Add x and y measurements

[quantumjim]

What is the expected behavior?

Qiskit natively supports z measurements using measure, but x and y basis measurements need to be done with a quick hack (reflecting how the hardware works).

However:

• We may want to support hardware that can directly perform these measurements
• It could help in the textbook and other educational materials to be able to talk about these measurements without first having to introduce the gates that are currently required to perform them.

I'd suggest measure_x and measure_y be added to QuantumCircuit, with measure_x compiling down to h followed by measure (except for relevant hardware).

[chriseclectic]

You should just be able to define a composite instruction for these. Eg

class MeasureX(Instruction):

    def __init__(self):
        super().__init__("measure_x", 1, 1, [])

    def broadcast_arguments(self, qargs, cargs):
         # copy from measure instruction

    def _define(self):
        from qiskit.extensions.standard.h import HGate
        from qiskit.circuit.measure import Measure

        q = QuantumRegister(1, 'q')
        c = ClassicalRegister(1, 'c')

        self.definition = [
            (HGate(), [q[0]], []),
            (Measure(), [q[0]], [c[0]])
            (HGate(), [q[0]], []),
        ]

class MeasureY(Instruction):

    def __init__(self):
        super().__init__("measure_y", 1, 1, [])

    def broadcast_arguments(self, qargs, cargs):
         # copy from measure instruction

    def _define(self):
        from qiskit.extensions.standard.h import HGate
        from qiskit.extensions.standard.s import SdgGate
        from qiskit.circuit.measure import Measure

        q = QuantumRegister(1, 'q')
        c = ClassicalRegister(1, 'c')

        self.definition = [
            (SdgGate(), [q[0]], []),
            (HGate(), [q[0]], []),
            (Measure(), [q[0]], [c[0]]),
            (HGate(), [q[0]], []),
            (SdgGate(), [q[0]], [])
        ]

Edited to include post measurement instruction to rotate to the correct collapsed state.

[Nisha-Banerjee]

Update for gosts_of_github_past

This issue may be solved by PR #5311 which is pending for review.

[jakelishman]

Update on this issue, since I've just closed another as a duplicate of it:

We remain interested in generalising the concept of measures that Qiskit can reason about, but it is not at all a trivial task nor a case of just adding a couple of new classes; there are many assumptions baked into the library in many places that measurements are Z-basis, and modifying that will have a lot of knock-on effects throughout the library.

For more context of other discussions that have happened since, see:

• Add Pauli measurements #4024
• Pauli measurement #5311
• Pauli-X measurement instruction #7716
• Pauli X, Y, Z measurement instructions #9269
• [WIP] introduce GroupingMeasure class #9750

[Cryoris]

We can now specify arbitrary Pauli product measurements (incl. X and Y measurements) using the PauliProductMeasurement, e.g.

>>> from qiskit.circuit import QuantumCircuit
>>> from qiskit.quantum_info import Pauli
>>> from qiskit.circuit.library import PauliProductMeasurement
>>> circuit = QuantumCircuit(1, 1)
>>> circuit.h(0)
>>> circuit.append(PauliProductMeasurement(Pauli("X")), [0], [0])
>>> circuit.draw()
   ┌───┐┌─────────┐
q: ┤ H ├┤0        ├
   └───┘│  PPM(X) │
c: ═════╡0        ╞
        └─────────┘
>>> circuit.decompose().draw()
     ┌────────────┐┌───┐┌─┐┌───┐
  q: ┤ U(π/2,0,π) ├┤ H ├┤M├┤ H ├
     └────────────┘└───┘└╥┘└───┘
c: 1/════════════════════╩══════
                         0

It's not super convenient to define X or Y basis measurements, but if we need we could e.g. directly construct PauliProductMeasurement("X") or something. Backends could report PauliProductMeasurement as supported instructions, though with this we couldn't specify that a backend would only support X-measurements or something specific like that.