What is a qubit?
A qubit is the smallest unit of quantum information. It can be in state 0, state 1, or a superposition of both states. A measurement always gives 0 or 1, and the probability of each outcome depends on the qubit’s state. A quantum computer calculates with many qubits.
Bit and qubit: the difference
A bit is like a light switch: on or off, 1 or 0. Every ordinary computer stores everything this way, from photos to web pages.
A qubit can also be 0 or 1. But it can also be in a state in between, a superposition. It contains both possibilities with certain weights, the amplitudes. One picture for this is a coin still spinning in the air. The picture has limits: unlike a coin, the weights of a qubit can be changed so that possibilities reinforce or cancel each other.
Try it: the qubit as a coin What happens when you measure?
When you measure a qubit, you never get “a bit of 0 and a bit of 1”, but always exactly one answer: 0 or 1. Which one is decided by chance, but with fixed probabilities. The probability is the squared magnitude of the amplitude. This is called the Born rule.
An example: after the H step, a qubit sits exactly between 0 and 1. Measure it 2,048 times, freshly prepared each time, and you get roughly 1,024 zeros and 1,024 ones. After the measurement the superposition is gone; the qubit is then really 0 or 1.
A qubit as an arrow: the Bloch sphere
Physicists like to draw a qubit as an arrow on a sphere. If the arrow points up, the qubit is 0; if it points down, it is 1. On the equator it is half and half.
Where the arrow points along the equator is called the phase. You cannot see it directly when measuring. But it decides whether possibilities reinforce or cancel in the next step. Calculation steps, called gates, turn the arrow: X flips it from top to bottom, H lays it on the equator, Z turns it there by half a circle.
What is a real qubit made of?
A qubit is not an off-the-shelf component but a tiny physical system that follows the rules of quantum physics. Today there are several types: superconducting circuits, trapped ions, neutral atoms, particles of light and defects in diamond.
Superconducting qubits run at about 10 to 20 millikelvin, colder than outer space. The groundwork for them was honoured with the 2025 Nobel Prize in Physics: in the 1980s, John Clarke, Michel Devoret and John Martinis showed that an entire electric circuit can behave in a quantum way. Qubits in diamond, called NV centres, on the other hand work in small systems even at room temperature.
All types have one thing in common: qubits are fragile. Heat, radiation and the smallest disturbances corrupt their state. That is why error correction is the biggest challenge in the field.
Why many qubits matter
A single qubit is easy to work out on paper. It gets exciting with many: to describe the state of n qubits completely, you need 2 to the power of n amplitudes. For 10 qubits that is 1,024; for 35 qubits it is already more than 34 billion.
That is exactly why quantum computers are interesting, and exactly why they are so hard to recompute on ordinary computers.
Frequently asked questions about qubits
What is a qubit, explained simply?
A qubit is the smallest unit of quantum information. It can be 0, 1 or a superposition of both. A measurement always gives 0 or 1, with probabilities that depend on the qubit’s state.
What is the difference between a bit and a qubit?
A bit is always exactly 0 or 1. Before measurement, a qubit can be in a superposition, and its possibilities can reinforce or cancel each other like waves. When measured, a qubit also gives only 0 or 1.
Is a qubit 0 and 1 at the same time?
That is a common simplification. More precisely: a qubit in superposition has one amplitude for 0 and one for 1. Only the measurement fixes a result, at random but with fixed probabilities.
What are qubits made of?
Of tiny physical systems, such as superconducting circuits, trapped ions, neutral atoms, particles of light or defects in diamond. Each type has its own advantages and drawbacks.
Why are qubits so fragile?
Because even the smallest disturbances from heat, radiation or neighbouring particles change their state. That is why many qubits are cooled strongly and protected with error correction.
- What is a quantum computer? Explained simply
- Superposition explained simply
- Entanglement explained simply
- Grover’s algorithm explained
- Can a quantum computer break encryption?
- Quantum computer simulator
Sources
- German Federal Government: Quantum computing briefly explained (German)
- Nobel Prize in Physics 2025: Clarke, Devoret, Martinis – quantum tunnelling in an electric circuit (7 October 2025)
As of 31 October 2026
Measure your first qubit.
In the coin lesson you put a qubit into superposition and measure it. No sign-up.