RECAP: CLASSICAL BITS

How a bit is stored in hardware

Always definite A bit is either 0 or 1, stored as charge, voltage or magnetism.

Reading is harmless Looking at a bit doesn't change it.

n bits Hold one of 2ⁿ patterns at a time.

Classical information is definite: you can always read it, copy it and check it.

QUBITS

A qubit holds a blend of 0 and 1

The state |ψ⟩ = α|0⟩ + β|1⟩, where α and β are amplitudes.

The odds Reading gives 0 with probability |α|² and 1 with probability |β|².

The catch Afterwards the qubit is left in whatever you read.

A qubit isn't 'secretly 0 or 1': its blend is real, and it disappears when you look.

PICTURE IT

North pole |0⟩.

South pole |1⟩.

Everywhere else A blend. The height sets the odds; the direction around is the phase.

This picture is called the Bloch sphere. The next unit explores it in detail.

MORE QUBITS

Three qubits Eight amplitudes at once: |000⟩ to |111⟩.

Ten qubits 1,024 amplitudes.

But measuring Gives back only n plain bits.

The power is in the amplitudes; the challenge is getting a useful answer out of them.

CHECK YOURSELF

Bits Definite, readable, copyable.

Qubits Blends that collapse when read, and can't be copied.

Both Give a 0 or a 1 when you read them, and can be built on chips.

Next: what superposition and measurement really mean.

UNIT 3 STUDY COMPLETE

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You can explain the difference between a bit and a qubit, and what measuring a qubit does.