I remember a paper/ video from ETH Zurich demonstrating this sometime around 2015. There's nothing particularly difficult about it if your motors and controllers are sufficiently overspecced and you're willing to let the aircraft start spinning (you sacrifice yaw control).

Edit: here it is, 2013 not 15. https://ethz.ch/en/news-and-events/eth-news/news/2013/12/new...