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physics
Interactive Course
Interactive chapters from intuition to mastery
Structured Lessons
Eight modules with formulas and self-checks
Quantum Brain
Navigate lessons, laws, gates, devices, and tools
Guided Experiments
Hands-on circuits that teach one idea each
simulator
Circuit Lab
Build circuits, run them, and see the results
Gate Reference
Quick reference for all quantum gates
wiringStudio
Cryostat Studio
3D cryostat design and simulation
Component Catalog
Browse all cryostat components
System Checks
Check your design for errors
Quantum FoundationsQubits and State Vectors0/6 mastered
Module overview
Quantum Foundations0/6 mastered
Qubits and State VectorsCore
Superposition and MeasurementCore
The Wavefunction
Operators, Eigenstates, Eigenvalues
The Schrodinger Equation
The Uncertainty Principle
Next module
Measurement, Phase, and Interference
Module 1
0/6
Quantum Foundations
Qubits and State VectorsCore
Superposition and MeasurementCore
The Wavefunction
Operators, Eigenstates, Eigenvalues
The Schrodinger Equation
The Uncertainty Principle
Next module
Measurement, Phase, and Interference
Home/Lessons/Quantum Foundations/Qubits and state vectors
1Lesson 1 of 6inQuantum Foundations

Qubits and state vectors

A qubit is a two-state quantum system described by amplitudes, not by a simple probability list.

Every gate, histogram bar, and Bloch sphere arrow in the simulator shows a different view of the same quantum state. If you understand what the state is, the rest of the interface becomes much easier to read.

1
The intuition
Start with the plain-language idea

A classical bit has a definite value: 0 or 1. A qubit is different. Before you measure it, its state is described by two amplitudes -- one for ∣0⟩ and one for ∣1⟩. These amplitudes are not probabilities. They are the deeper quantities that produce probabilities when you square their sizes. They also carry phase, which is why a qubit is richer than a random coin flip.

2
See it concretely
A real example before the abstraction

Think of a recipe versus a meal. The quantum state is the recipe -- it tells you what outcomes are possible and how they relate. The measurement result (0 or 1) is the meal you actually serve. The recipe contains more information than any single meal.

3
Tempting but wrong
The mistake most people make
Tempting but wrong
It is tempting to think a qubit is secretly either 0 or 1, and we just do not know which. That is not what is happening. The state genuinely encodes amplitudes for both outcomes. This is not a statement about our ignorance -- it is a different kind of description.
4
The precise version
Now with the formal detail
∣ψ⟩=α∣0⟩+β∣1⟩,∣α∣2+∣β∣2=1

The symbols α and β are complex amplitudes. Their squared magnitudes give the measurement probabilities in the standard basis: P(0) = |α|² and P(1) = |β|². Because α and β are complex numbers, they also encode phase information. Two states can have the same measurement probabilities but different phases, which means they will behave differently in future operations.

Check your understanding
What is the difference between an amplitude and a probability?
Think about this against what you just read.
Why can two qubit states have the same measurement probabilities but still behave differently?
Think about this against what you just read.
Try it yourself

Open the simulator and see this concept in action. Watch how the state changes and compare it to what you just learned.

▶ See a qubit state↗ MIT OCW 8.04: lecture notes↗ Nielsen and Chuang, Quantum Computation and Quantum Information
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What makes a qubit state richer than an ordinary probability list?
1 of 6 in Quantum Foundations
Next
Superposition and Measurement
Why a qubit can be in multiple states until you measure it
~8 min