Execution Tutorial - Part 2

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Expectation Values and Parameterized Quantum Programs

This tutorial covers the basics of measuring observables expressed as linear combinations of Pauli strings and executing a parameterized quantum program using Classiq via Python SDK. Alternatively, you can use the Classiq IDE web page to execute quantum algorithms.

A parameterized quantum program is a quantum circuit with adjustable parameters, such as angles in rotation gates, that can be tuned to alter the circuit's behavior. Think of it like tuning a camera with adjustable settings: the camera (the circuit) stays the same, but adjusting the settings (parameters) changes the captured images (outputs). In quantum computing, tuning these parameters helps identify the configuration that yields the most useful results. These programs are particularly useful in quantum machine learning and optimization, where the goal is to find the best parameter set.

First, we create a parameterized quantum program using two qubits. The program applies an X gate, a parameterized RY rotation, and a CX gate. The rotation angle is controlled by a variable called angle.

from classiq import *


@qfunc
def main(angle: CReal, x: Output[QBit], y: Output[QBit]) -> None:
    allocate(x)
    allocate(y)
    X(x)
    RY(angle, x)
    CX(x, y)


qprog = synthesize(main)
show(qprog)

Quantum program link: https://platform.classiq.io/circuit/31H8AszBLntOf5Y5z53i5BKLUak


gio: https://platform.classiq.io/circuit/31H8AszBLntOf5Y5z53i5BKLUak?login=True&version=0.89.0: Operation not supported

The first thing we can do is to sample the outputs of the quantum program for a given parameter. For example, \(\pi / 2\).

We can now execute the quantum program and obtain a sample of output states using ExecutionSession. To do this, we define the parameter values using a dictionary.

import numpy as np

# Set angle parameter to pi/2 for sampling
parameter = {"angle": np.pi / 2}

After generating the ExecutionSession, it is possible to show the counts for this particular parameter value:

with ExecutionSession(qprog) as es:
    first_sample = es.sample(parameter)

print("Counts for angle = pi/2", first_sample.counts)

Counts for angle = pi/2 {'11': 1026, '00': 1022}

Running your circuit with different parameter values helps explore how the output changes, revealing trends or minima in a cost function. This is especially useful in quantum optimization. As an example, we’ll evaluate the circuit over 50 values of angle from \(0\) to \(2\pi\).

# Create a list of 50 angle values from 0 to 2π
angles_list = np.linspace(0, 2 * np.pi, 50)
parameters_list = [{"angle": angles} for angles in angles_list]

# Execute batch sampling over all angles
with ExecutionSession(qprog) as es:
    second_sample = es.batch_sample(parameters_list)

The result of batch_sample is a list of results, one for each angle value.

Therefore, we can analyze the data from each parameter on angles_list. For example, an interesting way of analyzing this data is to plot the number of counts of the states \(|00\rangle\) and \(|11\rangle\) as functions of angle:

# extract counts of |11> and |00> for each result

pops_00 = [pops.counts.get("00", 0) for pops in second_sample]
pops_11 = [pops.counts.get("11", 0) for pops in second_sample]

import matplotlib.pyplot as plt

plt.figure(figsize=(8, 5))
plt.plot(angles_list / np.pi, pops_00, label='Counts of "00"')
plt.plot(angles_list / np.pi, pops_11, label='Counts of "11"', linestyle="-.")

plt.xlabel(r"$\mathrm{Angle} \; (\pi)$")
plt.ylabel("Counts")
plt.legend()
plt.show()

Measuring Pauli Strings

Measuring observables from a quantum program turns out to be necessary when you want to obtain information that can't be accessed only from the populations of states. For this end, you can measure Pauli Strings using Classiq. As an example, if we want to measure how close the output of our system is to the Bell state:

\[|\Phi^+ \rangle = \frac{1}{\sqrt{2}} \left( |00\rangle + |11\rangle \right),\]

it is possible measure the expected value of its projection:

\[P(\Phi^+) = |\Phi^+ \rangle \langle \Phi^+ | = \frac{1}{2} \left( |00\rangle + |11\rangle \right) \left( \langle00| + \langle 11| \right) = \frac{1}{2} \left ( |00\rangle \langle 00| + |00\rangle \langle 11 | + |11 \rangle \langle 00 | + |11\rangle \langle |11\right).\]

The projector operator, by its turn, can be represented as a Pauli string:

\[P(\Phi^+) = \frac{1}{4} \left( II + XX - YY + ZZ \right)\]

