Simplify decomposition of controlled eigengates with global phase #7383
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daxfohl
changed the title
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## main #7383 +/- ##
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Coverage 98.68% 98.68%
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Files 1112 1112
Lines 97641 97707 +66
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+ Hits 96359 96426 +67
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| @property | ||
| def is_identity(self) -> bool: | ||
| return ( | ||
| not protocols.is_parameterized(self._coefficient) and abs(self._coefficient - 1.0) == 0 |
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This was moved from three_qubit_gates.py line 120, but now that I look at it again, it seems equivalent to self.coefficient == 1. Will try that and see if it works. If so, that's simple enough that the method probably doesn't even need to exist, and can just be inlined.
| assert isinstance(gate0, cirq.EigenGate) | ||
| assert gate0.global_shift == 0 | ||
| assert gate0.exponent == exponent | ||
| if exponent * 2 / 3 % 2 != 0: |
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Could be simplified to if exponent % 3:
| @pytest.mark.parametrize('exponent', [0, 0.5, 2, 3, -0.5, -2, -3, sympy.Symbol('s')]) | ||
| def test_decompose_with_extracted_phases(gate_type: type, exponent: cirq.TParamVal) -> None: | ||
| context = cirq.DecompositionContext(cirq.SimpleQubitManager(), extract_global_phases=True) | ||
| gate = gate_type(exponent=exponent, global_shift=2 / 3) |
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Make 2/3 a variable since it's used multiple times, and refer back to the docstring of _extract_phase to explain why it's interesting.
| control_qubits = list(qubits[: self.num_controls()]) | ||
| controlled_sub_gate = self.sub_gate.controlled( | ||
| self.num_controls(), self.control_values, self.control_qid_shape | ||
| ) | ||
| # Prefer the subgate controlled version if available | ||
| if self != controlled_sub_gate: | ||
| return controlled_sub_gate.on(*qubits) | ||
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| # Try decomposing the subgate next. | ||
| # Extract global phases from decomposition, as controlled phases decompose easily. |
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Move this comment five lines down.
| @@ -802,3 +802,19 @@ def test_controlled_global_phase_matrix_gate_decomposes(num_controls, angle, con | |||
| decomposed = cirq.decompose(cg_matrix(*all_qubits)) | |||
| assert not any(isinstance(op.gate, cirq.MatrixGate) for op in decomposed) | |||
| np.testing.assert_allclose(cirq.unitary(cirq.Circuit(decomposed)), cirq.unitary(cg_matrix)) | |||
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| def test_simplified_controlled_phased_eigengate_decomposition() -> None: | |||
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Add the other affected gates?
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Actually it could be made more generic and parameterized: the decomposition should always be equal to the decomposition of the equivalent gate without global shift, plus a final Z gate.
Fixes #7238
DecomposeContextto configure extraction of terminal gates' global phases into distinct GlobalPhaseGates.a. This flag defaults to False for backward compatibility.
a. Added some convenience methods in GlobalPhaseGate to help.
b. Updated some unrelated existing code to use these convenience methods for clarity.
ControlledGate.decomposeto attempt decompose the subgate before attempting the brute-force approach.a. This is what ultimately simplifies the decomposition result.
b. Step 2 also allowed removal of the entire CZPowGate instanceof block there, since it was about extracting global phase.