The speed of light is the cosmic speed limit, according to physicists' best understanding: no information can be carried at a greater rate, no matter what method is used. But an analogous speed limit seems to exist within materials, where the interactions between particles are typically very short-range and motion is far slower than light-speed. A new set of experiments and simulations by Marc Cheneau and colleagues have identified this maximum velocity, which has implications for quantum entanglement and quantum computations.
In non-relativistic systems, where particle speeds are much less than the speed of light, interactions still occur very quickly, and they often involve lots of particles. As a result, measuring the speed of interactions within materials has been difficult. The theoretical speed limit is set by the Lieb-Robinson bound, which describes how a change in one part of a system propagates through the rest of the material. In this new study, the Lieb-Robinson bound was quantified experimentally for the first time, using a real quantum gas.
Within a lattice (such as a crystalline solid), a particle primarily interacts with its nearest neighbors. For example, the spin of an electron in a magnetically susceptible material depends mainly on the orientation of the spins of its neighbors on each side. Flipping one electron's spin will affect the electrons nearest to it.
But the effect also propagates throughout the rest of the material—other spins may themselves flip, or experience a change in energy resulting from the original electron's behavior. These longer range interactions can be swamped out by extraneous effects, like lattice vibrations. But it's possible to register them in very cold systems, as lattice vibrations die out near absolute zero.
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.
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