Fully discrete 2.5-GHz balanced homodyne detector with 66-dB common-mode rejection ratio

We present a high-performance, fully discrete balanced homodyne detector (BHD) with a 2.5 GHz bandwidth, exceptional gain flatness, and superior common-mode rejection ratio. The design leverages a Rogers 4350B-based printed circuit board, a broadband multi-section RF choke bias network, and inter-stage π-section attenuators, complemented by precise photodiode matching and phase-magnitude compensation to ensure signal integrity and suppress common-mode noise. Experimental results show a -3 dB bandwidth of 2.5 GHz and 1 dB gain flatness up to 2.4 GHz-significantly exceeding our prior 1.55 GHz/1.4 GHz implementation and representing state-of-the-art performance among discrete BHDs. At 16 mW optical input, the detector delivers a 16.5 dB signal-to-noise ratio at 2 GHz while maintaining excellent linearity, and achieves a 66 dB common-mode rejection ratio at the same frequency. This cost-effective and flexible BHD is ideally suited for high-speed continuous-variable quantum key distribution and quantum random number generation.