A quantum frequency is the rate at which things vibrate. It can be measured using a very sensitive sensor that looks for vibrations in the electromagnetic spectrum. This sensor could allow the US military to pick up communication signals that cover a wider range of wavelengths than current technology can with a single antenna.
The sensor works by harnessing the strange rules of quantum physics. In classical physics, particles like electrons have definite states, but in quantum mechanics they can exist in a superposition of many different energy states at once. Detecting the vibration of those particles can reveal their frequencies, which in turn can be used to calculate a signal’s frequency.
This new sensor uses a technique called quantum frequency conversion (QFC), which produces a stream of single photons that can be detected with high accuracy. The team combined a single semiconductor quantum dot emitting photons in the visible part of the spectrum with a strong classical pump beam in a nonlinear crystal to create a directional flow of single-photons. The researchers measured the photons’ quantum nature with time-correlated single photon counting, which reveals how much of the individual photons have been ‘converted’ to a new frequency state.
Traditionally, QFC has struggled with noise in the form of spectrally broadened fluctuations. To combat this, the UNSW scientists designed a special three-stage filter to narrow the noise bandwidth to 12 nm. The first stage involved two etalons, the BF2 and a FBG, which work together to reduce the noise bandwidth by 39%.