Merge pull request #1327 from qiboteam/ZNE_get_noisy_circuit

Added global unitary folding for get_noisy_circuits

src/qibo/models/error_mitigation.py

@@ -52,51 +52,77 @@ def get_gammas(noise_levels, analytical: bool = True):
     return zne_coefficients
 
 
-def get_noisy_circuit(circuit, num_insertions: int, insertion_gate: str = "CNOT"):
+def get_noisy_circuit(
+    circuit, num_insertions: int, global_unitary_folding=True, insertion_gate=None
+):
     """Standalone function to generate the noisy circuit with the inverse gate pairs insertions.
 
     Args:
         circuit (:class:`qibo.models.circuit.Circuit`): circuit to modify.
-        num_insertions (int): number of insertion gate pairs to add.
-        insertion_gate (str, optional): gate to be used in the insertion.
-            If ``"RX"``, the gate used is :math:``RX(\\pi / 2)``.
-            Default is ``"CNOT"``.
+        num_insertions (int): number of insertion gate pairs / global unitary folds to add.
+        global_unitary_folding (bool): If ``True``, noise is increased by global unitary folding.
+            If ``False``, local unitary folding is used. Defaults to ``True``.
+        insertion_gate (str, optional): gate to be folded in the local unitary folding.
+            If ``RX``, the gate used is :math:``RX(\\pi / 2)``. Otherwise, it is the ``CNOT`` gate.
 
     Returns:
-        :class:`qibo.models.Circuit`: circuit with the inserted gate pairs.
+        :class:`qibo.models.Circuit`: circuit with the inserted gate pairs or with global folding.
     """
-    if insertion_gate not in ("CNOT", "RX"):  # pragma: no cover
-        raise_error(
-            ValueError,
-            "Invalid insertion gate specification. Please select between 'CNOT' and 'RX'.",
-        )
-    if insertion_gate == "CNOT" and circuit.nqubits < 2:  # pragma: no cover
-        raise_error(
-            ValueError,
-            "Provide a circuit with at least 2 qubits when using the 'CNOT' insertion gate. "
-            + "Alternatively, try with the 'RX' insertion gate instead.",
-        )
 
-    i_gate = gates.CNOT if insertion_gate == "CNOT" else gates.RX
+    from qibo import Circuit  # pylint: disable=import-outside-toplevel
+
+    if global_unitary_folding:
+
+        copy_c = Circuit(**circuit.init_kwargs)
+        for g in circuit.queue:
+            if not isinstance(g, gates.M):
+                copy_c.add(g)
+
+        noisy_circuit = copy_c
 
-    theta = np.pi / 2
-    noisy_circuit = circuit.__class__(**circuit.init_kwargs)
+        for _ in range(num_insertions):
+            noisy_circuit += copy_c.invert() + copy_c
 
-    for gate in circuit.queue:
-        noisy_circuit.add(gate)
+        for m in circuit.measurements:
+            noisy_circuit.add(m)
 
-        if isinstance(gate, i_gate):
-            if insertion_gate == "CNOT":
-                control = gate.control_qubits[0]
-                target = gate.target_qubits[0]
-                for _ in range(num_insertions):
-                    noisy_circuit.add(gates.CNOT(control, target))
-                    noisy_circuit.add(gates.CNOT(control, target))
-            elif gate.init_kwargs["theta"] == theta:
-                qubit = gate.qubits[0]
-                for _ in range(num_insertions):
-                    noisy_circuit.add(gates.RX(qubit, theta=theta))
-                    noisy_circuit.add(gates.RX(qubit, theta=-theta))
+    else:
+        if insertion_gate is None or insertion_gate not in (
+            "CNOT",
+            "RX",
+        ):  # pragma: no cover
+            raise_error(
+                ValueError,
+                "Invalid insertion gate specification. Please select between 'CNOT' and 'RX'.",
+            )
+        if insertion_gate == "CNOT" and circuit.nqubits < 2:  # pragma: no cover
+            raise_error(
+                ValueError,
+                "Provide a circuit with at least 2 qubits when using the 'CNOT' insertion gate. "
+                + "Alternatively, try with the 'RX' insertion gate instead.",
+            )
+
+        i_gate = gates.CNOT if insertion_gate == "CNOT" else gates.RX
+
+        theta = np.pi / 2
+        noisy_circuit = Circuit(**circuit.init_kwargs)
+
+        for gate in circuit.queue:
+            noisy_circuit.add(gate)
+
+            if isinstance(gate, i_gate):
+                if insertion_gate == "CNOT":
+                    control = gate.control_qubits[0]
+                    target = gate.target_qubits[0]
+                    for _ in range(num_insertions):
+                        noisy_circuit.add(gates.CNOT(control, target))
+                        noisy_circuit.add(gates.CNOT(control, target))
+                elif insertion_gate == "RX":
+                    qubit = gate.qubits[0]
+                    theta = gate.init_kwargs["theta"]
+                    for _ in range(num_insertions):
+                        noisy_circuit.add(gates.RX(qubit, theta=theta))
+                        noisy_circuit.add(gates.RX(qubit, theta=-theta))
 
     return noisy_circuit
 
@@ -108,6 +134,7 @@ def ZNE(
     noise_model=None,
     nshots=10000,
     solve_for_gammas=False,
+    global_unitary_folding=True,
     insertion_gate="CNOT",
     readout=None,
     qubit_map=None,
@@ -129,9 +156,10 @@ def ZNE(
         nshots (int, optional): Number of shots. Defaults to :math:`10000`.
         solve_for_gammas (bool, optional): If ``True``, explicitly solve the
             equations to obtain the ``gamma`` coefficients. Default is ``False``.
-        insertion_gate (str, optional): gate to be used in the insertion.
-            If ``"RX"``, the gate used is :math:``RX(\\pi / 2)``.
-            Defaults to ``"CNOT"``.
+        global_unitary_folding (bool, optional): If ``True``, noise is increased by global unitary folding.
+            If ``False``, local unitary folding is used. Defaults to ``True``.
+        insertion_gate (str, optional): gate to be folded in the local unitary folding.
+            If ``RX``, the gate used is :math:``RX(\\pi / 2)``. Otherwise, it is the ``CNOT`` gate.
         readout (dict, optional): a dictionary that may contain the following keys:
 
             *    ncircuits: int, specifies the number of random circuits to use for the randomized method of readout error mitigation.
@@ -162,7 +190,10 @@ def ZNE(
     expected_values = []
     for num_insertions in noise_levels:
         noisy_circuit = get_noisy_circuit(
-            circuit, num_insertions, insertion_gate=insertion_gate
+            circuit,
+            num_insertions,
+            global_unitary_folding,
+            insertion_gate=insertion_gate,
         )
         val = get_expectation_val_with_readout_mitigation(
             noisy_circuit,

tests/test_models_error_mitigation.py

@@ -95,7 +95,8 @@ def get_circuit(nqubits, nmeas=None):
     ],
 )
 @pytest.mark.parametrize("solve", [False, True])
-def test_zne(backend, nqubits, noise, solve, insertion_gate, readout):
+@pytest.mark.parametrize("GUF", [False, True])
+def test_zne(backend, nqubits, noise, solve, GUF, insertion_gate, readout):
     """Test that ZNE reduces the noise."""
     if backend.name == "tensorflow":
         import tensorflow as tf
@@ -128,6 +129,7 @@ def test_zne(backend, nqubits, noise, solve, insertion_gate, readout):
         noise_model=noise,
         nshots=10000,
         solve_for_gammas=solve,
+        global_unitary_folding=GUF,
         insertion_gate=insertion_gate,
         readout=readout,
         backend=backend,