Quantum superposition.

Quantum superposition is a fundamental principle of quantum mechanics that says a quantum system — like an electron or photon — can exist in multiple states at the same time until it is measured.

Core Concept:

In classical physics, something is either A or B.
In quantum mechanics, it can be A and B at the same time — that's superposition.

Example:

Let’s say an electron can spin up or down.
Before measurement, it's not in one or the other — it's in a superposition of both:

|ψ⟩ = ½ (spin-up) + ½ (spin-down)

This means the electron doesn’t have a definite spin direction until you measure it. Once you do, it "collapses" into either spin-up or spin-down.

Famous Analogy: Schrödinger’s Cat

Imagine a cat in a sealed box with a quantum-triggered poison:

• If a quantum event happens → the cat dies.

• If not → the cat lives.

Until you look inside the box, quantum theory says the cat is in a superposition of both alive and dead states.
Once you observe it, the cat becomes either alive or dead.

It's not the cat that’s weird — it’s how the quantum particle behaves.

What Superposition Really Means:

• It's not that particles are flipping between states.

• They truly exist in all allowed states simultaneously until observed.

Applications of Superposition:

• Quantum computing: A quantum bit (qubit) can be 0, 1, or both at once — massively increasing computing power.

• Quantum sensing: Superposition allows for extreme sensitivity to changes in the environment.

Key Takeaway:

Quantum superposition means particles can be in multiple possible states simultaneously.
Measurement forces the system into one of the possible outcomes, collapsing the superposition.