Game

Qubit Factory

A quantum circuit, drawn as a production line. Press Run and a qubit rides the belt through the gate machines while the tank on its side shows the odds of reading 1. At the end the Reader looks, and it has to commit to an answer. Sample ×100 repeats the run a hundred times so the statistics stop being a claim and start being a bar chart. Ten exhibits, no score. The circuit diagram and the explanation for each one open once you have run it.

Runs entirely in your browser, with no sign-in and nothing to install. It makes sound while the belt is moving; there is a mute button in the control panel. Your progress through the exhibits is remembered locally on this device. Open it full-screen.

The ten exhibits

Each one is a short circuit with a point to make, and they build on each other. Exhibits 8 and 9 are the controls for 4 and 5: the same hardware and the same histograms without the quantum part, so the surprising results have something to be measured against. The narrator line under the floor changes with the qubit's actual state, not just with the machine it passed, so what you read is what the engine computed. Finish an exhibit and its circuit diagram and full explanation open up inside the game.

  1. 1 The Flipper X gate

    A fresh qubit reads 0. One machine turns it into a 1. The plainest thing a gate can do, and the baseline for everything after it.

  2. 2 The Splitter Hadamard gate

    The qubit leaves half on 0 and half on 1. The Reader still has to answer with one of them, and the readout fills up 50/50 over many runs.

  3. 3 The Hidden Dial Z gate, then interference

    The Dial changes nothing you can measure. Then a second Splitter turns that invisible change into an answer that is always 1. This is phase, and it is the part of a qubit that a coin does not have.

  4. 4 The Linker CNOT gate

    Two belts. Split the first qubit, then chain them. Neither belt has an answer of its own any more, but the pair always agrees: 00 or 11, never 01 or 10.

  5. 5 The Proof testing exhibit 4

    Exhibit 4 could have been a trick: hide one coin flip, put a copy on each belt, and they would always agree too. Spinning both belts tells the difference. A shared coin flip would scatter across all four outcomes. This does not.

  6. 6 Undone H twice

    Two Splitters in a row and the qubit is back where it started. Every quantum gate can be run backwards. Measurement is the one step that cannot.

  7. 7 Quarter Turn S gate

    Exhibit 3 with a quarter turn of phase instead of a half. The answer is no longer certain, it is an even 50/50. Interference is a dial, not a switch.

  8. 8 Just a Copy CNOT, definite input

    The same Linker as exhibit 4, fed a definite qubit rather than a split one. It prints 11 every time and no coupling ever forms. Two qubits that always agree are not necessarily entangled.

  9. 9 Independent H on both belts

    Two Splitters, no Linker, all four outcomes at about 25% each. This is what unrelated randomness looks like, and it is the histogram exhibits 5 and 10 are measured against.

  10. 10 Always Disagree a phase inside the pair

    One Dial added inside the entangled pair changes nothing you can measure, until both belts are spun. Then the readout inverts: only 01 and 10, never 00 or 11.

What each machine is, in the usual notation

The factory names things after what they do so you can watch before you have the vocabulary. Here is the translation, because every course, textbook and framework uses the second column.

On the factory floorIn a circuit diagramWhat it does to the qubit
Parts hopper the initial state |0⟩ Every run starts from the same place: a qubit that reads 0.
Flipper X gate Swaps 0 and 1. The quantum NOT.
Splitter Hadamard gate Puts a definite 0 or 1 into an even superposition.
Dial Z gate Changes the relative phase and nothing else. Invisible on its own, until a later Splitter turns it into interference you can measure.
Linker CNOT gate Flips the second qubit when the first reads 1. Fed a superposition, it produces entanglement.
Reader measurement Forces one answer and destroys the superposition. Run it again and you may get the other one.
Chain between the belts a Bell state Drawn only when the two qubits genuinely cannot be described separately.

What the picture does and does not claim

The tank on the canister is the probability of reading 1, so a half-full tank is a real 50/50 and not an artistic flourish. The gauge underneath is the relative phase, and it is deliberately blank when the state has no relative phase to report: a definite 0, a definite 1, or a qubit that is entangled with the other belt. Reporting a confident zero there would teach the wrong thing.

Two honest limits. This is a simulator with no noise, so nothing decoheres and every run is perfect, which real hardware is not. And a single qubit's state is fully described by a point on a sphere rather than a tank and a dial, so once you are comfortable here, the Bloch sphere simulator is the picture to graduate to. To write the same circuits as code, the circuit builder exports Qiskit, and the Python playground runs it in the browser.

Where to go next