Add Circuit Optimizer for sqrt-iSWAP (#4224)

Follow up to #4213.  Fixes #4083.  (Also contains a one-line change to fix #4225.)

I'm open to name suggestions for `MergeInteractionsToSqrtIswap`.

cirq-core/cirq/__init__.py

@@ -318,6 +318,7 @@
     merge_single_qubit_gates_into_phased_x_z,
     merge_single_qubit_gates_into_phxz,
     MergeInteractions,
+    MergeInteractionsToSqrtIswap,
     MergeSingleQubitGates,
     single_qubit_matrix_to_gates,
     single_qubit_matrix_to_pauli_rotations,

cirq-core/cirq/circuits/circuit.py

@@ -2312,7 +2312,7 @@ def _pick_inserted_ops_moment_indices(
         frontier = defaultdict(lambda: 0)
     moment_indices = []
     for op in operations:
-        op_start = max(start, max(frontier[q] for q in op.qubits))
+        op_start = max(start, max((frontier[q] for q in op.qubits), default=0))
         moment_indices.append(op_start)
         for q in op.qubits:
             frontier[q] = max(frontier[q], op_start + 1)

cirq-core/cirq/optimizers/__init__.py

@@ -60,6 +60,10 @@
     MergeInteractions,
 )
 
+from cirq.optimizers.merge_interactions_to_sqrt_iswap import (
+    MergeInteractionsToSqrtIswap,
+)
+
 from cirq.optimizers.merge_single_qubit_gates import (
     merge_single_qubit_gates_into_phased_x_z,
     merge_single_qubit_gates_into_phxz,

cirq-core/cirq/optimizers/merge_interactions.py

@@ -12,10 +12,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-"""An optimization pass that combines adjacent single-qubit rotations."""
+"""An optimization pass that combines adjacent series of gates on two qubits."""
 
 from typing import Callable, List, Optional, Sequence, Tuple, cast, TYPE_CHECKING
 
+import abc
 import numpy as np
 
 from cirq import circuits, ops, protocols
@@ -25,19 +26,25 @@
     import cirq
 
 
-class MergeInteractions(circuits.PointOptimizer):
-    """Combines series of adjacent one and two-qubit gates operating on a pair
-    of qubits."""
+class MergeInteractionsAbc(circuits.PointOptimizer, metaclass=abc.ABCMeta):
+    """Combines series of adjacent one- and two-qubit, non-parametrized gates
+    operating on a pair of qubits."""
 
     def __init__(
         self,
         tolerance: float = 1e-8,
-        allow_partial_czs: bool = True,
         post_clean_up: Callable[[Sequence[ops.Operation]], ops.OP_TREE] = lambda op_list: op_list,
     ) -> None:
+        """
+        Args:
+            tolerance: A limit on the amount of absolute error introduced by the
+                construction.
+            post_clean_up: This function is called on each set of optimized
+                operations before they are put into the circuit to replace the
+                old operations.
+        """
         super().__init__(post_clean_up=post_clean_up)
         self.tolerance = tolerance
-        self.allow_partial_czs = allow_partial_czs
 
     def optimization_at(
         self, circuit: circuits.Circuit, index: int, op: ops.Operation
@@ -63,10 +70,8 @@ def optimization_at(
         if not switch_to_new and old_interaction_count <= 1:
             return None
 
-        # Find a max-3-cz construction.
-        new_operations = two_qubit_decompositions.two_qubit_matrix_to_operations(
-            op.qubits[0], op.qubits[1], matrix, self.allow_partial_czs, self.tolerance, False
-        )
+        # Find a (possibly ideal) decomposition of the merged operations.
+        new_operations = self._two_qubit_matrix_to_operations(op.qubits[0], op.qubits[1], matrix)
         new_interaction_count = len(
             [new_op for new_op in new_operations if len(new_op.qubits) == 2]
         )
@@ -82,12 +87,29 @@ def optimization_at(
             new_operations=new_operations,
         )
 
