To determine the fundamental sensitivity limit of an optical receiver one must have a complete quantum mechanical description of the incoming signals, even if those signals are from “classical” sources of light, i.e., ones whose photo-detection statistics with conventional receivers can be explained correctly using the shot-noise theory. In this work, we calculate the fundamental (quantum) limit for discriminating between two types of classical states in the low photon number regime: a coherent state, a pure-state quantum description of ideal (coherent) laser light with Poisson distributed photon statistics, and a thermal state, a (incoherent) mixed state with Bose-Einstein photon statistics. The Helstrom bound for discrimination error probability for single mode measurement is computed along with error probability bounds for direct detection, coherent homodyne detection and the Kennedy receiver. The generalized Kennedy (GK) receiver is shown to closely approach the Helstrom limit. We experimentally validate these results by generating coherent and thermal light and demonstrate that for signal strengths n¯s > 0.01 photons/mode, using a GK receiver with a quasi-photon-number resolving detector, we can approach quantum-limited discrimination performance, out-performing the discrimination capability achievable with traditional optical sensors. We demonstrate ∼ 17 dB improvement in discrimination sensitivity over direct detection using a GK receiver, and an improvement of 17% in error probability over coherent detection at a mean signal photon number n¯s = 0.4 photons/mode.
|Original language||English (US)|
|State||Published - Dec 13 2019|
ASJC Scopus subject areas