PSWAP gate implementation in Lightning Qubit

[tomlqc]

Important Note

⚠️ This issue is part of an internal assignment and not meant for external contributors

Context

PennyLane has support for many quantum gates natively at the Python level, but often we may require faster implementations. lightning.qubit is a fully C++ backed simulator library that can be called from PennyLane, with optimal implementations of many highly-used gates. Implementing more optimized gates in the C++ backend would boost the performance of this simulator.

Requirements

The PSWAP gate has to be implemented in the lightning.qubit C++ backend. For more information about the PSWAP gate, please refer to PennyLane doc.

• Add a method for the PSWAP gate to the lightning_qubit/gates/cpu_kernels/GateImplementationsLM.hpp file. Implement 2 variants:

  • naive application of the matrix PSWAP to the state vector
  • similar to applySWAP() in GateImplementationsLM.hpp

• Add a GateOpToMemberFuncPtr struct in lightning_qubit/gates/OpToMemberFuncPtr.hpp similar to the struct added for SWAP.

• Add C++ tests for the PSWAP gate to Test_GateImplementations_Nonparam.cpp. You can follow the same style used for other gates in this test file.

• Update the list of supported gates in pennylane_lightning/lightning_qubit/lightning_qubit.py.

• Add Python tests in tests/test_gates.py to check the correctness of this gate against PennyLane qml.PSWAP. (you may reuse existing tests, but add some if coverage requires it)

• Create a pull-request in the PennyLane Lightning repository and ensure to complete all the necessary steps outlined in the PR template. Please keep the checklist at the top of the PR description.

• Compare the performance of both PSWAP implementations for varying number of qubits and upload the results to the pull-request for further discussions.

• Mark the PR ready for review.

Don't hesitate to ask for clarification or raise any concerns regarding the issue. We'll be happy to discuss with you!