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Fix #855: don't call cirq.unitary() on non-unitary matrices #857

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Merged
mhucka merged 4 commits into from
Aug 12, 2025
Merged

Fix #855: don't call cirq.unitary() on non-unitary matrices #857

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e141008
Fix #855: don't call cirq.unitary() on non-unitary matrices
mhucka Aug 10, 2025
a82b6ec
Fix formatting complaint from black
mhucka Aug 10, 2025
3576529
Add comment explaining what's happening
mhucka Aug 11, 2025
4f45403
Update qsimcirq_test.py
mhucka Aug 12, 2025
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22 changes: 16 additions & 6 deletions qsimcirq_tests/qsimcirq_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1057,7 +1057,17 @@ def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)

def _mixture_(self):
return [(prob, cirq.unitary(op)) for prob, op, in self._prob_op_pairs]
# Cirq's mixture() function in mixture_protocol.py returns tuples of
# the form (probability, unitary operation). It does this by applying
# Cirq's unitary() function to the second elements of the tuples
# returned from here. Now, the values in self._prob_op_pairs will be
# tuples of the form (probability, NoiseStep). NoiseStep defines a
# _unitary_() method that simply returns the array as-is. Thus, when
# Cirq's mixture() function gets the value returned here and calls
# unitary() on those NoiseStep objects, the values unitary() returns
# will not actually be unitary. This is done knowingly. The nonunitary
# values are eventually normalized in test_multi_qubit_noise().
return [(prob, op) for prob, op, in self._prob_op_pairs]
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@dstrain115 dstrain115 Aug 11, 2025

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Is _mixture a standard protocol for qsim or is this only used for this test? This is changing the return type for this method.

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@dstrain115 dstrain115 Aug 11, 2025

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Oh, I didn't realize op was already a np.array.

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@mhucka mhucka Aug 11, 2025

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Hmm. It's true that the 2nd element in the tuples in self._prob_op_pairs are actually NoiseStep objects. Lemme see if I can improve this.

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@mhucka mhucka Aug 11, 2025

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OK, with the way this all routed between different functions, I can't find a way to avoid changing this return type without rewriting test_multi_qubit_noise(). Given that this object class is only used in this file and only for that one function, that seems like it would be poor ROI.

I added a comment explaning what is happening. @dstrain115 can you check that it makes sense?

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@mhucka mhucka Aug 11, 2025

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Oh, I didn't realize op was already a np.array.

Actuallly, I was wrong about that in my earlier comment (which I deleted too slowly to save you typing that reply -- sorry about that). At this point in the flow of things, it's not an ndarray. The whole flow is a bit more complicated, which hopefully the new comment clarifies.



@pytest.mark.parametrize(
Expand Down Expand Up @@ -2025,11 +2035,11 @@ def test_qsimcirq_identity_expectation_value():
for w, pauli in objs:
pauli = pauli[::-1]
hamiltonian += float(w) * cirq.PauliString(
cirq.I(cirq.LineQubit(i))
if p == "I"
else cirq.Z(cirq.LineQubit(i))
if p == "Z"
else None
(
cirq.I(cirq.LineQubit(i))
if p == "I"
else cirq.Z(cirq.LineQubit(i)) if p == "Z" else None
)
for i, p in enumerate(pauli)
)

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