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Quantum Functions

Quantum functions are operations that modify the state of quantum object. The quantum objects are passed to the function as arguments of quantum types. In addition, quantum functions can take classical arguments and function arguments.

The following example demonstrates how to define a Qmod function. Function rotate applies a some given phase to a qubit.

qfunc rotate<r: real>(qv: qbit) {
  PHASE<r * pi>(qv);
from classiq import CReal, qfunc, QBit
from classiq.qmod.symbolic import pi

def rotate(r: CReal, qv: QBit) -> None:
    PHASE(theta=r * pi, target=qv)


The signature of a function determines the function's name and the arguments it expects. The body of a function is the specification of its implementation as a sequence of statements.

qfunc name [ < classical-args > ] ( quantum-args ) { statements }

classical-args is a list of zero-or-more comma-separated declarations in the form:

name : classical-type

quantum-args is a list of zero-or-more comma-separated declarations in the form:

[ output | input ] name : quantum-type

A quantum function is defined with a regular Python function decorated with @qfunc.

The @qfunc decorator relies on Python's type-hint mechanism to extract the Qmod argument declarations from the Python argument list. Type-hints for aguments are mandatory, and they can only be of Qmod types. In Python there is no separation or ordering requirement between classical and quantum arguments.

Direction modifiers for quantum arguments are represented with the generic classes Input and Output.


  • A function definition introduces a new function symbol into the global namespace.
  • Arguments can be used as variables in the body of the function.
  • Classical arguments can be used as variables in the declaration of subsequent argument types in the signature of the function.
  • The direction modifiers input and ouput may be used to specify input-only and output-only quantum arguments respectively.

Statements can do one of the following:

  • Call other quantum functions
  • Declare local quantum variables
  • Assign expressions to quantum variables
  • Bind quantum variables to other quantum variables

Qmod functions can also take functions as arguments. For details on this capability, see Operators.


Example 1 - Function Declarations

The following example demonstrates function declarations:

qfunc foo<n: int>(qba: qbit[2*n]) {
  // ...

qfunc bar(x: qnum, y: qnum, output res: qnum) {
  // ...
from classiq import CInt, QArray, QBit, QNum, Output, qfunc

def foo(n: CInt, qba: QArray[QBit, "2*n"]) -> None:

def bar(x: QNum, y: QNum, res: Output[QNum]) -> None:

Note that when classical arguments are used to specify subsequent arguments, as in the case of qba being a qubit array of size 2*n, the expression is specified as a string literal because the Python variable n is not in scope.

Example 2 - Function Definitions

The following example demonstrates a simple function definition. In its body it calls the built-in function H() and then iteratively under repeat the function PHASE() (for more on repeat see Classical Control Flow)

qfunc foo<n: int>(qv: qbit) {
  repeat (index: n) {
    PHASE<(index / n) * pi>(qv);

A function decorated with @qfunc is executed by the Python interpreter to construct the body of the Qmod function. Python functions corresponding to Qmod statements inject the respective statements into the constructed function.

from classiq import CInt, QBit, H, PHASE, allocate, repeat, qfunc
from classiq.qmod.symbolic import pi

def foo(n: CInt, qv: QBit) -> None:
    repeat(3, lambda i: PHASE(theta=(i / n) * pi, target=qv))