GSoC 2021 final blog post

During last 10 weeks I have been participating in the Google Summer of Code 2021. This post will be a permanent link for the work done during this time. My mentors have been Eric Giguère, Jake Lishman, Shahnawaz Ahmed and Simon Cross. I would like to thank them all for their help during this project. I really enjoyed working with you all and I always felt very welcome in QuTiP’s community.

My project has consisted on developing the python package qutip-tensorflow, a plugin for QuTiP that provides support for linear algebra operations using TensorFlow as backend. QuTiP is a useful package when it comes to working with quantum systems, and we wanted to provide extra functionality by leveraging some of TensoFlow’s strengths, namely GPU computation and auto differentiation. The project has been possible thanks to previous work on QuTiP to abstract the numerical operations into a dispatching system. This allows having different data representations of the main class in QuTiP, the Qobj.

Although most of the work has been focused on creating the new qutip-tensorflow package (which can be accessed here), it has also required to adapt some of QuTiP’s functions to allow auto differentiation to work. I will now explain in a little more detail what my contributions have been.

Contributions to qutip-tensorflow

• The TfTensor class (PR #8, merged): This PR includes the base class that wraps around a tensorflow.Tensor to be compatible with QuTiP. It also includes the to and create specialisations for TfTensor, allowing QuTiP’s Qobj to represent its data with the TfTensor class.

• Specialisations (PR #13- #17, #20, #21. All merged.): These pull requests add most of the specialisations required to make the TfTensor class work seamlessly in QuTiP. The specialisations are functions that define a operation for a particular combination of inputs. When these specialisations are registered in QuTiP, it allows the Qobj class backed with a TfTensor to operate using TensorFlow in the backend. They act as a translation from TensorFlow to QuTiP.

• Specialisations in review process (PR #22 and PR #24, in review). These specialisations are still in review and include both the different normalization operations and reshape operations that QuTiP makes use of.

• Example notebook (PR #25, in review). This PR includes an example showing a potential use case for qutip-tensoflow. It consists on the optimizaiton of a random unitary gate to perform a specific operation. It shows how to work with TensorFlow’s optimizers and QuTiP’s Qobj.

• Benchmark code (PR #4): This includes as set of benchmarks that help assessing for which system sizes can qutip-tensorflow’s GPU operations provide a meaningful improvement.

• README update (PR #26) This includes an update of the README page in GitHub with installation instructions and a description of qutip-tensorflow’s features.

Contributions to QuTiP

• Extending expect to return different types (PR #1636, in review process): Some of the functions in QuTiP assume that the specialisations return an instance of number.Number. However, for auto differentiation to work, it is necessary to return a tensorflow.Tensor. This pull request addresses this issue in the qutip.expect function. It should be noted that there are still a few other places in the code where the same assumption is done and hence they should be updated.

• Allowing arbitrary scalar multiplication times Qobj (PR #1620, merged): This PR improves __mul__ implementation to allow Qobj instances to be multiplied by tf.Variables. This is an extremely useful feature for auto-differentiation as most common use cases in optimization rely on scalar variables.

• Extending specialisation test suite (PR #1622, #1626 and #1630, merged. PR #1635 and #1637 in review): This PR extend the already existing test suite for specialisation functions. This test suite is also used in qutip-tensor flow and facilitates enormously the development of new data layers. Some of these PR also address a few bugs that were found during the development of these tests.

What is next?

I believe qutip-tensorflow will become a useful tool for developers working in the areas of quantum optimal control and machine learning for quantum computing. However, there are still a few improvements that will make it more attractive:

• Missing specialisations. There are still some missing specialisations in qutip-tensorflow. This will not be a problem in most of the cases as, when an operation in QuTiP is required for which an specialisation does not exist, QuTiP will automatically convert a data type into another for which the specialisation is known. However, this means that you lose the auto differentiation feature with that operations. Most of the specialisations are already included but there are still a few missing (see issue #28).

• Support for tensorflow.function. TensorFlow provides this decorator for JIT compiling functions in Python. Extending this function in qutip-tensorflow to work seamlessly with Qobj will further extend qutip-tensorflow capabilities.

• Support for tensorflow.keras.Model and batched operations. If qutip-tensorflow wants to stand out in the quantum machine learning field it will be necessary to support batched operations, as these improve the efficiency of common operations. Similarly, seamless integrations with Keras Model class will facilitate creating new machine learning models.