Quantum computing Qubits

A bit has to choose.
A qubit can be
both at once.

Today's computers think in 0s and 1s. Quantum computers work with the space in between. Let's explore what that means, one idea at a time, with things you can try yourself.

0 and 1 at the same time
Click to measure
Scroll to begin
Start here

Everything a computer does is built from one tiny choice.

A bit is the smallest piece of information. It can be a 0 or a 1. Off or on. A photo, a song, even this sentence: it's all just billions of these tiny switches, each set one way or the other.

A classical bit
0
OFF

Click the board to flip it. A normal bit only has one value at a time: 0 or 1, off or on. It can't be both at once.

Superposition

A qubit doesn't have to pick.

A quantum bit, or qubit, can hold 0 and 1 at the same time. That's called superposition. It isn't an average or a guess. The qubit really is both possibilities, right up until you look. Then it settles on a single answer.

A quantum coin
0
1
A coin that's sitting still is a plain bit. Give it a spin and it becomes both faces at once.
The state of a qubit

You can point a qubit anywhere.

Here's a map of everything a single qubit can be. The top is a sure 0. The bottom is a sure 1. Every point between them is a different blend. Drag to aim the arrow. The closer it points to a pole, the more likely the qubit lands on that value when you measure it.

Drag to aim the qubit
Use the arrow keys to tilt the qubit. Hold Shift with Left or Right to rotate around the sphere. Home points to 0; End points to 1.
Pointing at |0⟩
0100%
10%

A spinning coin sits exactly on the middle line, a perfect 50 / 50. Most of the power of quantum computing comes from choosing these in-between points on purpose.

Why it matters

Each qubit you add doubles what it can explore.

Add one classical bit and you get one more switch. Add one qubit and you double the number of combinations it can hold at the same time. It starts slow, then gets almost impossible to picture.

Combinations held at once
4 qubits
16
sixteen combinations at once
A classical computer would line these up and check them one by one.
Entanglement

Two qubits can share one fate.

Qubits can be linked so their results always agree, even after you carry them far apart. Measure one and you know the other in the same instant. Einstein called this "spooky." A century later, we still haven't found a way around it. It's just how the world works up close.

Prepared together, then carried far apart
?Qubit Ain the lab
?Qubit Bside by side
drag me anywhere
Use Left and Right to move Qubit B across the stage, and Up and Down to move it vertically. Home moves it to the left edge; End moves it to the right edge.
When two qubits are entangled, their answers match every single time.
A head to head

Finding a needle in a haystack.

Say you're looking for one marked box among a hundred and twenty-eight, with no clues. A classical computer has to open them one at a time. A quantum computer can nudge the odds toward the right box with each round, and get there in far fewer tries.

128 boxes · one is marked

Classical

opens one box at a time
Boxes opened0

Quantum

tilts the odds each round
Rounds used0

What's ahead

The problems quantum computers are built for.

These aren't everyday tasks. They're a handful of really tough problems where an ordinary computer can't realistically check every possibility.

Medicine

Designing new drugs by simulating real molecules.

Materials and energy

Better batteries, cleaner fuels, new materials.

Security

Why today's codes may need to change.

Chemistry and climate

Modeling reactions ordinary computers can't.

Where we are

Quantum computers won't replace your laptop.

They aren't faster at email, video, or games. They're built for a small set of really tough problems, and it's still early days. Today's qubits are fragile and make mistakes. Keeping them stable is one of the trickiest parts of making quantum computers work.

But the ideas are real, and the machines keep getting better. They could help us answer some of our biggest questions. Those strange rules you just played with aren't science fiction. They're what researchers are building on right now.

Learn more