[QHC-700] Expand transpiler

docs/releases/changelog-dev.md

@@ -233,6 +233,10 @@
 
 - Added a `save_plot=True` parameter to the `plotS21()` method of `ExperimentResults`. When enabled (default: True), the plot is automatically saved in the same directory as the experiment results. [#819](https://github.com/qilimanjaro-tech/qililab/pull/819)
 
+- Improved the transpiler, by making it more modular, and adding a `gate_cancellation()` stage before the transpilation to natives, this stage can be skipped, together with the old `optimize_transpilation()`, if the flag `optimize=False` is passed.
+
+[#823](https://github.com/qilimanjaro-tech/qililab/pull/823)
+
 ### Breaking changes
 
 - Renamed the platform's `execute_anneal_program()` method to `execute_annealing_program()` and updated its parameters. The method now expects `preparation_block` and `measurement_block`, which are strings used to retrieve blocks from the `Calibration`. These blocks are inserted before and after the annealing schedule, respectively.

src/qililab/digital/circuit_optimizer.py

@@ -0,0 +1,263 @@
+# Copyright 2023 Qilimanjaro Quantum Tech
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""CircuitOptimizer class"""
+
+from copy import deepcopy
+
+from qibo import Circuit, gates
+
+from qililab import digital
+from qililab.settings.digital.digital_compilation_settings import DigitalCompilationSettings
+
+from .native_gates import Drag
+
+
+class CircuitOptimizer:
+    """Optimizes a circuit, cancelling redundant gates."""
+
+    def __init__(self, digital_compilation_settings: DigitalCompilationSettings):  # type: ignore # ignore typing to avoid importing platform and causing circular imports
+        self.digital_compilation_settings = digital_compilation_settings
+
+    @classmethod
+    def run_gate_cancellations(cls, circuit: Circuit) -> Circuit:
+        """Main method to run the gate cancellations. Currently only consists of cancelling pairs of hermitian gates.
+
+        Can/Might be extended in the future to include more complex gate cancellations.
+
+        Args:
+            circuit (Circuit): circuit to optimize.
+
+        Returns:
+            Circuit: optimized circuit.
+        """
+        return cls.cancel_pairs_of_hermitian_gates(circuit)
+
+    @classmethod
+    def cancel_pairs_of_hermitian_gates(cls, circuit: Circuit) -> Circuit:
+        """Optimizes circuit by cancelling adjacent hermitian gates.
+
+        Args:
+            circuit (Circuit): circuit to optimize.
+
+        Returns:
+            Circuit: optimized circuit.
+        """
+        # Initial and final circuit gates lists, from which to, one by one, after checks, pass non-cancelled gates:
+        circ_list: list[tuple] = cls._get_circuit_gates(circuit)
+
+        # We want to do the sweep circuit cancelling gates least once always:
+        previous_circ_list = deepcopy(circ_list)
+        output_circ_list = cls._sweep_circuit_cancelling_pairs_of_hermitian_gates(circ_list)
+
+        # And then keep iterating, sweeping over the circuit (cancelling gates) each time, until there is full sweep without any cancellations:
+        while output_circ_list != previous_circ_list:
+            previous_circ_list = deepcopy(output_circ_list)
+            output_circ_list = cls._sweep_circuit_cancelling_pairs_of_hermitian_gates(output_circ_list)
+
+        # Create optimized circuit, from the obtained non-cancelled list:
+        return cls._create_circuit(output_circ_list, circuit.nqubits)
+
+    def optimize_transpilation(self, circuit: Circuit) -> list[gates.Gate]:
+        """Optimizes transpiled circuit by applying virtual Z gates.
+
+        This is done by moving all RZ to the left of all operators as a single RZ. The corresponding cumulative rotation
+        from each RZ is carried on as phase in all drag pulses left of the RZ operator.
+
+        Virtual Z gates are also applied to correct phase errors from CZ gates.
+
+        The final RZ operator left to be applied as the last operator in the circuit can afterwards be removed since the last
