Add 'u' as alias for 'u3' gate. Required for compatibility with OpenQ…

…ASM 2.

The `u` gate is in the header for OpenQASM2: https://github.com/Qiskit/qiskit/blob/main/qiskit/qasm/libs/qelib1.inc

In particular, it is produced by `qiskit.qasm2.dumps()` for circuits generated by `qiskit.circuit.random.random_circuit`.

doc/notebooks/gates.ipynb

@@ -141,7 +141,7 @@
     "draw_zx_diagram(1, 'rz(0.125*pi) q;')  # can also use 'p' or 'u1'\n",
     "\n",
     "draw_zx_diagram(1, 'u2(0.125*pi,0.125*pi) q[0];')\n",
-    "draw_zx_diagram(1, 'u3(0.125*pi,0.125*pi,0.125*pi) q[0];')"
+    "draw_zx_diagram(1, 'u3(0.125*pi,0.125*pi,0.125*pi) q[0];')  # can also use 'u'"
    ]
   },
   {

pyzx/circuit/gates.py

@@ -1251,6 +1251,7 @@ def to_graph(self, g, q_mapper, c_mapper):
     "CCZ": CCZ,
     "U2": U2,
     "U3": U3,
+    "U": U3,
     "CU3": CU3,
     "CU": CU,
     "CRX": CRX,
@@ -1286,6 +1287,7 @@ def to_graph(self, g, q_mapper, c_mapper):
     "u1": ZPhase,
     "u2": U2,
     "u3": U3,
+    "u": U3,
     "cu3": CU3,
     "cu": CU,
     "cx": CNOT,

pyzx/circuit/qasmparser.py

@@ -21,7 +21,7 @@
 from typing import List, Dict, Tuple, Optional
 
 from . import Circuit
-from .gates import Gate, qasm_gate_table, XPhase, YPhase, ZPhase, NOT, U2, U3
+from .gates import Gate, qasm_gate_table, U2, U3
 from ..utils import settings
 
 
@@ -193,7 +193,7 @@ def parse_command(self, c: str, registers: Dict[str,Tuple[int,int]]) -> List[Gat
             elif name == 'u2':
                 if len(phases) != 2: raise TypeError("Invalid specification {}".format(c))
                 gates.append(U2(argset[0],phases[0],phases[1]))
-            elif name == 'u3':
+            elif name in ('u3', 'u'):
                 if len(phases) != 3: raise TypeError("Invalid specification {}".format(c))
                 gates.append(U3(argset[0],phases[0],phases[1],phases[2]))
             elif name in ('cx', 'CX', 'cy', 'cz', 'ch', 'csx', 'swap'):

tests/test_qasm.py

@@ -196,7 +196,7 @@ def compare_gate_matrix_with_qiskit(gates, num_qubits: int, num_angles: int, qas
                     self.assertEqual(c.qubits, round_trip.qubits)
                     self.assertListEqual(c.gates, round_trip.gates)
 
-        # Test standard gates common to both OpenQASM 2 and 3.
+        # Test standard gates common to both qelib1.inc (OpenQASM 2) and stdgates.inc (OpenQASM 3).
         compare_gate_matrix_with_qiskit(
             ['x', 'y', 'z', 'h', 's', 'sdg', 't', 'tdg', 'sx'], 1, 0)
         compare_gate_matrix_with_qiskit(['u1', 'p', 'rx', 'ry', 'rz'], 1, 1)
@@ -209,14 +209,15 @@ def compare_gate_matrix_with_qiskit(gates, num_qubits: int, num_angles: int, qas
         compare_gate_matrix_with_qiskit(['ccx', 'cswap'], 3, 0)
         compare_gate_matrix_with_qiskit(['cu'], 2, 4)
 
-        # Test standard gates added to OpenQASM 3.
+        # Test standard gates added to stdgates.inc.
         compare_gate_matrix_with_qiskit(['cphase'], 2, 1, [3])
 
-        # Test standard gates removed from OpenQASM 3.
+        # Test standard gates removed from stdgates.inc.
         compare_gate_matrix_with_qiskit(['sxdg'], 1, 0, [2])
         compare_gate_matrix_with_qiskit(['csx'], 2, 0, [2])
         compare_gate_matrix_with_qiskit(['cu1', 'rxx', 'rzz'], 2, 1, [2])
         compare_gate_matrix_with_qiskit(['cu3'], 2, 3, [2])
+        compare_gate_matrix_with_qiskit(['u'], 1, 3, [2])
 
     @unittest.skipUnless(QuantumCircuit, "qiskit needs to be installed for this test")
     def test_qiskit_transpile_pyzx_optimization_round_trip(self):