Default Bloq.get_ctrl_system: Use And ladder to reduce multiple controls to single control (#1456)

* default ctrl system: use And ladder to reduce controls to a single control bit

* fix tests

* test: `XGate().controlled(...)`

---------

Co-authored-by: Matthew Harrigan <mpharrigan@google.com>

Author: anurudhp

qualtran/_infra/bloq.py

@@ -394,7 +394,12 @@ def _my_add_controlled(
             add_controlled: A function with the signature documented above that the system
                 can use to automatically wire up the new control registers.
         """
-        from qualtran import Controlled
+        from qualtran import Controlled, CtrlSpec
+        from qualtran.bloqs.mcmt.controlled_via_and import ControlledViaAnd
+
+        if ctrl_spec != CtrlSpec():
+            # reduce controls to a single qubit
+            return ControlledViaAnd.make_ctrl_system(self, ctrl_spec=ctrl_spec)
 
         return Controlled.make_ctrl_system(self, ctrl_spec=ctrl_spec)
 

qualtran/_infra/gate_with_registers_test.py

@@ -151,8 +151,10 @@ def test_gate_with_registers_decompose_from_context_auto_generated():
 
 
 def test_non_unitary_controlled():
+    from qualtran.bloqs.mcmt.controlled_via_and import ControlledViaAnd
+
     bloq = BloqWithDecompose()
-    assert bloq.controlled(control_values=[0]) == Controlled(bloq, CtrlSpec(cvs=0))
+    assert bloq.controlled(control_values=[0]) == ControlledViaAnd(bloq, CtrlSpec(cvs=0))
 
 
 @pytest.mark.notebook

qualtran/bloqs/basic_gates/identity_test.py

@@ -15,16 +15,14 @@
 import numpy as np
 import pytest
 import sympy
-from attrs import frozen
 
-from qualtran import Bloq, BloqBuilder, CtrlSpec, QInt, QUInt, Signature, Soquet, SoquetT
+from qualtran import BloqBuilder
 from qualtran.bloqs.basic_gates import OneState
 from qualtran.bloqs.basic_gates.identity import _identity, _identity_n, _identity_symb, Identity
 from qualtran.simulation.classical_sim import (
     format_classical_truth_table,
     get_classical_truth_table,
 )
-from qualtran.symbolics import SymbolicInt
 from qualtran.testing import execute_notebook
 
 
@@ -95,42 +93,6 @@ def test_identity_controlled():
     assert Identity(n).controlled() == Identity(n + 1)
 
 
-@frozen
-class TestIdentityDecomposition(Bloq):
-    """helper to test Identity.get_ctrl_system"""
-
-    bitsize: SymbolicInt
-
-    @property
-    def signature(self) -> 'Signature':
-        return Signature.build(q=self.bitsize)
-
-    def build_composite_bloq(self, bb: 'BloqBuilder', q: Soquet) -> dict[str, 'SoquetT']:
-        q = bb.add(Identity(self.bitsize), q=q)
-        q = bb.add(Identity(self.bitsize), q=q)
-        return {'q': q}
-
-
-@pytest.mark.parametrize("n", [4, sympy.Symbol("n")])
-@pytest.mark.parametrize(
-    "ctrl_spec",
-    [
-        CtrlSpec(cvs=(np.array([1, 0, 1]),)),
-        CtrlSpec(qdtypes=(QUInt(3), QInt(3)), cvs=(np.array(0b010), np.array(0b001))),
-    ],
-)
-def test_identity_get_ctrl_system(n: SymbolicInt, ctrl_spec: CtrlSpec):
-    m = ctrl_spec.num_qubits
-
-    bloq = TestIdentityDecomposition(n)
-    ctrl_bloq = bloq.controlled(ctrl_spec)
-
-    _ = ctrl_bloq.decompose_bloq()
-
-    _, sigma = ctrl_bloq.call_graph()
-    assert sigma == {Identity(n + m): 2}
-
-
 @pytest.mark.notebook
 def test_notebook():
     execute_notebook('identity')

qualtran/bloqs/basic_gates/x_basis_test.py

@@ -92,3 +92,17 @@ def test_x_truth_table():
 0 -> 1
 1 -> 0"""
     )
+
+
+def test_controlled_x():
+    from qualtran import CtrlSpec, QUInt
+    from qualtran.bloqs.basic_gates import CNOT
+    from qualtran.bloqs.mcmt import And
+
+    def _keep_and(b):
+        return isinstance(b, And)
+
+    n = 8
+    bloq = XGate().controlled(CtrlSpec(qdtypes=QUInt(n), cvs=1))
+    _, sigma = bloq.call_graph(keep=_keep_and)
+    assert sigma == {And(): n - 1, CNOT(): 1, And().adjoint(): n - 1, XGate(): 4 * (n - 1)}

qualtran/bloqs/qsp/generalized_qsp_test.py

@@ -201,7 +201,7 @@ def catch_rotations(bloq: Bloq) -> Bloq:
 
     expected_sigma: dict[Bloq, int] = {arbitrary_rotation: degree + 1}
     if degree > negative_power:
-        expected_sigma[Controlled(U, CtrlSpec(cvs=0))] = degree - negative_power
+        expected_sigma[U.controlled(ctrl_spec=CtrlSpec(cvs=0))] = degree - negative_power
     if negative_power > 0:
         expected_sigma[Controlled(U.adjoint(), CtrlSpec())] = min(degree, negative_power)
     if negative_power > degree: