Fix to classical registers in to_qasm

src/qibo/models/_openqasm.py

@@ -170,9 +170,7 @@ def to_circuit(
             density_matrix,
             wire_names=self._construct_wire_names(),
         )
-        for gate in gates:
-            circ.add(gate)
-        self._reorder_registers(circ.measurements)
+        circ.add(self._merge_measurements(gates))
         return circ
 
     def _get_measurement(self, measurement):
@@ -267,11 +265,27 @@ def _def_gate(self, definition):
         gates = [self._get_gate(gate) for gate in definition.body]
         self.defined_gates.update({name: CustomQASMGate(name, gates, qubits, args)})
 
-    def _reorder_registers(self, measurements):
-        """Reorders the registers of the provided :class:`qibo.gates.measurements.M`
-        gates according to the classical registers order defined in the QASM program."""
-        for meas in measurements:
-            meas.target_qubits = [self.c_registers[meas.register_name].pop(0)]
+    def _merge_measurements(self, gates):
+        """Merges separated measurements of a same register into a single one.
+        This is needed because qibo doesn't allow to separetely define two measurements in a same register:
+
+        # not allowed
+        c.add(gates.M(0, register="m0"))
+        c.add(gates.M(1, register="m0"))
+        """
+        updated_queue = []
+        for gate in gates:
+            if isinstance(gate, qibo.gates.M):
+                if gate.register_name in self.c_registers:
+                    updated_queue.append(
+                        qibo.gates.M(
+                            *self.c_registers.pop(gate.register_name),
+                            register_name=gate.register_name,
+                        )
+                    )
+            else:
+                updated_queue.append(gate)
+        return updated_queue
 
     def _construct_wire_names(self):
         """Builds the wires names from the declared quantum registers."""

src/qibo/models/circuit.py

@@ -667,13 +667,11 @@ def add(self, gate):
                     nreg = self.queue.nmeasurements - 1
                     gate.register_name = f"register{nreg}"
                 else:
-                    registers = self.measurement_tuples
-                    for register, qubits in registers.items():
-                        intersection = set(qubits).intersection(set(gate.target_qubits))
-                        if len(intersection) > 0:
+                    name = gate.register_name
+                    for mgate in self.measurements:
+                        if name == mgate.register_name:
                             raise_error(
-                                KeyError,
-                                f"Qubits {tuple(intersection)} already measured in register `{register}`.",
+                                KeyError, f"Register {name} already exists in circuit."
                             )
 
                 gate.result.circuit = self
@@ -1156,16 +1154,14 @@ def to_qasm(self):
         code += [f"qreg q[{self.nqubits}];"]
 
         # Set measurements
-        for measurement in self.measurements:
-            reg_name = measurement.register_name
-            reg_qubits = measurement.target_qubits
-            if not reg_name.islower():
+        for register, qubits in self.measurement_tuples.items():
+            if not register.islower():
                 raise_error(
                     NameError,
                     "OpenQASM does not support capital letters in "
-                    + f"register names but {reg_name} was used",
+                    + f"register names but {register} was used",
                 )
-            code.append(f"creg {reg_name}[{len(reg_qubits)}];")
+            code.append(f"creg {register}[{len(qubits)}];")
 
         # Add gates
         for gate in self.queue:
@@ -1186,10 +1182,9 @@ def to_qasm(self):
             code.append(f"{name} {qubits};")
 
         # Add measurements
-        for measurement in self.measurements:
-            reg_name = measurement.register_name
-            for i, q in enumerate(measurement.target_qubits):
-                code.append(f"measure q[{q}] -> {reg_name}[{i}];")
+        for register, qubits in self.measurement_tuples.items():
+            for i, q in enumerate(qubits):
+                code.append(f"measure q[{q}] -> {register}[{i}];")
 
         return "\n".join(code)
 

tests/test_models_circuit_qasm.py

@@ -188,11 +188,18 @@ def test_crotations():
         target = c.to_qasm()
 
 
-def test_measurements():
+@pytest.mark.parametrize(
+    "measurements",
+    [
+        [gates.M(0, 1)],
+    ],
+)
+def test_measurements(measurements):
     c = Circuit(2)
     c.add(gates.X(0))
     c.add(gates.Y(1))
-    c.add(gates.M(0, 1))
+    for m in measurements:
+        c.add(m)
     target = f"""// Generated by QIBO {__version__}
 OPENQASM 2.0;
 include "qelib1.inc";