Cosmic Axions Revealed via Amplified Modulation of Ellipticity of Laser (CARAMEL)

Abstract

We propose a new axion dark matter detection strategy that employs optical readout of laser-beam ellipticity modulations caused by axion-induced electric fields in a microwave cavity, using electro-optic (EO) crystals, enhanced by externally injected radio-frequency (rf) power. Building upon the variance-based probing method~\cite{Omarov_2023}, we extend this concept to the optical domain: a weak probe laser interacts with an EO crystal coupled to the resonant microwave cavity field at cryogenic temperatures, and the axion-induced electric field is revealed through induced ellipticity. The injected rf signal coherently interferes with that of the axion field, amplifying the optical response and significantly improving sensitivity. While our EO-based method employs a Fabry-Pérot resonator, we do not require Michelson interferometers. Our method hence enables compact, high-frequency axion searches across the $0.5$--$50\,\mathrm{GHz}$ range. Operating at cryogenic temperatures not only suppresses thermal backgrounds but, critically, allows the probing method to mitigate quantum noise. This approach offers a scalable path forward for axion detection over the $\sim(\text{few}--200)\,μ\mathrm{eV}$ mass range -- covering the preferred parameter space for post-inflationary Peccei--Quinn axion dark matter -- using compact, tunable systems. \noindent Published in \textit{Phys. Rev. D} \textbf{113}, 032012 (2026).\\ DOI: 10.1103/PhysRevD.113.032012. This version includes further experimental details.

Figures (4)

• Figure 1: The current status of the axion to two photon coupling in the mass range 1-300 $\mu$eV, corresponding to the frequency range 0.2-70 GHz (from Ref. AxionLimits, ADMX Asztalos2010ADMX:2018ghoADMX:2019uokADMX:2021nhdBartram:2024ovwADMX:2025vom, ADMX-Sidecar ADMX:2018ogsBartram:2021ysp, ADMX-SLIC Crisosto:2019fcj, CAPP Lee:2020cfjJeong:2020cwzCAPP:2020utbYoon:2022gzpLee:2022mncarticle:CAPP-PACE-JPA12TB-PRLYang:2023yryKim:2022hmgarticle:Younggeun_Kim2024article:Ahn-PRX2023Bae:2024kmy, CAST CAST:2007jpsCAST:2017uphCAST:2024eil, CAST-CAPP Adair:2022rtw, GrAHal Grenet:2021vbb, HAYSTAC Brubaker:2016ktlHAYSTAC:2018rwyHAYSTAC:2020kwvHAYSTAC:2023camHAYSTAC:2024jch, MADMAX Garcia:2024xzc, ORGAN McAllister:2017lkbQuiskamp:2022pksQuiskamp:2023ehrQuiskamp:2024oet, QUAX Alesini:2019ajtAlesini:2020vnyAlesini:2022lnpQUAX:2023gopQUAX:2024fut, RADES CAST:2020rlfAhyoune:2024klt, RBF Panfilis1987Wuensch:1989sa, TASEH TASEH:2022vvu, UF HagmannHagmann:1996qd). CARAMEL aims to facilitate probing the frequency range of 0.5-50 GHz, or $\sim$ (2-200) $\mu$eV, with better than DFSZ sensitivity within the next five years. CARAMEL can potentially cover the preferred post-inflationary axion parameter space, corresponding to 40-180 $\mu$eV Buschmann:2021sdq, with better than DFSZ sensitivity using presently available technical capabilities. The projected sensitivity curve in this mass range corresponds to a total scanning time of approximately $2\times10^{7}$ s, which includes both data acquisition (3 s) and cavity retuning (3 s) per frequency step, assuming frequency steps of half the axion linewidth. The projection further assumes a constant axion-to-photon conversion power of $10^{-21}$ W and a cavity quality factor of $Q_c=10^6$ across the indicated mass range (equivalent to $10^{-23}$ W for a quality factor of $Q_c=10^4$).
• Figure 2: Signal-to-noise ratio (SNR) as a function of $Q_c$ and $T$ at different operating frequencies. The transition from the quantum regime to the classical regime becomes apparent around 480 mK, where the thermal photon occupation number begins to exceed the vacuum (zero-point) contribution. The cavity volume is kept constant at 3.7 liters. The axion to photon conversion power is kept at $10^{-23}$ W assuming $Q_c=10^4$, $t=3$ s, and scaled appropriately for different cavity quality factor values; see Appendix B and D.
• Figure 3: Signal-to-noise ratio (SNR) as a function of $Q_c$ and $T$ at different operating frequencies. The transition from the quantum regime to the classical regime becomes apparent around 24 mK for 0.5 GHz (red), and 2.4 K for 50 GHz (blue), where the thermal photon occupation number begins to exceed the vacuum (zero-point) contribution. The cavity volume is kept constant at 3.7 liters. The axion to photon conversion power is kept at $10^{-23}$ W assuming $Q_c=10^4$ and scaled appropriately for different cavity quality factor values; see text.
• Figure 4: Schematic of the quantum readout system (QRS) used in the feasibility analysis. The microwave cavity is placed at the lowest required operating temperature (typically $0.2$–$1~\mathrm{K}$ depending on the target frequency), while the QRS is mounted on the top plate and coupled to the cavity via a short, impedance-matched RF cable. The electro-optic (EO) crystal is embedded between two high-reflectivity mirrors that form the Fabry–Pérot resonator and is probed by a linearly polarized laser. A polarizer, quarter-wave plate (QWP), and analyzer (oriented at $45^\circ$ relative to the polarizer) convert the axion-induced ellipticity modulation into a detectable polarization rotation. The resulting signal is read out using balanced photodiodes (PDs) at room temperature (RT), which measure the normalized differential output $(I_1(t)-I_2(t))/(I_1(t)+I_2(t))$. Since the required cavity temperatures do not need to fall below $0.2$–$1~\mathrm{K}$ across the targeted frequency range, the laser-induced absorption in the EO crystal must remain below $\sim10\%$ of the available cooling power at that temperature. Depending on the available laboratory infrastructure, the EO/FP stage may alternatively be positioned at the $4~\mathrm{K}$ plate, remaining electrically coupled to the cavity while being only weakly thermally anchored to it.