Constructing Cliffords from quantum circuits with other Cliffords

qiskit/quantum_info/operators/symplectic/clifford_circuits.py

@@ -91,6 +91,18 @@ def _append_operation(clifford, operation, qargs=None):
             raise QiskitError("Invalid qubits for 2-qubit gate.")
         return _BASIS_2Q[name](clifford, qargs[0], qargs[1])
 
+    # If gate is a Clifford, we can either unroll the gate using the "to_circuit"
+    # method, or we can compose the Cliffords directly. Experimentally, for large
+    # cliffords the second method is considerably faster.
+
+    # pylint: disable=cyclic-import
+    from qiskit.quantum_info import Clifford
+
+    if isinstance(gate, Clifford):
+        composed_clifford = clifford.compose(gate, qargs=qargs, front=False)
+        clifford.tableau = composed_clifford.tableau
+        return clifford
+
     # If not a Clifford basis gate we try to unroll the gate and
     # raise an exception if unrolling reaches a non-Clifford gate.
     # TODO: We could also check u3 params to see if they

qiskit/transpiler/passes/optimization/collect_cliffords.py

@@ -56,7 +56,21 @@ def __init__(self, do_commutative_analysis=False, split_blocks=True, min_block_s
         )
 
 
-clifford_gate_names = ["x", "y", "z", "h", "s", "sdg", "cx", "cy", "cz", "swap"]
+clifford_gate_names = [
+    "x",
+    "y",
+    "z",
+    "h",
+    "s",
+    "sdg",
+    "cx",
+    "cy",
+    "cz",
+    "swap",
+    "clifford",
+    "linear_function",
+    "pauli",
+]
 
 
 def _is_clifford_gate(node):

releasenotes/notes/improve-collect-cliffords-f57aeafe95460b18.yaml

@@ -0,0 +1,47 @@
+---
+upgrade:
+  - |
+    Upgrading :class:`~CollectCliffords` transpiler pass to collect and combine blocks
+    of "clifford gates" into :class:`qiskit.quantum_info.Clifford` objects, where the
+    "clifford gates" may now also include objects of type :class:`.LinearFunction`,
+    :class:`qiskit.quantum_info.Clifford`, and :class:`~qiskit.circuit.library.PauliGate`.
+    As an example::
+
+        from qiskit.circuit import QuantumCircuit
+        from qiskit.circuit.library import LinearFunction, PauliGate
+        from qiskit.quantum_info.operators import Clifford
+        from qiskit.transpiler.passes import CollectCliffords
+        from qiskit.transpiler import PassManager
+
+        # Create a Clifford
+        cliff_circuit = QuantumCircuit(2)
+        cliff_circuit.cx(0, 1)
+        cliff_circuit.h(0)
+        cliff = Clifford(cliff_circuit)
+
+        # Create a linear function
+        lf = LinearFunction([[0, 1], [1, 0]])
+
+        # Create a pauli gate
+        pauli_gate = PauliGate("XYZ")
+
+        # Create a quantum circuit with the above and also simple clifford gates.
+        qc = QuantumCircuit(4)
+        qc.cz(0, 1)
+        qc.append(cliff, [0, 1])
+        qc.h(0)
+        qc.append(lf, [0, 2])
+        qc.append(pauli_gate, [0, 2, 1])
+        qc.x(2)
+
+        # Run CollectCliffords transpiler pass
+        qct = PassManager(CollectCliffords()).run(qc)
+
+    All the gates will be collected and combined into a single Clifford. Thus the final
+    circuit consists of a single Clifford object.
+
+fixes:
+  - |
+    Restoring the functionality to construct :class:`qiskit.quantum_info.Clifford`
+    objects from quantum circuits containing other :class:`qiskit.quantum_info.Clifford`
+    objects.

test/python/quantum_info/operators/symplectic/test_clifford.py

@@ -31,6 +31,8 @@
     XGate,
     YGate,
     ZGate,
+    LinearFunction,
+    PauliGate,
 )
 from qiskit.exceptions import QiskitError
 from qiskit.quantum_info import random_clifford
@@ -432,6 +434,58 @@ def test_from_circuit_with_conditional_gate(self):
         with self.assertRaises(QiskitError):
             Clifford(qc)
 
