Measurement: Remove targets and classical_store by mudit06mah · Pull Request #386 · qutip/qutip-qip

mudit06mah · 2026-04-17T19:03:05Z

Description
This PR removes targets and classical_store from Measurement class.

Changes

Deprecates(Removes) targets and classical_store from Measurement class
Changes measurement_comp_basis() to accept targets as arg
Changes _apply_measurement to accept targets and classical_store as arg
Modifies related test cases

Related issues or PRs
Part of #331

AI Usage Disclosure
To fix docstring grammar

mudit06mah · 2026-04-17T19:17:04Z

-    final_measurement = Measurement("start", targets=[2])
-    initial_measurement = Measurement("start", targets=[0])
+    final_measurement = Measurement("start")
+    initial_measurement = Measurement("start")


I was curious as to why both of these measurements have the name "start", it is the same in other tests as well so It is intentional I think.

The name argument in Measurement object doesn't matter and should be removed.

Ideally both of these should be

Suggested change

initial_measurement = Measurement("start")

final_measurement = Mz

initial_measurement = Mz

If someone wants to define a custom measurement operator, it should be via inheritance

I haven't yet introduced measurement operators, but we can remove the name prop from the class I guess

Sounds good.

@Mayank447 I have left the name prop but removed assigning the passed the name prop in the __init__ method, so when operators are introduced they can have the name prop to "MZ', "MX", etc.

Instead of defining the measurement operators "Mz", "Mx" as name prop, we can make Measurement class an abstract class and define any measurement operators by inheriting the Measurement. This is what we follow for Gate class and in future plan to do the same for device classes. You don't need to make any change for this in this PR.

I see Measurement() being repeated at quite a few places. What you can do is to define Mz = Measurement() in measurement.py, import that wherever qc.add_measurement is being called use Mz instead of Measurement(). This will fix the interface for the other files, we won't have to touch them. Later all our future changes like making Mz inherit from Measurement etc. would only need to be made in measurement.py

we can make Measurement class an abstract class and define any measurement operators

Yes that is what I meant as well, I am not removing the name prop right now and leaving it to use for the Mz and other operator classes which will be abstracted by Measurement.

Nice idea about defining Mz, I have made new changes according to that :)

Mayank447 · 2026-04-20T00:44:10Z

Hi, thanks for this PR. I admit I am a bit lost. All the changes seem to be squashed in a single commit, could you explain a bit more the reasoning behind the different changes.

mudit06mah · 2026-04-20T03:43:44Z

Hey @Mayank447, since the changes weren't that big I only made one commit.

This PR focuses on removing targets and classical_store from the Measurement class.
Since measurement_comp_basis was accessing Measurement.targets I had to pass it from _apply_measurementwhich uses the 'classical_store` so I passed both targets and classical_store from the step function.
I changed the test cases accordingly.

BoxiLi · 2026-04-20T06:11:21Z

Glad to see this coming, just wanna add that, please let me know if you find it better to create a separate dev branch for this. E.g. it is hard or cumbersome to keep the tests passing for this refactor in every single PR.

mudit06mah · 2026-04-20T06:27:23Z

@BoxiLi I did fix all the tests for this PR, didn't appear to be too cumbersome but maybe when I introduced measurement operators which might cause a lot of changes in tests, though the PRs could very well just be bigger in size. I guess you can make that call :)

Mayank447 · 2026-04-27T05:34:06Z

Doctest appears to be failing. @mudit06mah could you make the same change in qip-basics.rst

…tor/targets

mudit06mah · 2026-04-27T17:40:35Z

@Mayank447 looks like I forgot to add the qip-basics.rst file in my commit, should be fine now :)

Mayank447

Good work!

Mayank447 · 2026-04-28T15:14:49Z

-            parameters are unpacked and added.
-        targets: list
+        measurement : :class:`.Measurement`
+            `Measurement` object


Suggested change

`Measurement` object

`Measurement` subclasses

Mayank447 · 2026-04-28T15:17:25Z

+        if isinstance(measurement, str):
+            warnings.warn(
+                "Passing a string name to add_measurement is deprecated. "
+                "Please pass an instantiated Measurement object instead.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+            measurement = Measurement(name=measurement)


Suggested change

if isinstance(measurement, str):

warnings.warn(

"Passing a string name to add_measurement is deprecated. "

"Please pass an instantiated Measurement object instead.",

DeprecationWarning,

stacklevel=2,

)

measurement = Measurement(name=measurement)

if isinstance(measurement, str):

warnings.warn(

"Passing a string name to add_measurement has been removed. "

"Defaulting to measurement in the Z- basis.",

DeprecationWarning,

stacklevel=2,

)

measurement = Mz

Object instantiation won't work since Measurement class will be abstracted in next couple of PRs.

