qml.pauli_measure¶

pauli_measure(pauli_word, wires, postselect=None)[source]¶

Perform a Pauli product measurement.

A Pauli product measurement (PPM) is the measurement of a tensor product of Pauli observables (X, Y, Z, and I).

The eigenvalue of this tensor product is one of 1 or -1, which is mapped to the 0 or 1 outcome of the PPM, respectively. After the measurement, the state collapses to the superpositions of all degenerate eigenstates corresponding to the measured eigenvalue.

Note

Circuits comprising pauli_measure are currently not executable on any backend. This function is only for analysis using the null.qubit device and potential future execution when a suitable backend is available.

See also

For more information on Pauli product measurements, check out the Quantum Compilation hub and catalyst.passes.ppm_compilation() for compiling these circuits with Catalyst.

Parameters:

pauli_word (str) – The Pauli word to measure.
wires (Wires) – The wires that the Pauli word acts on.
postselect (Optional[int]) – The postselection value, one of 0 or 1. It determines which subspace of degenerate eigenstates to postselect after a Pauli product measurement. None by default.

Returns:

A reference to the future result of the Pauli product measurement

Return type:

MeasurementValue

Raises:

ValueError – if the Pauli word has characters other than X, Y and Z.
ValueError – if the number of wires does not match the length of the Pauli word.

Example:

The following example illustrates how to include a Pauli product measurement (PPM) in a circuit by specifiying the Pauli word and the wires it acts on.

@qml.qnode(qml.device("null.qubit", wires=3))
def circuit():
    qml.Hadamard(0)
    qml.Hadamard(2)

    ppm = qml.pauli_measure(pauli_word="XY", wires=[0, 2])
    qml.cond(ppm, qml.X)(wires=1)

    return qml.expval(qml.Z(0))

The X operation on wire 1 will be applied conditionally on the value of the PPM outcome:

>>> print(qml.draw(circuit)())
0: ──H─╭┤↗X├────┤  <Z>
1: ────│──────X─┤
2: ──H─╰┤↗Y├──║─┤
         ╚════╝

Additionally, the number of PPM operations in a circuit can be easily inspected with specs() where they are denoted as a PauliMeasure gate type:

>>> print(qml.specs(circuit)()['resources'])
Total wire allocations: 3
Total gates: 4
Circuit depth: 3
Gate types:
  Hadamard: 2
  PauliMeasure: 1
  Conditional(PauliX): 1
Measurements:
  expval(PauliZ): 1

code/api/pennylane.pauli_measure

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qml.pauli_measure¶

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