Although the classical descrip­tion of light as a wave phenomenon is rarely questioned, the physical origins of some optical effects are. A team of researchers at Tampere University have brought the discussion on one funda­mental wave effect, i.e., the debate around the anomalous behaviour of focused light waves, to the quantum domain. The researchers have been able to show that quantum waves behave signi­ficantly differently from their classical counterparts and can be used to increase the precision of distance measure­ments. Their findings also add to the discussion on physical origin of the anomalous focusing behaviour. 

“Interestingly, we started with an idea based on our earlier results and set out to structure quantum light for enhanced measure­ment precision. However, we then realised that the underlying physics of this application also contributes to the long debate about the origins of the Gouy phase anomaly of focused light fields.”, explains Robert Fickler, group leader of the Experi­mental Quantum Optics group at Tampere University.

Over the last decades, methods for struc­turing light fields down on the single photon level have vastly matured and led to a myriad of novel findings. In addition, a better of optics’ founda­tions has been achieved. However, the physical origin of why light behaves in such an unexpected way when going through a focus, the Gouy phase anomaly, is still often debated. This is despite its widespread use and importance in optical systems. The novelty of the current study is now to put the effect into the quantum domain.

“When developing the theory to describe our experi­mental results, we realized after a long debate that the Gouy phase for quantum light is not only different than the standard one, but its origin can be linked to another quantum effect. This is just like what was speculated in an earlier work,” adds Doctoral researcher Markus Hiekkamäki.

In the quantum domain, the anomalous behaviour is sped up when compared to classical light. As the Gouy phase behaviour can be used to determine the distance a beam of light has propa­gated, the speed up of the quantum Gouy phase could allow for an improvement in the precision of measuring distances.

With this new under­standing at hand, the researchers are planning to develop novel techniques to enhance their measurement abilities such that it will be possible to measure more complex beams of structured photons. The team expects that this will help them push forward the appli­cation of the observed effect, and poten­tially bring to light more differences between quantum and classical light fields. (Source: Tampere U.)

Reference: M. Hiekkamäki et al.: Observation of the quantum Gouy phase, Nat. Phot., online 6 October 2022; DOI: 10.1038/s41566-022-01077-w

Link: Photonics Laboratory, Physics Unit, Tampere University, Tampere, Finland