Mid-circuit measurements and conditional operations #2211

antalszava · 2022-02-17T22:41:17Z

Context:
Certain quantum circuits include measuring qubits in the middle of the circuit and conditioning operations in the circuit on the classical information obtained during measurement. Example use cases include quantum teleportation and error correction schemes.

Description of the Change:

This PR adds the user interface and a transform to allow mid-circuit measurements and conditional operations to be defined in PennyLane.

It adds two user-facing functionality:

A new qml.measure top-level function to perform a mid-circuit measurement on a single qubit;
A qml.cond top-level function that can be used to apply operations conditioned on measurement outcomes.

Further, a transform applying the deferred measurement principle is added.

    @qml.qnode(dev)
    @qml.defer_measurements
    def qnode_conditional_op_on_zero(x, y):
        qml.RY(x, wires=1)
        qml.CNOT(wires=[0, 1])
        m_0 = qml.measure(1)
        qml.cond(m_0 == 0, qml.RY)(y, wires=0)
        return qml.probs(wires=[0])
    pars = np.array([0.643, 0.246], requires_grad=True)

>>> qnode_conditional_op_on_zero(*pars)
tensor([0.98645017, 0.01354983], requires_grad=True)

Benefits:

Mid-circuit measurement and conditional operations can be defined in quantum functions;
PennyLane applies the defer_measurements transform in cases when a device doesn't support the former capabilities.

Possible Drawbacks:
N/A

Related GitHub Issues:
N/A

…gration tests; add lightning to the list of tested devices

trbromley

Thanks @antalszava @puzzleshark - I've left a final few comments/suggestions, but overall approved! 🟢 Great job everyone, it's very exciting to get this feature added 🚀

doc/introduction/measurements.rst

trbromley · 2022-03-03T19:29:02Z

doc/introduction/measurements.rst

+for the ``0`` measurement outcome..
+
+PennyLane implements the deferred measurement principle to transform
+conditional operations with the :func:`~.pennylane.transforms.defer_measurements` quantum


This link still won't work for some reason 🤔

Thanks for the catch! I'll be trying out other ones. 🤔

It seems that having qfunc_transform as the decorator doesn't allow such references 🤔 See #2283.

doc/introduction/measurements.rst

trbromley · 2022-03-03T19:45:42Z

pennylane/measurements.py

+    measurement_id = str(uuid.uuid4())[:8]
+    MeasurementProcess(MidMeasure, wires=wire, id=measurement_id)
+    return MeasurementValue(measurement_id)


Why do we need to make an ID, rather than, e.g., having MeasurementValue contain a MeasurementProcess?

Good question! We were thinking about this too.

having MeasurementValue contain a MeasurementProcess?

This seemed the most practical, yet, it is a bit confusing that the value object contains a measurement. Sam mentioned , however, that having an ID would be required when we do serialization of QNodes with mid-circuit measurements, so sticking with the original ID generation method seemed like a good approach.

pennylane/transforms/condition.py

pennylane/transforms/defer_measurements.py

trbromley · 2022-03-03T20:01:04Z

tests/transforms/test_defer_measurements.py

+        def teleportation_circuit(rads):
+
+            # Create Alice's secret qubit state
+            qml.RY(rads, wires=0)
+
+            # create an EPR pair with wires 1 and 2. 1 is held by Alice and 2 held by Bob
+            qml.Hadamard(wires=1)
+            qml.CNOT(wires=[1, 2])
+
+            # Alice sends her qubits through a CNOT gate.
+            qml.CNOT(wires=[0, 1])
+
+            # Alice then sends the first qubit through a Hadamard gate.
+            qml.Hadamard(wires=0)
+
+            # Alice measures her qubits, obtaining one of four results, and sends this information to Bob.
+            m_0 = qml.measure(0)
+            m_1 = qml.measure(1)
+
+            # Given Alice's measurements, Bob performs one of four operations on his half of the EPR pair and
+            # recovers the original quantum state.
+            qml.cond(m_1, qml.RX)(math.pi, wires=2)
+            qml.cond(m_0, qml.RZ)(math.pi, wires=2)
+
+            return qml.probs(wires=2)


Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com>

…eAI/pennylane into conditional-ops-alpha-sanitized

* implement and test __invert__ * add else_op to qml.cond * using inversion in an integration * format * copy measurement under the hood when else; else test * format * err tests * format * docstrings * isort * test_condition * docstrings * format * no need for separate logic for ops * refactor wrapper as per Josh's suggestion from code review * Update tests/transforms/test_defer_measurements.py Co-authored-by: Josh Izaac <josh146@gmail.com> * two unit tests for checking qml.cond queuing * add queue unit tests * format * invert docstring * rename arguments * module docstring * module docstring: note where integration tests are * changelog * Update pennylane/transforms/condition.py Co-authored-by: Josh Izaac <josh146@gmail.com> Co-authored-by: Josh Izaac <josh146@gmail.com>

puzzleshark added 30 commits January 13, 2022 19:31

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antalszava added 2 commits March 3, 2022 12:59

adjust the logic in defer_measurements to allow m == 0 case; add inte…

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…gration tests; add lightning to the list of tested devices

format

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trbromley approved these changes Mar 3, 2022

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antalszava mentioned this pull request Mar 4, 2022

Extend qml.cond #2275

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antalszava and others added 18 commits March 4, 2022 09:02

Update doc/introduction/measurements.rst

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Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com>

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Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com>

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Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com>

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Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com>

Merge branch 'master' into conditional-ops-alpha-sanitized

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def meas link

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antalszava mentioned this pull request Mar 4, 2022

Move mid-circuit measurement and conditional op transform into device #2282

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antalszava mentioned this pull request Mar 4, 2022

[BUG] Unable to reference some decorated top-lvl functions #2283

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antalszava merged commit 0499c3b into master Mar 4, 2022

antalszava deleted the conditional-ops-alpha-sanitized branch March 4, 2022 20:16

antalszava mentioned this pull request Mar 4, 2022

Fix conditional operation linting #2284

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josh146 mentioned this pull request Mar 14, 2022

Adds Measurement Capabilities to default.qubit #1093

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soosub mentioned this pull request Aug 2, 2022

Support for multiple mid-circuit measurements #2884

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Mid-circuit measurements and conditional operations #2211

Mid-circuit measurements and conditional operations #2211

antalszava commented Feb 17, 2022 •

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Mid-circuit measurements and conditional operations #2211

Mid-circuit measurements and conditional operations #2211

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antalszava commented Feb 17, 2022 •

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