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1. Sagnac Interferometer for Two-Dimensional Spectroscopy in the Pump-Probe Geometry

   https://doi.org/10.1007/978-3-319-13242-6  

   Park, Samuel D.; Courtney, Trevor L.; Baranov, Dmitry; Cho, Byungmoon; Jonas, David M. (January 2015, Ultrafast Phenomena XIX)

   An intrinsically phase-stable Sagnac interferometer is introduced for enhanced sensitivity detection in partially collinear two-dimensional spectroscopy in the short-wave IR. The sensitivity and phase accuracy of the apparatus are demonstrated on the dye IR-26. 

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2. Sagnac interferometry for high-sensitivity optical measurements of spin-orbit torque

   Karimeddiny, Saba; Cham, Thow Min; Ralph, Daniel C.; Luo, Yunqiu Kelly (September 2021, ArXivorg)

   null (Ed.)

   We adapt Sagnac interferometry for magneto-optic Kerr effect measurements of spin-orbit-torque-induced magnetic tilting in thin-film magnetic samples. The high sensitivity of Sagnac interferometry permits for the first time optical quantification of spin-orbit torque from small-angle magnetic tilting of samples with perpendicular magnetic anisotropy (PMA). We find significant disagreement between Sagnac measurements and simultaneously-performed harmonic Hall (HH) measurements of spin-orbit torque on Pt/Co/MgO and Pd/Co/MgO samples with PMA. The Sagnac results for PMA samples are consistent with both HH and Sagnac measurements for the in-plane geometry, so we conclude that the conventional analysis framework for PMA HH measurements is flawed. We suggest that the explanation for this discrepancy is that although magnetic-field induced magnetic tilting in PMA samples can produce a strong planar Hall effect, when tilting is instead generated by spin-orbit torque it produces negligible change in the planar Hall signal. This very surprising result demonstrates an error in the most-popular method for measuring spin-orbit torques in PMA samples, and represents an unsolved puzzle in understanding the planar Hall effect in magnetic thin films. 

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3. Measuring the angular momentum of a neutron using Earth's rotation

   https://doi.org/10.1103/PhysRevResearch.7.013046  

   Geerits, Niels; Sponar, Stephan; Steffen, Kyle E; Snow, William M; Parnell, Steven R; Mauri, Giacomo; Smith, Gregory N; Dalgliesh, Robert M; de_Haan, Victor (January 2025, Physical Review Research)

   A coupling between Earth's rotation and orbital angular momentum (OAM), known as the Sagnac effect, is observed in entangled neutrons produced using a spin-echo interferometer. After correction for instrument systematics the measured coupling is within 5% of theory, with an uncertainty of 7.2%. The OAM in our setup is transverse to the propagation direction and scales linearly with neutron wavelength (4–12.75 Å), so the Sagnac coupling can be varied without mechanically rotating the device, which avoids systematic errors present in previous experiments. The detected transverse OAM of our beam corresponds to $4098\pm 295\hslash {\AA }^{-1},{10}^5$times lower than in the previous neutron experiments. This demonstrates the feasibility of using the Sagnac effect to definitively measure neutron OAM and paves the way towards a future observation of the quantum Sagnac effect. Published by the American Physical Society2025 

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4. System Design of a Cold Atom Gyroscope based on Interfering Matter-wave Solitons

   https://doi.org/10.1109/INERTIAL48129.2020.9090099  

   Patil, Y. S.; Cheung, H. F.; Bhave, S. A.; Vengalattore, M. (March 2020, 2020 IEEE International Symposium on Inertial Sensors and Systems (INERTIAL 2020))

   We propose a novel implementation of a trapped- atom Sagnac gyroscope based on the interference between matter- wave solitons confined around an optical microring resonator. Our integrated nanophotonic approach to trapped atom interferometry combines the long-term stability and quantum-limited sensitivity of ultracold matter-wave interferometers with the robustness, scalability and low power operation of nanophotonic architectures. The use of optical microresonators for atomic confinement ensures disorder-free symmetric waveguides for the confined atoms, a high degree of vibration insensitivity owing to the reciprocal structure of the waveguide, and enhanced bias and scale-factor stability via concurrent feedback stabilization of the microresonator. We have performed detailed quantum simulations based on demonstrated experimental parameters to confirm stable dispersion-free propagation of matter-wave solitons around the microresonator and the appearance of high contrast interference fringes due to the accrued Sagnac phase shift. We estimate the shot-noise limited rotation sensitivity of this gyroscope to be 0.8urad/s/rt.Hz for single-loop propagation of the solitons around a microring of radius 1 mm, with the possibility of substantial improvements via multiloop propagation of the solitons, fabrication of microring resonators of larger diameter, and the use of quantum-correlated states such as spin- squeezed quantum states. The proposed device illustrates the benefits of harnessing quantum many-body states such as matter- wave solitons for quantum-enhanced inertial sensing applications. 

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5. Sagnac-type neutron displacement-noise-free interferometeric gravitational-wave detector

   https://doi.org/10.1088/1361-6382/ad42fa  

   Kawasaki, Yuki; Iwaguchi, Shoki; Ishikawa, Tomohiro; Nishizawa, Atsushi; Kitaguchi, Masaaki; Yamagata, Yutaka; Chen, Yanbei; Wu, Bin; Shimizu, Ryuma; Umemura, Kurumi; et al (May 2024, Classical and Quantum Gravity)

   Abstract The detection of low-frequency gravitational waves on Earth requires the reduction of displacement noise, which dominates the low-frequency band. One method to cancel test mass displacement noise is a neutron displacement-noise-free interferometer (DFI). This paper proposes a new neutron DFI configuration, a Sagnac-type neutron DFI, which uses a Sagnac interferometer in place of the Mach–Zehnder interferometer. We demonstrate that a sensitivity of the Sagnac-type neutron DFI is higher than that of a conventional neutron DFI with the same interferometer scale. This configuration is particularly significant for neutron DFIs with limited space for construction and limited flux from available neutron sources. 

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