{"record":{"id":"5c176fbbf28d1951","repo":"TheAlgorithms/Python","slug":"number-of-qubits-too-large-to-simulate-10","errorCode":null,"errorMessage":"number of qubits too large to simulate(>10).","messagePattern":"number of qubits too large to simulate\\(>10\\)\\.","errorType":"validation","errorClass":"ValueError","httpStatus":null,"severity":"error","filePath":"quantum/q_fourier_transform.py","lineNumber":65,"sourceCode":"        ...\n    TypeError: number of qubits must be a integer.\n    >>> quantum_fourier_transform(100)\n    Traceback (most recent call last):\n        ...\n    ValueError: number of qubits too large to simulate(>10).\n    >>> quantum_fourier_transform(0.5)\n    Traceback (most recent call last):\n        ...\n    ValueError: number of qubits must be exact integer.\n    \"\"\"\n    if isinstance(number_of_qubits, str):\n        raise TypeError(\"number of qubits must be a integer.\")\n    if number_of_qubits <= 0:\n        raise ValueError(\"number of qubits must be > 0.\")\n    if math.floor(number_of_qubits) != number_of_qubits:\n        raise ValueError(\"number of qubits must be exact integer.\")\n    if number_of_qubits > 10:\n        raise ValueError(\"number of qubits too large to simulate(>10).\")\n\n    qr = QuantumRegister(number_of_qubits, \"qr\")\n    cr = ClassicalRegister(number_of_qubits, \"cr\")\n\n    quantum_circuit = QuantumCircuit(qr, cr)\n\n    counter = number_of_qubits\n\n    for i in range(counter):\n        quantum_circuit.h(number_of_qubits - i - 1)\n        counter -= 1\n        for j in range(counter):\n            quantum_circuit.cp(np.pi / 2 ** (counter - j), j, counter)\n\n    for k in range(number_of_qubits // 2):\n        quantum_circuit.swap(k, number_of_qubits - k - 1)\n\n    # measure all the qubits","sourceCodeStart":47,"sourceCodeEnd":83,"githubUrl":"https://github.com/TheAlgorithms/Python/blob/f5988cc09713315817df6a7e327e258013a94440/quantum/q_fourier_transform.py#L47-L83","documentation":"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.","triggerScenarios":"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.","commonSituations":"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.","solutions":["Reduce the qubit count to 10 or fewer, e.g. quantum_fourier_transform(10).","If n comes from user input or config, clamp or validate it: n = min(n, 10) or assert 1 <= n <= 10 before calling.","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.","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)."],"exampleFix":"# before\nresult = quantum_fourier_transform(user_n)  # user_n = 12 -> ValueError\n\n# after\nif user_n > 10:\n    raise SystemExit(\"QFT demo supports at most 10 qubits for local simulation\")\nresult = quantum_fourier_transform(user_n)","handlingStrategy":"validation","validationCode":"def safe_qft(n):\n    if not isinstance(n, int) or isinstance(n, bool):\n        raise TypeError('number_of_qubits must be int')\n    if not 1 <= n <= 10:\n        raise ValueError('number_of_qubits must be between 1 and 10 for local simulation')\n    return quantum_fourier_transform(n)","typeGuard":"def is_simulatable_qubit_count(n) -> bool:\n    return isinstance(n, int) and not isinstance(n, bool) and 1 <= n <= 10","tryCatchPattern":"try:\n    counts = quantum_fourier_transform(n)\nexcept ValueError as e:\n    # covers >10, <=0, and non-integer messages; check str(e) if you need to distinguish\n    print(f'Skipping QFT: {e}')","preventionTips":["Clamp user-provided qubit counts to 1..10 before calling simulation code.","Treat the cap as a memory limit: 2^n grows fast; do not fork-and-raise it on a laptop.","Prefer real hardware or cloud simulators for registers above 10 qubits."],"tags":["quantum","qiskit","simulation","input-validation","precondition"],"backgroundTag":null,"analyzedSha":"f5988cc09713315817df6a7e327e258013a94440","analyzedAt":"2026-08-14T17:30:07.041Z","schemaVersion":2},"datasetVersion":"2026-08-15T22:17:37.221Z"}