+    @abc.abstractmethod
     def _may_keep_old_op(self, old_op: 'cirq.Operation') -> bool:
         """Returns True if the old two-qubit operation may be left unchanged
         without decomposition."""
-        if self.allow_partial_czs:
-            return isinstance(old_op.gate, ops.CZPowGate)
-        return isinstance(old_op.gate, ops.CZPowGate) and old_op.gate.exponent == 1
+
+    @abc.abstractmethod
+    def _two_qubit_matrix_to_operations(
+        self,
+        q0: 'cirq.Qid',
+        q1: 'cirq.Qid',
+        mat: np.ndarray,
+    ) -> Sequence['cirq.Operation']:
+        """Decomposes the merged two-qubit gate unitary into the minimum number
+        of two-qubit gates.
+
+        Args:
+            q0: The first qubit being operated on.
+            q1: The other qubit being operated on.
+            mat: Defines the operation to apply to the pair of qubits.
+
+        Returns:
+            A list of operations implementing the matrix.
+        """
 
     def _op_to_matrix(
         self, op: ops.Operation, qubits: Tuple['cirq.Qid', ...]
@@ -130,7 +152,7 @@ def _op_to_matrix(
 
     def _scan_two_qubit_ops_into_matrix(
         self, circuit: circuits.Circuit, index: Optional[int], qubits: Tuple['cirq.Qid', ...]
-    ) -> Tuple[List[ops.Operation], List[int], np.ndarray]:
+    ) -> Tuple[Sequence[ops.Operation], List[int], np.ndarray]:
         """Accumulates operations affecting the given pair of qubits.
 
         The scan terminates when it hits the end of the circuit, finds an
@@ -181,3 +203,55 @@ def _flip_kron_order(mat4x4: np.ndarray) -> np.ndarray:
         for j in range(4):
             result[order[i], order[j]] = mat4x4[i, j]
     return result
+
+
+class MergeInteractions(MergeInteractionsAbc):
+    """Combines series of adjacent one- and two-qubit, non-parametrized gates
+    operating on a pair of qubits and replaces each series with the minimum
+    number of CZ gates."""
+
+    def __init__(
+        self,
+        tolerance: float = 1e-8,
+        allow_partial_czs: bool = True,
+        post_clean_up: Callable[[Sequence[ops.Operation]], ops.OP_TREE] = lambda op_list: op_list,
+    ) -> None:
+        """
+        Args:
+            tolerance: A limit on the amount of absolute error introduced by the
+                construction.
+            allow_partial_czs: Enables the use of Partial-CZ gates.
+            post_clean_up: This function is called on each set of optimized
+                operations before they are put into the circuit to replace the
+                old operations.
+        """
+        super().__init__(tolerance=tolerance, post_clean_up=post_clean_up)
+        self.allow_partial_czs = allow_partial_czs
+
+    def _may_keep_old_op(self, old_op: 'cirq.Operation') -> bool:
+        """Returns True if the old two-qubit operation may be left unchanged
+        without decomposition."""
+        if self.allow_partial_czs:
+            return isinstance(old_op.gate, ops.CZPowGate)
+        return isinstance(old_op.gate, ops.CZPowGate) and old_op.gate.exponent == 1
+
+    def _two_qubit_matrix_to_operations(
+        self,
+        q0: 'cirq.Qid',
+        q1: 'cirq.Qid',
+        mat: np.ndarray,
+    ) -> Sequence['cirq.Operation']:
+        """Decomposes the merged two-qubit gate unitary into the minimum number
+        of CZ gates.
+
+        Args:
+            q0: The first qubit being operated on.
+            q1: The other qubit being operated on.
+            mat: Defines the operation to apply to the pair of qubits.
+
+        Returns:
+            A list of operations implementing the matrix.
+        """
+        return two_qubit_decompositions.two_qubit_matrix_to_operations(
+            q0, q1, mat, self.allow_partial_czs, self.tolerance, False
+        )

