TheAlgorithms/Python · error · ValueError

number of qubits too large to simulate(>10).

Error message

number of qubits too large to simulate(>10).

What it means

Raised by quantum_fourier_transform in quantum/q_fourier_transform.py when number_of_qubits exceeds 10. The function builds and executes a full-state Qiskit circuit with 10000 shots, so the qubit count is capped because the state vector grows as 2^n and simulation cost explodes beyond ~10 qubits. This is a hard precondition guard, not a Qiskit failure: the check fires before any circuit is constructed. Note the earlier guards already rejected strings, non-positive, and non-integer values, so this specific error means the value passed all of those but is simply too large.

Source

Thrown at quantum/q_fourier_transform.py:65

        ...
    TypeError: number of qubits must be a integer.
    >>> quantum_fourier_transform(100)
    Traceback (most recent call last):
        ...
    ValueError: number of qubits too large to simulate(>10).
    >>> quantum_fourier_transform(0.5)
    Traceback (most recent call last):
        ...
    ValueError: number of qubits must be exact integer.
    """
    if isinstance(number_of_qubits, str):
        raise TypeError("number of qubits must be a integer.")
    if number_of_qubits <= 0:
        raise ValueError("number of qubits must be > 0.")
    if math.floor(number_of_qubits) != number_of_qubits:
        raise ValueError("number of qubits must be exact integer.")
    if number_of_qubits > 10:
        raise ValueError("number of qubits too large to simulate(>10).")

    qr = QuantumRegister(number_of_qubits, "qr")
    cr = ClassicalRegister(number_of_qubits, "cr")

    quantum_circuit = QuantumCircuit(qr, cr)

    counter = number_of_qubits

    for i in range(counter):
        quantum_circuit.h(number_of_qubits - i - 1)
        counter -= 1
        for j in range(counter):
            quantum_circuit.cp(np.pi / 2 ** (counter - j), j, counter)

    for k in range(number_of_qubits // 2):
        quantum_circuit.swap(k, number_of_qubits - k - 1)

    # measure all the qubits

View on GitHub (pinned to f5988cc097)

Solutions

1. Reduce the qubit count to 10 or fewer, e.g. quantum_fourier_transform(10).
2. If n comes from user input or config, clamp or validate it: n = min(n, 10) or assert 1 <= n <= 10 before calling.
3. If you genuinely need > 10 qubits, run on real quantum hardware or a cloud simulator (e.g. IBM Quantum runtime) instead of this local Aer-based function.
4. Fork the local function and remove/raise the cap only if you have the memory for a 2^n state vector (11 qubits is already 2048 amplitudes per shot).

Example fix

# before
result = quantum_fourier_transform(user_n)  # user_n = 12 -> ValueError

# after
if user_n > 10:
    raise SystemExit("QFT demo supports at most 10 qubits for local simulation")
result = quantum_fourier_transform(user_n)

def safe_qft(n):
    if not isinstance(n, int) or isinstance(n, bool):
        raise TypeError('number_of_qubits must be int')
    if not 1 <= n <= 10:
        raise ValueError('number_of_qubits must be between 1 and 10 for local simulation')
    return quantum_fourier_transform(n)

def is_simulatable_qubit_count(n) -> bool:
    return isinstance(n, int) and not isinstance(n, bool) and 1 <= n <= 10

try:
    counts = quantum_fourier_transform(n)
except ValueError as e:
    # covers >10, <=0, and non-integer messages; check str(e) if you need to distinguish
    print(f'Skipping QFT: {e}')

When it happens

Trigger: Calling quantum_fourier_transform(11), quantum_fourier_transform(100), or any integer-valued input greater than 10 (e.g. quantum_fourier_transform(10.0) passes the floor check and then 10.0 is fine, but 11.0 or 12 triggers it). Also triggered when number_of_qubits comes from a config value, CLI arg, or loop parameter that was never clamped.

Common situations: Porting textbook QFT examples (which often use small n) to larger registers; parameter sweeps that iterate n over a range including values > 10; copying Shor's algorithm or quantum phase estimation demos that assume many qubits; misunderstanding that the cap is about local simulator memory, not the algorithm itself.

Related errors

• number of qubits must be a integer.
• number of qubits must be > 0.
• k integer must be greater or equal to zero.
• All numbers must be positive
• Expected a_coeffs to have {self.order + 1} elements for {sel

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/5c176fbbf28d1951. Report an issue: GitHub.