Bravyi-Kitaev implementation (NVIDIA#35)

This is the implementation of the Bravyi-Kitaev fermionic transformation. The was largely a translation from the OpenFermion BK python implementation into C++:

https://github.com/quantumlib/OpenFermion/blob/master/src/openfermion/transforms/opconversions/bravyi_kitaev.py

libs/solvers/include/cudaq/solvers/operators/molecule/fermion_compilers/bravyi_kitaev.h

@@ -0,0 +1,29 @@
+/*******************************************************************************
+ * Copyright (c) 2024 NVIDIA Corporation & Affiliates.                         *
+ * All rights reserved.                                                        *
+ *                                                                             *
+ * This source code and the accompanying materials are made available under    *
+ * the terms of the Apache License 2.0 which accompanies this distribution.    *
+ ******************************************************************************/
+#pragma once
+
+#include "cudaq/solvers/operators/molecule/fermion_compiler.h"
+
+namespace cudaq::solvers {
+
+/// @brief Helper function used by the Bravyi-Kitaev transformation.
+cudaq::spin_op seeley_richard_love(std::size_t i, std::size_t j,
+                                   std::complex<double> coef, int n_qubits);
+
+/// @brief Map fermionic operators to spin operators via the
+/// Bravyi-Kitaev transformation.
+class bravyi_kitaev : public fermion_compiler {
+public:
+  cudaq::spin_op generate(const double constant, const cudaqx::tensor<> &hpq,
+                          const cudaqx::tensor<> &hpqrs,
+                          const cudaqx::heterogeneous_map &options) override;
+
+  CUDAQ_EXTENSION_CREATOR_FUNCTION(fermion_compiler, bravyi_kitaev)
+};
+CUDAQ_REGISTER_TYPE(bravyi_kitaev)
+} // namespace cudaq::solvers

libs/solvers/lib/CMakeLists.txt

@@ -15,6 +15,7 @@ add_library(cudaq-solvers SHARED
   operators/molecule/drivers/pyscf_driver.cpp
   operators/molecule/fermion_compilers/fermion_compiler.cpp
   operators/molecule/fermion_compilers/jordan_wigner.cpp
+  operators/molecule/fermion_compilers/bravyi_kitaev.cpp
   operators/molecule/molecule.cpp
   operators/graph/max_cut.cpp
   operators/graph/clique.cpp