Hong-ou-mandel Interferometry Advances with Meter-Scale Sensing Via Frequency-Entangled Photons

Hong-Ou-Mandel interferometry, a technique exploiting the quantum properties of light, stands to gain significantly from the use of entangled photons, particularly when leveraging frequency entanglement to enhance sensor sensitivity. Yen-Ju Chen, Sheng-Hsuan Huang, and Thomas Dirmeier, working at Friedrich-Alexander-Universität Erlangen-Nürnberg, alongside colleagues including Kaisa Laiho from the German Aerospace Center and Dmitry Strekalov and Andrea Aiello from the Max Planck Institute for the Science of Light, have developed a new source of narrowband frequency-entangled photons that overcomes limitations of previous designs. Their innovative approach utilises resonant parametric down-conversion within a crystalline whispering gallery mode resonator, achieving a dramatically extended dynamic range, scaling from millimetres to metres, compared to existing technologies. This breakthrough enables high-contrast interference with sub-picosecond resolution and promises substantial advancements in precision sensing and quantum information processing.

This source generates photon pairs exhibiting strong frequency correlation, a crucial property for enhancing the visibility of interference effects in this fundamental quantum optics experiment. The team achieves this by employing a carefully designed nonlinear crystal and a narrow bandwidth pump laser, ensuring efficient generation of entangled photons with a highly correlated frequency spectrum. Precise control over the phase matching conditions within the nonlinear crystal optimises the generation of entangled photon pairs while minimising unwanted background noise.

Spectral filtering techniques further refine the frequency correlation of the generated photons, resulting in a significantly improved signal-to-noise ratio. This refined source allows for more precise measurements of the Hong-Ou-Mandel effect, enabling investigations into the fundamental nature of quantum interference. The source offers enhanced capabilities for exploring fundamental quantum phenomena and advancing the field of quantum technologies.

Whispering Gallery Resonator for Entangled Photons

Scientists engineered a novel narrowband frequency-entangled photon source utilizing resonant parametric down-conversion within a crystalline whispering gallery mode resonator, achieving a meter-scale dynamic range for sensing applications. This breakthrough overcomes limitations of previous sources, which relied on non-resonant processes restricting coherence length and sensing range to the sub-millimeter scale. The team fabricated a whispering gallery mode resonator from z-cut 5% magnesium oxide-doped lithium niobate, carefully stabilizing its temperature to satisfy type-I phase-matching conditions for efficient down-conversion. The experimental setup involved pumping the resonator with a continuous-wave laser from both clockwise and counterclockwise directions, employing beam splitters to monitor reflectance spectra and optimize down-conversion efficiency.

Generated signal and idler photons were coupled out and separated before being recombined to prepare the frequency-entangled state. To implement Hong-Ou-Mandel interference, scientists utilized a half-wave plate and polarizing beam splitter, achieving efficient recombination and subsequent interference. The team demonstrated high-contrast beating with sub-picosecond resolution in the Hong-Ou-Mandel experiment using highly nondegenerate frequency-entangled photon pairs. Coincidence counts were measured under destructive interference, and pump laser leakage was suppressed using filters. Measurements confirmed the quality of the generated entangled state.

Hong-Ou-Mandel interferometry with entangled photons exhibits distinctive quantum features. By introducing frequency entanglement into this interference, the characteristic dip is modulated, enhancing the sensitivity of sensors. Previous frequency-entangled photon sources relied on non-resonant processes, limiting sensing dynamic range. This work demonstrates a narrowband frequency-entangled photon source based on resonant parametric down-conversion.

Meter-Scale Entangled Photons from Whispering Galleries

Researchers have developed a new source of narrowband frequency-entangled photons based on a crystalline whispering gallery mode resonator. This innovative approach overcomes limitations found in previous sources, which restricted sensing capabilities to sub-millimeter scales, by achieving a significantly extended dynamic range of up to a meter. The team demonstrated high-contrast beating with sub-picosecond resolution in a Hong-Ou-Mandel experiment, showcasing the source’s ability to generate photons with a narrow spectral bandwidth and substantial frequency detuning. This advancement enables more precise and sensitive measurements in several areas, including time-resolved sensing of coherent photon transfer and precise in-fiber sensing for security applications in optical networks. The researchers acknowledge that the performance of their system is currently limited by the stability and compactness of the resonator, and future work could focus on integrating the technology onto photonic integrated circuits. Further development promises to unlock new opportunities for narrowband frequency-entangled photon pairs in the broader field of quantum information processing technologies, potentially contributing to scalable quantum key distribution networks and other quantum applications.