Mayank447 · 2026-04-28T15:22:14Z

+        if targets is None or classical_store is None:
+            raise ValueError("'targets' and 'classical_store' must not be None")
+


Suggested change

if targets is None or classical_store is None:

raise ValueError("'targets' and 'classical_store' must not be None")

if not isinstance(targets, IntSequence):

raise ValueError("'targets' must be a sequence of positive integer")

if not isinstance(classical_store, IntSequence):

raise ValueError("'classical_store' must be a sequence of positive integer")

Also need to check for each targets, classical_store is >=0 and less than num_qubits for the circuit. I wrote some helper functions for this, they were used in add_gate. You can reuse those functions.

Mayank447 · 2026-04-28T15:24:48Z

 from qutip.measurement import measurement_statistics
 from qutip_qip.operations import expand_operator

+__all__ = ["Measurement", "Mz"]


Suggested change

__all__ = ["Measurement", "Mz"]

__all__ = ["Mz"]

Measurement is already exported in __init__.py. Although not relevant to this PR, later measurement.py can be renamed to measurementclass.py and a directory measurement can be created which stores all the standard measurements like Pauli basis, POVMs

Mayank447 · 2026-04-29T06:31:58Z

-                    raise ValueError("Index of a qubit must be an integer")
-
-    def measurement_comp_basis(self, state):
+    def measurement_comp_basis(self, state, targets):


This should be made a classmethod

Mayank447 · 2026-04-29T06:34:02Z

+        targets : tuple of int
+            The indices of the qubits to be measured.
+        classical_store : tuple of int
+            The indices of the classical registers where the measurement
+            results will be stored.


Suggested change

targets : tuple of int

The indices of the qubits to be measured.

classical_store : tuple of int

The indices of the classical registers where the measurement

results will be stored.

qubits : tuple of int

The indices of the qubits to be measured.

cbits : tuple of int

The indices of the classical registers where the measurement

results will be stored.

Same change needs to be made in the function.

Mayank447 · 2026-04-29T06:36:16Z

+        initial_measurement = Mz
+        _, initial_probabilities = initial_measurement.measurement_comp_basis(


Suggested change

initial_measurement = Mz

_, initial_probabilities = initial_measurement.measurement_comp_basis(

_, initial_probabilities = Mz.measurement_comp_basis(

Mayank447 · 2026-04-29T06:36:29Z

+        final_measurement = Mz
+        _, final_probabilities = final_measurement.measurement_comp_basis(


Suggested change

final_measurement = Mz

_, final_probabilities = final_measurement.measurement_comp_basis(

_, final_probabilities = Mz.measurement_comp_basis(

Same change needs to be made at few more places in this file as well.

Mayank447 · 2026-05-03T19:01:17Z

    "get_controlled_gate",
    "AngleParametricGate",
    "Measurement",
+    "Mz",


Suggested change

"Mz",

Why are you exporting Mz in both measurement and operation's __init__.py.

Oh yeah, sorry I guess I got a bit confused since I was thinking on where to keep and where to remove it, I have removed it now from operations.__init__

Mayank447

LGTM, just two minor changes.

Mayank447 · 2026-05-03T19:32:13Z

-
-    def measurement_comp_basis(self, state):
+    @classmethod
+    def measurement_comp_basis(cls, state, targets):


Suggested change

def measurement_comp_basis(cls, state, targets):

def measurement_comp_basis(cls, state, qubits):

Same change in function definition.

Mayank447 · 2026-05-03T19:34:47Z

    def add_measurement(
        self,
-        measurement: str | Measurement,
+        measurement: Measurement,


Suggested change

measurement: Measurement,

measurement: Type[Measurement],

Measurement -> instantiated object from Measurement class, Type[Measurement] -> a Measurement subclass

mudit06mah · 2026-05-03T19:51:52Z

@Mayank447 I have made the necessary changes now, could you please take a look?

Mayank447 · 2026-05-03T20:01:23Z

LGTM

Copilot

Pull request overview

This PR refactors how measurements are represented and attached to circuits by removing targets/classical_store state from the Measurement object and passing these via MeasurementInstruction/add_measurement arguments, aligning with the ongoing gate/instruction refactor (#331).

