From bits to qubits
What changes when a bit can be more than off or on, and why a qubit is not a dimmer switch.
Lesson 1 of 7 about 9 min
Everything is switches
Every photo on your phone, every song, every message you send is stored as a long line of tiny switches. Each switch is either off or on, and we write that as 0 or 1. One switch is called a bit.
On its own, a bit doesn't say much. But eight bits in a row can stand for a letter. Try it: these eight switches spell one character.
Bits are simple, and that's their strength
A bit is always exactly 0 or exactly 1. You can look at it as often as you like and it never changes. You can also copy it perfectly, millions of times a second. That's what happens every time you forward a photo.
Your turn
Meet the qubit
A qubit (short for “quantum bit”) also has two special states, |0⟩ and |1⟩. You met it in the warm-up as an arrow of length 1: pointing one way it's |0⟩, the other way |1⟩, and it can point anywhere in between.
So is a qubit just a dimmer switch, a light that can be half on? No. And this is the single most important thing to get right about qubits. Compare them side by side:
A dimmer switch
Look to see its brightness.
A qubit
Look to see what it shows.
So, more than a bit?
Your turn
Here's where that power comes from. These four ideas are the whole of this level, and much of quantum computing:
Superposition
In-between states
A qubit can be set anywhere between 0 and 1, not just at the ends.
Measurement
Random answers
Reading it still gives a plain 0 or 1, picked by chance, and the reading changes it.
Interference
Amounts that cancel
The hidden amounts inside it can cancel out, like waves. Bits can't do that.
No-cloning
No copies
You can copy a bit as often as you like. Nobody can copy an unknown qubit. Not even in theory.
What is a qubit made of?
A qubit isn't one particular object. It's a job description: anything with two clearly different states that can also be in between can be a qubit. Companies around the world build them in very different ways. Here are four of them.
Superconducting circuit
IBM, Google
A tiny loop of metal, cooled to about 0.01 K so electricity flows with no resistance. Its two lowest energy levels are 0 and 1.
Trapped ion
Quantinuum, IonQ
A single charged atom, held still in a vacuum by electric fields. Two of its energy levels are 0 and 1.
Neutral atom
QuEra, Pasqal
Atoms held in place by tightly focused laser beams, called optical tweezers. Again, two energy levels are 0 and 1.
Photon
PsiQuantum, Xanadu
A particle of light. One way to make a qubit: 0 and 1 are the direction its wave wiggles in, side to side or up and down (its polarisation).
Your turn
Why all the fuss?
One qubit takes two numbers to describe (how much 0, how much 1). Two qubits take four. Every qubit you add doubles it, just like the coins in the warm-up.
1,024
numbers to keep track of
16 KB
of memory
Easy. Any phone could hold it.
What you now know
- A bit is a switch: always exactly 0 or 1, and easy to read and copy.
- A qubit has two special states, |0⟩ and |1⟩, and can be set anywhere in between.
- It is not a dimmer: reading it always gives a plain 0 or 1, with odds set by where it points.
- Reading one qubit gives one bit. Its power comes from superposition, measurement and interference.
- Real qubits are made from superconducting circuits, single atoms and ions, and particles of light.
Next: superposition. What does “in between 0 and 1” really mean? (Hint: not what most headlines say.)