shiavm006 opened a new issue, #632:
URL: https://github.com/apache/mahout/issues/632
### Description
The `execute_circuit()` function in `qiskit_backend.py` uses an incorrect
check to determine if measurements already exist in the circuit. It checks
`circuit.cregs` (classical registers) instead of checking if measurements are
actually present. This causes duplicate measurements to be added every time
`execute_circuit()` is called multiple times on the same circuit, corrupting
the circuit state and producing incorrect results.
### Steps to Reproduce
1. Create a circuit and apply some gates:on
qumat = QuMat(backend_config)
qumat.create_empty_circuit(2)
qumat.apply_hadamard_gate(0)
qumat.apply_cnot_gate(0, 1)
2. Execute the circuit multiple times:
results1 = qumat.execute_circuit() # First execution - adds measurements
results2 = qumat.execute_circuit() # Second execution - ADDS DUPLICATE
MEASUREMENTS!3. Observe that the circuit now has duplicate measurement
operations
### Expected Behavior
The circuit should check if measurements already exist using the proper
Qiskit API (e.g., checking circuit data for measurement operations) and only
add measurements once, even if `execute_circuit()` is called multiple times.
### Actual Behavior
Current code in `qumat/qiskit_backend.py` lines 90-91:
if not circuit.cregs:
circuit.measure_all()This checks for classical registers (`cregs`), not
measurements. Since `measure_all()` automatically creates classical registers,
the check `not circuit.cregs` will always be True on the first call, but may
incorrectly evaluate on subsequent calls, leading to duplicate measurements.
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