The standard quantum limit for sensing is related to the quantum noise from coherent detection of two conjugate quantities such as amplitude and phase. One way to beat this limit is with a technique called squeezing which increases the uncertainty along one axis in exchange for reducing it along the complementary axis. Another way to beat the limit is to use incoherent detection which is essentially phase insensitive. In the limit of low photon occupation number, single photon detection can achieve much better sensitivity than the standard quantum limit. I will describe several applications of single photon detection and the associated technology development including: i) microwave single photon counting detectors for axion searches, ii) single photon counting intensity interferometry with small telescopes and iii) deep space and interstellar optical communication.
E. Battistelli