Remove redundant gates after depth-optimal synthesis. (#336)

## Description

Remove redundant gates in a post-processing step after depth-optimal
synthesis

Fixes #325

## Checklist:

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- [x] The pull request only contains commits that are related to it.
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include/cliffordsynthesis/CliffordSynthesizer.hpp

@@ -152,6 +152,7 @@ class CliffordSynthesizer {
                        const Configuration& config);
   static void updateResults(const Configuration& config,
                             const Results& newResults, Results& currentResults);
+  void        removeRedundantGates();
 };
 
 } // namespace cs

pyproject.toml

@@ -106,7 +106,7 @@ testpaths = ["test/python"]
 addopts = ["-ra", "--strict-markers", "--strict-config", "--showlocals"]
 log_cli_level = "INFO"
 xfail_strict = true
-filterwarnings = ["error"]
+filterwarnings = ["error", 'ignore:Conversion.*to a scalar is deprecated.*:DeprecationWarning:qiskit:']
 
 [tool.coverage.run]
 source = ["mqt.qmap"]

src/cliffordsynthesis/CliffordSynthesizer.cpp

@@ -6,11 +6,14 @@
 #include "cliffordsynthesis/CliffordSynthesizer.hpp"
 
 #include "LogicTerm/Logic.hpp"
+#include "QuantumComputation.hpp"
+#include "cliffordsynthesis/Tableau.hpp"
 #include "utils/logging.hpp"
 
 #include <chrono>
 #include <fstream>
 #include <future>
+#include <memory>
 #include <thread>
 
 namespace cs {
@@ -226,6 +229,12 @@ void CliffordSynthesizer::depthOptimalSynthesis(
     // fixed depth limit and the goal to minimize the number of gates.
     minimizeGatesFixedDepth(config);
   }
+
+  if (!initialTableau.hasDestabilizers()) {
+    // If destabilizers aren't considered, the synthesis might include gates
+    // that have no impact on the final tableau, so we can remove them
+    removeRedundantGates();
+  }
 }
 
 void CliffordSynthesizer::minimizeGatesFixedDepth(EncoderConfig config) {
@@ -518,4 +527,22 @@ CliffordSynthesizer::synthesizeSubcircuit(
   synth.initResultCircuitFromResults();
   return synth.resultCircuit;
 }
+
+void CliffordSynthesizer::removeRedundantGates() {
+  Tableau prev = initialTableau;
+  Tableau curr = initialTableau;
+  initResultCircuitFromResults();
+  qc::QuantumComputation reducedResult(resultCircuit->getNqubits());
+
+  for (auto& gate : *resultCircuit) {
+    curr.applyGate(gate.get());
+    if (prev != curr) {
+      prev.applyGate(gate.get());
+      reducedResult.emplace_back(std::move(gate));
+    }
+  }
+
+  results.setResultCircuit(reducedResult);
+  results.setSingleQubitGates(reducedResult.getNsingleQubitOps());
+}
 } // namespace cs