refactoring: make CircuitTransformer into an abstract interface (quantumlib#142)

* refactor out an abstract interface for circuit transformers and allow constructing QuantumBoard with an optional custom transformer.

* fix pytket cirq extension import after new version of pytket

Author: weinstein

Diff for: recirq/qaoa/placement.py

@@ -6,7 +6,7 @@
 import networkx as nx
 import numpy as np
 import pytket
-import pytket.cirq
+import pytket.extensions.cirq
 from pytket.circuit import Node, Qubit
 from pytket.passes import SequencePass, RoutingPass, PlacementPass
 from pytket.predicates import CompilationUnit, ConnectivityPredicate
@@ -87,7 +87,7 @@ def place_on_device(circuit: cirq.Circuit,
         initial_map: Initial placement of qubits
         final_map: The final placement of qubits after action of the circuit
     """
-    tk_circuit = pytket.cirq.cirq_to_tk(circuit)
+    tk_circuit = pytket.extensions.cirq.cirq_to_tk(circuit)
     tk_device = _device_to_tket_device(device)
 
     unit = CompilationUnit(tk_circuit, [ConnectivityPredicate(tk_device)])
@@ -103,7 +103,7 @@ def place_on_device(circuit: cirq.Circuit,
                      for n1, n2 in unit.initial_map.items()}
     final_map = {tk_to_cirq_qubit(n1): tk_to_cirq_qubit(n2)
              for n1, n2 in unit.final_map.items()}
-    routed_circuit = pytket.cirq.tk_to_cirq(unit.circuit)
+    routed_circuit = pytket.extensions.cirq.tk_to_cirq(unit.circuit)
 
     return routed_circuit, initial_map, final_map
 

Diff for: recirq/quantum_chess/circuit_transformer.py

@@ -22,19 +22,31 @@
 
 
 class CircuitTransformer:
+    """Abstract interface for circuit transformations.
+
+    For example: NamedQubit -> GridQubit transformations.
+    """
+    def __init__(self):
+        pass
+
+    def transform(self, circuit: cirq.Circuit) -> cirq.Circuit:
+        """Applies the transformation to the circuit."""
+        return None
+
+
+class ConnectivityHeuristicCircuitTransformer(CircuitTransformer):
     """Optimizer that will transform a circuit using NamedQubits
     and transform it to use GridQubits.  This will use a breadth-first
     search to find a suitable mapping into the specified grid.
 
     It will then transform all operations to use the new qubits.
     """
-
     def __init__(self, device: cirq.Device):
+        super().__init__()
         self.device = device
         self.mapping = None
         self.starting_qubit = self.find_start_qubit(device.qubits)
         self.qubit_list = device.qubits
-        super().__init__()
 
     def qubits_within(self, depth: int, qubit: cirq.GridQubit,
                       qubit_list: Iterable[cirq.GridQubit]) -> int:
@@ -54,7 +66,8 @@ def qubits_within(self, depth: int, qubit: cirq.GridQubit,
             c += self.qubits_within(depth - 1, qubit + diff, qubit_list)
         return c
 
-    def find_start_qubit(self, qubit_list: List[cirq.Qid],
+    def find_start_qubit(self,
+                         qubit_list: List[cirq.Qid],
                          depth=3) -> Optional[cirq.GridQubit]:
         """Finds a reasonable starting qubit to start the mapping.
 
@@ -278,7 +291,7 @@ def qubit_mapping(self,
         self.mapping = mapping
         return mapping
 
-    def optimize_circuit(self, circuit: cirq.Circuit) -> cirq.Circuit:
+    def transform(self, circuit: cirq.Circuit) -> cirq.Circuit:
         """ Creates a new qubit mapping for a circuit and transforms it.
 
         This uses `qubit_mapping` to create a mapping from the qubits
@@ -299,10 +312,9 @@ class SycamoreDecomposer(cirq.PointOptimizer):
     Currently supported are controlled ISWAPs with a single control
     and control-X gates with multiple controls (TOFFOLI gates).:w
     """
-
-    def optimization_at(self, circuit: cirq.Circuit, index: int,
-                        op: cirq.Operation
-                        ) -> Optional[cirq.PointOptimizationSummary]:
+    def optimization_at(
+            self, circuit: cirq.Circuit, index: int,
+            op: cirq.Operation) -> Optional[cirq.PointOptimizationSummary]:
         if len(op.qubits) > 3:
             raise ValueError(f'Four qubit ops not yet supported: {op}')
         new_ops = None

Diff for: recirq/quantum_chess/circuit_transformer_test.py

@@ -1,5 +1,5 @@
 # Copyright 2020 Google
-# 
+#
 # 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
@@ -33,59 +33,59 @@
 @pytest.mark.parametrize('device',
                          (cirq.google.Sycamore23, cirq.google.Sycamore))
 def test_single_qubit_ops(device):
-    transformer = ct.CircuitTransformer(device)
+    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
     c = cirq.Circuit(cirq.X(a1), cirq.X(a2), cirq.X(a3))
     transformer.qubit_mapping(c)
-    c = transformer.optimize_circuit(c)
+    c = transformer.transform(c)
     device.validate_circuit(c)
 
