✨ Implement two-qubit gate decomposition pass

CMakeLists.txt

@@ -23,6 +23,14 @@ include(PreventInSourceBuilds)
 include(PackageAddTest)
 include(Cache)
 include(AddMQTCoreLibrary)
+include(FetchContent)
+
+FetchContent_Declare(
+  Eigen
+  GIT_REPOSITORY https://gitlab.com/libeigen/eigen.git
+  GIT_TAG 3.4.1
+  GIT_SHALLOW TRUE)
+FetchContent_MakeAvailable(Eigen)
 
 option(BUILD_MQT_CORE_BINDINGS "Build the MQT Core Python bindings" OFF)
 if(BUILD_MQT_CORE_BINDINGS)

mlir/include/mlir/Dialect/MQTOpt/Transforms/Passes.h

@@ -35,6 +35,7 @@ enum class RoutingMethod : std::uint8_t { Naive, AStar };
 #include "mlir/Dialect/MQTOpt/Transforms/Passes.h.inc" // IWYU pragma: export
 
 void populateGateEliminationPatterns(mlir::RewritePatternSet& patterns);
+void populateGateDecompositionPatterns(mlir::RewritePatternSet& patterns);
 void populateMergeRotationGatesPatterns(mlir::RewritePatternSet& patterns);
 void populateSwapReconstructionAndElisionPatterns(
     mlir::RewritePatternSet& patterns);

mlir/include/mlir/Dialect/MQTOpt/Transforms/Passes.td

@@ -43,6 +43,15 @@ def GateElimination : Pass<"gate-elimination", "mlir::ModuleOp"> {
   }];
 }
 
+def GateDecomposition : Pass<"gate-decomposition", "mlir::ModuleOp"> {
+  let dependentDialects = [ "mlir::arith::ArithDialect", "mqt::ir::opt::MQTOptDialect" ];
+  let summary = "This pass performs various gate decompositions to translate quantum gates being used.";
+  let description = [{
+    Decomposes series of operations that operate on up to two qubits into a sequence of up to three
+    two-qubit basis gates and single-qubit operations.
+  }];
+}
+
 def MergeRotationGates : Pass<"merge-rotation-gates", "mlir::ModuleOp"> {
   let summary = "This pass searches for consecutive applications of rotation gates that can be merged.";
   let description = [{

mlir/lib/Dialect/MQTOpt/Transforms/CMakeLists.txt

@@ -7,7 +7,7 @@
 # Licensed under the MIT License
 
 get_property(dialect_libs GLOBAL PROPERTY MLIR_DIALECT_LIBS)
-set(LIBRARIES ${dialect_libs} MQT::CoreIR)
+set(LIBRARIES ${dialect_libs} MQT::CoreIR Eigen3::Eigen)
 add_compile_options(-fexceptions)
 
 file(GLOB_RECURSE TRANSFORMS_SOURCES *.cpp)

mlir/lib/Dialect/MQTOpt/Transforms/GateDecomposition.cpp

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2023 - 2025 Chair for Design Automation, TUM
+ * Copyright (c) 2025 Munich Quantum Software Company GmbH
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ * Licensed under the MIT License
+ */
+
+#include "mlir/Dialect/MQTOpt/Transforms/Passes.h"
+
+#include <mlir/IR/PatternMatch.h>
+#include <mlir/Support/LLVM.h>
+#include <mlir/Transforms/GreedyPatternRewriteDriver.h>
+#include <utility>
+
+namespace mqt::ir::opt {
+
+#define GEN_PASS_DEF_GATEDECOMPOSITION
+#include "mlir/Dialect/MQTOpt/Transforms/Passes.h.inc"
+
+/**
+ * @brief This pass attempts to collect as many operations as possible into a
+ *        4x4 unitary matrix and then decompose it into 1q rotations and 2q
+ *        basis gates.
+ */
+struct GateDecomposition final
+    : impl::GateDecompositionBase<GateDecomposition> {
+
+  void runOnOperation() override {
+    // Get the current operation being operated on.
+    auto op = getOperation();
+    auto* ctx = &getContext();
+
+    // Define the set of patterns to use.
+    mlir::RewritePatternSet patterns(ctx);
+    populateGateDecompositionPatterns(patterns);
+
+    // Configure greedy driver
+    mlir::GreedyRewriteConfig config;
+    config.setUseTopDownTraversal(true);
+
+    // Apply patterns in an iterative and greedy manner.
+    if (mlir::failed(
+            mlir::applyPatternsGreedily(op, std::move(patterns), config))) {
+      signalPassFailure();
+    }
+  }
+};
+
+} // namespace mqt::ir::opt