Superposition Explained
What superposition actually claims, and the wording to avoid.
What Is a Qubit?
BeginnerQuantum foundations6 min read
By QuantumO Editorial · Scientific reviewer: not yet assigned · Published · Updated
In short
A qubit is the basic unit of quantum information: a two-level quantum system whose state can be a combination of the outcomes 0 and 1. Unlike a bit, a qubit carries amplitudes that can interfere, and measuring it yields 0 or 1 with probabilities set by those amplitudes.
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Visit the channelHow is a qubit different from a bit?
A bit is one of two values. A qubit is described by two amplitudes, one for the outcome 0 and one for the outcome 1. Those amplitudes are numbers that can be negative or complex, and their squared sizes must add to one.
That extra structure is what interference uses. It is also why a qubit cannot be read out fully: a single measurement returns one bit of information and disturbs the state.
Picturing a single qubit
The Bloch sphere places every single-qubit state on the surface of a sphere: poles for the definite outcomes, everything else in between. Gates become rotations. The picture is genuinely useful, but it only covers one qubit — it cannot represent entanglement between two.
What qubits are made of
Superconducting circuits, trapped ions, neutral atoms, photons, spins in silicon, and defects in diamond are all used as qubits. Each trades off speed, coherence time, connectivity, and manufacturability differently.
No platform has clearly won. The mathematics is identical across them; the engineering is not.
Analogy: a dimmer switch is not enough
People often describe a qubit as a dimmer switch between off and on. That captures the continuous range but misses the essential part: dimmer settings cannot cancel each other, whereas quantum amplitudes can. Use the analogy for intuition about continuity, not about behaviour.
Common misconceptions
- Claim: A qubit stores far more data than a bit.
- More accurately: You can only extract one bit per qubit measured. The advantage is in how states evolve before measurement, not in storage capacity.
Important terminology
- Qubit
- A qubit is the basic unit of quantum information: a two-level quantum system that can hold a combination of the outcomes 0 and 1.
- Bloch sphere
- The Bloch sphere is a picture of every possible state of a single qubit as a point on the surface of a sphere.
- Classical bit
- A classical bit is a unit of information that is either 0 or 1 at any moment.
- Amplitude
- An amplitude is the number a quantum state assigns to each possible outcome; its size squared gives the probability of that outcome.
Frequently asked questions
- What is a qubit?
- A qubit is a two-level quantum system used as the basic unit of quantum information. Its state assigns amplitudes to the outcomes 0 and 1.
- How is a qubit different from a bit?
- A bit is definitely 0 or 1. A qubit holds amplitudes for both outcomes that can interfere, and measuring it returns a single outcome with a probability set by those amplitudes.
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Sources and further reading
Keep going
Measurement and Probability
How amplitudes become outcomes, and why quantum experiments are run repeatedly.
Quantum Hardware Basics
The main qubit platforms and the engineering trade-offs between them.
Intermediate7 min read