First glimpse inside a macroscopic quantum state

In a recent study published in Physical Review Letters and highlighted by the magazine Science News, the research group led by ICREA Prof at ICFO Morgan Mitchell has detected, for the first time, entanglement among individual photon pairs in a beam of squeezed light.

Quantum entanglement is always related to the microscopic world, but it also has striking macroscopic effects, such as the squeezing of light or superconductivity, a physical phenomenon that allows high-speed trains to levitate. Squeezed light is not physically compressed but it is it manipulated in such a way that one of its properties is super well defined, for example its polarization. Compared with normal light, laser light, composed of independent photons, has an extremely small but nonzero polarization uncertainty. This uncertainty or "quantum noise" is directly linked to the existence of photons, the smallest energy quanta of light. Now, squeezed light has an uncertainty that is farther below this level. Therefore, in optical communications, squeezed light can help transmit much weaker signals with the same signal to noise ratio and the same light power. It can also be used to distribute secret keys to two distant parties through quantum cryptography.