Nine qubits hold one protected qubit between them. Click any of the nine to damage it, and watch which of the eight checks notice.
The thing to try
Break one qubit. Some checks light up, and the encoded qubit survives — the damage is found and undone. Try every qubit: it always works. That is what this code promises.
Now break two. Sometimes it still survives. Sometimes it does not, and the page tells you the repair was the wrong one. Exactly half of the thirty-six pairs defeat it.
Then break these three: top-left, middle-left, bottom-left. Nothing lights up at all — every check reports that everything is fine — and the encoded qubit is destroyed.
That last case is the one worth sitting with.
Why a silent failure is the whole point
The checks do not measure the qubits. Measuring a quantum state destroys it, so that is not available. Each check measures a relationship between qubits — whether an even or an odd number of them have flipped — which reveals where damage is without revealing what the qubits hold.
The consequence is that a check pattern is a shadow of the error, not the error. Several different errors cast exactly the same shadow. So the decoder cannot look up the answer; it has to pick the most likely explanation and act on it. Usually it is right. Sometimes it is wrong, and it makes things worse.
And some errors cast no shadow at all. A chain of three flips crossing the lattice is invisible to every check and changes the answer the qubit holds. The shortest such chain is three, and that number is what people mean by the code's distance.
What the Sparsr device is doing
Each check asks a parity question: of the four qubits I watch, have an even or an odd number flipped?
That is one Sparsr instruction. The check picks out its qubits with an AND against a
4,096-bit register, and the machine reduces that whole register to a single parity bit as part
of the same operation. Eight checks, eight instructions.
The decoding is not on the device, and this page will not pretend it is. Turning a check pattern into a best guess is a sixteen-entry lookup here, and it runs in your browser. Decoding is the expensive part of error correction at scale — it is where the research effort goes — and Sparsr is not what makes it cheap. What Sparsr makes cheap is the parity extraction, and that is what is running.
One click, 4,096 experiments
The kernel is built to run 4,096 independent copies of the experiment at once, one per bit of a wide register. That is what this machine is for: sampling millions of noise trajectories to find out how often a code fails.
You are describing one. So your error is copied into all 4,096 shots, and the instruction count on the page is the cost of answering your question 4,096 times over. It is an honest number for a slightly silly use of the machine.
The code
The distance-3 rotated surface code: nine data qubits in a 3×3 grid, eight stabilizer checks, one encoded qubit. It is the layout essentially every current hardware experiment uses, and with its eight measurement ancillas counted it is the one often called Surface-17.
The ancillas are not modelled here. This tracks where errors are, not the circuit that measures them — a circuit-level model would add the ancillas, their gates, and the errors those introduce, which is a much larger thing than this page.
What this does not tell you
Nothing about speed. The processor here is a software model running in your browser, so how long a click takes is a property of your laptop rather than of Sparsr. The instruction counts are exact; there is deliberately no timing figure beside them.
Nothing about a real quantum computer's error rates. You choose the errors here. A real device gets them from physics, at rates this page does not model.