At the Quantum Research Center’s (QRC) Quantum Communications lab (QComms), researchers are building a new generation of secure communications devices that harness the quantum properties of light. Specifically, these devices work by manipulating quantum entanglement applied to pairs of photons, or light particles. Recently, the QRC team achieved an important technological milestone, demonstrating the violation of a Bell inequality using pairs of polarized entangled photons.
Since its discovery, quantum entanglement has evolved from a philosophical curiosity to become a valuable engineering resource. Dr. James Grieve, Director of QComms, explains that physicists first became interested in the fundamentals of quantum entanglement as they worked out the mechanics of the quantum world in the early 20th century. They found that to properly describe subatomic systems, they had to draw on what is known as a quantum state. A quantum state describes the probability of measuring a certain property of a particle, like its position, momentum, color, or spin. Physicists were surprised to discover that these properties can exist in apparently contradictory combinations, known as quantum superpositions. Even more surprising was the observation that these superpositions can span multiple particles, which share a common “inseparable” quantum state. They called these phenomena “entanglement”.
When particles exist within an entangled state, observations recorded on one of them will automatically provide information about the others, regardless of the distance between them. Any action to one particle will immediately impact the others in the entangled system. QRC’s researchers are working to harness these effects in ultra-secure key distribution schemes to enhance secure communication.
Bell’s theory addressed the non-locality of entangled particles, by studying the correlations observable in the outcomes of distant experiments, when they are described with any local references. Bell recognized that quantum mechanics predicts outcomes that violate this inequality, with important consequences. When experiments first recorded evidence of these violations in the 70s and 80s, it became clear that the physics of our world must be somehow non-local.
Today, violation of a Bell inequality serves as an important benchmark for an entangled photon source, providing a kind of “quantum health check”. With this latest milestone, the QRC team can ensure the application-readiness of their devices.