In everyday life, events happen in a fixed sequence: one thing happens before another. In quantum physics, however, nature can behave in surprising ways. One such phenomenon is indefinite causal order, where the order of two events is not fixed but exists in a quantum superposition. Rather than occurring in one sequence or the other, both orders can effectively happen at the same time until the system is measured. This unusual feature has attracted growing interest because it can enable tasks that are impossible in conventional physics.
Dr. Lorenzo M. Procopio, researcher at the Institute of Photonic Quantum Systems (PhoQS) in the Mesoscopic Quantum Optics group of Prof. Dr. Tim Bartley, and colleagues show how indefinite causal order can be used in a cavity quantum electrodynamics system to create strong quantum links, known as entanglement, between two distant light fields that never directly interact. They demonstrate that this quantum effect allows light to be exchanged coherently between the cavities without changing the state of the atom that mediates the interaction and even enables the creation of highly nonclassical states of light called NOON states. These findings reveal indefinite causal order as a powerful new tool for controlling light–matter interactions and advancing future quantum technologies.
The paper was published in the journal “Physical Review Research” and can be found under the following link: https://journals.aps.org/prresearch/abstract/10.1103/wgr6-1g5m
Background information on indefinite causal order: https://physicsworld.com/a/indefinite-causal-order-how-quantum-physics-is-challenging-our-understanding-of-cause-and-effect/
