Quantifying Effective Heterodyne Detection Efficiency with SI-Traceable Standards

TL;DR

This paper introduces an SI-traceable method for accurately calibrating heterodyne detection efficiency in coherent optical receivers, crucial for quantum communication and sensing, validated through experiments on free-space and fiber-coupled systems.

Contribution

It presents a novel, SI-traceable calibration protocol for heterodyne detection efficiency using shot-noise measurements and spectrum analyzer data, applicable to practical optical systems.

Findings

Protocol validated on free-space receiver with consistent results

Robust calibration demonstrated on fiber-coupled receiver

Provides a traceable, uncertainty-bounded calibration framework

Abstract

Accurate calibration of coherent optical receivers is essential for reliable performance assessment in coherent communications, precision and quantum sensing, and continuous-variable quantum key distribution (CV-QKD), where the effective detection efficiency directly impacts channel parameter estimation. We present a methodology traceable to the International System of Units (SI) to determine the effective heterodyne detection efficiency of balanced receivers using shot-noise-referenced measurements. The protocol relies on two observables acquired with an electrical spectrum analyzer: the heterodyne beat-note power and the local oscillator shot-noise variance, with explicit treatment of the analyzer's equivalent noise bandwidth (ENBW). The photon flux in the signal path is referenced to SI units via calibrated radiometric standards. We first validate the protocol on a free-space receiver, demonstrating consistency with an independently constructed optical loss chain across a wide range of signal powers and under controlled, calibrated attenuation. Extending the same estimator to a fiber-coupled, polarization-maintaining balanced receiver confirms that the protocol is robust for practical coherent-receiver architectures and intermediate frequencies in the MHz range. These results establish a traceable, uncertainty-bounded framework for real-time receiver calibration, providing a practical route for CV-QKD and other coherent optical systems.