In the field of quantum metrology, scientists are developing novel measurement schemes that benefit from quantum features and are more precise and sensitive than classical conven­tional methods. The team of researchers from Tampere University, Finland, and the National Research Council of Canada has now shown how a simple and powerful technique – two-photon N00N states – can be used to create spatially structured quantum states of light that can go beyond the classical limit in rotation esti­mation. 

“Our experimental results demons­trate a simple but powerful way of custom-tailoring two-photon quantum states and holds promise for applications that can achieve high measurement preci­sions. The simplicity of our method opens a path to creating a measurement system that beats the classical estimation limit with current techno­logies”, explains Markus Hiekkamäki. The method utilizes a fundamental quantum feature, i.e., the interference between two photons, which is often termed photon bunching. In contrast to the more common photon bunching into the same physical path, the novel scheme leads to a bunching into the same spatial structure. 

“In our case, the quantum inter­ference results in an entangled state of two photons. Because of the quantum nature of the realized state, the entangled photon pair gives a better measure­ment precision when compared to the same spatial shape imprinted on a similar amount of single photons or laser light. Using a counter-intuitive quantum response, we were able to show that it will be possible to achieve measurement precisions at the absolute quantum limit “, says Robert Fickler, leader of the Experi­mental Quantum Optics group at Tampere University.

Besides rotational measure­ments, the method allows the generation of a large variety of different quantum states for trans­verse-spatial modes. Hence, it could also be utilized in measure­ments of many different types of systems as well as in funda­mental tests of multi-photon quantum states of light. After demons­trating the advantage in rotational estimation, the researchers are now planning on using the method to shed new light on another funda­mental property of waves called the Gouy phase. In addition, they study how it could be extended into quantum-enhanced measure­ment schemes in multiple degrees of freedom. (Source: U. Tampere)

Reference: M. Hiekkamäki et al.: Photonic Angular Superresolution Using Twisted N00N States, Phys. Rev. Lett. 127, 263601 (2021); DOI: 10.1103/PhysRevLett.127.263601

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