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Default atol for sub_state_vector is too picky for default precision of simulator #4786

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Strilanc opened this issue Dec 29, 2021 · 0 comments · Fixed by #4877
Closed

Default atol for sub_state_vector is too picky for default precision of simulator #4786

Strilanc opened this issue Dec 29, 2021 · 0 comments · Fixed by #4877
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area/simulation area/state-vector-simulator kind/bug-report Something doesn't seem to work. triage/accepted A consensus emerged that this bug report, feature request, or other action should be worked on

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@Strilanc
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The following raises an exception instead of finishing successfully:

import cirq
a, b = cirq.LineQubit.range(2)
final_state_vector = cirq.Simulator().simulate(cirq.Circuit(
    cirq.H(a),
    cirq.H(b),
    cirq.CZ(a, b),
    cirq.measure(a),
)).final_state_vector.reshape((2, 2))
print(cirq.sub_state_vector(final_state_vector, [0]))

Giving atol=1e-6 to the sub_state_vector method fixes it. The default of atol=1e-8 seems to be too strict given that this simple example fails it.
@Strilanc Strilanc added the kind/bug-report Something doesn't seem to work. label Dec 29, 2021
@viathor viathor added area/simulation area/state-vector-simulator triage/accepted A consensus emerged that this bug report, feature request, or other action should be worked on labels Jan 12, 2022
@tonybruguier tonybruguier mentioned this issue Jan 23, 2022
Merged
CirqBot pushed a commit that referenced this issue Jan 26, 2022
@tonybruguier
Increase atol for sub_state_vector() (#4877)
64e2eac 
Attempts to fix #4786
rht pushed a commit to rht/Cirq that referenced this issue May 1, 2023
@tonybruguier @rht
Increase atol for sub_state_vector() (quantumlib#4877)
8708b9a 
Attempts to fix quantumlib#4786
harry-phasecraft pushed a commit to PhaseCraft/Cirq that referenced this issue Oct 31, 2024
@tonybruguier
Increase atol for sub_state_vector() (quantumlib#4877)
2cb5e58 
Attempts to fix quantumlib#4786
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Increase atol for sub_state_vector() tonybruguier/Cirq
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@Strilanc @viathor