What is a qubit?
A qubit is the basic unit of information in a quantum computer. Like an ordinary bit, it gives 0 or 1 when you read it. Unlike a bit, before it's read it can be in a state that combines 0 and 1 with two numbers called amplitudes, which set the chance of each answer and can strengthen or cancel each other during a calculation. Reading it gives one answer and ends that combined state.
Bit versus qubit
| Bit | Qubit | |
|---|---|---|
| Before you read it | Definitely 0 or definitely 1 | 0, 1, or a combination of both, described by two amplitudes |
| When you read it | You get the value it holds | You get 0 or 1, with chances set by the amplitudes |
| After you read it | Unchanged | Left in the state you read: the combination is gone |
| Copying | Easy | An unknown qubit state can't be copied |
What the amplitudes mean
A qubit's state is written α|0⟩ + β|1⟩. The chance of reading 0 is |α|² and the chance of reading 1 is |β|², and the two always add up to 1. Amplitudes can be negative, or complex numbers, which is what lets them cancel out when combined. That is the root of interference, and it's why a qubit is more than a coin with a hidden side.
What a qubit is made of
Any physical system with two distinct states that can be controlled precisely can serve as a qubit. Today's main designs use superconducting circuits, trapped ions, neutral atoms, photons or electron spins in silicon. Each is a trade-off between speed, error rates and how easily it scales. All of them are fragile: stray heat, light or vibration can disturb the state, which is why logical qubits are built from many physical ones.
Many qubits
Describing n qubits takes up to 2n amplitudes: about a thousand for 10 qubits and about a million for 20. That's why simulating large, highly entangled quantum states on ordinary computers quickly becomes impractical, although some special kinds of state can still be simulated efficiently. But reading n qubits still gives only n bits, so the size of the state isn't an answer you can read out. Algorithms have to steer the amplitudes so the right answer is likely when you measure.
Common confusions
- "A qubit is 0 and 1 at the same time." A useful shorthand, but it hides the important part: the amplitudes, which can cancel. A qubit isn't two bits.
- "A qubit stores unlimited information." No. Read on its own, one qubit gives at most one bit of information.
- "More qubits means a better computer." Only if they're reliable. Error rates matter as much as the count.
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 (how sources are used). Checked against these sources on 19 October 2026. Spotted an error? Email hello@plainquantum.com and it will be corrected, with a note.
- 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
- BookProgramming Quantum ComputersEric Johnston, Nic Harrigan and Mercedes Gimeno-Segovia, O'Reilly, 2019
- Peer-reviewed paperQuantum Computing in the NISQ era and beyondJohn Preskill, Quantum, 2018