Add exact tensor network C++ backend to LT

.github/CHANGELOG.md

@@ -2,8 +2,8 @@
 
 ### New features since last release
 
-* Add native N-controlled gate/matrix operations and adjoint support to `lightning.kokkos`.
-  [(#950)](https://github.com/PennyLaneAI/pennylane-lightning/pull/950)
+* Add Exact Tensor Network C++ backend to `lightning.tensor`.
+  [(#977)](https://github.com/PennyLaneAI/pennylane-lightning/pull/977)
 
 * Add native N-controlled generators and adjoint support to `lightning.gpu`'s single-GPU backend.
   [(#970)](https://github.com/PennyLaneAI/pennylane-lightning/pull/970)
@@ -12,6 +12,9 @@
   [(#960)](https://github.com/PennyLaneAI/pennylane-lightning/pull/960)
   [(#999)](https://github.com/PennyLaneAI/pennylane-lightning/pull/999)
 
+* Add native N-controlled gate/matrix operations and adjoint support to `lightning.kokkos`.
+  [(#950)](https://github.com/PennyLaneAI/pennylane-lightning/pull/950)
+
 * Add native N-controlled gates support to `lightning.gpu`'s single-GPU backend.
   [(#938)](https://github.com/PennyLaneAI/pennylane-lightning/pull/938)
 
@@ -55,6 +58,9 @@
 * Update Kokkos version support to 4.4.1 and enable Lightning-Kokkos[CUDA] C++ tests on CI.
   [(#1000)](https://github.com/PennyLaneAI/pennylane-lightning/pull/1000)
 
+* Add C++ unit tests for Exact Tensor Network backends.
+  [(#998)](https://github.com/PennyLaneAI/pennylane-lightning/pull/998)
+
 * Add native BLAS support to the C++ layer via dynamic `scipy-openblas32` loading.
   [(#995)](https://github.com/PennyLaneAI/pennylane-lightning/pull/995)
 

CMakeLists.txt

@@ -72,9 +72,8 @@ endif()
 foreach(BACKEND ${PL_BACKEND})
     message(STATUS "PL_BACKEND: ${BACKEND}")
     if("${BACKEND}" STREQUAL "lightning_tensor")
-        set(PL_TENSOR_METHOD "mps" CACHE STRING "PennyLane LightningTensor MPS simulator.")
         set(PL_TENSOR_BACKEND "cutensornet" CACHE STRING "PennyLane LightningTensor backed by cutensornet")
-        set(PL_TENSOR "${PL_BACKEND}_${PL_TENSOR_METHOD}_${PL_TENSOR_BACKEND}")
+        set(PL_TENSOR "${PL_BACKEND}_${PL_TENSOR_BACKEND}")
     endif()
 endforeach()
 

pennylane_lightning/core/src/simulators/lightning_tensor/tncuda/CMakeLists.txt

@@ -17,7 +17,7 @@ include("${pennylane_lightning_SOURCE_DIR}/cmake/support_pltensortncuda.cmake")
 findCUDATK(lightning_external_libs)
 findCutensornet(lightning_external_libs)
 
-set(LTENSOR_MPS_FILES  MPSTNCuda.cpp CACHE INTERNAL "" FORCE)
+set(LTENSOR_MPS_FILES  MPSTNCuda.cpp ExactTNCuda.cpp MPOTNCuda.cpp CACHE INTERNAL "" FORCE)
 
 add_library(${PL_BACKEND} STATIC ${LTENSOR_MPS_FILES})
 

pennylane_lightning/core/src/simulators/lightning_tensor/tncuda/ExactTNCuda.cpp

@@ -0,0 +1,19 @@
+// Copyright 2024 Xanadu Quantum Technologies Inc.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "ExactTNCuda.hpp"
+
+// explicit instantiation
+template class Pennylane::LightningTensor::TNCuda::ExactTNCuda<float>;
+template class Pennylane::LightningTensor::TNCuda::ExactTNCuda<double>;

pennylane_lightning/core/src/simulators/lightning_tensor/tncuda/ExactTNCuda.hpp

@@ -0,0 +1,70 @@
+// Copyright 2024 Xanadu Quantum Technologies Inc.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/**
+ * @file MPSTNCuda.hpp
+ * MPS class with cuTensorNet backend. Note that current implementation only
+ * support the open boundary condition.
+ */
+
+#pragma once
+
+#include <vector>
+
+#include "DevTag.hpp"
+#include "TNCuda.hpp"
+#include "TNCudaBase.hpp"
+#include "TensorCuda.hpp"
+#include "Util.hpp"
+
+/// @cond DEV
+namespace {
+using namespace Pennylane::LightningGPU;
+using namespace Pennylane::LightningTensor::TNCuda;
+using namespace Pennylane::LightningTensor::TNCuda::Util;
+} // namespace
+/// @endcond
+
+namespace Pennylane::LightningTensor::TNCuda {
+
+/**
+ * @brief Managed memory Exact Tensor Network class using cutensornet high-level
+ * APIs.
+ *
+ * @tparam Precision Floating-point precision type.
+ */
+
+template <class Precision>
+class ExactTNCuda final : public TNCuda<Precision, ExactTNCuda<Precision>> {
+  private:
+    using BaseType = TNCuda<Precision, ExactTNCuda>;
+
+  public:
+    constexpr static auto method = "exacttn";
+
+    using CFP_t = decltype(cuUtil::getCudaType(Precision{}));
+    using ComplexT = std::complex<Precision>;
+    using PrecisionT = Precision;
+
+  public:
+    ExactTNCuda() = delete;
+
+    explicit ExactTNCuda(std::size_t numQubits) : BaseType(numQubits) {}
+
+    explicit ExactTNCuda(std::size_t numQubits, DevTag<int> dev_tag)
+        : BaseType(numQubits, dev_tag) {}
+
+    ~ExactTNCuda() = default;
+};
+} // namespace Pennylane::LightningTensor::TNCuda