What Is a Qubit? Superposition and Measurement Explained
How a qubit behaves
Before measurement, a qubit holds a combination of 0 and 1 described by two numbers called amplitudes. Measuring it forces a single result, and the amplitudes decide the probability of getting 0 or 1. This is the heart of superposition.
How qubits are built
- Superconducting circuits: tiny electrical circuits cooled close to absolute zero.
- Trapped ions: charged atoms held by electric fields and controlled with lasers.
- Photonic qubits: particles of light steered through optical chips.
- Neutral atoms: uncharged atoms held in place by focused laser beams.
Each approach has trade-offs. See types of quantum computers.
Why qubits are fragile
Heat, vibration and stray fields can destroy a qubit's state. This is called decoherence. Much of quantum engineering is about keeping qubits stable long enough to finish a calculation, then fixing the errors that still creep in. See quantum error correction.
Physical versus logical qubits
A physical qubit is the real hardware. A logical qubit is a more reliable qubit built from many physical ones using error correction. Useful large-scale machines are expected to need many physical qubits per logical qubit.
A worked example with real numbers
Suppose a qubit has amplitudes 0.6 for the 0 state and 0.8 for the 1 state. Square each one to get the odds: 0.36 and 0.64, which add to 1. Measure it and you see 0 about 36 times in 100 and 1 about 64 times in 100. Amplitudes can also be negative or complex, and that is what lets them cancel each other, the effect behind interference. A fuller picture of a single qubit is the Bloch sphere.
Comparing the main qubit types
| Type | Strength | Challenge |
|---|---|---|
| Superconducting | Fast gates, chip manufacturing | Needs a dilution refrigerator, noise |
| Trapped ion | High accuracy, any-to-any links | Slower gates, scaling the trap |
| Neutral atom | Large arrays | Speed and atom loss |
| Photonic | Networking friendly | Photon loss |
Physical qubits in numbers
In Google's 2024 error correction experiment, one logical qubit of the largest code (distance 7) used 101 of the Willow chip's 105 physical qubits. Its logical qubit lasted about 2.4 times longer than the best single physical qubit on the chip. That is a good snapshot of the overhead: roughly a hundred physical qubits to make one better one. Later designs aim to cut that cost. Read how resource estimates work and decoherence, T1 and T2.
Common mistakes
- Believing a qubit stores "infinite" information. You can only read out one bit per qubit when you measure.
- Comparing qubit counts across designs without checking accuracy.
- Assuming a qubit is a tiny ball. It is a two-level system, however it is built.
Sources
- Google Quantum AI: Quantum error correction below the surface code threshold (arXiv 2408.13687)
- Google: Willow chip announcement
- Nobel Prize in Physics 2025 press release
Try it: a one qubit simulator
Frequently asked questions
How many qubits does a quantum computer need?
It depends on the task. Qubit count alone is misleading because quality matters too. Large useful machines are expected to need many error corrected logical qubits.
Is a qubit a particle?
A qubit is a controllable two-level quantum system. It can be built from an atom, an ion, a photon or a tiny superconducting circuit.
Can a qubit be copied?
No. The no-cloning theorem says an unknown quantum state cannot be copied exactly. This is one reason quantum error correction is hard.
What is the difference between a qubit and a bit in practice?
A bit is always 0 or 1. A qubit's state is described by amplitudes, and you only get a 0 or 1 when you measure. Algorithms use the amplitudes before measurement.
How long does a qubit last?
It depends on the type. In Google's 2024 chip the median physical qubit held its state for roughly 85 microseconds, while trapped ions can last far longer. See decoherence.
Do qubits need to be cold?
Superconducting ones do, close to absolute zero. Ions and neutral atoms use vacuum and lasers instead. See how quantum computers are cooled.
What is a qutrit?
A three-level version of a qubit. Some research uses them, but almost all machines today use two-level qubits.
Keep reading
- Superposition Explained in Plain English
Superposition lets a qubit hold a blend of 0 and 1. Here is what it really means, what it does not mean, and why it matters. - Quantum Entanglement Explained Simply
Entanglement links qubits so they share a joint state. Learn what it is, what it is not, and why quantum computers use it. - Quantum Error Correction Explained
Qubits are fragile, so quantum computers need error correction. Learn how logical qubits are built and why this is the key challenge. - Types of Quantum Computers: Superconducting, Ion, Photonic and More
A guide to the main ways quantum computers are built, with the strengths and trade-offs of each approach. - The Bloch Sphere Explained: How to Picture a Qubit
The Bloch sphere is a globe that shows every possible state of one qubit. Here is how to read it, with simple analogies and common mistakes. - Decoherence and Noise Explained: What T1 and T2 Mean
Why qubits lose their quantum behavior. A plain guide to decoherence, T1 energy relaxation and T2 dephasing, and why they limit quantum computers.
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