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Jul 1, 2020
Merged

Only override control when global phase is zero #3113

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507a0c1
Only override control when global phase is zero
dabacon Jun 26, 2020
380cd8d
Combine doc string paragraphs
dabacon Jun 29, 2020
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28 changes: 20 additions & 8 deletions cirq/ops/common_gates.py
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Original file line number Diff line number Diff line change
Expand Up @@ -134,13 +134,16 @@ def controlled(self,
This behavior only occurs when the last control qubit is a default-type
control qubit. A default-type control qubit is one with shape of 2 (not
a generic qudit) and where the control is satisfied by the qubit being
ON, as opposed to OFF.
ON, as opposed to OFF. Note also that this only transforms into a
CXPowGate (or controlled version of that gate) if the global shift on
the XPowGate is 0, otherwise it produces a normal ControlledGate.

(Note that a CXPowGate is, by definition, a controlled-XPowGate.)
"""
result = super().controlled(num_controls, control_values,
control_qid_shape)
if (isinstance(result, controlled_gate.ControlledGate) and
if (self._global_shift == 0 and
isinstance(result, controlled_gate.ControlledGate) and
result.control_values[-1] == (1,) and
result.control_qid_shape[-1] == 2):
return cirq.CXPowGate(exponent=self._exponent,
Expand Down Expand Up @@ -452,13 +455,16 @@ def controlled(self,
This behavior only occurs when the last control qubit is a default-type
control qubit. A default-type control qubit is one with shape of 2 (not
a generic qudit) and where the control is satisfied by the qubit being
ON, as opposed to OFF.
ON, as opposed to OFF. Note also that this only transforms into a
CZPowGate (or controlled version of that gate) if the global shift of
the ZPowGate is 0, otherwise it produces a normal ControlledGate.

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Looks like the above two paragraphs can actually be combined more concisely into a single paragraph listing the preconditions for the specialization to occur.

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Combined for this and all other gates.

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I was suggesting more along the lines of:

This method will specialize the control only if:
1. global shift is 0
2. the last of the controls is a default-type control qubit (followed by the clarifying what that means)

It would make the prerequisites clearer to the reader. But it's a minor documentation nit and not worth blocking the PR.

(Note that a CZPowGate is, by definition, a controlled-ZPowGate.)
"""
result = super().controlled(num_controls, control_values,
control_qid_shape)
if (isinstance(result, controlled_gate.ControlledGate) and
if (self._global_shift == 0 and
isinstance(result, controlled_gate.ControlledGate) and
result.control_values[-1] == (1,) and
result.control_qid_shape[-1] == 2):
return cirq.CZPowGate(exponent=self._exponent,
Expand Down Expand Up @@ -803,13 +809,16 @@ def controlled(self,
This behavior only occurs when the last control qubit is a default-type
control qubit. A default-type control qubit is one with shape of 2 (not
a generic qudit) and where the control is satisfied by the qubit being
ON, as opposed to OFF.
ON, as opposed to OFF. Note also that this only transforms into a
CCZPowGate (or controlled version of that gate) if the global shift of
the CZPowGate is 0, otherwise it produces a normal ControlledGate.

(Note that a CCZPowGate is, by definition, a controlled-CZPowGate.)
"""
result = super().controlled(num_controls, control_values,
control_qid_shape)
if (isinstance(result, controlled_gate.ControlledGate) and
if (self._global_shift == 0 and
isinstance(result, controlled_gate.ControlledGate) and
result.control_values[-1] == (1,) and
result.control_qid_shape[-1] == 2):
return cirq.CCZPowGate(exponent=self._exponent,
Expand Down Expand Up @@ -969,13 +978,16 @@ def controlled(self,
This behavior only occurs when the last control qubit is a default-type
control qubit. A default-type control qubit is one with shape of 2 (not
a generic qudit) and where the control is satisfied by the qubit being
ON, as opposed to OFF.
ON, as opposed to OFF. Note also that this only transforms into a
CCXPowGate (or controlled version of that gate) if the global shift of
the CXPowGate is 0, otherwise it produces a normal ControlledGate.

(Note that a CCXPowGate is, by definition, a controlled-CXPowGate.)
"""
result = super().controlled(num_controls, control_values,
control_qid_shape)
if (isinstance(result, controlled_gate.ControlledGate) and
if (self._global_shift == 0 and
isinstance(result, controlled_gate.ControlledGate) and
result.control_values[-1] == (1,) and
result.control_qid_shape[-1] == 2):
return cirq.CCXPowGate(exponent=self._exponent,
Expand Down
29 changes: 29 additions & 0 deletions cirq/ops/common_gates_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -183,6 +183,35 @@ def test_specialized_control(input_gate, specialized_output):
input_gate, num_controls=3, control_qid_shape=(3, 2, 4))


@pytest.mark.parametrize(
'gate, specialized_type',
[(cirq.ZPowGate(global_shift=-0.5, exponent=0.5), cirq.CZPowGate),
(cirq.CZPowGate(global_shift=-0.5, exponent=0.5), cirq.CCZPowGate),
(cirq.XPowGate(global_shift=-0.5, exponent=0.5), cirq.CXPowGate),
(cirq.CXPowGate(global_shift=-0.5, exponent=0.5), cirq.CCXPowGate)])
def test_no_specialized_control_for_global_shift_non_zero(
gate, specialized_type):
assert not isinstance(gate.controlled(), specialized_type)


@pytest.mark.parametrize(
'gate, matrix',
[(cirq.ZPowGate(global_shift=-0.5, exponent=1), np.diag([1, 1, -1j, 1j])),
(cirq.CZPowGate(global_shift=-0.5,
exponent=1), np.diag([1, 1, 1, 1, -1j, -1j, -1j, 1j])),
(cirq.XPowGate(global_shift=-0.5, exponent=1),
np.block([[np.eye(2), np.zeros(
(2, 2))], [np.zeros(
(2, 2)), np.array([[0, -1j], [-1j, 0]])]])),
(cirq.CXPowGate(global_shift=-0.5, exponent=1),
np.block([[np.diag([1, 1, 1, 1, -1j, -1j]),
np.zeros((6, 2))],
[np.zeros(
(2, 6)), np.array([[0, -1j], [-1j, 0]])]]))])
def test_global_phase_controlled_gate(gate, matrix):
np.testing.assert_equal(cirq.unitary(gate.controlled()), matrix)


def test_rot_gates_eq():
eq = cirq.testing.EqualsTester()
gates = [
Expand Down