SparsePauliOp

qc/qiskit qc/math

Qiskit’s sparse representation of a weighted sum of Pauli strings (e.g. Z, ZZ, XIZY), used to define observables for measurement (e.g. with Estimator).

from qiskit.quantum_info import SparsePauliOp
observable = SparsePauliOp('Z', coeffs=[1.0])   # single-qubit Z

Common trap: must be passed as a plain object, not wrapped in a list, for methods like .apply_layout() to work. Also: the Pauli string length must match the circuit’s qubit count (one letter = one qubit).

Related

• Pauli Operators
• Transpilation — .apply_layout() is used after transpiling to remap an observable onto physical qubits
• Qiskit API Gotchas — this “plain object, not a list” trap is one entry in the running gotchas list

Self-Check

• What is a SparsePauliOp actually representing, in plain language?
• Why does the Pauli string length have to match the circuit’s qubit count?
• Why does an observable need .apply_layout() after Transpilation?