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Combine Pauli measurements and postselection #7492

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Combine Pauli measurements and postselection #7492

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ddddddanni
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@github-actions github-actions bot added the size: XL lines changed >1000 label Jul 15, 2025
eliottrosenberg
eliottrosenberg requested changes Jul 15, 2025
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Thanks, Danni! This is a really great start! In addition to the comments below, it would be great if we could return the fraction of the bitstrings that survive the postselection, and maybe even more granular information in addition, like how many violate each of the symmetries. Maybe we can also optionally return the raw bitstrings to the users in case they want to do more analysis on them.

cirq-core/cirq/contrib/paulistring/pauli_string_measurement_with_readout_mitigation.py
together.
postselection_symmetries: A dictionary mapping Pauli strings or Pauli sums to
expected values for postselection symmetries. The
circuit is the eigenvector of each Pauli string or
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-> "The final state generated by the circuit is an eigenvector ... with the specified eigenvalues."

cirq-core/cirq/contrib/paulistring/pauli_string_measurement_with_readout_mitigation.py

circuit: circuits.FrozenCircuit
pauli_strings: list[ops.PauliString] | list[list[ops.PauliString]]
postselection_symmetries: dict[ops.PauliString | ops.PauliSum, int]
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Maybe this should default to None?

cirq-core/cirq/contrib/paulistring/pauli_string_measurement_with_readout_mitigation.py

circuit: circuits.FrozenCircuit
pauli_strings: list[ops.PauliString] | list[list[ops.PauliString]]
postselection_symmetries: dict[ops.PauliString | ops.PauliSum, int]
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I realized that if I try to construct a dictionary with PauliSums as keys, I get the error TypeError: unhashable type: 'PauliSum'. Do you have any suggestions of what to do here? Should we use tuples or make a new data structure?

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Curious is PauliSum just a combination of PauliStrings? If so, can we remove the PauliSum from the postselection_symmetries: dict[ops.PauliString | ops.PauliSum, int], and make it postselection_symmetries: dict[ops.PauliString, int] instead?
For users who originally want to make

psum = cirq.PauliSum.from_pauli_strings([
    pauli_1,
    pauli_2,
    pauli_3
])
postselection_symmetries[psum] = 1

they can split the psum into paulistrings and make the postselection_symmetries dict by

postselection_symmetries[pauli_1] = 1
postselection_symmetries[pauli_2] = 1
etc

Wdyt?

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@eliottrosenberg eliottrosenberg Aug 4, 2025 •
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There are some cases where the symmetry has to be specified by a PauliSum. For example, if total number is conserved, then $\sum_i Z_i$ is conserved, but each $Z_i$ individually is not necessarily.

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Ahh got it, thanks for the clarification! In this case, I think we can define postselection_symmetries as a list of tuples such as Sequence[tuple[cirq.PauliString | cirq.PauliSum, int]]?

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Thanks, Danni! This is a really great start! In addition to the comments below, it would be great if we could return the fraction of the bitstrings that survive the postselection, and maybe even more granular information in addition, like how many violate each of the symmetries. Maybe we can also optionally return the raw bitstrings to the users in case they want to do more analysis on them.

I actually have

@attrs.frozen
class PostFilteringSymmetryCalibrationResult:
    """Result of post-selection symmetry calibration.
    Attributes:
        raw_bitstrings: The raw bitstrings obtained from the measurement.
        filtered_bitstrings: The bitstrings after applying post-selection symmetries.
    """

    raw_bitstrings: np.ndarray
    filtered_bitstrings: np.ndarray

Maybe I can add:

  1. fraction_of_survived_bistrings: float
  2. symmetry_to_filtered_bistrings: dict{paulistring|paulisum, list[bitstrings]}?

eliottrosenberg
eliottrosenberg reviewed Jul 15, 2025
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cirq-core/cirq/contrib/paulistring/pauli_string_measurement_with_readout_mitigation.py
def measure_pauli_strings_with_symmetries(
sampler: work.Sampler,
circuits_to_pauli_params: list[CircuitToPauliStringsParameters],
pauli_measurement_circuits: list[circuits.Circuit],
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Do we need this input given that we have circuits_to_pauli_params?

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I guess the user doesn't use this function directly, so it's ok

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eliottrosenberg commented Jul 15, 2025 •
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@ddddddanni ah, I missed that I can do .calibration_result.filtered_bitstrings and .calibration_result.calibration_result.filtered_bitstrings. That's great! Thanks!

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mhucka commented Oct 13, 2025

@ddddddanni Just wanted to gently ask what the status of this is. No pressure; I know this is a draft.

@mhucka mhucka moved this to Waiting in Follow-up tracker Oct 13, 2025
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@ddddddanni Just wanted to gently ask what the status of this is. No pressure; I know this is a draft.

Thanks for checking!! I was working on the other PRs and Q3 okrs. I plan to get back to this PR this week.

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mhucka commented Oct 14, 2025

@ddddddanni No problem at all. Thanks for your quick reply!

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@eliottrosenberg eliottrosenberg eliottrosenberg requested changes

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