Retail companies are increasingly turning to robots to gain a competitive edge. Several large store chains in the US, including Walmart, Target and Lowe's, are using robots to do jobs once carried out by employees. It's part of an effort by traditional brick-and-mortar stores to improve the experience of their shoppers in the face of competition from online retailers. But robotics experts warn the technology still has a long way to go. Steve Mort reports from San Francisco.