We investigate the photon noise produced by a phase-diffusion radiation source based on a Josephson junction oscillator and detected by a cold-electron bolometer receiver. The oscillator generates a signal with a narrow spectral linewidth, situating it in an intermediate regime between coherent source (synthesizer) and broadband thermal radiation (“chaotic” source). We experimentally observe a crossover of the photon-noise scaling from proportional to $\sqrt{P}$ to proportional to $P$ at low radiation power with an unexpectedly large bunching term contribution proportional to $P$. This behavior results from the conversion of frequency fluctuations of the radiation source into amplitude fluctuations via the frequency-dependent response of the detector. Namely, the maximal photon noise corresponds not to the maximal accepted power but to the maximal slope of the receiver response. Our results demonstrate that detector-source interaction can modify the observed photon statistics and reveal a regime of photon-noise detection relevant for radiation sources with finite linewidths.

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• Cosmic microwave background
• Optoelectronics
• Quantum fluctuations & noise
• Solid-state detectors
• Superconducting devices
• Astrophysical electromagnetic fields
• Bolometers
• Terahertz techniques