Bits and qubits
Lesson 6 of 23 in the course “Quantum”. Free, with animations and a quiz.
A bit is a switch, a qubit is a sphere
An ordinary bit is a switch: 0 or 1, nothing else. A qubit is the quantum version of a bit. Before measurement it has two amplitudes: one for 0 and one for 1. It’s handy to draw them as a point on a sphere rather than two positions. But remember the key rule from level one: when measured, a qubit also gives only 0 or 1. The whole trick is what happens to the amplitudes before measurement.
The Bloch sphere
The state of one qubit can be shown as an arrow on a sphere, invented by physicist Felix Bloch. The north pole is surely 0, the south pole is surely 1. The closer the arrow is to a pole, the higher the chance of getting it when measured. On the equator it’s exactly 50/50. And turning around the equator, like longitude on a globe, is called phase. That’s the sign of the amplitude from the superposition lesson. In the trainer you can spin the sphere with gate buttons.
Phase: invisible but decisive
Take two points on the equator: “plus” and “minus”. Both have exactly 50/50 chances of 0 and 1, and a direct measurement can’t tell them apart. But they’re different states: in “minus” the amplitude for 1 has a minus sign. Apply the H gate before measuring, and “plus” turns into a sure 0 while “minus” turns into a sure 1. That’s why phase matters: quantum algorithms hide the answer in exactly these signs, then reveal it with interference.
Why qubits are hard to fake
To describe n qubits on an ordinary computer you need 2 to the power n amplitudes, because every combination of 0s and 1s gets its own. 10 qubits: 1,024 numbers, no big deal. 50 qubits: about a quadrillion numbers, too much for a laptop’s memory. 300 qubits: more numbers than atoms in the observable universe. That’s why big quantum circuits can’t simply be computed on a supercomputer, and that’s their strength.
But you can only read n bits
Here’s the hype trap: if there are a quadrillion numbers inside, isn’t that a quadrillion answers? No. Measure 50 qubits and you get exactly 50 ordinary bits, while all the amplitudes collapse. Physicists even proved it as a theorem: you can’t pull more than n bits of information out of n qubits. So the power of qubits isn’t storage. It’s how an algorithm makes wrong answers cancel each other, so the right one comes up when you measure.
What is a qubit physically?
A qubit can be anything with two clear quantum states that can be isolated and controlled. The superconducting circuit from the 2025 Nobel: Google and IBM build on it. A single ion held by an electric field: Quantinuum and IonQ. A neutral atom held by a laser beam: QuEra and Atom Computing. A photon: PsiQuantum and Xanadu. Each option has its pros and cons; we’ll go through them in the hardware level.
Check yourself
- A qubit is on the Bloch sphere’s equator. What will measurement give?
Correct answer: 0 or 1, roughly equally often. The equator means 50/50. Each measurement gives 0 or 1, and over a hundred tries they come out roughly even. - How do you answer “50 qubits store a quadrillion answers”?
Correct answer: You can only read out 50 bits. Describing 50 qubits takes a quadrillion amplitudes, but measurement gives exactly 50 bits. That’s a proven theorem, not a temporary problem. - What’s the difference between the “plus” and “minus” states?
Correct answer: The sign of the amplitude, the phase. Both are 50/50, but “minus” has a minus sign on the amplitude for 1. The H gate turns “plus” into 0 and “minus” into 1, so the difference is real.