Nothing travels faster than light in a vacuum, obviously. And while scientists knew that quantum particles interact with one another at a slower speed, they had trouble measuring the speed at which that happens. Until now that is.
According to an article published by Nature Magazine (and told through Ars Technica), an experiment run by Marc Cheneau was able to prove the velocity ceiling for quantum particles.
In the experiment described in Nature, the researchers begin with a simple one-dimensional quantum gas consisting of atoms in an optical lattice. This type of trap is made by crossing laser beams so that they interfere and create a standing-wave pattern; by adjusting the power output of the lasers, the trap can be made deeper or shallower. Optical lattices are much simpler than crystal lattices, as the atoms are not involved in chemical bonding.
By rapidly increasing the depth of the optical lattice, the researchers create what is known as a quenched system. You can think of this as analogous to plunging a hot forged piece of metal into water to cool it quickly. Before the change, the atoms are in equilibrium; after the change, they are highly excited.
As in many other strongly interacting systems, these excitations take the form of quasiparticles that can travel through the lattice. neighbouring quasiparticles begin with their quantum states entangled, but propagate rapidly in opposite directions down the lattice. As in all entangled systems, the states of the quasiparticles remain correlated even as the separation between them grows. By measuring the distance between the excitations as a function of time, the real velocity of the quasiparticles' propagation can be measured. As measured, it is more than twice the speed of sound in the system.
Picture it this way: Imagine that 10 quantum particles were dominoes, and they were standing in a straight line. You couldn't knock over the first one and expect the 10th one to instantaneously fall over. The reaction would have to travel through the other eight first over an elapsed time.
What does that mean in the scheme of things? Well it serves as an important confirmation for those working in quantum field theory, who are attempting to prove that elements of special relativity do, in fact, operate within the realm of quantum physics. As Ars Technica points out, the velocity could not have been determined using calculations that exist solely within the quantum realm. And namely it gives weight to the speed limit theory set forth by the "Lieb-Robinson bound". Having this information about the max speed of quantum interactions within a given material will help them model more accurate experiments by which they can prove their hypothesis. [Ars Technica]