.
Try it: does the quantum search find your code? Do quantum computers already exist?
Yes, but none yet that solve useful everyday problems better than ordinary computers. Companies and research institutions around the world are building quantum computers with different technologies: superconducting circuits, trapped ions, neutral atoms or particles of light.
A milestone was Google’s “Willow” chip with 105 qubits in December 2024. According to Google, its error rate falls as more qubits work together on error correction, and a special benchmark ran in under five minutes. Such benchmarks are deliberately chosen to be hard for ordinary computers and have no practical use of their own yet.
The biggest hurdle is errors. Qubits are sensitive to heat, radiation and the smallest disturbances. Many useful applications therefore need error-corrected qubits built from many physical qubits. The whole industry is working on this.
How many qubits do you need, and why is simulation so hard?
Every additional qubit doubles the memory an ordinary computer needs for an exact simulation. For 35 qubits that is more than 34 billion complex numbers, 512 GiB in double precision. For 50 qubits it would already be 16 pebibytes, more than fits into a supercomputer.
At Qubitwerk we research exactly this limit. In a research test we computed a complete 35-qubit circuit on an ordinary PC with an Intel Core i9 and an RTX 3090. For twelve mixed test jobs our research prototype needed 12.23 seconds, the widely used tool Qiskit Aer 24.58 seconds on the same PC. All measurement conditions and checks are on our research page.
Simulators matter because they make quantum programs testable before they run on real hardware, and because they show from which point a quantum computer really has an advantage.
Try a quantum computer yourself
You do not need to study physics to understand how a quantum computer calculates. On Qubitwerk you can try it without signing up: qubits as coins, waves that reinforce or cancel, entangled twin coins, Grover’s treasure search and the combination lock.
In the circuit playground you build your own quantum circuits with up to three qubits, and in the diamond lab you control a simulated qubit with microwave pulses. For schools we are developing a free app for iPad and iPhone that makes the topic tangible in a single lesson.
Frequently asked questions about quantum computers
How does a quantum computer work, explained simply?
It calculates with qubits that contain several possibilities at once before measurement. A quantum program changes these possibilities so that wrong answers cancel out and the right one is reinforced. Measurement then returns the result you are looking for with high probability.
What is a qubit?
A qubit is the smallest unit of computation in a quantum computer. Unlike a bit it is not simply 0 or 1 before measurement but in a superposition of both values. Only measurement gives a definite result.
Is a quantum computer faster than an ordinary computer?
Only for certain problems with suitable quantum programs, such as simulating molecules or factoring large numbers. For everyday tasks the ordinary computer is faster and cheaper.
Can a quantum computer break passwords or encryption today?
No. According to current estimates this would need far larger, error-corrected quantum computers than exist today. Even so, the switch to quantum-safe encryption, standardised by NIST in August 2024, has already begun.
Can you already buy a quantum computer?
Some manufacturers sell or rent quantum computers to research institutions and companies, usually via the cloud. There are no devices for private users, but you can simulate quantum programs in your browser, for example on Qubitwerk.
Can I try a quantum computer myself?
Yes, with a simulator. On Qubitwerk you can try qubits, entanglement and a quantum search without signing up and build your own circuits. Our PC simulates the calculations.
Why is it so hard to simulate quantum computers?
Because every additional qubit doubles the memory needed. 35 qubits already need 512 GiB in double precision, 50 qubits would need 16 pebibytes.
Sources
- Bundesregierung: Quantencomputing kurz erklärt
- Google: Meet Willow, our state-of-the-art quantum chip (Dezember 2024)
- NIST: First 3 Finalized Post-Quantum Encryption Standards (13. August 2024)
- Craig Gidney: How to factor 2048 bit RSA integers with less than a million noisy qubits (arXiv, Mai 2025)
As of 31 October 2026
Try it yourself now.
Start with a single qubit or set your code on the combination lock straight away. No sign-up.