aqt_device.py

# Copyright 2019 The Cirq Developers
#
# 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
#
#     https://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.

"""Current device parameters for the AQT/UIBK ion trap device

The device is based on a linear calcium ion string with
arbitrary connectivity. For more information see:

[https://quantumoptics.at/en/publications/journal-articles.html](https://quantumoptics.at/en/publications/journal-articles.html){:.external}

[https://iopscience.iop.org/article/10.1088/1367-2630/15/12/123012/meta](https://iopscience.iop.org/article/10.1088/1367-2630/15/12/123012/meta){:.external}

The native gate set consists of the local gates: X, Y, and XX entangling gates
"""

import json
from enum import Enum
from typing import Any, cast, Dict, Iterable, List, Optional, Sequence, Set, Tuple, Union

import networkx as nx
import numpy as np

import cirq
from cirq_aqt import aqt_device_metadata

gate_dict = {'X': cirq.X, 'Y': cirq.Y, 'Z': cirq.Z, 'MS': cirq.XX, 'R': cirq.PhasedXPowGate}


class OperationString(Enum):
    """String representations of operations supported by AQT resources."""

    MS = "MS"
    """Cirq: XXPowGate, AQT: RXX gate."""

    Z = "Z"
    """Cirq: ZPowGate, AQT: RZ gate."""

    R = "R"
    """Cirq: PhasedXPowGate, AQT: R gate."""

    MEASURE = "Meas"
    """Measurement gate."""


def get_op_string(op_obj: cirq.Operation) -> str:
    """Find the string representation for a given gate or operation.

    Args:
        op_obj: Gate or operation object. Gate must be one of: XXPowGate,
            ZPowGate, PhasedXPowGate, or MeasurementGate.

    Returns:
        String representing the gate operations.

    Raises:
        ValueError: If the gate is not one of the supported gates.
    """
    if isinstance(op_obj.gate, cirq.XXPowGate):
        op_str = OperationString.MS.value
    elif isinstance(op_obj.gate, cirq.ZPowGate):
        op_str = OperationString.Z.value
    elif isinstance(op_obj.gate, cirq.PhasedXPowGate):
        op_str = OperationString.R.value
    elif isinstance(op_obj.gate, cirq.MeasurementGate):
        op_str = OperationString.MEASURE.value
    else:
        raise ValueError(f'Got unknown gate on operation: {op_obj}.')
    return str(op_str)


class AQTNoiseModel(cirq.NoiseModel):
    """A noise model for the AQT ion trap"""

    def __init__(self):
        self.noise_op_dict = get_default_noise_dict()

    def noisy_moment(
        self, moment: cirq.Moment, system_qubits: Sequence[cirq.Qid]
    ) -> List[cirq.Operation]:
        """Returns a list of noisy moments.

        The model includes
        - Depolarizing noise with gate-dependent strength
        - Crosstalk  between neighboring qubits

        Args:
            moment: ideal moment
            system_qubits: List of qubits

        Returns:
            List of ideal and noisy moments
        """
        noise_list = []
        for op in moment.operations:
            op_str = get_op_string(op)
            if op_str not in self.noise_op_dict:
                break
            noise_op = self.noise_op_dict[op_str]
            for qubit in op.qubits:
                noise_list.append(noise_op.on(qubit))
            noise_list += self.get_crosstalk_operation(op, system_qubits)

        return list(moment) + noise_list

    def get_crosstalk_operation(
        self, operation: cirq.Operation, system_qubits: Sequence[cirq.Qid]
    ) -> List[cirq.Operation]:
        """Returns a list of operations including crosstalk

        Args:
            operation: Ideal operation
            system_qubits: Tuple of line qubits

        Returns:
            List of operations including crosstalk
        """
        cast(Tuple[cirq.LineQubit], system_qubits)
        num_qubits = len(system_qubits)
        xtlk_arr = np.zeros(num_qubits)
        idx_list = []
        for qubit in operation.qubits:
            idx = system_qubits.index(qubit)
            idx_list.append(idx)
            neighbors = [idx - 1, idx + 1]
            for neigh_idx in neighbors:
                if neigh_idx >= 0 and neigh_idx < num_qubits:
                    xtlk_arr[neigh_idx] = self.noise_op_dict['crosstalk']

        for idx in idx_list:
            xtlk_arr[idx] = 0
        xtlk_op_list = []
        op_str = get_op_string(operation)
        gate = cast(cirq.EigenGate, gate_dict[op_str])

