Release 0.6.0

[pablolh]

Almost a complete rewrite. No functional change except it's faster.

Added

• Ability to easily implement any quantum gate by simply giving an arbitrary unitary matrix in Gates.h
• Performance speedup using a single thread
• Some proper linear algebra primitives using template metaprogramming
• Abseil-cpp dependency for flat_hash_map
• Ability to compile the simulator with an set maximum number of qubits - in the future this will be using templates as well to switch data structures based on the input quantum circuit

Changed

• Quantum state is stored as a sparse array implemented using a flat_hash_map from Abseil-cpp
• C++20 in place of C++14,
• Code style: camel case
• Fixed CI for proper coverage release/debug and testing a single Python version

Removed

• OpenMP and multithreading.
• SSE, AVX and other intrinsics.

[pablolh]

@oschusler this PR makes it possible to build and run on ARM

[oschusler]

I see you removed support for 3.7? Why exactly?

[pablolh]

Morning Olaf,

Sorry for not mentioning it, I've been a bit messy with my commit. I included a refactoring to the CI.

• The tests workflow no longer tests all Python versions, only the latest one. On the other hand, more tests are included (C++ tests with different compilers and different build types Debug or Release). This has no effect on the released artifacts.
• As you noticed, the Python 3.7 wheel is no longer published. I did this to reduce the load on the Github CI, since I believe that nobody uses 3.7, which has its official end-of-life in June this year. When I was pushing a lot of changes to Github, my workflows would not run immediately because of the load of building that many Python versions, so I had to cancel runs manually which was inconvenient. If QI or someone else depends on Python 3.7, I suggest upgrading to 3.8+ but if that is not possible I can add that version back.

Other things that I did not mention:

• Error model is no longer implemented. That means this page no longer works. I realized that this might be a blocker, but error models would anyway need to be reimplemented with https://github.com/QuTech-Delft/qx-simulator/issues/107

• The classical not gate operating on the measurement register described here is also no longer implemented. I can add it back shortly.

  I suggest then we can merge this PR, then I create a separate PR when there is time for the error models and the classical not gate. Meanwhile QI doesn't have to upgrade. But that makes it possible to experiment with ARM64 it need be.