+    def test_from_circuit_with_other_clifford(self):
+        """Test initialization from circuit containing another clifford."""
+        cliff = random_clifford(1, seed=777)
+        qc = QuantumCircuit(1)
+        qc.append(cliff, [0])
+        cliff1 = Clifford(qc)
+        self.assertEqual(cliff, cliff1)
+
+    def test_from_circuit_with_multiple_cliffords(self):
+        """Test initialization from circuit containing multiple clifford."""
+        cliff1 = random_clifford(2, seed=777)
+        cliff2 = random_clifford(2, seed=999)
+
+        # Append the two cliffords to circuit and create the clifford from this circuit
+        qc1 = QuantumCircuit(3)
+        qc1.append(cliff1, [0, 1])
+        qc1.append(cliff2, [1, 2])
+        expected_cliff1 = Clifford(qc1)
+
+        # Compose the two cliffords directly
+        qc2 = QuantumCircuit(3)
+        expected_cliff2 = Clifford(qc2)
+        expected_cliff2 = Clifford.compose(expected_cliff2, cliff1, qargs=[0, 1], front=False)
+        expected_cliff2 = Clifford.compose(expected_cliff2, cliff2, qargs=[1, 2], front=False)
+        self.assertEqual(expected_cliff1, expected_cliff2)
+
+    def test_from_circuit_with_all_types(self):
+        """Test initialization from circuit containing various Clifford-like objects."""
+
+        # Construct objects that can go onto a Clifford circuit.
+        # These include regular clifford gates, linear functions, Pauli gates, other Clifford,
+        # and even circuits with other clifford objects.
+        linear_function = LinearFunction([[0, 1], [1, 1]])
+        pauli_gate = PauliGate("YZ")
+        cliff = random_clifford(2, seed=777)
+        qc = QuantumCircuit(2)
+        qc.cx(0, 1)
+        qc.append(random_clifford(1, seed=999), [1])
+
+        # Construct a quantum circuit with these objects and convert it to clifford
+        circuit = QuantumCircuit(3)
+        circuit.h(0)
+        circuit.append(linear_function, [0, 2])
+        circuit.cz(0, 1)
+        circuit.append(pauli_gate, [2, 1])
+        circuit.append(cliff, [0, 1])
+        circuit.swap(0, 2)
+        circuit.append(qc, [0, 1])
+
+        # Check that Clifford can be constructed from such circuit.
+        Clifford(circuit)
+
 
 @ddt
 class TestCliffordSynthesis(QiskitTestCase):

test/python/transpiler/test_clifford_passes.py

@@ -16,6 +16,7 @@
 import numpy as np
 
 from qiskit.circuit import QuantumCircuit, Gate
+from qiskit.circuit.library import LinearFunction, PauliGate
 from qiskit.converters import dag_to_circuit, circuit_to_dag
 from qiskit.dagcircuit import DAGOpNode
 from qiskit.transpiler.passes import HighLevelSynthesis
@@ -536,6 +537,122 @@ def test_do_not_merge_conditional_gates(self):
         # Make sure that the condition on the middle gate is not lost
         self.assertIsNotNone(qct.data[1].operation.condition)
 
