article published Dennis Scharwald author 559070009-0007-5654-5412 Polina Sharapova author 60286 975 department 15 department 623 department 569 department 170 department 35 department 429 department 27 department 34 department 502 department Multimode quantum light has promising applications in many areas of physics, such as quantum communications and quantum computing. However, its multimode nature also makes it challenging to measure its properties. Recently [I. Barakat et al., Optica Quantum 3, 36 (2025)], a technique for the simultaneous measurement of squeezing of multiple broadband modes based on a phase-sensitive amplification approach was experimentally implemented using a setup that effectively corresponds to an SU(1,1) interferometer. Here, we aim to provide a complete theoretical analysis of the modal structure of (generally unbalanced) SU(1,1) interferometers and a detailed theoretical formal derivation of the framework for this technique. Utilizing the joint Schmidt decomposition of the transfer functions, we investigate the shape and phase profiles of the modes of the SU(1,1) interferometer and its components [parametric down-conversion (PDC) sections] for different parametric gain regimes. We discover a complicated interplay between the PDC modes and the modes of the entire interferometer, and analyze it by using their overlap coefficients as a similarity measure. Finally, we develop a rigorous processing method for the aforementioned multimode squeezing measurement technique and discuss necessary approximations to make this method experimentally feasible. American Physical Society (APS)2026 eng 2643-156410.1103/ph64-ts39 83 Scharwald, Dennis, and Polina Sharapova. “Characterization of Spatial Schmidt Modes in High-Gain SU(1,1) Interferometers.” <i>Physical Review Research</i> 8, no. 3 (2026). <a href="https://doi.org/10.1103/ph64-ts39">https://doi.org/10.1103/ph64-ts39</a>. D. Scharwald, P. Sharapova, Physical Review Research 8 (2026). D. Scharwald and P. Sharapova, “Characterization of spatial Schmidt modes in high-gain SU(1,1) interferometers,” <i>Physical Review Research</i>, vol. 8, no. 3, Art. no. 033139, 2026, doi: <a href="https://doi.org/10.1103/ph64-ts39">10.1103/ph64-ts39</a>. Scharwald, D., &#38; Sharapova, P. (2026). Characterization of spatial Schmidt modes in high-gain SU(1,1) interferometers. <i>Physical Review Research</i>, <i>8</i>(3), Article 033139. <a href="https://doi.org/10.1103/ph64-ts39">https://doi.org/10.1103/ph64-ts39</a> @article{Scharwald_Sharapova_2026, title={Characterization of spatial Schmidt modes in high-gain SU(1,1) interferometers}, volume={8}, DOI={<a href="https://doi.org/10.1103/ph64-ts39">10.1103/ph64-ts39</a>}, number={3033139}, journal={Physical Review Research}, publisher={American Physical Society (APS)}, author={Scharwald, Dennis and Sharapova, Polina}, year={2026} } Scharwald D, Sharapova P. Characterization of spatial Schmidt modes in high-gain SU(1,1) interferometers. <i>Physical Review Research</i>. 2026;8(3). doi:<a href="https://doi.org/10.1103/ph64-ts39">10.1103/ph64-ts39</a> Scharwald, Dennis, and Polina Sharapova. “Characterization of Spatial Schmidt Modes in High-Gain SU(1,1) Interferometers.” <i>Physical Review Research</i>, vol. 8, no. 3, 033139, American Physical Society (APS), 2026, doi:<a href="https://doi.org/10.1103/ph64-ts39">10.1103/ph64-ts39</a>. 666652026-08-05T11:51:21Z2026-08-05T15:06:31Z