Add global Molmer-Sorenson gate (#3149)

* add global ms gate

* clean up

* Update qiskit/extensions/standard/ms.py

Co-Authored-By: Kevin Krsulich <[email protected]>

* Update qiskit/extensions/standard/ms.py

Co-Authored-By: Kevin Krsulich <[email protected]>

* clean up

qiskit/extensions/standard/__init__.py

@@ -45,3 +45,4 @@
 from .cu3 import Cu3Gate
 from .rzz import RZZGate
 from .rxx import RXXGate
+from .ms import MSGate

qiskit/extensions/standard/ms.py

@@ -0,0 +1,53 @@
+# -*- coding: utf-8 -*-
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2017.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+# pylint: disable=invalid-name
+
+"""
+Molmer-Sorensen gate.
+"""
+
+
+from qiskit.circuit import Gate
+from qiskit.circuit import QuantumCircuit
+from qiskit.circuit import QuantumRegister
+from qiskit.extensions.standard.rxx import RXXGate
+
+
+class MSGate(Gate):
+    """Global Molmer-Sorensen gate."""
+
+    def __init__(self, n_qubits, theta):
+        """Create new MS gate."""
+        super().__init__("ms", n_qubits, [theta])
+
+    def _define(self):
+        definition = []
+        q = QuantumRegister(self.num_qubits, "q")
+        rule = []
+        for i in range(self.num_qubits):
+            for j in range(i+1, self.num_qubits):
+                rule += [(RXXGate(self.params[0]), [q[i], q[j]], [])]
+
+        for inst in rule:
+            definition.append(inst)
+        self.definition = definition
+
+
+def ms(self, theta, qubits):
+    """Apply MS to q1 and q2."""
+    return self.append(MSGate(len(qubits), theta), qubits)
+
+
+QuantumCircuit.ms = ms

qiskit/transpiler/passes/ms_basis_decomposer.py

@@ -31,25 +31,21 @@ class MSBasisDecomposer(TransformationPass):
     """
 
     supported_input_gates = (U3Gate, CnotGate)
-    supported_basis_names = ('rx', 'ry', 'rxx')
+    supported_basis_names = ('rx', 'ry', 'rxx', 'ms')
 
     def __init__(self, basis):
         """
         Args:
-            basis (list[str]): Target basis names, e.g. `['rx', 'ry', 'rxx']` .
+            basis (list[str]): Target basis names, e.g. `['rx', 'ry', 'rxx', 'ms']` .
 
         Raises:
-            QiskitError: if target basis is not [ 'rx', 'ry', 'rxx' ]
+            QiskitError: if target basis is not [ 'rx', 'ry', 'rxx', 'ms' ]
 
         """
         super().__init__()
 
         self.basis = basis
-        self.requires = [Unroller(['u3', 'cx'])]
-
-        if set(basis) != {gate_name for gate_name in self.supported_basis_names}:
-            raise QiskitError("Cannot unroll the circuit to the given basis, %s. "
-                              % str(self.basis))
+        self.requires = [Unroller(list(set(basis).union(['u3', 'cx'])))]
 
     def run(self, dag):
         """Replace U3,CX nodes in input dag with equivalent Rx,Ry,Rxx gates.
@@ -70,10 +66,12 @@ def run(self, dag):
         for node in dag.op_nodes():
             basic_insts = ['measure', 'reset', 'barrier', 'snapshot']
             if node.name in basic_insts:
-                # TODO: this is legacy behavior.Basis_insts should be removed that these
+                # TODO: this is legacy behavior. basic_insts should be removed and these
                 #  instructions should be part of the device-reported basis. Currently, no
                 #  backend reports "measure", for example.
                 continue
+            if node.name in self.basis:  # If already a base, ignore.
+                continue
 
             if not isinstance(node.op, self.supported_input_gates):
                 raise QiskitError("Cannot convert the circuit to the given basis, %s. "

qiskit/transpiler/transpile_circuit.py

@@ -48,12 +48,11 @@ def transpile_circuit(circuit, transpile_config):
         device_insts = set(transpile_config.basis_gates).union(basic_insts)
 
         ms_basis_swap = None
-        if (
-                'rxx' in transpile_config.basis_gates
-                and not device_insts >= circuit.count_ops().keys()
-        ):
+        if 'rxx' in transpile_config.basis_gates and \
+                not device_insts >= circuit.count_ops().keys():
             ms_basis_swap = transpile_config.basis_gates
-            transpile_config.basis_gates = ['u3', 'cx']
+            transpile_config.basis_gates = list(set(['u3', 'cx']).union(
+                transpile_config.basis_gates))
 
         level = transpile_config.optimization_level
         if level is None: