Fixes for device wire handling

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 * Made plugin device compatible with new PennyLane wire management.
   [(#37)](https://github.com/PennyLaneAI/pennylane-cirq/pull/37)
+  [(#42)](https://github.com/PennyLaneAI/pennylane-cirq/pull/42)
 
   One can now specify any string or number as a custom wire label,
   and use these labels to address subsystems on the device:
@@ -21,7 +22,7 @@
 
 This release contains contributions from (in alphabetical order):
 
-Maria Schuld
+Josh Izaac, Nathan Killoran, Maria Schuld
 
 ---
 

pennylane_cirq/cirq_device.py

@@ -32,11 +32,15 @@
 ~~~~~~~~~~~~
 """
 import abc
+from collections.abc import Iterable  # pylint: disable=no-name-in-module
+from collections import OrderedDict
+
 import cirq
 import numpy as np
 import pennylane as qml
 from pennylane import QubitDevice
 from pennylane.operation import Operation
+from pennylane.wires import Wires
 
 from ._version import __version__
 from .cirq_operation import CirqOperation
@@ -51,12 +55,17 @@ class CirqDevice(QubitDevice, abc.ABC):
             or strings (``['ancilla', 'q1', 'q2']``).
         shots (int): Number of circuit evaluations/random samples used
             to estimate expectation values of observables. Shots need to be >= 1.
-        qubits (List[cirq.Qubit]): a list of Cirq qubits that are used
-            as wires. The wire number corresponds to the index in the list.
-            By default, an array of ``cirq.LineQubit`` instances is created.
+        qubits (List[cirq.Qubit]): A list of Cirq qubits that are used
+            as wires. By default, an array of ``cirq.LineQubit`` instances is created.
+            Wires are mapped to qubits using Cirq's internal mechanism for ordering
+            qubits. For example, if ``wires=2`` and ``qubits=[q1, q2]``, with
+            ``q1>q2``, then the wire indices 0 and 1 are mapped to q2 and q1, respectively.
+            If the user provides their own wire labels, e.g., ``wires=["alice", "bob"]``, and the
+            qubits are the same as the previous example, then "alice" would map to qubit q2
+            and "bob" would map to qubit q1.
     """
 
-    name = "Cirq Abstract PennyLane plugin baseclass"
+    name = "Cirq Abstract PennyLane plugin base class"
     pennylane_requires = ">=0.11.0"
     version = __version__
     author = "Xanadu Inc"
@@ -69,23 +78,31 @@ class CirqDevice(QubitDevice, abc.ABC):
     short_name = "cirq.base_device"
 
     def __init__(self, wires, shots, analytic, qubits=None):
-        super().__init__(wires, shots, analytic)
-
-        self.circuit = None
 
-        device_wires = self.map_wires(self.wires)
+        if not isinstance(wires, Iterable):
+            # interpret wires as the number of consecutive wires
+            wires = range(wires)
+        num_wires = len(wires)
 
         if qubits:
-            if wires != len(qubits):
+            if num_wires != len(qubits):
                 raise qml.DeviceError(
                     "The number of given qubits and the specified number of wires have to match. Got {} wires and {} qubits.".format(
                         wires, len(qubits)
                     )
                 )
-
-            self.qubits = qubits
         else:
-            self.qubits = [cirq.LineQubit(wire) for wire in device_wires.labels]
+            qubits = [cirq.LineQubit(idx) for idx in range(num_wires)]
+
+        # cirq orders the subsystems based on a total order defined on qubits.
+        # For consistency, this plugin uses that same total order
+        self._unsorted_qubits = qubits
+        self.qubits = sorted(qubits)
+
+        super().__init__(wires, shots, analytic)
+
+        self.circuit = None
+        self.cirq_device = None
 
         # Add inverse operations
         self._inverse_operation_map = {}
@@ -149,7 +166,10 @@ def reset(self):
         # pylint: disable=missing-function-docstring
         super().reset()
 
-        self.circuit = cirq.Circuit()
+        if self.cirq_device:
+            self.circuit = cirq.Circuit(device=self.cirq_device)
+        else:
+            self.circuit = cirq.Circuit()
 
     @property
     def observables(self):
@@ -226,3 +246,10 @@ def apply(self, operations, **kwargs):
         # Diagonalize the given observables
         for operation in rotations:
             self._apply_operation(operation)
+
+    def define_wire_map(self, wires):  # pylint: disable=missing-function-docstring
+        cirq_order = np.argsort(self._unsorted_qubits)
+        consecutive_wires = Wires(cirq_order)
+
+        wire_map = zip(wires, consecutive_wires)
+        return OrderedDict(wire_map)

pennylane_cirq/simulator_device.py

@@ -132,9 +132,11 @@ def apply(self, operations, **kwargs):