Changes:

Introduces a shared Z-basis measurement object (Mz) and updates call sites to pass Mz into QubitCircuit.add_measurement.
Updates measurement execution to use instruction-provided qubit/cbit indices (simulator changes) and adapts measurement_comp_basis to accept qubits as an argument.
Updates tests, QASM/Qiskit conversion, and documentation examples to use the new measurement API.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/test_renderer.py	Switches circuit measurement usage from string names to `Mz`.
tests/test_qasm.py	Updates QASM tests to use `Mz` and pass explicit `qubits` into `measurement_comp_basis`.
tests/test_measurement.py	Updates measurement tests to call `Mz.measurement_comp_basis(state, qubits=...)`.
tests/test_circuit.py	Updates circuit tests to add measurements via `Mz` and adjusts assertions around instruction operations.
src/qutip_qip/qiskit/utils/converter.py	Converts Qiskit `measure` instructions into `add_measurement(Mz, ...)`.
src/qutip_qip/qasm.py	QASM import final pass now adds measurements using `Mz`.
src/qutip_qip/operations/measurement.py	Deprecates `Measurement` initialization args, makes `measurement_comp_basis` take qubits, and defines singleton `Mz`.
src/qutip_qip/circuit/simulator/matrix_mul_simulator.py	Updates measurement application to use instruction qubits/cbits and new `measurement_comp_basis` signature.
src/qutip_qip/circuit/circuit.py	Refactors `add_measurement` to accept measurement objects and normalizes targets/cbits to sequences with bounds checks.
src/qutip_qip/algorithms/phase_flip.py	Updates syndrome measurement to use `Mz`.
src/qutip_qip/algorithms/bit_flip.py	Updates syndrome measurement to use `Mz`.
doc/source/qip-basics.rst	Updates measurement examples/expected output to reflect `Mz` usage (but still contains at least one remaining string-based example).

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mudit06mah · 2026-05-04T09:08:36Z

+            The collapsed state after the measurement.
        """
-        states, probabilities = operation.measurement_comp_basis(self.state)
+        states, probabilities = operation.measurement_comp_basis(self.state, qubits)


I have removed the state argument from the _apply_measurement.py function.

Mayank447 · 2026-05-03T20:19:33Z

@@ -61,6 +51,8 @@ def measurement_comp_basis(self, state):
        state : ket or oper
                state to be measured on specified by
                ket vector or density matrix
+        qubits : list or tuple of int
+                The indices of the qubits to be measured.

        Returns
        -------
@@ -74,7 +66,7 @@ def measurement_comp_basis(self, state):
        """

        n = int(np.log2(state.shape[0]))
-        target = self.targets[0]
+        target = qubits[0]
        if target < n:
            op0 = basis(2, 0) * basis(2, 0).dag()
            op1 = basis(2, 1) * basis(2, 1).dag()
@@ -83,8 +75,7 @@ def measurement_comp_basis(self, state):
            raise ValueError("target is not valid")

        measurement_ops = [
-            expand_operator(op, dims=[2] * n, targets=self.targets)
-            for op in measurement_ops
+            expand_operator(op, dims=[2] * n, targets=qubits) for op in measurement_ops
        ]


Ignore this, expanding Measurement beyond one qubit is unrrelated to this PR.

+    def __init__(self, name=None, targets=None, index=None, classical_store=None):
        """
        Create a measurement with specified parameters.
        """
+        if name is not None:
+            warnings.warn(
+                "'name' argument in Measurement has been deprecated.",
+                DeprecationWarning,
+                stacklevel=2,
+            )


Mayank447 · 2026-05-03T20:21:21Z

 .. plot::
  :include-source:

  from qutip_qip.circuit import QubitCircuit
  from qutip_qip.operations.gates import H, CX, ISWAP

  # create the quantum circuit
  qc = QubitCircuit(2, num_cbits=1)
  qc.add_gate(CX, controls=0, targets=1)
  qc.add_gate(H, targets=1)
  qc.add_gate(ISWAP, targets=[0,1])
  qc.add_measurement("M0", targets=1, classical_store=0)

  qc.draw("matplotlib", dpi=300)

 **Customization Examples**:

 .. plot::
  :include-source:

  from qutip_qip.circuit import QubitCircuit
  from qutip_qip.operations.gates import H, CX, ISWAP
+  from qutip_qip.operations.measurement import Mz



I missed this earlier, check line 300. There might be more instances.

