In a groundbreaking development, physicists have unveiled a novel type of quantum entanglement, termed as frequency-domain photon number-path entanglement. This pioneering discovery revolves around a cutting-edge device known as a frequency beam splitter. Remarkably, this device can modify the frequency of photons, achieving a remarkable 50% rate of success.

Historically, the focus has been on spatial-domain photon number-path entanglement, which has been instrumental in advancing quantum metrology and information science.

The principle behind this is the formation of NOON states, a unique arrangement of photons that are either collectively in one path or another. This arrangement has facilitated breakthroughs such as super-resolution imaging beyond conventional boundaries, improved quantum sensors, and the creation of quantum computing algorithms for highly phase-sensitive tasks.

Recently published in the journal Light: Science & Applications, a research team led by Professor Heedeuk Shin from the Physics Department at Pohang University of Science and Technology in Korea, has successfully engineered entangled states within the frequency domain. This achievement parallels the spatial-domain NOON states but introduces a transformative twist: photons are not split between paths but are instead distributed across two distinct frequencies.

This innovation has culminated in the formation of a two-photon NOON state within a single-mode fiber, demonstrating the capability for two-photon interference with twice the resolution compared to its single-photon equivalent. This signifies a leap in stability and opens up new possibilities for future technological applications.