         # pylint: disable=missing-function-docstring
         super().apply(operations, **kwargs)
 
-        # We apply an identity gate to all wires, otherwise Cirq would ignore
+        # TODO: remove the need for this hack by keeping better track of unused wires
+        # We apply identity gates to all wires, otherwise Cirq would ignore
         # wires that are not acted upon
-        self.circuit.append(cirq.IdentityGate(len(self.qubits))(*self.qubits))
+        for q in self.qubits:
+            self.circuit.append(cirq.IdentityGate(1)(q))
 
         if self.analytic:
             self._result = self._simulator.simulate(self.circuit, initial_state=self._initial_state)

tests/test_cirq_device.py

@@ -19,6 +19,7 @@
 
 import cirq
 import pennylane as qml
+from pennylane.wires import Wires
 import pytest
 import numpy as np
 
@@ -51,8 +52,8 @@ def test_default_init_of_qubits(self):
         assert dev.qubits[1] == cirq.LineQubit(1)
         assert dev.qubits[2] == cirq.LineQubit(2)
 
-    def test_outer_init_of_qubits(self):
-        """Tests that giving qubits as parameters to CirqDevice works."""
+    def test_outer_init_of_qubits_ordered(self):
+        """Tests that giving qubits as parameters to CirqDevice works when the qubits are already ordered consistently with Cirq's convention."""
 
         qubits = [
             cirq.GridQubit(0, 0),
@@ -63,10 +64,21 @@ def test_outer_init_of_qubits(self):
 
         dev = CirqDevice(4, 100, False, qubits=qubits)
         assert len(dev.qubits) == 4
-        assert dev.qubits[0] == cirq.GridQubit(0, 0)
-        assert dev.qubits[1] == cirq.GridQubit(0, 1)
-        assert dev.qubits[2] == cirq.GridQubit(1, 0)
-        assert dev.qubits[3] == cirq.GridQubit(1, 1)
+        assert dev.qubits == qubits
+
+    def test_outer_init_of_qubits_unordered(self):
+        """Tests that giving qubits as parameters to CirqDevice works when the qubits are not ordered consistently with Cirq's convention."""
+
+        qubits = [
+            cirq.GridQubit(0, 1),
+            cirq.GridQubit(1, 0),
+            cirq.GridQubit(0, 0),
+            cirq.GridQubit(1, 1),
+        ]
+
+        dev = CirqDevice(4, 100, False, qubits=qubits)
+        assert len(dev.qubits) == 4
+        assert dev.qubits == sorted(qubits)
 
     def test_outer_init_of_qubits_error(self):
         """Tests that giving the wrong number of qubits as parameters to CirqDevice raises an error."""
@@ -85,6 +97,60 @@ def test_outer_init_of_qubits_error(self):
             dev = CirqDevice(3, 100, False, qubits=qubits)
 
 
+class TestCirqDeviceIntegration:
+    """Integration tests for Cirq devices"""
+
+    def test_outer_init_of_qubits_with_wire_number(self):
+        """Tests that giving qubits as parameters to CirqDevice works when the user provides a number of wires."""
+
+        unordered_qubits = [
+            cirq.GridQubit(0, 1),
+            cirq.GridQubit(1, 0),
+            cirq.GridQubit(0, 0),
+            cirq.GridQubit(1, 1),
+        ]
+
+        dev = qml.device("cirq.simulator", wires=4, qubits=unordered_qubits)
+        assert len(dev.qubits) == 4
+        assert dev.qubits == sorted(unordered_qubits)
+
+    def test_outer_init_of_qubits_with_wire_label_strings(self):
+        """Tests that giving qubits as parameters to CirqDevice works when the user also provides custom string wire labels."""
+
+        unordered_qubits = [
+            cirq.GridQubit(0, 1),
+            cirq.GridQubit(1, 0),
+            cirq.GridQubit(0, 0),
+            cirq.GridQubit(1, 1),
+        ]
+
+        user_labels = ["alice", "bob", "charlie", "david"]
+        sort_order = [2,0,1,3]
+
+        dev = qml.device("cirq.simulator", wires=user_labels, qubits=unordered_qubits)
+        assert len(dev.qubits) == 4
+        assert dev.qubits == sorted(unordered_qubits)
+        assert all(dev.map_wires(Wires(label)) == Wires(idx) for label, idx in zip(user_labels, sort_order))
+
+    def test_outer_init_of_qubits_with_wire_label_ints(self):
+        """Tests that giving qubits as parameters to CirqDevice works when the user also provides custom integer wire labels."""
+
+        unordered_qubits = [
+            cirq.GridQubit(0, 1),
+            cirq.GridQubit(1, 0),
+            cirq.GridQubit(0, 0),
+            cirq.GridQubit(1, 1),
+        ]
+
+        user_labels = [-1,1,66,0]
+        sort_order = [2,0,1,3]
+
+        dev = qml.device("cirq.simulator", wires=user_labels, qubits=unordered_qubits)
+        assert len(dev.qubits) == 4
+        assert dev.qubits == sorted(unordered_qubits)
+        assert all(dev.map_wires(Wires(label)) == Wires(idx) for label, idx in zip(user_labels, sort_order))
+
+
 @pytest.fixture(scope="function")
 def cirq_device_1_wire(shots):
     """A mock instance of the abstract Device class"""