        if len(operation.qubits) == 1:
            for idx in xtlk_arr.nonzero()[0]:
                exponent = operation.gate.exponent  # type:ignore
                exponent = exponent * xtlk_arr[idx]
                xtlk_op = operation.gate.on(system_qubits[idx]) ** exponent  # type:ignore
                xtlk_op_list.append(xtlk_op)
        elif len(operation.qubits) == 2:
            for op_qubit in operation.qubits:
                for idx in xtlk_arr.nonzero()[0]:
                    exponent = operation.gate.exponent  # type:ignore
                    exponent = exponent * xtlk_arr[idx]
                    xtlk_op = gate.on(op_qubit, system_qubits[idx]) ** exponent
                    xtlk_op_list.append(xtlk_op)
        return xtlk_op_list


class AQTSimulator:
    """A simulator for the AQT device."""

    def __init__(
        self,
        num_qubits: int,
        circuit: cirq.Circuit = cirq.Circuit(),
        simulate_ideal: bool = False,
        noise_dict: Optional[Dict] = None,
    ):
        """Initializes the AQT simulator.

        Args:
            num_qubits: Number of qubits.
            circuit: Optional, circuit to be simulated.
                Last moment needs to be a measurement over all qubits with key 'm'
            simulate_ideal: If True, an ideal, noiseless, circuit will be simulated.
            noise_dict: A map from gate to noise to be applied after that gate. If None, uses
                a default noise model.
        """
        self.circuit = circuit
        self.num_qubits = num_qubits
        self.qubit_list = cirq.LineQubit.range(num_qubits)
        if noise_dict is None:
            noise_dict = get_default_noise_dict()
        self.noise_dict = noise_dict
        self.simulate_ideal = simulate_ideal

    def generate_circuit_from_list(self, json_string: str):
        """Generates a list of cirq operations from a json string.

        The default behavior is to add a measurement to any qubit at the end
        of the circuit as there are no measurements defined in the AQT API.

        Args:
            json_string: json that specifies the sequence.
        """
        self.circuit = cirq.Circuit()
        json_obj = json.loads(json_string)
        gate: Union[cirq.PhasedXPowGate, cirq.EigenGate]
        for circuit_list in json_obj:
            op_str = circuit_list[0]
            if op_str == 'R':
                gate = cast(cirq.PhasedXPowGate, gate_dict[op_str])
                theta = circuit_list[1]
                phi = circuit_list[2]
                qubits = [self.qubit_list[i] for i in circuit_list[3]]
                self.circuit.append(gate(phase_exponent=phi, exponent=theta).on(*qubits))
            else:
                gate = cast(cirq.EigenGate, gate_dict[op_str])
                angle = circuit_list[1]
                qubits = [self.qubit_list[i] for i in circuit_list[2]]
                self.circuit.append(gate.on(*qubits) ** angle)
        # TODO: Better solution for measurement at the end.
        # Github issue: https://github.com/quantumlib/Cirq/issues/2199
        self.circuit.append(cirq.measure(*[qubit for qubit in self.qubit_list], key='m'))

    def simulate_samples(self, repetitions: int) -> cirq.Result:
        """Samples the circuit.

        Args:
            repetitions: Number of times the circuit is simulated.

        Returns:
            Result from Cirq.Simulator.

        Raises:
            RuntimeError: Simulate called without a circuit.
        """
        if self.simulate_ideal:
            noise_model = cirq.NO_NOISE
        else:
            noise_model = AQTNoiseModel()

        if self.circuit == cirq.Circuit():
            raise RuntimeError('Simulate called without a valid circuit.')

        sim = cirq.DensityMatrixSimulator(noise=noise_model)

        result = sim.run(self.circuit, repetitions=repetitions)

        return result


@cirq.value_equality
class AQTDevice(cirq.Device):
    """Ion trap device with qubits having all-to-all connectivity and placed on a line."""

    def __init__(
        self,
        measurement_duration: 'cirq.DURATION_LIKE',
        twoq_gates_duration: 'cirq.DURATION_LIKE',
        oneq_gates_duration: 'cirq.DURATION_LIKE',
        qubits: Iterable[cirq.LineQubit],
    ) -> None:
        """Initializes the description of an ion trap device.

        Args:
            measurement_duration: The maximum duration of a measurement.
            twoq_gates_duration: The maximum duration of a two qubit operation.
            oneq_gates_duration: The maximum duration of a single qubit
            operation.
            qubits: Qubits on the device, identified by their x location.

