ExperimentsExperiment 11 · Quantum information

Quantum Noise

Bit flip, phase flip, depolarizing — ideal vs noisy distributions.

① What is the problem?

What happens to a quantum circuit on imperfect hardware?

② How does a classical computer approach it?

Classical bits are extremely reliable; errors are rare and easy to correct.

③ How does the quantum approach differ?

Qubits are fragile: interactions with the environment cause bit flips, phase flips and loss of coherence.

Tip: select any gate to learn what it does.

Purity Tr(ρ²)

1.000

Purity Tr(ρ²)

0.705

Real qubits are delicate. Heat, vibration and stray signals can flip them or scramble their phase. That makes answers less reliable.

④ What is happening mathematically?

Channels act on the density matrix: ρ → Σ Kᵢ ρ Kᵢ†. Purity Tr(ρ²) drops below 1 as noise mixes the state.

⑤ What does the simulation show?

A Bell circuit simulated with an exact density matrix, noise after every gate, and sampled shots.

⑥ What did we learn?

Noise spreads probability into wrong outcomes; phase noise can be invisible in some measurements yet destroy interference.

⑦ What can you experiment with?

Phase flip at 50% — why does the histogram look perfect while the purity drops?

📓 Record: my lab notebook

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