+        operation is going to be a measurement, which is performed on the Z basis and is therefore invariant under rotations
+        around the Z axis.
+
+        This last step can also be seen from the fact that an RZ operator applied on a single qubit, with no operations carried
+        on afterwards induces a phase rotation. Since phase is an imaginary unitary component, its absolute value will be 1
+        independent on any (unitary) operations carried on it.
+
+        Mind that moving an operator to the left is equivalent to applying this operator last so
+        it is actually moved to the _right_ of ``Circuit.queue`` (last element of list).
+
+        For more information on virtual Z gates, see https://arxiv.org/abs/1612.00858
+
+        Args:
+            circuit (Circuit): circuit with native gates, to optimize.
+
+        Returns:
+            list[gates.Gate] : list of re-ordered gates
+        """
+        nqubits: int = circuit.nqubits
+        ngates: list[gates.Gate] = circuit.queue
+
+        supported_gates = ["rz", "drag", "cz", "wait", "measure"]
+        new_gates = []
+        shift = dict.fromkeys(range(nqubits), 0)
+        for gate in ngates:
+            if gate.name not in supported_gates:
+                raise NotImplementedError(f"{gate.name} not part of native supported gates {supported_gates}")
+            if isinstance(gate, gates.RZ):
+                shift[gate.qubits[0]] += gate.parameters[0]
+            # add CZ phase correction
+            elif isinstance(gate, gates.CZ):
+                gate_settings = self.digital_compilation_settings.get_gate(
+                    name=gate.__class__.__name__, qubits=gate.qubits
+                )
+                control_qubit, target_qubit = gate.qubits
+                corrections = next(
+                    (
+                        event.pulse.options
+                        for event in gate_settings
+                        if (
+                            event.pulse.options is not None
+                            and f"q{control_qubit}_phase_correction" in event.pulse.options
+                        )
+                    ),
+                    None,
+                )
+                if corrections is not None:
+                    shift[control_qubit] += corrections[f"q{control_qubit}_phase_correction"]
+                    shift[target_qubit] += corrections[f"q{target_qubit}_phase_correction"]
+                new_gates.append(gate)
+            else:
+                # if gate is drag pulse, shift parameters by accumulated Zs
+                if isinstance(gate, Drag):
+                    # create new drag pulse rather than modify parameters of the old one
+                    gate = Drag(gate.qubits[0], gate.parameters[0], gate.parameters[1] - shift[gate.qubits[0]])
+
+                # append gate to optimized list
+                new_gates.append(gate)
+
+        return new_gates
+
+    @staticmethod
+    def _get_circuit_gates(circuit: Circuit) -> list[tuple]:
+        """Get the gates of the circuit.
+
+        Args:
+            circuit (qibo.models.Circuit): Circuit to get the gates from.
+
+        Returns:
+            list[tuple]: List of gates in the circuit. Where each gate is a tuple of ('name', 'init_args', 'initi_kwargs').
+        """
+        return [(type(gate).__name__, gate.init_args, gate.init_kwargs) for gate in circuit.queue]
+
+    @staticmethod
+    def _create_gate(gate_class: str, gate_args: list | int, gate_kwargs: dict) -> gates.Gate:
+        """Converts a tuple representation of qibo gate (name, qubits) into a Gate object.
+
+        Args:
+            gate_class (str): The class name of the gate. Can be any Qibo or Qililab supported class.
+            gate_args (list | int): The qubits the gate acts on.
+            gate_kwargs (dict): The kwargs of the gate.
+
+        Returns:
+            gates.Gate: The qibo Gate object.
+        """
+        # Solve Identity gate, argument int issue:
+        gate_args = [gate_args] if isinstance(gate_args, int) else gate_args
+
+        return (
+            getattr(digital, gate_class)(*gate_args, **gate_kwargs)
