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QPY: Bug fix in Rust implementation of SparsePauliObservableElemPack #16788

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3a22fe5
Bug fix in Rust implementation of `SparsePauliObservableElemPack`
gadial Aug 18, 2026
5ff1a11
Release note
gadial Aug 18, 2026
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16 changes: 8 additions & 8 deletions crates/qpy/src/formats.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -506,21 +506,21 @@ pub struct SparsePauliOpListElemPack {
#[derive(Debug)]
pub struct SparsePauliObservableElemPack {
pub num_qubits: u32,
#[bw(calc = coeff_data.len() as u64)]
#[bw(calc = (coeff_data.len() * std::mem::size_of::<f64>()) as u64)]

@Cryoris Cryoris Aug 19, 2026

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Aren't the coeffs Complex64? Or why is f64 the correct size?

@gadial gadial Aug 20, 2026

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Look at _write_elem_sparse in the python code (the source of the format, in this case). coeffs are stored using

coeff_data = struct.pack(
            f"!{len(coeffs)*2}d", *(val for coeff in coeffs for val in (coeff.real, coeff.imag))
        )

So each coeff is stored using two consecutive f64 values comprising the original Complex64.

In the rust code we do the same, on pack_sparse_pauli_op:

let coeff_data = sparse_observable
            .coeffs()
            .iter()
            .flat_map(|coeff| [coeff.re, coeff.im])
            .collect();

pub coeff_data_size: u64,
#[bw(calc = bitterm_data.len() as u64)]
#[bw(calc = (bitterm_data.len() * std::mem::size_of::<u16>()) as u64)]

@Cryoris Cryoris Aug 19, 2026

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I think there's something I'm not understanding, I thought this would correspond to the size of BitTerm which is u8 -- could you explain where these sizes are coming from?

@gadial gadial Aug 20, 2026

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This is a bug in the original python implementation, and is actually one of the things we're gonna fix in QPY18, you can see it on the list at #15524. Even more, this bug was found as part of the fix...

The python code used bitterm_data = struct.pack(f"!{len(bitterms)}H", *bitterms); instead of H it should have been a B.

pub bitterm_data_size: u64,
#[bw(calc = inds_data.len() as u64)]
#[bw(calc = (inds_data.len() * std::mem::size_of::<u32>()) as u64)]
pub inds_data_size: u64,
#[bw(calc = bounds_data.len() as u64)]
#[bw(calc = (bounds_data.len() * std::mem::size_of::<u64>()) as u64)]
pub bounds_data_size: u64,
#[br(count = coeff_data_size)]
#[br(count = coeff_data_size / std::mem::size_of::<f64>() as u64)]
pub coeff_data: Vec<f64>, // complex numbers stored in format [re1, im1, re2, im2,...]
#[br(count = bitterm_data_size)]
#[br(count = bitterm_data_size / std::mem::size_of::<u16>() as u64)]
pub bitterm_data: Vec<u16>,
#[br(count = inds_data_size)]
#[br(count = inds_data_size / std::mem::size_of::<u32>() as u64)]
pub inds_data: Vec<u32>,
#[br(count = bounds_data_size)]
#[br(count = bounds_data_size / std::mem::size_of::<u64>() as u64)]
pub bounds_data: Vec<u64>,
}

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6 changes: 6 additions & 0 deletions releasenotes/notes/fix-qpy-sparse_pauli-095b5c3e6ef4f5e8.yaml
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@@ -0,0 +1,6 @@
---
fixes:
- |
Fixed a bug with the QPY serialization and deserialization of instructions
containing :class:`.SparseObservable` data.
Fixed `#16722 <https://github.com/Qiskit/qiskit/issues/16722>`__.
15 changes: 14 additions & 1 deletion test/python/qpy/test_roundtrip.py
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Expand Up @@ -22,7 +22,7 @@
from qiskit.circuit.random import random_circuit
from qiskit.circuit.parameter import Parameter
from qiskit.circuit.parametervector import ParameterVector
from qiskit.quantum_info import SparsePauliOp
from qiskit.quantum_info import SparsePauliOp, SparseObservable
from qiskit.circuit.classical import expr
from qiskit.synthesis import LieTrotter
from qiskit.qpy.common import QPY_RUST_READ_MIN_VERSION, QPY_RUST_WRITE_MIN_VERSION, QPY_VERSION
Expand Down Expand Up @@ -283,3 +283,16 @@ def test_literal_integers_in_for(self, version, write_with, read_with):
with qc.for_loop((2, 5, (1 << 60))) as _:
qc.x(0)
self.assert_roundtrip_equal(qc, version=version, read_with=read_with, write_with=write_with)

@all_qpy_combinations(17)
def test_evolutiongate_sparse_observable(self, version, write_with, read_with):
"""Test loading a circuit with an evolution gate over a SparseObservable works.

``SparseObservable`` support was added to QPY in version 17.
"""
op = SparseObservable.from_list([("XIX", 0.1), ("ZIZ", 0.3)])
evo = PauliEvolutionGate(op, time=2, synthesis=LieTrotter(reps=2))

qc = QuantumCircuit(op.num_qubits)
qc.append(evo, range(op.num_qubits))
self.assert_roundtrip_equal(qc, version=version, read_with=read_with, write_with=write_with)