Tying quantum knots

As topologically stable objects in field theories, knots have been put forward to explain various persistent phenomena in systems ranging from atoms and molecules to cosmic textures in the universe. Recent experiments have reported the observation of knots in different classical contexts. However, no experimental observation of knots has yet been reported in quantum matter. Here we demonstrate the experimental creation and detection of knot solitons in the order parameter of a spinor Bose-Einstein condensate. The observed texture corresponds to a topologically nontrivial element of the third homotopy group and exhibits the celebrated Hopf fibration, which unites many seemingly unrelated physical phenomena. Our work calls for future studies of the stability and dynamics knot solitons in the quantum regime.