First Results of the Laser-Interferometric Detector for Axions (LIDA)

We present the operating principle and the first observing run of a novel kind of direct detector for axions and axionlike particles in the galactic halo. Sensitive to the polarisation rotation of linearly polarised laser light induced by an axion field, our experiment is the first detector of its kind collecting scientific data. We discuss our peak sensitivity of $1.51\times {10}^{-10}{\mathrm{GeV}}^{-1}$ (95% confidence level) to the axion-photon coupling strength in the axion mass range of 1.97–2.01 neV which is, for instance, motivated by supersymmetric grand-unified theories. We also report on effects that arise in our high-finesse in-vacuum cavity at an unprecedented optical continuous-wave intensity of $4.7\mathrm{MW}/{\mathrm{cm}}^2$. Our detector already belongs to the most sensitive direct searches within its measurement band, and our results pave the way towards surpassing the current sensitivity limits even of astrophysical observations in the mass range from ${10}^{-8}$ down to ${10}^{-16}\mathrm{eV}$ via quantum-enhanced laser interferometry, especially with the potential of scaling our detector up to kilometer length.

Figure 1

Simplified schematic of the experimental setup on the left, details of the main laser source and of the signal readout on the right. Red beam: pump field, orange beam: signal field, orange-dashed beam: planned squeezed field, electro-optic modulator (EOM), Faraday isolator (FI), nonplanar ring laser (NPRO), polarizing beam splitter (PBS), photodetector (PD), radio frequency (rf) generator. The installation of a squeezed light source as indicated is planned.

Figure 2

Sensitivity to the axion-photon coupling coefficient $g_{a\gamma }$ that LIDA reached during the first observing run, dependent on the axion mass and measurement frequency, at the 95% confidence level of the full data. Both curves are compared to the predictions of the shot-noise limited model from Eq. (5), and to the constraint set by the CAST detector [19].

Figure 3

Bottom: time series of the circulating intracavity power when the detector is disturbed. It typically relocks automatically, however, often reaching a final state with less circulating power than initially. The CCD camera pictures show the corresponding fields transmitted through the cavity. Top: amplitude spectral densities (ASD) of the electronic dark noise and shot noise in the demodulated output signal. The difference between their incoherent sum and the initial readout noise (corresponding to the cavity state of the first 20 s of the time series) is caused by technical laser noise. The post-relock readout noise (corresponding to the cavity state of the last 20 s) is significantly increased over the full measurement band, especially at lower frequencies. 1 and 2 are two examples as the post-relock noise varies.