Making MPI not required

CMakeLists.txt

@@ -54,7 +54,7 @@ set(CppMicroServices_INCLUDE_DIR ${CppMicroServices_BASE_DIR}/include/cppmicrose
 set(CppMicroServices_LIBRARY_DIR ${CppMicroServices_BASE_DIR}/lib)
 
 # We require MPI
-find_package(MPI REQUIRED)
+find_package(MPI)
 
 # We require Boost
 find_package(Boost COMPONENTS system program_options filesystem python REQUIRED)

quantum/gate/utils/SimulatedQubits.hpp

@@ -35,6 +35,7 @@
 #include <complex>
 #include "Tensor.hpp"
 #include <bitset>
+#include <unsupported/Eigen/KroneckerProduct>
 
 namespace xacc {
 
@@ -59,6 +60,8 @@ protected:
 	 */
 	QubitState bufferState;
 
+	Eigen::MatrixXcd Z;
+
 public:
 
 	/**
@@ -70,6 +73,17 @@ public:
 	SimulatedQubits(const std::string& str, const int N) :
 			AcceleratorBuffer(str, N), bufferState((int) std::pow(2, N)) {
 		bufferState(0) = 1.0;
+		Eigen::MatrixXcd z(2,2), temp;
+		z << 1, 0, 0, -1;
+
+		temp = z;
+		for (int i = 0; i < N-1; i++) {
+			temp = Eigen::kroneckerProduct(z, temp).eval();
+		}
+
+		Z = temp;
+
+		assert(temp.rows() == std::pow(2,N));
 	}
 
 	/**
@@ -126,6 +140,12 @@ public:
 		bufferState = st;
 	}
 
+	virtual const double getExpectationValueZ() {
+		auto data = bufferState.createReference().first.data();
+		Eigen::VectorXcd state = Eigen::Map<Eigen::VectorXcd, Eigen::Unaligned>(data, bufferState.size());
+		return std::real(state.transpose().dot(Z * state));
+	}
+
 	/**
 	 * Print the state to the provided output stream.
 	 *

xacc/accelerator/AcceleratorBuffer.hpp

@@ -172,7 +172,7 @@ public:
 	 *
 	 * @return expVal The expectation value
 	 */
-	virtual const double getExpectationValueZ() const {
+	virtual const double getExpectationValueZ() {
 		std::stringstream ss;
 		double aver = 0.;
 		long n_measurements = measurements.size();