Compute a Kraus representation from the Choi matrix #4549
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mpharrigan
approved these changes
Oct 5, 2021
| if choi.shape != (d * d, d * d): | ||
| raise ValueError(f"Invalid Choi matrix shape, expected {(d * d, d * d)}, got {choi.shape}") | ||
| if not np.allclose(choi, choi.T.conj()): | ||
| raise ValueError("Choi matrix must be Hermitian") |
cirq-core/cirq/qis/channels.py
Outdated
| if not np.allclose(choi, choi.T.conj()): | ||
| raise ValueError("Choi matrix must be Hermitian") | ||
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| w, v = np.linalg.eig(choi) |
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The new
cirq.choi_to_krausfunction is like the existingcirq.kraus_to_choibut in the opposite direction.Note that the former chooses one of many admissible Kraus representations while the latter maps all valid Kraus representations to the Choi. Therefore,
cirq.kraus_to_choi(cirq.choi_to_kraus(c)) == c, but it is not necessarily the case thatcirq.choi_to_kraus(cirq.kraus_to_choi(ks)) == ks. Check out section and retraction if you like this sort of thing.Example future use-cases: channel comparisons, obtaining Kraus representation of a noisy circuit.
Some context: #3248.