Entanglement explained simply
Entanglement links two or more qubits so that their measurement results are connected. Each single result is random, but together they always match, in a way that no prior agreement can explain. For quantum computers, entanglement is one of the most important ingredients.
Two coins that stick together
Imagine two coins you toss at the same time. Normally all four results are possible: heads-heads, heads-tails, tails-heads and tails-tails. With two entangled qubits it is different.
In a quantum computer, a simple entangled pair is made with two steps: H puts the first qubit into superposition, an if-then step (CNOT) links it to the second. If you then measure both, you only get 00 or 11, each about half of the time. 01 and 10 never occur. This pair is called a Bell state.
Try it: two entangled coins Not simply agreed in advance
You might think the two qubits had “agreed” on their result long ago, like two gloves packed into separate boxes: whoever finds the left one knows immediately that the right one is in the other box.
In 1964 the physicist John Bell showed that this explanation can be tested. If entangled particles are measured in different directions, prior agreement and quantum physics predict different results. Experiments since the 1970s have confirmed quantum physics again and again. For this, Alain Aspect, John Clauser and Anton Zeilinger received the 2022 Nobel Prize in Physics.
No messages faster than light
Albert Einstein called entanglement “spooky action at a distance”. Indeed, the results remain connected even when the particles are far apart.
You still cannot send a message this way. Each single result is random and cannot be controlled. The connection only becomes visible when both series of measurements are compared by ordinary means.
Why does a quantum computer need entanglement?
Without entanglement, a quantum computer would just be a collection of single qubits, each easy to recompute. Only entangled, shared states make the calculation so rich that an ordinary computer quickly reaches its limits. Circuits with little entanglement can therefore often be simulated with more compact methods.
Entanglement is also part of many other quantum technologies: in error correction, which spreads information across many qubits, in quantum teleportation and in some methods of quantum cryptography.
Frequently asked questions about entanglement
What is entanglement, explained simply?
Entanglement links qubits so that their measurement results are connected. Each single result is random, but together they always match.
How is entanglement created in a quantum computer?
For example with two steps: H puts one qubit into superposition, a CNOT gate links it to a second qubit. The result is a Bell state in which only 00 or 11 is measured.
Can entanglement be used to communicate faster than light?
No. The single measurement results are random and cannot be controlled. The connection only shows when the results are compared by ordinary means.
What was the 2022 Nobel Prize in Physics awarded for?
Alain Aspect, John Clauser and Anton Zeilinger received it for experiments with entangled particles of light. They showed that entanglement cannot be explained by prior agreement and laid foundations for quantum information.
What is a Bell state?
The simplest entangled state of two qubits. A measurement gives either 00 or 11, each with probability one half.
- What is a quantum computer? Explained simply
- What is a qubit?
- Superposition explained simply
- Grover’s algorithm explained
- Can a quantum computer break encryption?
- Quantum computer simulator
Sources
- Nobel Prize in Physics 2022: Aspect, Clauser, Zeilinger – experiments with entangled photons (4 October 2022)
- German Federal Government: Quantum computing briefly explained (German)
As of 31 October 2026
Entangle two qubits.
In the twin-coin lesson you see which pairs occur when measuring. No sign-up.