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Repeat

The repeat concept of the QMOD language is just like a classical for loop but for quantum objects. That is, when one wants to repeat a quantum operation several times, e.g. to do a bit-wise operation, they can just use the repeat concept of QMOD.

With the Classiq Python SDK package repeat is accessed as a function that has 2 arguments:

  1. count: int or CInt - how many times to repeat the operation;
  2. iteration: QCallable - what operation to repeat;

The argmuent that is passed as the iteration is a function by itself. Therefore it leverages the concept of lambda functions in Python (read more here).

In the following example, two quantum states \(|\psi \rangle\) and \(| \phi \rangle\) are initialized with 5 qubits each. Then an Hadamard gate is applied on all the qubits of \(|\psi \rangle\), i.e. \(H^{\otimes 5}|\psi \rangle\), and a NOT gate on all the qubits of \(|\phi \rangle\), i.e. \(X^{\otimes 5}|\phi \rangle\). Finally, using the repeat function, a bit-wise SWAP is performed between \(|\phi \rangle\) and \(|\psi \rangle\).

Phase Kickback High Level
from classiq import (
    SWAP,
    H,
    Output,
    QArray,
    QBit,
    X,
    allocate,
    apply_to_all,
    create_model,
    qfunc,
    repeat,
    show,
    synthesize,
)


@qfunc
def main(psi: Output[QArray[QBit]], phi: Output[QArray[QBit]]):

    allocate(num_qubits=5, out=psi)
    allocate(num_qubits=5, out=phi)

    apply_to_all(H, psi)
    apply_to_all(X, phi)

    repeat(count=psi.len, iteration=lambda i: SWAP(phi[i], psi[i]))


qprog = synthesize(create_model(main))
show(qprog)
Opening: https://platform.classiq.io/circuit/5b8e48b5-93c5-4068-8ce7-3ba10b2a814b?version=0.41.0.dev39%2B79c8fd0855

The native QMOD version of the above code:

// Repeat Example

qfunc main(output psi: qbit[], output phi: qbit[]) {
  allocate<5>(psi);
  allocate<5>(phi);

  apply_to_all<H>(psi);
  apply_to_all<X>(phi);

  repeat(i:psi.len) {
    SWAP(phi[i], psi[i]);
  }
}
from classiq import write_qmod

write_qmod(create_model(main), "repeat")