+    def test_collect_with_cliffords(self):
+        """Make sure that collecting Clifford gates and replacing them by Clifford
+        works correctly when the gates include other cliffords."""
+
+        # Create a Clifford over 2 qubits
+        cliff_circuit = QuantumCircuit(2)
+        cliff_circuit.cx(0, 1)
+        cliff_circuit.h(0)
+        cliff = Clifford(cliff_circuit)
+
+        qc = QuantumCircuit(3)
+        qc.h(0)
+        qc.append(cliff, [1, 0])
+        qc.cx(1, 2)
+
+        # Collect clifford gates from the circuit (in this case all the gates must be collected).
+        qct = PassManager(CollectCliffords()).run(qc)
+        self.assertEqual(len(qct.data), 1)
+
+        # Make sure that the operator for the initial quantum circuit is equivalent to the
+        # operator for the collected clifford.
+        op1 = Operator(qc)
+        op2 = Operator(qct)
+        self.assertTrue(op1.equiv(op2))
+
+    def test_collect_with_linear_functions(self):
+        """Make sure that collecting Clifford gates and replacing them by Clifford
+        works correctly when the gates include LinearFunctions."""
+
+        # Create a linear function over 2 qubits
+        lf = LinearFunction([[0, 1], [1, 0]])
+
+        qc = QuantumCircuit(3)
+        qc.h(0)
+        qc.append(lf, [1, 0])
+        qc.cx(1, 2)
+
+        # Collect clifford gates from the circuit (in this case all the gates must be collected).
+        qct = PassManager(CollectCliffords()).run(qc)
+        self.assertEqual(len(qct.data), 1)
+
+        # Make sure that the operator for the initial quantum circuit is equivalent to the
+        # operator for the collected clifford.
+        op1 = Operator(qc)
+        op2 = Operator(qct)
+        self.assertTrue(op1.equiv(op2))
+
+    def test_collect_with_pauli_gates(self):
+        """Make sure that collecting Clifford gates and replacing them by Clifford
+        works correctly when the gates include PauliGates."""
+
+        # Create a pauli gate over 2 qubits
+        pauli_gate = PauliGate("XY")
+
+        qc = QuantumCircuit(3)
+        qc.h(0)
+        qc.append(pauli_gate, [1, 0])
+        qc.cx(1, 2)
+
+        # Collect clifford gates from the circuit (in this case all the gates must be collected).
+        qct = PassManager(CollectCliffords()).run(qc)
+        self.assertEqual(len(qct.data), 1)
+
+        # Make sure that the operator for the initial quantum circuit is equivalent to the
+        # operator for the collected clifford.
+        op1 = Operator(qc)
+        op2 = Operator(qct)
+        self.assertTrue(op1.equiv(op2))
+
+    def test_collect_with_all_types(self):
+        """Make sure that collecting Clifford gates and replacing them by Clifford
+        works correctly when the gates include all possible clifford gate types."""
+
+        cliff_circuit0 = QuantumCircuit(1)
+        cliff_circuit0.h(0)
+        cliff0 = Clifford(cliff_circuit0)
+
+        cliff_circuit1 = QuantumCircuit(2)
+        cliff_circuit1.cz(0, 1)
+        cliff_circuit1.s(1)
+        cliff1 = Clifford(cliff_circuit1)
+
+        lf1 = LinearFunction([[0, 1], [1, 1]])
+        lf2 = LinearFunction([[0, 1, 0], [1, 0, 0], [0, 0, 1]])
+
+        pauli_gate1 = PauliGate("X")
+        pauli_gate2 = PauliGate("YZX")
+
+        qc = QuantumCircuit(3)
+        qc.h(0)
+        qc.cx(0, 1)
+        qc.append(cliff0, [1])
+        qc.cy(0, 1)
+
+        # not a clifford gate (separating the circuit)
+        qc.rx(np.pi / 2, 0)
+
+        qc.append(pauli_gate2, [0, 2, 1])
+        qc.append(lf2, [2, 1, 0])
+        qc.x(0)
+        qc.append(pauli_gate1, [1])
+        qc.append(lf1, [1, 0])
+        qc.h(2)
+        qc.append(cliff1, [1, 2])
+
+        # Collect clifford gates from the circuit (we should get two Clifford blocks separated by
+        # the RX gate).
+        qct = PassManager(CollectCliffords()).run(qc)
+        self.assertEqual(len(qct.data), 3)
+
+        # Make sure that the operator for the initial quantum circuit is equivalent to the
+        # operator for the circuit with the collected cliffords.
+        op1 = Operator(qc)
+        op2 = Operator(qct)
+        self.assertTrue(op1.equiv(op2))
+
 
 if __name__ == "__main__":
     unittest.main()