You have seen the whole factory floor.
You have now watched a bit flip, a superposition, phase turning into interference, and two
qubits that only have an answer together. That is the conceptual core of every quantum circuit
you will meet. The next step is writing one:
build the same Bell state in Qiskit,
which is exhibit 4 in about fifteen lines of Python, or read
what entanglement actually means now that you have seen it
rather than been told about 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 five exhibits
Each one is a short circuit with a point to make, and they build on each other. 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.
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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.
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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.
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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.
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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.
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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.
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 floor | In a circuit diagram | What 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.