Define separate classes for CSIGN and CZ

This makes sure sure that gate defined using CSIGN will always be recognized by isinstance(gate, CSIGN).
To compare the generated gate it is better to use the fidelity measure.

src/qutip_qip/operations/gateclass.py

@@ -1026,9 +1026,39 @@ def get_compact_qobj(self):
         return cnot()
 
 
+class CZ(_OneControlledGate):
+    """
+    Controlled Z gate. Identical to the CSIGN gate.
+
+    Examples
+    --------
+    >>> from qutip_qip.operations import CZ
+    >>> CSIGN(0, 1).get_compact_qobj() # doctest: +NORMALIZE_WHITESPACE
+    Quantum object: dims = [[2, 2], [2, 2]], shape = (4, 4), type = oper, isherm = True
+    Qobj data =
+    [[ 1.  0.  0.  0.]
+     [ 0.  1.  0.  0.]
+     [ 0.  0.  1.  0.]
+     [ 0.  0.  0. -1.]]
+    """
+
+    def __init__(self, controls, targets, **kwargs):
+        self.target_gate = Z
+        super().__init__(
+            targets=targets,
+            controls=controls,
+            target_gate=self.target_gate,
+            **kwargs,
+        )
+        self.name = "CZ"
+
+    def get_compact_qobj(self):
+        return csign()
+
+
 class CSIGN(_OneControlledGate):
     """
-    Controlled Z gate.
+    Controlled CSIGN gate. Identical to the CZ gate.
 
     Examples
     --------
@@ -1056,10 +1086,6 @@ def get_compact_qobj(self):
         return csign()
 
 
-CZ = CSIGN
-CZ.__doc__ = CZ.__doc__
-
-
 class CPHASE(_OneControlledGate):
     r"""
     CPHASE gate.

tests/test_circuit.py

@@ -399,7 +399,7 @@ def test_single_qubit_gates(self):
 
         assert qc.gates[8].name == "T"
         assert qc.gates[7].name == "S"
-        assert qc.gates[6].name == "CSIGN"
+        assert qc.gates[6].name == "CZ"
         assert qc.gates[5].name == "CT"
         assert qc.gates[4].name == "Z"
         assert qc.gates[3].name == "CS"