+            if gate_class in {"Drag", "Wait"}
+            else getattr(gates, gate_class)(*gate_args, **gate_kwargs)
+        )
+
+    @classmethod
+    def _create_circuit(cls, gates_list: list[tuple], nqubits: int) -> Circuit:
+        """Converts a list of gates (name, qubits) into a qibo Circuit object.
+
+        Args:
+            gates_list (list[tuple]): List of gates in the circuit. Where each gate is a tuple of ('name', 'init_args', 'initi_kwargs')
+            nqubits (int): Number of qubits in the circuit.
+
+        Returns:
+            Circuit: The qibo Circuit object.
+        """
+        # Create optimized circuit, from the obtained non-cancelled list:
+        output_circuit = Circuit(nqubits)
+        for gate, gate_args, gate_kwargs in gates_list:
+            qibo_gate = cls._create_gate(gate, gate_args, gate_kwargs)
+            output_circuit.add(qibo_gate)
+
+        return output_circuit
+
+    @classmethod
+    def _sweep_circuit_cancelling_pairs_of_hermitian_gates(cls, circ_list: list[tuple]) -> list[tuple]:
+        """Cancels adjacent gates in a circuit.
+
+        Args:
+            circ_list (list[tuple]): List of gates in the circuit. Where each gate is a tuple of ('name', 'init_args', 'initi_kwargs')
+
+        Returns:
+            list[tuple]: List of gates in the circuit, after cancelling adjacent gates. Where each gate is a tuple of ('name', 'init_args', 'initi_kwargs')
+        """
+        # List of gates, that are available for cancellation:
+        hermitian_gates: list = ["H", "X", "Y", "Z", "CNOT", "CZ", "SWAP"]
+
+        output_circ_list: list[tuple] = []
+
+        while circ_list:  # If original circuit list, is empty or has one gate remaining, we are done:
+            if len(circ_list) == 1:
+                output_circ_list.append(circ_list[0])
+                break
+
+            # Gate of the original circuit, to find a match for:
+            gate, gate_args, gate_kwargs = circ_list.pop(0)
+            gate_qubits = cls._extract_qubits(gate_args)  # Assuming qubits are the first two args
+
+            # If gate is not hermitian (can't be cancelled), add it to the output circuit and continue:
+            if gate not in hermitian_gates:
+                output_circ_list.append((gate, gate_args, gate_kwargs))
+                continue
+
+            subend = False
+            for i in range(len(circ_list)):
+                # Next gates, to compare the original with:
+                comp_gate, comp_args, comp_kwargs = circ_list[i]
+                comp_qubits = cls._extract_qubits(comp_args)  # Assuming qubits are the first two args
+
+                # Simplify duplication, if same gate and qubits found, without any other in between:
+                if gate == comp_gate and gate_args == comp_args and gate_kwargs == comp_kwargs:
+                    circ_list.pop(i)
+                    break
+
+                # Add gate, if there is no other gate that acts on the same qubits:
+                if i == len(circ_list) - 1:
+                    output_circ_list.append((gate, gate_args, gate_kwargs))
+                    break
+
+                # Add gate and leave comparison_gate loop, if we find a gate in common qubit, that prevents contraction:
+                for gate_qubit in gate_qubits:
+                    if gate_qubit in comp_qubits:
+                        output_circ_list.append((gate, gate_args, gate_kwargs))
+                        subend = True
+                        break
+                if subend:
+                    break
+
+        return output_circ_list
+
+    @staticmethod
+    def _extract_qubits(gate_args: list | int) -> list:
+        """Extract qubits from gate_args.
+
+        Args:
+            gate_args (list | int): The arguments of the gate.
+
+        Returns:
+            list: The qubits of the gate in an iterable.
+        """
+        # Assuming qubits are the first one or two args:
+        if isinstance(gate_args, int):
+            return [gate_args]
+        return gate_args if len(gate_args) <= 2 else gate_args[:2]