The qubit — both, until you look
A switch that is neither on nor off
The switches on the binary board were either on or off. The switch on this board tilts. Work through the four boards in order, and on the last one you will do for yourself what a quantum computer does to find an answer.
Board 1 · A qubit tilts
The bulb on the left changes when you tap it. The qubit on the right you tilt by turning the needle with your finger. Leave it near the middle and press Look once several times, then Look 100 times.
An ordinary switch (a bit)
A qubit
offonoff 100% · on 0%
What you have just done will be explained here.
What you learn — a bit holds one answer. A qubit holds the possibility of each answer. Yet look once and it is always on or off, one or the other. The tilt shows itself only over many looks.
Board 2 · Two half turns
Flip the coin on the left twice, and press Half turn twice for the qubit on the right. The coin stays half and half, but the qubit comes back to where it started.
A coin (for comparison)
HeadsFlips: 0 · chance of heads 100%
A qubit
A bar is the size and direction of a possibility. Right is +, left is −.
off 100% · on 0%
What you have just done will be explained here.
What you learn — a coin is half and half however many times you flip it. A qubit turned twice comes back to the start. That is because a possibility has a direction: the two possibilities heading for the on side met as + and − and cancelled each other. This cancelling is the quantum computer's weapon. It is used on Board 4.
Board 3 · Tie two together
Press Separately, half and half and then Look 100 times; then Tie together and Look 100 times again. Watch how the four columns change.
Not prepared yet
What you have just done will be explained here.
As more qubits are tied together
8 qubits → possibilities held in one state: 256
What you learn — tie them together and the moment you look at one, the other is settled, however far apart they are. And while 8 bits hold one of 256 values, 8 tied qubits hold all 256 possibilities at once. That is why, with only a few hundred qubits, an ordinary computer cannot even imitate them.
Board 4 · So what does a quantum computer do?
Only one of four chests holds the treasure. We do not know which. All we can do is open a chest and look.
On the left, open the chests one at a time. On the right, press ① to ④ in order.
An ordinary computer
It opens the chests one at a time to check.
Chests opened: 0
A quantum computer
It spreads the four chests across two qubits as possibilities.
Not started yet
What you have just done will be explained here.
What you learn — after the marking in ②, the four chests were all still at 25%. Marking changes only a direction. Look in that state and any chest may come up. The flip in ③ makes the treasure chest, the one pointing the other way, grow while the rest cancel. So a single look brings up the treasure. A quantum computer is not a machine that knows all the answers; it is a machine that cancels wrong answers.
The story behind the tilted switch
What you just did by hand is written up in these pieces.
A bit stores an answer; a qubit stores the possibilities of answers. Those possibilities have direction, so they can cancel one another, and when several are tied together one state holds a power of two of them. A quantum computer uses these two facts to arrange a calculation so that the possibilities of wrong answers cancel and only the right one is left.