[I] [Bug] No qubit index validation allows out of bounds access causing cryptic backend errors [mahout]

["shiavm006 (via GitHub)" <gi...@apache.org>]

shiavm006 opened a new issue, #624:
URL: https://github.com/apache/mahout/issues/624

   ### Description
   None of the gate application methods in `qumat.py` validate that qubit indices are within valid bounds (0 to num_qubits-1). This allows users to apply gates to non-existent qubits, causing cryptic backend-specific errors instead of clear validation messages at the frontend layer.
   
   ### Steps to Reproduce
   1. Create a 2-qubit circuit: `qumat.create_empty_circuit(2)`
   2. Try to apply a gate to qubit index 5 (out of bounds): `qumat.apply_hadamard_gate(5)`
   3. Try to apply a gate to negative index: `qumat.apply_cnot_gate(-1, 0)`
   4. Observe backend-specific cryptic errors instead of clear validation
   
   ### Expected Behavior
   Qumat should validate qubit indices at the frontend layer and raise clear `ValueError` or `IndexError` with descriptive messages like:
   ValueError: qubit index 5 out of range for circuit with 2 qubits (valid range: 0-1)
   
   
   ### Actual Behavior
   **Current code** (example from `apply_hadamard_gate` in `qumat/qumat.py` lines 94-105):
   def apply_hadamard_gate(self, qubit_index):
       """Apply a Hadamard gate to the specified qubit."""
       self._ensure_circuit_initialized()
       self.backend_module.apply_hadamard_gate(self.circuit, qubit_index)
       #No validation that 0 <= qubit_index < self.num_qubits**Backend-specific errors** that result:
   - **Qiskit**: `CircuitError: "Index 5 out of range for size 2"`
   - **Cirq**: May create new LineQubit silently or crash during execution
   - **Braket**: Backend-specific validation errors
   


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Re: [I] [Bug] No qubit index validation allows out of bounds access causing cryptic backend errors [mahout]

["guan404ming (via GitHub)" <gi...@apache.org>]

guan404ming closed issue #624: [Bug] No qubit index validation  allows out of bounds access causing cryptic backend errors
URL: https://github.com/apache/mahout/issues/624


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Re: [I] [Bug] No qubit index validation allows out of bounds access causing cryptic backend errors [mahout]

["guan404ming (via GitHub)" <gi...@apache.org>]

guan404ming commented on issue #624:
URL: https://github.com/apache/mahout/issues/624#issuecomment-3528766842

   Nice catch, to add validation for this sounds like a great idear.


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