A new quantum ripple-carry addition circuit
  We present a new linear-depth ripple-carry quantum addition circuit. Previous addition circuits required linearly many ancillary qubits; our new adder uses only a single ancillary qubit. Also, our circuit has lower depth and fewer gates than previous ripple-carry adders. 
Steven A. Cuccaro, Thomas G. Draper, Samuel A. Kutin, and David Petrie   Moulton