"We have figured out a way of creating waves that can force a floating object to move against the direction of the wave," said Dr Horst Punzmann, from the Research School of Physics and Engineering, who led the project.
http://phys.org/news/2014-08-physicists-tractor.html#jCp
http://phys.org/news/2014-08-physicists-tractor.html#jCp