From 06c030e4f34a352cf933ab3ec0cdd9a7b9913fe1 Mon Sep 17 00:00:00 2001
From: "Christopher J. Wood" <cjwood@us.ibm.com>
Date: Mon, 27 Apr 2020 20:07:49 -0400
Subject: [PATCH] Add Cifford.from_label method (#4222)

* Add Cifford.from_label method

* Add release note

* Add test

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
---
 .../operators/symplectic/clifford.py          | 55 ++++++++++++++++++-
 .../clifford-from-label-4ac8a987109916ef.yaml |  6 ++
 .../operators/symplectic/test_clifford.py     | 12 ++++
 3 files changed, 72 insertions(+), 1 deletion(-)
 create mode 100644 releasenotes/notes/clifford-from-label-4ac8a987109916ef.yaml

diff --git a/qiskit/quantum_info/operators/symplectic/clifford.py b/qiskit/quantum_info/operators/symplectic/clifford.py
index b82f39f342f2..e7f3b99ad3d6 100644
--- a/qiskit/quantum_info/operators/symplectic/clifford.py
+++ b/qiskit/quantum_info/operators/symplectic/clifford.py
@@ -15,11 +15,12 @@
 Clifford operator class.
 """
 # pylint: disable=invalid-name, abstract-method
-
+import re
 import numpy as np
 
 from qiskit.exceptions import QiskitError
 from qiskit.circuit import QuantumCircuit, Instruction
+from qiskit.extensions.standard import IGate, XGate, YGate, ZGate, HGate, SGate
 from qiskit.quantum_info.operators.base_operator import BaseOperator
 from qiskit.quantum_info.operators.operator import Operator
 from qiskit.quantum_info.operators.scalar_op import ScalarOp
@@ -389,6 +390,58 @@ def from_circuit(circuit):
         _append_circuit(clifford, circuit)
         return clifford
 
+    @staticmethod
+    def from_label(label):
+        """Return a tensor product of single-qubit Clifford gates.
+
+        Args:
+            label (string): single-qubit operator string.
+
+        Returns:
+            Clifford: The N-qubit Clifford operator.
+
+        Raises:
+            QiskitError: if the label contains invalid characters.
+
+        Additional Information:
+            The labels correspond to the single-qubit Cliffords are
+
+            * - Label
+              - Stabilizer
+              - Destabilizer
+            * - ``"I"``
+              - +Z
+              - +X
+            * - ``"X"``
+              - -Z
+              - +X
+            * - ``"Y"``
+              - -Z
+              - -X
+            * - ``"Z"``
+              - +Z
+              - -X
+            * - ``"H"``
+              - +X
+              - +Z
+            * - ``"S"``
+              - +Z
+              - +Y
+        """
+        # Check label is valid
+        label_gates = {
+            'I': IGate(), 'X': XGate(), 'Y': YGate(),
+            'Z': ZGate(), 'H': HGate(), 'S': SGate()
+        }
+        if re.match(r'^[IXYZHS\-+]+$', label) is None:
+            raise QiskitError('Label contains invalid characters.')
+        # Initialize an identity matrix and apply each gate
+        num_qubits = len(label)
+        op = Clifford(np.eye(2 * num_qubits, dtype=np.bool))
+        for qubit, char in enumerate(reversed(label)):
+            _append_circuit(op, label_gates[char], qargs=[qubit])
+        return op
+
     # ---------------------------------------------------------------------
     # Internal helper functions
     # ---------------------------------------------------------------------
diff --git a/releasenotes/notes/clifford-from-label-4ac8a987109916ef.yaml b/releasenotes/notes/clifford-from-label-4ac8a987109916ef.yaml
new file mode 100644
index 000000000000..8b4047fb4b25
--- /dev/null
+++ b/releasenotes/notes/clifford-from-label-4ac8a987109916ef.yaml
@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Add :meth:`~qiskit.quantum_info.Clifford.from_label` method to the
+    :class:`qiskit.quantum_info.Clifford` class for initializing as the
+    tensor product of single-qubit I, X, Y, Z, H, or S gates.
diff --git a/test/python/quantum_info/operators/symplectic/test_clifford.py b/test/python/quantum_info/operators/symplectic/test_clifford.py
index fc472d9b2514..4d7c27247de7 100644
--- a/test/python/quantum_info/operators/symplectic/test_clifford.py
+++ b/test/python/quantum_info/operators/symplectic/test_clifford.py
@@ -788,6 +788,18 @@ def test_dict_round_trip(self, num_qubits):
         value = Clifford.from_dict(target.to_dict())
         self.assertEqual(value, target)
 
+    def test_from_label(self):
+        """Test from_label method"""
+        label = 'IXYZHS'
+        CI = Clifford(IGate())
+        CX = Clifford(XGate())
+        CY = Clifford(YGate())
+        CZ = Clifford(ZGate())
+        CH = Clifford(HGate())
+        CS = Clifford(SGate())
+        target = CI.tensor(CX).tensor(CY).tensor(CZ).tensor(CH).tensor(CS)
+        self.assertEqual(Clifford.from_label(label), target)
+
 
 if __name__ == '__main__':
     unittest.main()