Therefore, if we want to measure the projector expected value for some output of the quantum circuit, say angle \(= \pi/5\), it is possible using estimate and ExecutionSession. For this, first we need to define the Hamiltonian to be measured:

projector_operator = 0.25 * (
    Pauli.I(0) * Pauli.I(1)
    + Pauli.X(0) * Pauli.X(1)
    - Pauli.Y(0) * Pauli.Y(1)
    + Pauli.Z(0) * Pauli.Z(1)
)

Now, using ExecutionSession, evaluate the expected value of the output:

parameter = {"angle": np.pi / 5}

with ExecutionSession(qprog) as es:
    first_estimate = es.estimate(projector_operator, parameter).value

print("Expected value for angle = pi/5: ", first_estimate)

Expected value for angle = pi/5:  (0.20458984375+0j)

The same can be done in batches, for example, if we want to plot a graph of fidelity between the output of the quantum program and the \(|\Phi^+\rangle\) state:

with ExecutionSession(qprog) as es:
    second_estimate = es.batch_estimate(projector_operator, parameters_list)

values = [measurement.value.real for measurement in second_estimate]
plt.figure(figsize=(8, 5))
plt.plot(angles_list / np.pi, values, label="Expectation value")

plt.xlabel(r"$\mathrm{Angle} \; (\pi)$")
plt.ylabel(r"$|\langle \Phi^+ | \psi \rangle |^2$")
plt.legend()
plt.show()

Retrieving Jobs Executed Using Execution Session

When executing a job that may take longer, or is on queue with other jobs on a hardware backend, it is interesting to have a way of retrieving this job in case it takes too long to execute. For this, it is possible to submit an ExecutionJob - which is associated to an ID - and retrieve its results later. To do so, it is only necessary to add the prefix submit to its original functions. As an example, we can submit two different jobs and retrieve its outputs by using its ID. For this, first we need to submit them:

with ExecutionSession(qprog) as execution_session:
    # These are the sampling jobs
    sample_job = execution_session.submit_sample(parameter)
    # These are the estimate jobs
    estimate_job = execution_session.submit_estimate(projector_operator, parameter)


# These are the job IDs for the respective jobs
sample_job_ID = sample_job.id
estimate_job_ID = estimate_job.id

Once you have the job ID, it is possible to retrieve its execution data using ExecutionJob. For example, here we retrieve a previously execution of estimate_job and compare it to the outputs of first_sample - they should be close:

# Retrieving the job from its ID
retrieved_estimate = ExecutionJob.from_id(estimate_job_ID)

print("Retrieved job result:", retrieved_estimate.result_value().value)

Retrieved job result: (0.211181640625+0j)

As expected, the retrieved job result is very close to the first sample, since they execute the same quantum circuit.

Application: Variational Quantum Circuit to Prepare a Bell State

In this additional session, we create a simple parameterized quantum algorithm that prepares the Bell State \(|\Phi^+\rangle\). For this, an ansatz with two parameters is constructed:

# Note that now angles are declared as a CArray[CReal, 2], where 2 represents its length


@qfunc
def main(angles: CArray[CReal, 2], x: Output[QBit], y: Output[QBit]) -> None:
    allocate(x)
    allocate(y)
    RX(angles[0], x)
    RY(angles[1], x)
    CX(x, y)


qprog_bell = synthesize(main)

Then define the function that is subject to classical optimization. In this case, we aim to maximize the expected value of the projector_operator. Therefore, we create a negative_coeffs_projector_operator to minimize:

negative_coeffs_projector_operator = (-1) * projector_operator

The final step is to perform the optimization of the cost function defined by the quantum ansatz. In this tutorial, the minimize method from ExecutionSession will be employed.

with ExecutionSession(qprog_bell) as es:
    res = es.minimize(
        negative_coeffs_projector_operator,
        initial_params={"angles": [0, 0]},
        max_iteration=200,
    )

coefficients = res[-1][1]
fidelity = -res[-1][0]
print("Fidelity =", fidelity, "Coefficients: ", coefficients)

Fidelity = 0.999267578125 Coefficients:  {'angles': [0.00507601286416629, 1.5209301348447444]}

These values corresponds to the quantum circuit that generates this Bell State using RX, RY, and CX gates.

Final Remarks

In this tutorial, we built a simple parameterized quantum circuit, explored sampling it with specific parameter values, and visualized how output probabilities vary with those parameters. At the end, a simple Variational Quantum Algorithm is presented to prepare a Bell State.

These techniques form the foundation for building and optimizing more complex quantum algorithms.