Completed single-qubit gate merge optimization pass

Signed-off-by: Thien Nguyen <nguyentm@ornl.gov>

quantum/plugins/optimizers/gate_merge/GateMergeOptimizer.cpp

@@ -32,15 +32,35 @@ void MergeSingleQubitGatesOptimizer::apply(std::shared_ptr<CompositeInstruction>
             auto fuser = xacc::getService<xacc::quantum::GateFuser>("default");
             fuser->initialize(tmpKernel);
             const Eigen::Matrix2cd uMat = fuser->calcFusedGate(1);
-            std::cout << "Unitary matrix: \n" << uMat << "\n";
             auto zyz = std::dynamic_pointer_cast<quantum::Circuit>(xacc::getService<Instruction>("z-y-z"));
             const bool expandOk = zyz->expand({ 
                 std::make_pair("unitary", uMat)
             });
-            
-            assert(zyz->nInstructions() == 3);
-            std::cout << "Decomposed: \n" << zyz->toString() << "\n";
-            // TODO: further optimize this sequence: remove trivial rotation (0, 2*pi, etc.)
+
+            // Optimized decomposed sequence:
+            const auto nbInstructionsAfter = zyz->nInstructions();
+            // A simplified sequence was found.
+            if (nbInstructionsAfter < sequence.size())
+            {
+                // Rewrite:
+                const size_t bitIdx = program->getInstruction(sequence[0])->bits()[0];
+                // Disable to remove:
+                const auto programLengthBefore = program->nInstructions();
+                for (const auto& instIdx: sequence)
+                {
+                    auto instrPtr = program->getInstruction(instIdx);
+                    instrPtr->disable();
+                }
+                program->removeDisabled();
+                const auto locationToInsert = sequence[0];
+                for (auto& newInst: zyz->getInstructions())
+                {
+                    newInst->setBits({bitIdx});
+                    program->insertInstruction(locationToInsert, newInst->clone());
+                }
+                const auto programLengthAfter = program->nInstructions();
+                assert(programLengthAfter < programLengthBefore);
+            }
         }
     }
 }

quantum/plugins/optimizers/gate_merge/tests/GateMergingTester.cpp

@@ -8,6 +8,7 @@
 TEST(GateMergingTester, checkSingleQubitSimple) 
 {
     auto c = xacc::getService<xacc::Compiler>("xasm");
+    // This is identity: H-X-H is equal to Z => Z-Z = I
     auto f = c->compile(R"(__qpu__ void test1(qbit q) {
         Z(q[0]);
         H(q[0]);
@@ -17,6 +18,8 @@ TEST(GateMergingTester, checkSingleQubitSimple)
 
     auto opt = xacc::getService<xacc::IRTransformation>("single-qubit-gate-merging");
     opt->apply(f, nullptr);
+    // No instruction after optimization
+    EXPECT_EQ(f->nInstructions(), 0);
 }
 
 TEST(GateMergingTester, checkSingleQubitStopAtTwoQubitGate) 
@@ -32,6 +35,12 @@ TEST(GateMergingTester, checkSingleQubitStopAtTwoQubitGate)
 
     auto opt = xacc::getService<xacc::IRTransformation>("single-qubit-gate-merging");
     opt->apply(f, nullptr);
+    // Becomes: Z (Rz(pi)) - CNOT - H
+    EXPECT_EQ(f->nInstructions(), 3);
+    EXPECT_EQ(f->getInstruction(0)->name(), "Rz");
+    EXPECT_NEAR(f->getInstruction(0)->getParameter(0).as<double>(), M_PI, 1e-6);
+    EXPECT_EQ(f->getInstruction(1)->name(), "CNOT");
+    EXPECT_EQ(f->getInstruction(2)->name(), "H");
 }
 
 int main(int argc, char **argv) {