The heterodyne sensing system for the ALPS II search for sub-eV weakly interacting particles

TL;DR

This paper details the heterodyne sensing scheme for ALPS II, a cutting-edge experiment searching for axion-like particles, focusing on optical layout, alignment, noise minimization, and calibration methods.

Contribution

It introduces the heterodyne sensing scheme for ALPS II, including design details and control strategies, marking the first deployment of this detection method in the experiment.

Findings

Design of the heterodyne sensing optical layout

Strategies for minimizing stray light and spurious signals

Calibration and control channels for detector sensitivity

Abstract

ALPS II, the Any Light Particle Search, is a second-generation Light Shining through a Wall experiment that hunts for axion-like particles. The experiment is currently transitioning from the design and construction phase to the commissioning phase, with science runs expected to start in 2021. ALPS II plans to use two different sensing schemes to confirm the potential detection of axion-like particles or to verify an upper limit on their coupling strength to two photons of $g_{a\gamma\gamma}\leq2\times10^{-11}\text{GeV}^{-1}$. This paper discusses a heterodyne sensing scheme (HET) which will be the first scheme deployed to detect the regenerated light. It presents critical details of the optical layout, the length and alignment sensing scheme, design features to minimize spurious signals from stray light, as well as several control and veto channels specific to HET which are needed to commission and operate the instrument and to calibrate the detector sensitivity.