parallel_experiment.py

# This code is part of Qiskit.
#
# (C) Copyright IBM 2021.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.
"""
Parallel Experiment class.
"""
from typing import List, Optional
import numpy as np

from qiskit import QuantumCircuit, ClassicalRegister
from qiskit.circuit import Clbit
from qiskit.providers.backend import Backend
from qiskit_experiments.exceptions import QiskitError
from .composite_experiment import CompositeExperiment, BaseExperiment
from .composite_analysis import CompositeAnalysis


class ParallelExperiment(CompositeExperiment):
    """Combine multiple experiments into a parallel experiment.

    Parallel experiments combine individual experiments on disjoint subsets
    of qubits into a single composite experiment on the union of those qubits.
    The component experiment circuits are combined to run in parallel on the
    respective qubits.

    Analysis of parallel experiments is performed using the
    :class:`~qiskit_experiments.framework.CompositeAnalysis` class which handles
    marginalizing the composite experiment circuit data into individual child
    :class:`ExperimentData` containers for each component experiment which are
    then analyzed using the corresponding analysis class for that component
    experiment.

    See :class:`~qiskit_experiments.framework.CompositeAnalysis`
    documentation for additional information.
    """

    def __init__(
        self,
        experiments: List[BaseExperiment],
        backend: Optional[Backend] = None,
        flatten_results: bool = True,
        analysis: Optional[CompositeAnalysis] = None,
        experiment_type: Optional[str] = None,
    ):
        """Initialize the analysis object.

        Args:
            experiments: a list of experiments.
            backend: Optional, the backend to run the experiment on.
            flatten_results: If True flatten all component experiment results
                             into a single ExperimentData container, including
                             nested composite experiments. If False save each
                             component experiment results as a separate child
                             ExperimentData container. This kwarg is ignored
                             if the analysis kwarg is used.
            analysis: Optional, the composite analysis class to use. If not
                      provided this will be initialized automatically from the
                      supplied experiments.
        """
        qubits = []
        for exp in experiments:
            qubits += exp.physical_qubits
        super().__init__(
            experiments,
            qubits,
            backend=backend,
            analysis=analysis,
            flatten_results=flatten_results,
            experiment_type=experiment_type,
        )

    def circuits(self):
        return self._combined_circuits(device_layout=False)

    def _transpiled_circuits(self):
        return self._combined_circuits(device_layout=True)

    def _combined_circuits(self, device_layout: bool) -> List[QuantumCircuit]:
        """Generate combined parallel circuits from transpiled subcircuits."""
        if not device_layout:
            # Num qubits will be computed from sub experiments
            num_qubits = len(self.physical_qubits)
        else:
            # Expand the number of qubits similar to how qiskit.transpile does
            # Here we progress from most to least specific way of specifying
            # the number of qubits: coupling_map->target->backend
            #
            # TODO: Behave more like a layout transpiler pass and set the
            # _layout property on the circuits. Doing this requires accessing
            # private attributes of Qiskit or possibly running a layout pass of
            # the transpiler if that can be done without too much overhead.
            num_qubits = 1 + max(self.physical_qubits)
            coupling_map = getattr(self.transpile_options, "coupling_map", None)
            target = getattr(self.transpile_options, "target", None)
            if coupling_map is not None:
                num_qubits = max(num_qubits, 1 + np.max(coupling_map))
            elif target is not None:
                num_qubits = max(num_qubits, target.num_qubits)
            elif self.backend:
                num_qubits = max(num_qubits, self._backend_data.num_qubits)

        joint_circuits = []
        sub_qubits = 0
        for exp_idx, sub_exp in enumerate(self._experiments):
            # Generate transpiled subcircuits
            sub_circuits = sub_exp._transpiled_circuits()

            # Qubit remapping for non-transpiled circuits
            if not device_layout:
                qubits = list(range(sub_qubits, sub_qubits + sub_exp.num_qubits))
                qargs_map = {q: qubits[i] for i, q in enumerate(sub_exp.physical_qubits)}
                sub_qubits += sub_exp.num_qubits
            else:
                qubits = list(sub_exp.physical_qubits)
                qargs_map = {q: q for q in sub_exp.physical_qubits}

            for circ_idx, sub_circ in enumerate(sub_circuits):
                if circ_idx >= len(joint_circuits):
                    # Initialize new joint circuit or extract
                    # existing circuit if already initialized
                    new_circuit = QuantumCircuit(num_qubits, name=f"parallel_exp_{circ_idx}")
                    new_circuit.metadata = {
                        "experiment_type": self._type,
                        "composite_index": [],
                        "composite_metadata": [],
                        "composite_qubits": [],
                        "composite_clbits": [],
                    }
                    joint_circuits.append(new_circuit)

                # Add classical registers required by subcircuit
                circuit = joint_circuits[circ_idx]
                num_clbits = circuit.num_clbits
                sub_clbits = sub_circ.num_clbits
                clbits = list(range(num_clbits, num_clbits + sub_clbits))
                if sub_clbits:
                    creg = ClassicalRegister(sub_clbits)
                    sub_cargs = [Clbit(creg, i) for i in range(sub_clbits)]
                    circuit.add_register(creg)
                else:
                    sub_cargs = []

                # Apply transpiled subcircuit
                # Note that this assumes the circuit was not expanded to use
                # any qubits outside the specified physical qubits
                for data in sub_circ.data:
                    inst = data.operation
                    qargs = data.qubits
                    cargs = data.clbits
                    mapped_cargs = [sub_cargs[sub_circ.find_bit(i).index] for i in cargs]
                    try:
                        mapped_qargs = [
                            circuit.qubits[qargs_map[sub_circ.find_bit(i).index]] for i in qargs
                        ]
                    except KeyError as ex:
                        # Instruction is outside physical qubits for the component
                        # experiment.
                        # This could legitimately happen if the subcircuit was
                        # explicitly scheduled during transpilation which would
                        # insert delays on all auxillary device qubits.
                        # We skip delay instructions to allow for this.
                        if inst.name == "delay":
                            continue
                        raise QiskitError(
                            "Component experiment has been transpiled outside of the "
                            "allowed physical qubits for that component. Check the "
                            "experiment is valid on the backends coupling map."
                        ) from ex
                    circuit._append(inst, mapped_qargs, mapped_cargs)

                # Add subcircuit metadata
                circuit.metadata["composite_index"].append(exp_idx)
                circuit.metadata["composite_metadata"].append(sub_circ.metadata)
                circuit.metadata["composite_qubits"].append(qubits)
                circuit.metadata["composite_clbits"].append(clbits)

                # Add the calibrations
                for gate, cals in sub_circ.calibrations.items():
                    for key, sched in cals.items():
                        circuit.add_calibration(gate, qubits=key[0], schedule=sched, params=key[1])

        return joint_circuits