Quantum Light Conversion Mapped with Unprecedented Precision Using New Technique
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Researchers have developed a novel technique for comprehensively characterising quantum frequency converters, devices crucial for manipulating light in emerging quantum technologies. Mateusz J Olszewski, Kasper Hecht Alexander, and Michael T M Woodley, all from the Centre for Photonics at the University of Bath, alongside Leah R Murphy, Peter J Mosley, Alex O C Davis et al., demonstrate a method that recovers the complete complex spectral transfer function of these converters by analysing spectral interference patterns created with a tunable light source. This work significantly advances the field by enabling detailed phase-sensitive measurements, revealing how frequency conversion and dispersion occur within the device, and providing a powerful new tool for optimising the performance of active photonic components. This work introduces two-tone tomography, an experimental method that recovers the complete spectral transfer function, or Green’s function, of a frequency converter by analysing spectral interference patterns generated by a tunable bichromatic probe. The research validates this approach through a proof-of-concept experiment utilising a frequency conversion module based on Bragg-scattering four-wave mixing in photonic crystal fibre. Current methods often only report conversion efficiencies for limited input states, providing an incomplete picture of the process. By separately reconstructing both the amplitude and phase of the spectral transfer function, two-tone tomography offers a more holistic understanding of how these devices manipulate optical signals. The approach requires no prior knowledge of the frequency converter’s characteristics beyond the input and output spectral ranges, making it broadly applicable. The core of this advancement lies in probing the frequency converter with a bichromatic signal, two closely spaced frequencies, and then spectrally resolving the resulting interference. Through Fourier-domain analysis of this interference, researchers can accurately determine the complex spectral transfer function, revealing both the intensity response and the crucial spectral phase. This phase information is particularly important, as it dictates how non-monochromatic inputs are processed and can reveal the internal dynamics of the frequency conversion process, such as variations in efficiency along the device’s length. Demonstrations using a Bragg-scattering four-wave mixing setup highlight the technique’s ability to identify the optimal input mode for maximizing conversion efficiency. Simulations and experimental data show that the spectral phase of the Green’s function directly influences the shape of the input pulse required for efficient conversion, with discrepancies potentially leading to efficiency losses exceeding 50%. Spectral resolution of the interference between the converted outputs of these two frequency components then allowed reconstruction of the properties of the frequency conversion process using Fourier-domain analysis. This approach separately determines both the amplitude and the phase of the spectral transfer function, effectively mapping the Green’s function of the frequency converter. This revealed the relative positions of regions exhibiting active frequency conversion and passive dispersive propagation within the module itself. By probing with a bichromatic signal, the work demonstrates a new approach to characterizing the performance of active devices with applications in emerging quantum technologies. This method recovers the complex spectral transfer function, or Green’s function, of a frequency converter by analyzing spectral interference resulting from a tunable bichromatic probe. The study validated this technique on a frequency conversion module utilizing Bragg-scattering four-wave mixing in photonic crystal fiber, successfully recovering information contained within the phase of the Green’s function. The work demonstrates the ability to reveal the relative positions of regions of active frequency conversion and passive dispersive propagation within the module itself. Specifically, the Green’s function, governing the input-output relation of single-photon quantum frequency conversion, was fully reconstructed, separately determining both amplitude and phase. This reconstruction required no prior knowledge of the frequency conversion process beyond the spectral span of the input and output signals. Analysis of the Green’s function revealed that the spectral phase significantly impacts quantum frequency conversion efficiency with non-monochromatic inputs. For instance, simulations using unchirped Gaussian pumps exhibited a nearly flat phase in the Green’s function, while linearly chirped pumps induced a quadratic spectral phase along the input and output frequency axes. Consequently, the optimal input mode for the chirped pump case acquires a quadratic spectral phase, demonstrating the importance of phase matching for efficient conversion. This method recovers the complex spectral transfer function, essentially a ‘Green’s function’, of a frequency converter by analysing spectral interference patterns created using a tunable bichromatic probe. The technique was successfully demonstrated using a frequency conversion module based on Bragg-scattering four-wave mixing within photonic crystal fibre. The recovered Green’s function reveals crucial information about the phase of the converted light, specifically identifying regions where frequency conversion actively occurs and where light undergoes passive, dispersive propagation within the module. This detailed characterization is critical for optimising conversion efficiency by tailoring input light to maximise the desired output pulse shape. Experimental results aligned with theoretical predictions, confirming the accuracy of the interferometric phase retrieval process. The current experiment was limited by the frequency shear of the probe light and distortions in radio frequency signals caused by substantial temporal delays. However, the authors indicate that improvements to existing methodologies could enhance measurement sensitivity by a considerable margin, potentially enabling reconstruction of quantum frequency conversion dynamics on femtosecond timescales. Future research will likely focus on applying this technique to diverse quantum frequency conversion modalities, including those based on different nonlinear optical interactions, and ultimately supporting the development of advanced quantum technologies such as long-distance quantum communication networks. 👉 More information 🗞 Phase-sensitive characterization of a quantum frequency converter by spectral interferometry 🧠 ArXiv: https://arxiv.org/abs/2602.06796 Tags: