Adds mixed-polarity multi-controlled gates #1104

glassnotes · 2021-02-22T21:23:58Z

Context: A common use case of multi-controlled operations is to control on values other than all qubits being in state |1>.

Description of the Change: This PR does two things:

Extends the existing ControlledQubitUnitary to accept a string of bits (or integer) that specifies the state of the control bits
Adds a new operation, MixedPolarityMultiControlledToffoli which is the special case of a multi-controlled NOT

Benefits: Makes multi-controlled operations more flexible without having to apply Pauli X's everywhere.

Possible Drawbacks:

"Mixed-polarity multi-controlled Toffoli" is the term I've encountered for these gates in the wild, but the name is quite long and may be confusing since it's a multi-controlled NOT. One option is to abbreviate this as MPMCT. An alternative is MultiControlledNOT.

Related GitHub Issues: None

pennylane/ops/qubit.py

codecov · 2021-02-23T18:31:02Z

Codecov Report

Merging #1104 (4a301e8) into master (59e074a) will decrease coverage by 0.00%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master    #1104      +/-   ##
==========================================
- Coverage   97.74%   97.74%   -0.01%     
==========================================
  Files         155      155              
  Lines       11760    11783      +23     
==========================================
+ Hits        11495    11517      +22     
- Misses        265      266       +1

Impacted Files	Coverage Δ
pennylane/devices/default_mixed.py	`100.00% <ø> (ø)`
pennylane/devices/default_qubit.py	`100.00% <ø> (ø)`
pennylane/ops/qubit.py	`97.38% <96.15%> (-0.07%)`	⬇️

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trbromley

Thanks! This is a really nice addition. When we have controlled templates/wires, would be nice to also think about having mixed polarity there too.

trbromley · 2021-02-24T22:46:47Z

.github/CHANGELOG.md

+- Added support for mixed-polarity multi-controlled operations in
+  `ControlledQubitUnitary` for arbitrary unitary operations, and special
+  case `MixedPolarityMultiControlledToffoli` for multi-controlled `NOT`
+  gates. [(#1104)](https://github.com/PennyLaneAI/pennylane/pull/1104)
+


Could we merge the ControlledQubitUnitary part into the bullet point above, and potentially say what mixed polarity means? There, we could have an example:

For example, we can create a multi-controlled T gate using:

T = qml.T._matrix() qml.ControlledQubitUnitary(T, control_wires=[0, 1, 3], wires=2, control_values="110")

Here, control wires 0 and 1 must be in the 1 state, while control wire 3 must be in the zero state for the unitary to be applied to wire 2.

Then this bullet point could be short and highlight MixedPolarityMultiControlledToffoli?

pennylane/ops/qubit.py

trbromley · 2021-02-24T22:54:36Z

pennylane/ops/qubit.py

+    Typically controlled operations apply a desired gate if the control qubits
+    are all in the state :math:`\vert 1\rangle`. However, there are some situations where
+    it is necessary to apply a gate conditioned on all qubits being in the
+    :math:`\vert 0\rangle` state, or some mix of the two.
+
+    The state on which to control can be changed by passing a value to
+    `control_values` as either a bit string, or an integer (which is converted
+    to binary representation in Big-Endian format).
+
+    For example, if we want to apply a single-qubit unitary to wire ``3``
+    conditioned on three wires where the first is in state ``0``, the second is
+    in state ``1``, and the third in state ``1``, we can write:
+
+    >>> qml.ControlledQubitUnitary(U, control_wires=[0, 1, 2], wires=3, control_values='011')
+
+    Or equivalently,
+
+    >>> qml.ControlledQubitUnitary(U, control_wires=[0, 1, 2], wires=3, control_values=3)
+
+    since ``011`` is 3 in binary.
+


Nice example!

pennylane/ops/qubit.py

trbromley · 2021-02-25T16:49:38Z

pennylane/ops/qubit.py

+    >>> qml.MixedPolarityMultiControlledToffoli(control_wires=[0, 1, 2, 3], wires=4, control_values='1110'])
+    >>> qml.MixedPolarityMultiControlledToffoli(control_wires=[0, 1, 2, 3], wires=4, control_values=14)


Not sure if this was as helpful here. Maybe they can defer to ControlledQubitUnitary docs?

tests/ops/test_qubit_ops.py

trbromley · 2021-02-25T18:40:13Z

tests/ops/test_qubit_ops.py

+        # The result of applying the mixed-polarity gate should be the same as
+        # if we conjugated the specified control wires with Pauli X and applied the
+        # "regular" ControlledQubitUnitary in between.


trbromley · 2021-02-25T18:54:52Z

tests/ops/test_qubit_ops.py

+        U = unitary_group.rvs(2 ** len(target_wires), random_state=3)
+
+        @qml.qnode(dev)
+        def circuit_mixed_polarity():


Minor point, but would be it interesting to initialize in a less standard state? E.g. we take the first column of another randomly generated unitary as the starting state?