cirq-core/cirq/optimizers/merge_interactions_to_sqrt_iswap.py

@@ -0,0 +1,104 @@
+# Copyright 2021 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""An optimization pass that combines adjacent series of gates on two qubits and
+outputs a circuit with SQRT_ISWAP or SQRT_ISWAP_INV gates."""
+
+from typing import Callable, Optional, Sequence, TYPE_CHECKING
+
+import numpy as np
+
+from cirq import ops
+from cirq.optimizers import two_qubit_to_sqrt_iswap, merge_interactions
+
+if TYPE_CHECKING:
+    import cirq
+
+
+class MergeInteractionsToSqrtIswap(merge_interactions.MergeInteractionsAbc):
+    """Combines series of adjacent one- and two-qubit, non-parametrized gates
+    operating on a pair of qubits and replaces each series with the minimum
+    number of SQRT_ISWAP gates.
+
+    See also: ``two_qubit_matrix_to_sqrt_iswap_operations``
+    """
+
+    def __init__(
+        self,
+        tolerance: float = 1e-8,
+        *,
+        required_sqrt_iswap_count: Optional[int] = None,
+        use_sqrt_iswap_inv: bool = False,
+        post_clean_up: Callable[[Sequence[ops.Operation]], ops.OP_TREE] = lambda op_list: op_list,
+    ) -> None:
+        """
+        Args:
+            tolerance: A limit on the amount of absolute error introduced by the
+                construction.
+            required_sqrt_iswap_count: When specified, each merged group of
+                two-qubit gates will be decomposed into exactly this many
+                sqrt-iSWAP gates even if fewer is possible (maximum 3).  Circuit
+                optimization will raise a ``ValueError`` if this number is 2 or
+                lower and synthesis of any set of merged interactions requires
+                more.
+            use_sqrt_iswap_inv: If True, optimizes circuits using
+                ``SQRT_ISWAP_INV`` gates instead of ``SQRT_ISWAP``.
+            post_clean_up: This function is called on each set of optimized
+                operations before they are put into the circuit to replace the
+                old operations.
+
+        Raises:
+            ValueError:
+                If ``required_sqrt_iswap_count`` is not one of the supported
+                values 0, 1, 2, or 3.
+        """
+        if required_sqrt_iswap_count is not None and not 0 <= required_sqrt_iswap_count <= 3:
+            raise ValueError('the argument `required_sqrt_iswap_count` must be 0, 1, 2, or 3.')
+        super().__init__(tolerance=tolerance, post_clean_up=post_clean_up)
+        self.required_sqrt_iswap_count = required_sqrt_iswap_count
+        self.use_sqrt_iswap_inv = use_sqrt_iswap_inv
+
+    def _may_keep_old_op(self, old_op: 'cirq.Operation') -> bool:
+        """Returns True if the old two-qubit operation may be left unchanged
+        without decomposition."""
+        if self.use_sqrt_iswap_inv:
+            return isinstance(old_op.gate, ops.ISwapPowGate) and old_op.gate.exponent == -0.5
+        return isinstance(old_op.gate, ops.ISwapPowGate) and old_op.gate.exponent == 0.5
+
+    def _two_qubit_matrix_to_operations(
+        self,
+        q0: 'cirq.Qid',
+        q1: 'cirq.Qid',
+        mat: np.ndarray,
+    ) -> Sequence['cirq.Operation']:
+        """Decomposes the merged two-qubit gate unitary into the minimum number
+        of SQRT_ISWAP gates.
+
+        Args:
+            q0: The first qubit being operated on.
+            q1: The other qubit being operated on.
+            mat: Defines the operation to apply to the pair of qubits.
+
+        Returns:
+            A list of operations implementing the matrix.
+        """
+        return two_qubit_to_sqrt_iswap.two_qubit_matrix_to_sqrt_iswap_operations(
+            q0,
+            q1,
+            mat,
+            required_sqrt_iswap_count=self.required_sqrt_iswap_count,
+            use_sqrt_iswap_inv=self.use_sqrt_iswap_inv,
+            atol=self.tolerance,
+            check_preconditions=False,
+        )