Constructing one or multiple Jobs (using Batch)

In Qaptiva, the quantum circuit can be viewed as routines preparing a quantum memory in a particular state. When simulating the circuit, the user will need to describe the result that he want to obtain from the quantum state. This is achieved by specifying the final measurement to be carried out on the final state of the register, i.e on the wavefunction \(|\psi\rangle\) that has been prepared by the quantum circuit.

Two types of final measurements are commonly used in quantum algorithms:

• “Sample”: Measuring the projection of the wavefunction on the computational basis (formally speaking, this means measuring qubits “along the Z axis”).

  For instance, if the final state is \(|\psi\rangle=(|00\rangle+|11\rangle)/\sqrt{2}\), measuring “in the computational basis” will yield the bitstring “00” in 50% (\(1/\sqrt{2}^2\)) of cases, and “11” the rest of the time. Because of the statistical nature of the measurement, many repeated measurements on the computational basis, yielding many different bitstrings or “samples”, are necessary to get an accurate estimate of the frequencies of each computational basis state (“00”: 50%, “01”: 0%, “10”: 0%, “11”: 50% in our case). We will call “nbshots” the number of such repetitions (with a classical emulator, one can in general emulate an infinite number of shots since one has access to the exact frequencies). Of course, one may decide to measure only a subset of qubits.

• “Observable”: Measuring the value of a quantum mechanical observable \(O\) in the final state.

  On average, such a value is given by the formula \(\langle O \rangle = \langle \psi | O |\psi \rangle\). While this average value can directly be computed using a classical computer, on an actual QPU, only a limited set of observables can be measured. In practice, most QPUs provide only “Z-axis” measurements, i.e, as described above, the possibility to measure the projection of the wavefunction on computational basis states. In order to perform the estimation of the average value of an observable, one thus needs to convert the computation of \(\langle O \rangle\) into a series of Z axis measurements (with possible modifications of the circuit), estimate bitstrings frequencies as described above (with a given number of shots or repetitions), and combine those subresults back into an estimate of \(\langle O \rangle\).

To contain these informations, the quantum jobs are wrapped in a Job class. In addition to the quantum circuit, the user can provide several options when constructing the quantum job:

• The type of final measurement to perform: standard measurement in computational basis SAMPLE or sampling of an observable OBS

• The number of shots to perform

• The qubits to be measured

• The format of the returned result: whether to aggregate the samples

• The amplitude threshold of the final measurement: states with amplitude having a norm lower than this threshold will be considered null

• The initial quantum state of the simulation

• In case of a variational job, the value of all the binded variables

In most cases, the job object is not meant to be constructed manually. Instead, it is generated from a Circuit using the to_job() method. The job can then be sent to the QPU to be simulated.

The Batch class contains a list of jobs, allowing several circuits to be sent to a QPU with only a single request. A batch can also include a meta_data field if the QPU accepts metadata so that it can be configured with batch related options.

The code block below shows a simple example using a job to simulate a circuit:

# Import a Quantum Processor Unit Factory (the default one)
from qat.qpus import get_default_qpu

# Create a Quantum Processor Unit
qpu = get_default_qpu()
# Create a job from a circuit
job = circuit.to_job(nbshots=1024, qubits=[1,7])

# Submit the job
result = qpu.submit(job)