Great idea -- this also helped me catch a bug in the tests, the Pauli X were being applied to the wrong control wires. It now works with arbitrary states prepared on both the control and target qubits 🚀

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

thisac

Really nice addition @glassnotes! I really like how this works. Just some questions/clarifications, but nothing major (I didn't have time to check all the tests due to the Gnocchi cooking class starting soon, but I'll do that as soon as I can). 🙂

thisac · 2021-02-25T22:11:15Z

.github/CHANGELOG.md

+  ```python
+  T = qml.T._matrix()
+  qml.ControlledQubitUnitary(T, control_wires=[0, 1, 3], wires=2, control_values="110")
+  ```
+
+  Here, the T gate will be applied to wire `2` if control wires `0` and `1` are in
+  state `1`, and control wire `3` is in state `0`. If no value is passed to
+  `control_values`, the gate will be applied if all control wires are in
+  the `1` state.


Nice example. Really clear! 💯

pennylane/ops/qubit.py

thisac · 2021-02-25T22:26:43Z

pennylane/ops/qubit.py

+    Or equivalently,
+
+    >>> qml.ControlledQubitUnitary(U, control_wires=[0, 1, 2], wires=3, control_values=3)
+
+    since ``011`` is 3 in binary.


Is it really necessary to be able to pass integers? I don't see any obvious use case for it (but I might be missing something). Also, it's not super-clear what control_wires=3 does while control_values='011' makes more sense. 🤔

Otherwise a really nice and clear example. 😄

The specific use-case I had in mind was lookup tables (like a quantum ROM) where you have integer or binary addresses with some stored contents. Also sometimes just it's more convenient to write an integer if there is a large number of control bits in some random-like sequence of 0 and 1 😄

Thanks! I think it's fine keeping it like this, especially if it's useful in certain cases, although I'm tend to prefer only having one type of input per argument. It could be a bit confusing to see control_values=42 in the wild. That, too me, would sound like there are 42 control values rather than what it's meant to be.

Yeah I definitely see how it would be confusing (especially if the integer is something like 10, is that meant to be 10, or 2?). Maybe there is a better way to enable the user to input either value - having two separate keyword arguments, control_bits and control_int would be one option, I suppose.

It avoids the confusion above, although now there are two arguments that basically do the same thing. Not sure if it might be better to just have string inputs allowed, and then the extra step to create it from an int would, if needed, would be on the user.

the extra step to create it from an int would

This is something I have to look up every time 😅 Okay I'm on board, as this will force me to remember.

thisac · 2021-02-25T23:06:16Z

pennylane/ops/qubit.py

+    @staticmethod
+    def _parse_control_values(control_wires, control_values):
+
+        control_int = control_values


Perhaps this statement can be put inside the if isinstance(control_values, int)?

thisac · 2021-02-25T23:08:24Z

pennylane/ops/qubit.py

+        return control_int
+
+
+class MultiControlledX(ControlledQubitUnitary):


Much better name! 👍

thisac · 2021-02-25T23:20:36Z

tests/ops/test_qubit_ops.py

+    def test_invalid_mixed_polarity_controls(
+        self, control_wires, wires, control_values, expected_error_message


Wouldn't it make more sense having these test only test the gates, e.g. simply

target_wires = Wires(wires) with pytest.raises(ValueError, match=expected_error_message): qml.ControlledQubitUnitary( X, control_wires=control_wires, wires=target_wires, control_values=control_values )

That would make them a bit more unit-test-like. 🤔

Co-authored-by: Theodor <theodor@xanadu.ai>

…ng QNode.

pennylane/ops/qubit.py

…into add-mpmct

trbromley

Thanks @glassnotes!

.github/CHANGELOG.md

tests/ops/test_qubit_ops.py

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

glassnotes · 2021-02-26T21:08:41Z

@trbromley how do we handle failure of codecov in this case?

trbromley · 2021-02-26T21:14:20Z

Is it failing in error? I think so - looks like it's saying the Alternative control values must be passed as a binary string. line is not covered, but I can see it covered in the tests.

If we're sure it's a mistake by codecov, we can ask @josh146 to merge anyway.

glassnotes · 2021-02-26T21:18:10Z

Is it failing in error? I think so - looks like it's saying the Alternative control values must be passed as a binary string. line is not covered, but I can see it covered in the tests.

If we're sure it's a mistake by codecov, we can ask @josh146 to merge anyway.

Very strange, it's definitely covered in test cases for both ControlledQubitUnitary and MultiControlledX. Anyways yes, we can go ahead and merge 👍

thisac

Looks great @glassnotes! Thanks for adding this. 🚀

.github/CHANGELOG.md

Co-authored-by: Josh Izaac <josh146@gmail.com>

Starting point for implementation.

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Fix f strings.

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Add MPMCT to docs.

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glassnotes marked this pull request as ready for review February 23, 2021 18:25

glassnotes changed the title ~~[WIP] Adds mixed-polarity multi-controlled gates~~ Adds mixed-polarity multi-controlled gates Feb 23, 2021

glassnotes requested review from trbromley and thisac February 23, 2021 18:26

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Adds mixed-polarity multi-controlled gates #1104

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		>>> qml.MixedPolarityMultiControlledToffoli(control_wires=[0, 1, 2, 3], wires=4, control_values='1110'])
		>>> qml.MixedPolarityMultiControlledToffoli(control_wires=[0, 1, 2, 3], wires=4, control_values=14)

		return control_int


		class MultiControlledX(ControlledQubitUnitary):

		def test_invalid_mixed_polarity_controls(
		self, control_wires, wires, control_values, expected_error_message

Adds mixed-polarity multi-controlled gates #1104

Adds mixed-polarity multi-controlled gates #1104

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