Since June 2012 Google recommends responsive design to ensure the consistency of user experience across all devices.
The February 26 Google announced on its blog:
“From April 21st, we will be extending the use of mobile compatibility as a ranking criterion. This change will affect mobile searches in all languages around the world and will have a significant impact on our search results. This will make it easier for users to get quality, relevant results that are optimized for their device. … Read the rest