Mayank447 · 2026-05-03T20:21:30Z

    def add_measurement(
        self,
-        measurement: str | Measurement,
+        measurement: Type[Measurement],
        targets: int | IntSequence,
        classical_store: int,


Ignore this

Mayank447 · 2026-05-03T20:22:15Z

+        measurement: Type[Measurement],
        targets: int | IntSequence,
        classical_store: int,
        index: None = None,


I agree with Copilot on this

Suggested change

measurement: Type[Measurement],

targets: int | IntSequence,

classical_store: int,

index: None = None,

measurement: Type[Measurement],

targets: int | IntSequence,

classical_store: int | IntSequence,

index: None = None,

Mayank447 · 2026-05-03T20:22:44Z

+        if targets is None or classical_store is None:
+            raise ValueError("'targets' and 'classical_store' must not be None")

-        if type(targets) is int:
-            targets = [targets]
+        targets = convert_type_input_to_sequence(int, "targets", targets)
+        classical_store = convert_type_input_to_sequence(
+            int, "classical_store", classical_store
+        )

-        if type(classical_store) is int:
-            classical_store = [classical_store]
+        check_limit("targets", targets, 0, self.num_qubits - 1)
+        check_limit("classical_store", classical_store, 0, self.num_cbits - 1)

        self._instructions.append(
            MeasurementInstruction(
-                operation=meas,
+                operation=measurement,
                qubits=tuple(targets),
                cbits=tuple(classical_store),
            )


mudit06mah · 2026-05-04T09:58:52Z

@BoxiLi Could you take a look as well?

Measurement: Remove targets and classical_store

7972b0c

mudit06mah commented Apr 17, 2026

View reviewed changes

Measurement: Remove name arg from init

423e6be

mudit06mah requested a review from Mayank447 April 21, 2026 13:16

Add Mz import

4985a07

mudit06mah force-pushed the measurement-refactor/targets branch from 210fa03 to 73c1ecd Compare April 26, 2026 13:56

Merge remote-tracking branch 'upstream/master' into measurement-refac…

f4bc91e

…tor/targets

mudit06mah force-pushed the measurement-refactor/targets branch from 73c1ecd to f4bc91e Compare April 27, 2026 17:39

Mayank447 suggested changes Apr 29, 2026

View reviewed changes

mudit06mah requested a review from Mayank447 May 2, 2026 20:47

Mayank447 reviewed May 3, 2026

View reviewed changes

mudit06mah force-pushed the measurement-refactor/targets branch from f2ad83a to 9ab3243 Compare May 3, 2026 19:23

Mayank447 approved these changes May 3, 2026

View reviewed changes

Fix imports and docstrings

4c93e05

mudit06mah force-pushed the measurement-refactor/targets branch from 9ab3243 to 4c93e05 Compare May 3, 2026 19:51

Mayank447 requested a review from Copilot May 3, 2026 20:13

Copilot started reviewing on behalf of Mayank447 May 3, 2026 20:13 View session

Copilot AI reviewed May 3, 2026

View reviewed changes

Removed state from add_measurement

b2867ee

Mayank447 approved these changes May 4, 2026

View reviewed changes

	initial_measurement = Measurement("start")
	final_measurement = Mz
	initial_measurement = Mz

		if targets is None or classical_store is None:
		raise ValueError("'targets' and 'classical_store' must not be None")

-        if targets is None or classical_store is None:
-            raise ValueError("'targets' and 'classical_store' must not be None")
+        if not isinstance(targets, IntSequence):
+            raise ValueError("'targets' must be a sequence of positive integer")
+         if not isinstance(classical_store, IntSequence):
+            raise ValueError("'classical_store' must be a sequence of positive integer")

		initial_measurement = Mz
		_, initial_probabilities = initial_measurement.measurement_comp_basis(

	initial_measurement = Mz
	_, initial_probabilities = initial_measurement.measurement_comp_basis(
	_, initial_probabilities = Mz.measurement_comp_basis(

		final_measurement = Mz
		_, final_probabilities = final_measurement.measurement_comp_basis(

	final_measurement = Mz
	_, final_probabilities = final_measurement.measurement_comp_basis(
	_, final_probabilities = Mz.measurement_comp_basis(

	def measurement_comp_basis(cls, state, targets):
	def measurement_comp_basis(cls, state, qubits):

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