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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