Hi @lynx cuQuantum itself is built for CUDA compute capability SM 7.0 and above. In this instance, the GTX 960 compute capability is SM 5.2 as reported by the warning message:
UserWarning: CUDA device is an unsupported version: (5, 2)
The RTX 3050 is SM 8.6 and the Titan RTX is SM 7.5, which means these should work fine, but the GTX 960 will not.
The NVIDIA GPU->compute capabilities can be found here. As mentioned by @CatalinaAlbornoz this is a limit imposed by cuStateVec, and so we cannot support the older generation of cards.
If you need to use an older card, we have a new device under development that can be compiled by the user for older GPU architectures here. Since this is under active development, we do not guarantee it will work in all of the same cases as lightning.gpu. Assuming you are running on Linux, and have an active CUDA installation, you can create a working install within the repository with:
BACKEND="CUDA" python -m pip install -e .
python -m pip install git+https://github.com/PennyLaneAI/pennylane.git@master
Though, I suggest using lightning.gpu where possible.