  An optical quantum sensor (OQS) based on lasers and alkali-metal atoms is a
sensitive ambient-temperature magnetometer that can be used in axion dark
matter search with an inductor-capacitor (LC) circuit at kHz and MHz
frequencies. We have previously investigated the sensitivity of an LC
circuit-OQS axion detector to ultralight axion dark matter that could be
achieved using a fT-noise OQS constructed in our lab. In this paper, we
investigate the sensitivity that could be potentially reached by an OQS
performing close to the fundamental quantum noise levels of 10
aT/$\sqrt{\text{Hz}}$. To take advantage of the quantum-limited OQS, the LC
circuit has to be made of a superconductor and cooled to low temperature of a
few K. After considering the intrinsic noise of the advanced axion detector and
characterizing possible background noises, we estimate that such an experiment
could probe benchmark QCD axion models in an unexplored mass range near 10 neV.
Reaching such a high sensitivity is a difficult task, so we have conducted some
preliminary experiments with a large-bore magnet and a prototype axion detector
consisting of a room-temperature LC circuit and a commercial OQS unit. This
paper describes the prototype experiment and its projected sensitivity to
axions in detail.
