adding VQE example, not adding to CMakeLists yet

examples/quantum/gate/vqe.cpp

+/***********************************************************************************
+ * Copyright (c) 2016, UT-Battelle
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *   * Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *   * Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in the
+ *     documentation and/or other materials provided with the distribution.
+ *   * Neither the name of the xacc nor the
+ *     names of its contributors may be used to endorse or promote products
+ *     derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> BE LIABLE FOR ANY
+ * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Contributors:
+ *   Initial API and implementation - Alex McCaskey
+ *   Extension to VQE algorithm - 29 MAR 2017
+ *
+ **********************************************************************************/
+#include "XACC.hpp"
+#include "FireTensorAccelerator.hpp"
+
+// Scaffold Kernel for Variational Quantum Eigensolver
+const std::string src("__qpu__ vqe(qbit qreg, int theta) {\n"
+        "   //Declare variables to store values\n"
+	"   cbit creg[1];\n"
+        "   \n"
+        "   // Declare operator to minimize\n"
+        "   // FIXME: This should be a string of Pauli operators\n"
+        "   \n"
+	"   // Initialize register using current paramterization \n"
+        "   // FIXME: This should depend on theta \n" 
+	"   X(qreg[0]);\n"
+        "   \n"
+        "   // Apply operator to minimize to current register state\n"
+	"   H(qreg[1]);\n"
+        "   \n"
+        "   // Read out result from register\n"
+	"   creg[0] = MeasZ(qreg[0]);\n"
+	"}\n");
+
+int main (int argc, char** argv) {
+
+	// Create a convenient alias for our simulator...
+	using CircuitSimulator = xacc::quantum::FireTensorAccelerator<6>;
+
+	// Create a reference to the 6 qubit simulation Accelerator
+	auto qpu = std::make_shared<CircuitSimulator>();
+
+        // Loop over QPU's and allocate resources for each thread
+	// Allocate 3 qubits, give them a unique identifier...
+	auto qubitReg = qpu->createBuffer("qreg", 3);
+	using QubitRegisterType = decltype(qubitReg);
+
+	// Construct a new XACC Program
+	xacc::Program quantumProgram(qpu, src);
+
+	// Build the program using Scaffold comipler
+	// and output the Graph Intermediate Representation
+	quantumProgram.build("--compiler scaffold "
+			"--writeIR vqe.xir");
+
+	// Retrieve the created kernel. It takes a 
+	// qubit register as input
+	auto vqe = quantumProgram.getKernel<QubitRegisterType>("vqe");
+
+        // Declare range of parameters to search for minimization of expectation value
+        float theta = 0.0;
+        float dtheta = 3.14159265353/10.0;
+        for(int j = 0; theta < 10; j++ ){
+
+          // Set value of theta
+          theta = j*dtheta;
+
+	  // Execute the kernel with the qubit register
+          // Pass current initialization parameters
+	  vqe(qubitReg, theta);
+
+          // Compose current value from last measurement results
+          // FIXME: Need to extract measurements values from QPU to compute
+          float current_value = 0.;
+
+          // Compare output of last instance with current best
+          // FIXME: This is part of the brute force search method; need to try better methods
+          if(current_minimum > current_value) {
+            current_minimum = current_value;
+          };
+
+	  // Pretty print the resultant state
+	  qubitReg->printBufferState(std::cout);
+
+        }; // theta
+
+        printf("Minimized value = %f at theta = %f\n", current_minimum, theta);
+
+	return 0;
+
+} // end VQE example
+