        Raises:
            TypeError: If not all the qubits supplied are `cirq.LineQubit`s.
        """
        if not all(isinstance(qubit, cirq.LineQubit) for qubit in qubits):
            raise TypeError(
                "All qubits were not of type cirq.LineQubit, instead were "
                f"{set(type(qubit) for qubit in qubits)}"
            )
        self.qubits = frozenset(qubits)

        graph = nx.Graph()
        graph.add_edges_from([(a, b) for a in qubits for b in qubits if a != b], directed=False)
        self._metadata = aqt_device_metadata.AQTDeviceMetadata(
            qubits=self.qubits,
            measurement_duration=measurement_duration,
            twoq_gates_duration=twoq_gates_duration,
            oneq_gates_duration=oneq_gates_duration,
        )

    @property
    def metadata(self) -> aqt_device_metadata.AQTDeviceMetadata:
        return self._metadata

    def validate_gate(self, gate: cirq.Gate):
        if gate not in self.metadata.gateset:
            raise ValueError(f'Unsupported gate type: {gate!r}')

    def validate_operation(self, operation):
        if not isinstance(operation, cirq.GateOperation):
            raise ValueError(f'Unsupported operation: {operation!r}')

        self.validate_gate(operation.gate)

        for q in operation.qubits:
            if not isinstance(q, cirq.LineQubit):
                raise ValueError(f'Unsupported qubit type: {q!r}')
            if q not in self.qubits:
                raise ValueError(f'Qubit not on device: {q!r}')

    def validate_circuit(self, circuit: cirq.AbstractCircuit):
        super().validate_circuit(circuit)
        _verify_unique_measurement_keys(circuit.all_operations())

    def at(self, position: int) -> Optional[cirq.LineQubit]:
        """Returns the qubit at the given position, if there is one, else None."""
        q = cirq.LineQubit(position)
        return q if q in self.qubits else None

    def _value_equality_values_(self) -> Any:
        return (self.metadata, self.qubits)

    def __str__(self) -> str:
        diagram = cirq.TextDiagramDrawer()

        for q in self.qubits:
            diagram.write(q.x, 0, str(q))
            for q2 in q.neighbors(self.qubits):
                diagram.grid_line(q.x, 0, q2.x, 0)

        return diagram.render(horizontal_spacing=3, vertical_spacing=2, use_unicode_characters=True)

    def __repr__(self) -> str:
        return (
            f'cirq_aqt.aqt_device.AQTDevice('
            f'measurement_duration={self.metadata.measurement_duration!r}, '
            f'twoq_gates_duration={self.metadata.twoq_gates_duration!r}, '
            f'oneq_gates_duration={self.metadata.oneq_gates_duration!r}, '
            f'qubits={sorted(self.qubits)!r}'
            f')'
        )

    def _repr_pretty_(self, p: Any, cycle: bool):
        """iPython (Jupyter) pretty print."""
        p.text("AQTDevice(...)" if cycle else self.__str__())


def get_aqt_device(num_qubits: int) -> Tuple[AQTDevice, List[cirq.LineQubit]]:
    """Returns an AQT ion device

    Args:
        num_qubits: number of qubits

    Returns:
         A tuple of AQTDevice and qubit_list
    """
    qubit_list = cirq.LineQubit.range(num_qubits)
    us = 1000 * cirq.Duration(nanos=1)
    ion_device = AQTDevice(
        measurement_duration=100 * us,
        twoq_gates_duration=200 * us,
        oneq_gates_duration=10 * us,
        qubits=qubit_list,
    )
    return ion_device, qubit_list


def get_default_noise_dict() -> Dict[str, Any]:
    """Returns the current noise parameters"""
    default_noise_dict = {
        OperationString.R.value: cirq.depolarize(1e-3),
        OperationString.Z.value: cirq.depolarize(0),
        OperationString.MS.value: cirq.depolarize(1e-2),
        'crosstalk': 0.03,
    }
    return default_noise_dict


def _verify_unique_measurement_keys(operations: Iterable[cirq.Operation]):
    seen: Set[str] = set()
    for op in operations:
        if isinstance(op.gate, cirq.MeasurementGate):
            meas = op.gate
            key = cirq.measurement_key_name(meas)
            if key in seen:
                raise ValueError(f'Measurement key {key} repeated')
            seen.add(key)