使用混合作业和 PennyLane 运行QAOA算法

在本节中，您将使用所学知识与参数化编译 PennyLane 一起编写实际的混合程序。您可以使用算法脚本来解决量子近似优化算法 (QAOA) 问题。该程序创建了一个与经典的 Max Cut 优化问题相对应的成本函数，指定了参数化的量子电路，并使用简单的梯度下降方法来优化参数，从而使成本函数最小化。在此示例中，为简单起见，我们在算法脚本中生成问题图，但对于更典型的用例，最佳做法是通过输入数据配置中的专用通道提供问题规范。该标志parametrize_differentiable默认为，True因此您可以自动从支持的QPUs参数化编译中获得提高运行时性能的好处。

import os
import json
import time

from braket.jobs import save_job_result
from braket.jobs.metrics import log_metric

import networkx as nx
import pennylane as qml
from pennylane import numpy as np
from matplotlib import pyplot as plt

def init_pl_device(device_arn, num_nodes, shots, max_parallel):
    return qml.device(
        "braket.aws.qubit",
        device_arn=device_arn,
        wires=num_nodes,
        shots=shots,
        # Set s3_destination_folder=None to output task results to a default folder
        s3_destination_folder=None,
        parallel=True,
        max_parallel=max_parallel,
        parametrize_differentiable=True, # This flag is True by default.
    )

def start_here():
    input_dir = os.environ["AMZN_BRAKET_INPUT_DIR"]
    output_dir = os.environ["AMZN_BRAKET_JOB_RESULTS_DIR"]
    job_name = os.environ["AMZN_BRAKET_JOB_NAME"]
    checkpoint_dir = os.environ["AMZN_BRAKET_CHECKPOINT_DIR"]
    hp_file = os.environ["AMZN_BRAKET_HP_FILE"]
    device_arn = os.environ["AMZN_BRAKET_DEVICE_ARN"]

    # Read the hyperparameters
    with open(hp_file, "r") as f:
        hyperparams = json.load(f)

    p = int(hyperparams["p"])
    seed = int(hyperparams["seed"])
    max_parallel = int(hyperparams["max_parallel"])
    num_iterations = int(hyperparams["num_iterations"])
    stepsize = float(hyperparams["stepsize"])
    shots = int(hyperparams["shots"])

    # Generate random graph
    num_nodes = 6
    num_edges = 8
    graph_seed = 1967
    g = nx.gnm_random_graph(num_nodes, num_edges, seed=graph_seed)

    # Output figure to file
    positions = nx.spring_layout(g, seed=seed)
    nx.draw(g, with_labels=True, pos=positions, node_size=600)
    plt.savefig(f"{output_dir}/graph.png")

    # Set up the QAOA problem
    cost_h, mixer_h = qml.qaoa.maxcut(g)

    def qaoa_layer(gamma, alpha):
        qml.qaoa.cost_layer(gamma, cost_h)
        qml.qaoa.mixer_layer(alpha, mixer_h)

    def circuit(params, **kwargs):
        for i in range(num_nodes):
            qml.Hadamard(wires=i)
        qml.layer(qaoa_layer, p, params[0], params[1])

    dev = init_pl_device(device_arn, num_nodes, shots, max_parallel)

    np.random.seed(seed)
    cost_function = qml.ExpvalCost(circuit, cost_h, dev, optimize=True)
    params = 0.01 * np.random.uniform(size=[2, p])

    optimizer = qml.GradientDescentOptimizer(stepsize=stepsize)
    print("Optimization start")

    for iteration in range(num_iterations):
        t0 = time.time()

        # Evaluates the cost, then does a gradient step to new params
        params, cost_before = optimizer.step_and_cost(cost_function, params)
        # Convert cost_before to a float so it's easier to handle
        cost_before = float(cost_before)

        t1 = time.time()

        if iteration == 0:
            print("Initial cost:", cost_before)
        else:
            print(f"Cost at step {iteration}:", cost_before)

        # Log the current loss as a metric
        log_metric(
            metric_name="Cost",
            value=cost_before,
            iteration_number=iteration,
        )

        print(f"Completed iteration {iteration + 1}")
        print(f"Time to complete iteration: {t1 - t0} seconds")

    final_cost = float(cost_function(params))
    log_metric(
        metric_name="Cost",
        value=final_cost,
        iteration_number=num_iterations,
    )

    # We're done with the hybrid job, so save the result.
    # This will be returned in job.result()
    save_job_result({"params": params.numpy().tolist(), "cost": final_cost})