Upconversion Loop Oscillator Axion Detection Experiment: A Precision Frequency Interferometric Axion Dark Matter Search with a Cylindrical Microwave Cavity

First experimental results from a room-temperature tabletop phase-sensitive axion haloscope experiment are presented. The technique exploits the axion-photon coupling between two photonic resonator oscillators excited in a single cavity, allowing low-mass axions to be upconverted to microwave frequencies, acting as a source of frequency modulation on the microwave carriers. This new pathway to axion detection has certain advantages over the traditional haloscope method, particularly in targeting axions below $1\mu \mathrm{eV}$ (240 MHz) in energy. At the heart of the dual-mode oscillator, a tunable cylindrical microwave cavity supports a pair of orthogonally polarized modes (${\mathrm{TM}}_{0,2,0}$ and ${\mathrm{TE}}_{0,1,1}$), which, in general, enables simultaneous sensitivity to axions with masses corresponding to the sum and difference of the microwave frequencies. However, in the reported experiment, the configuration was such that the sum frequency sensitivity was suppressed, while the difference frequency sensitivity was enhanced. The results place axion exclusion limits between 7.44–19.38 neV, excluding a minimal coupling strength above $5\times {10}^{-7}1/\mathrm{GeV}$, after a measurement period of two and a half hours. We show that a state-of-the-art frequency-stabilized cryogenic implementation of this technique, ambitious but realizable, may achieve the best limits in a vast range of axion space.

Figure 1

UPLOAD-CMC schematic with frequency noise readout system. Loop oscillators support the microwave modes in the main resonator allowing axion conversion while a frequency counter records $|f_2-f_1|$. The TM mode acts as the readout mode with constant $f_2$. The frequency noise is continuously scanned with a high-$Q$ frequency discriminator (FD) based on a duplicate cavity, creating the frequency-phase dispersion required for voltage to frequency noise conversion of the mixer output.

Figure 2

Top left: Filtered spectral density of fractional frequency noise at Fourier frequencies about the normal mode frequency, $f_{\mathrm{TM}}$, for $f_{\mathrm{TM}}=8.9989\mathrm{GHz}$ and $f_{\mathrm{TE}}=9.00241\mathrm{GHz}$. Bottom left: The second decade of data, with inverse power law fit. Decades were analyzed independently. Top right: Residuals from the spectral density of fractional frequency noise fit, examinable as axion-induced excesses. Bottom right: Standardized residuals (henceforth referred to as “search spectrum amplitudes”), ready to be interrogated with the axion-search procedure.

Figure 3

The 95% confidence exclusion zone for $g_{a\gamma \gamma }$ (in natural units) for the measured mass range (top), with CAST’s helioscope limits and popular QCD axion models in green (KSVZ and DSFZ) [1, 50], blue (conventional ALP misalignment), and red (ALP cogenesis) [51, 52]. Projected upconversion limits are dashed, illustrating best limits throughout the range in 1 Hz tuning steps (30 days per Hz): UPLOAD-CMC-II-RT, a copper resonator with frequency stabilized (FS) loops at room temperature (RT); and UPLOAD-CMC-II-Cryo, cryogenic Nb with FS loops. These dashed limits represent sensitivity at a uniform Fourier offset, and so it would be temporally infeasible to scan the entire range at the represented level. Since a real stationary measurement has sensitivity varying within the Fourier range, we also present examples of 30 day measurements covering 1 MHz in Fourier space, in bold. Projections are based on readily constructible setups with available equipment and standard techniques. With a similar setup as for UPLOAD-CMC-II-Cryo, in UPLOAD-CMC-III-Cryo we expect to reach QCD axion limits at very low upconversion energies ($<1.8\mathrm{kHz}$, i.e., $7.7\times {10}^{-12}\mathrm{eV}$) by tuning the orthogonal modes to coincide in resonant frequency (to reach the sensitivity illustrated in the left subplot, 1.3 years of data acquisition is necessary). Excluded axion space by ADMX [13, 14, 15, 53], ORGAN [16], ABRACADABRA [54], ADMX-SLIC [55], HAYSTAC [56], UF [57], CAPP-8TB [58], and RBF [59] is also represented.