 
 @pytest.mark.parametrize('device',
                          (cirq.google.Sycamore23, cirq.google.Sycamore))
 def test_single_qubit_with_two_qubits(device):
-    transformer = ct.CircuitTransformer(device)
+    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
     c = cirq.Circuit(cirq.X(a1), cirq.X(a2), cirq.X(a3),
                      cirq.ISWAP(a3, a4) ** 0.5)
     transformer.qubit_mapping(c)
-    device.validate_circuit(transformer.optimize_circuit(c))
+    device.validate_circuit(transformer.transform(c))
 
 
 @pytest.mark.parametrize('device',
                          (cirq.google.Sycamore23, cirq.google.Sycamore))
 def test_three_split_moves(device):
-    transformer = ct.CircuitTransformer(device)
+    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
     c = cirq.Circuit(qm.split_move(a1, a2, b1), qm.split_move(a2, a3, b3),
                      qm.split_move(b1, c1, c2))
     transformer.qubit_mapping(c)
-    device.validate_circuit(transformer.optimize_circuit(c))
+    device.validate_circuit(transformer.transform(c))
 
 
 @pytest.mark.parametrize('device',
                          (cirq.google.Sycamore23, cirq.google.Sycamore))
 def test_disconnected(device):
-    transformer = ct.CircuitTransformer(device)
+    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
     c = cirq.Circuit(qm.split_move(a1, a2, a3), qm.split_move(a3, a4, d1),
                      qm.split_move(b1, b2, b3), qm.split_move(c1, c2, c3))
     transformer.qubit_mapping(c)
-    device.validate_circuit(transformer.optimize_circuit(c))
+    device.validate_circuit(transformer.transform(c))
 
 
 @pytest.mark.parametrize('device',
                          (cirq.google.Sycamore23, cirq.google.Sycamore))
 def test_move_around_square(device):
-    transformer = ct.CircuitTransformer(device)
+    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
     c = cirq.Circuit(qm.normal_move(a1, a2), qm.normal_move(a2, b2),
                      qm.normal_move(b2, b1), qm.normal_move(b1, a1))
     transformer.qubit_mapping(c)
-    device.validate_circuit(transformer.optimize_circuit(c))
+    device.validate_circuit(transformer.transform(c))
 
 
 @pytest.mark.parametrize('device',
                          (cirq.google.Sycamore23, cirq.google.Sycamore))
 def test_split_then_merge(device):
-    transformer = ct.CircuitTransformer(device)
+    transformer = ct.ConnectivityHeuristicCircuitTransformer(device)
     c = cirq.Circuit(qm.split_move(a1, a2, b1), qm.split_move(a2, a3, b3),
                      qm.split_move(b1, c1, c2), qm.normal_move(c1, d1),
                      qm.normal_move(a3, a4), qm.merge_move(a4, d1, a1))
     transformer.qubit_mapping(c)
-    device.validate_circuit(transformer.optimize_circuit(c))
+    device.validate_circuit(transformer.transform(c))

Diff for: recirq/quantum_chess/quantum_board.py

@@ -24,7 +24,7 @@
     square_to_bit,
     xy_to_bit,
 )
-import recirq.quantum_chess.circuit_transformer as circuit_transformer
+import recirq.quantum_chess.circuit_transformer as ct
 import recirq.quantum_chess.enums as enums
 import recirq.quantum_chess.move as move
 import recirq.quantum_chess.quantum_moves as qm
@@ -60,6 +60,8 @@ class CirqBoard:
             an error or post-selects the result away.
         noise_mitigation: Threshold of samples to overcome in order
             to be considered not noise.
+        transformer: The CircuitTransformer to use to convert the board's
+            NamedQubit circuit into a GridQubit circuit.
     """
 
     def __init__(self,
@@ -68,11 +70,14 @@ def __init__(self,
                  device: Optional[cirq.Device] = None,
                  error_mitigation: Optional[
                      enums.ErrorMitigation] = enums.ErrorMitigation.Nothing,
-                 noise_mitigation: Optional[float] = 0.0):
+                 noise_mitigation: Optional[float] = 0.0,
+                 transformer: Optional[ct.CircuitTransformer] = None):
         self.device = device
         self.sampler = sampler
         if device is not None:
-            self.transformer = circuit_transformer.CircuitTransformer(device)
+            self.transformer = (
+                transformer
+                or ct.ConnectivityHeuristicCircuitTransformer(device))
         self.with_state(init_basis_state)
         self.error_mitigation = error_mitigation
         self.noise_mitigation = noise_mitigation
@@ -179,11 +184,9 @@ def sample_with_ancilla(self, num_samples: int
             # Translate circuit to grid qubits and sqrtISWAP gates
             if self.device is not None:
                 # Decompose 3-qubit operations
-                circuit_transformer.SycamoreDecomposer().optimize_circuit(
-                    measure_circuit)
+                ct.SycamoreDecomposer().optimize_circuit(measure_circuit)
                 # Create NamedQubit to GridQubit mapping and transform
-                measure_circuit = self.transformer.optimize_circuit(
-                    measure_circuit)
+                measure_circuit = self.transformer.transform(measure_circuit)
 
                 # For debug, ensure that the circuit correctly validates
                 self.device.validate_circuit(measure_circuit)