A global team of researchers from Leibniz University Hannover (Germany) and the University of Strathclyde in Glasgow (United Kingdom) has disproved a previously held assumption concerning the impact of multiphoton components in interference effects of thermal fields (e.g. sunlight) and parametric single photons (generated in non-linear crystals). “We experimentally proved that the interference effect between thermal light and parametric single photons also results in quantum interference with the background field. For that reason, the background cannot simply be neglected and subtracted from calculations, as has been the case to date,” says Prof. Dr. Michael Kues, Head of the Institute of Photonics and member of the Board of the PhoenixD Cluster of Excellence at Leibniz University Hannover.
The leading scientist was PhD student Anahita Khodadad Kashi, who performs research on photonic quantum information processing on the Institute of Photonics. She investigated how the visibility of the so-called Hong-Ou-Mandel effect, a quantum interference effect, is affected by multiphoton contamination. “With our experiment, we now have disproved the previously valid assumption that multiphoton components would only impair visibility and might subsequently be subtracted within the calculation,” says Khodadad Kashi and continues: “We’ve discovered a brand new fundamental characteristic that was not considered in previous calculations. Our newly developed model can predict the quantum interference and we will measure this effect in an experiment.”
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The scientists got here across their discovery while carrying out an experiment within the laser laboratory. They obtained a negative result after they initially followed the unique calculation method. “However the result would have been physically not possible,” says Khodadad Kashi. Together, the team began troubleshooting the experimental setup and the calculation model.
“When an experiment seems very different from what is anticipated, scientists start questioning previous assumptions and look for brand new explanations,” says Kues. They jointly developed their recent theory of quantum interference of thermal fields with parametric single photons. Quantum researcher Lucia Caspani from the University of Strathclyde in Glasgow was the primary to check the approach. As the following step, Khodadad Kashi presented her theory and the experimental results at international conferences, including Photonics West in San Francisco, the world’s largest specialist conference for optics and photonics, attracting around 22,000 participants. There, she discussed her model with other scientists and received confirmation of her results. The journal Physical Review Letters has now published the team’s research.
With the brand new theory and the experimental verification, Kues’ team has made a crucial contribution to a greater understanding of quantum phenomena. “The findings may very well be essential for quantum key distribution, which is obligatory for secure communications in the longer term, specifically how quantum interference effects are interpreted for the generation of secret keys,” says Khodadad Kashi. Nonetheless, many questions remain unanswered, says Kues: “Little research has been done into multiphoton effects, so quite a lot of work continues to be needed.”
The research was supported by the European Research Council inside the Starting-Grant-Project QFreC. Prof. Dr. Michael Kues is head of the Institute of Photonics and a board member of the Cluster of Excellence PhoenixD: Photonics, Optics, and Engineering — Innovation across Disciplines at Leibniz University Hannover, Germany. The PhoenixD research cluster comprises around 120 scientists working on novel integrated optics. The German Research Foundation (DFG) funds PhoenixD with about 52 million euros from 2019 to 2025.
