When
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TITLE
Search for scalar ultralight dark matter using cryogenic optical cavities
ABSTRACT
One of the biggest challenges in fundamental physics is understanding the microscopic nature of dark matter. Scalar ultralight dark matter (ULDM) is a well-motivated extension to the standard model (SM) of particle physics, hypothesized to couple to SM parameters like the electron mass and the fine-structure constant, thereby inducing coherent oscillations in the size of macroscopic solids at the ULDM Compton frequency.
This talk presents the first experimental demonstration of a novel optomechanical ULDM sensor that employs two cryogenic sapphire Fabry-Pérot optical cavities. This apparatus leverages the cavities’ longitudinal mechanical resonances to achieve sensitivity to differential cavity length variations induced by ULDM.
A four-day observation period with this sensor achieved an improvement of up to two orders of magnitude in the limits to ULDM coupling to the SM for the ULDM’s Compton frequencies ranging from 5 kHz to 100 kHz. This was demonstrated for both the galactic halo and the Earth-bound relaxion halo models. This work represents a crucial step towards future upgrades, which are projected to yield an improvement of up to five orders of magnitude over a wider frequency range (100 Hz to 1 MHz), ultimately surpassing the theoretical naturalness threshold.
Publication: Tejas Deshpande, Andra Ionescu, Nicholas Miller, Zhiyuan Wang, Gerald Gabrielse, Andrew A. Geraci, and Tim Kovachy. “Demonstration That Differential Length Changes of Optical Cavities Are a Sensitive Probe for Ultralight Dark Matter.” Physical Review Letters 135, no. 26 (2025): 261001.
BIO
Tejas Deshpande earned his Bachelor of Engineering in Honors Electrical Engineering with a minor in Physics from McGill University in 2011. He completed his PhD in Experimental Condensed Matter Physics at Caltech in 2019 under Prof. David Hsieh, developing tools to probe order in single crystals via electron and photon spectroscopy. His doctoral research focused specifically on mapping surface Majorana modes in three-dimensional time-reversal invariant topological superconductors.
Following his PhD, Dr. Deshpande became a Postdoctoral Scholar at Northwestern University's Center for Fundamental Physics, led by Prof. Gerald Gabrielse, where he led the design and construction of an ultralight dark matter detector utilizing vibration-isolated cryogenic optical cavities. In Prof. Tim Kovachy's lab, he also led the development of a two-meter strontium atom interferometer designed to measure Newton's gravitational constant to under 10 parts per million. Additionally, as part of the MAGIS (Matter-wave Atomic Gradiometer Interferometric Sensor) collaboration, he was involved in the construction of MAGIS-100—the world's first 100-meter atom interferometer at Fermilab. This instrument serves as a testbed for future kilometer-scale gravitational wave (GW) detectors in a frequency range bridging existing and planned optical detectors.
Within the MAGIS collaboration, Dr. Deshpande led the Northwestern team's development of ultrastable, frequency-agile, high-pulse-repetition-rate laser systems. He also contributed to the development of quantum control techniques optimizing laser pulses for high atom-photon interaction efficiency—both of which are critical to achieving the target GW sensitivities. Ultimately, both the Northwestern and Fermilab interferometers will perform precision fundamental physics tests exploring the interplay of quantum mechanics and gravity.
Currently, Dr. Deshpande is a Postdoctoral Scholar at Northern Arizona University in Prof. Ryan Behunin’s group, leading the development of a millikelvin optomechanical system to study spin-phonon coupling in diamond acoustic resonators using stimulated Brillouin spectroscopy.