Segmentation fault in cudaq.observe() when the state/ket is not a power of 2

[wsttiger]

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Describe the bug

When a vector of size N is passed to cudaq.observe() and N is not a power of 2, the software segfaults rather that give a descriptive error message.

Steps to reproduce the bug

import numpy as np
import cudaq
from cudaq import spin

cudaq.set_target('qpp-cpu')

n_size = 7

@cudaq.kernel
def initial_state(vec: list[float]):
    q = cudaq.qvector(vec)

x = np.random.randn(n_size)
x = x / np.linalg.norm(x)

psi = cudaq.get_state(initial_state, x)
print(f'psi: {psi}')
print(f'x: {x}')

H = -5.0 + 2.5 * spin.x(0) * spin.y(1) + 0.23 * spin.z(1) + 0.11 * spin.z(0)

result = cudaq.observe(initial_state, H, x).expectation()
print(f'result: {result}')

Expected behavior

Should give a meaningful error message to alert the user of the power of 2 requirement

Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment

• CUDA-Q version:
• Python version:
• C++ compiler:
• Operating system:

Suggestions

No response

[bmhowe23]

It would be advantageous if the error check could also handle a mismatch in data length vs the number of qubits. I.e. throw an error if the user provides a vector of length 4 to initialize 4 qubits. If possible, this may need to be done in the simulators themselves (like in the addQubitsToState calls where state is non-null.

The exact number to compare against would need to be simulator-dependent. I.e. 2^n for state-vector-based simulators and 2^(2*n) for density matrix based simulators.