Soonjae Lee

Photonic integrated circuits (PICs) are emerging as a key enabler for compact, rugged and mass-producible quantum key distribution (QKD) terminals, dramatically reducing size, weight, power and cost while facilitating co-packaging with classical transceivers. However, inevitable fabrication asymmetries and on-chip optical components such as modulator, attenuator, and wavelength division multiplexer (WDM) introduce polarization dependent loss (PDL) of up to several decibels. In polarization-based QKD that employs multiple polarization states, such intrinsic PDL can distort those states and thereby degrade overall system performance. The impact is particularly severe in polarization-based reference frame independent (RFI) QKD, where the six polarization states must remain mutually unbiased and orthogonal within each basis so that the secret key rate depends only on the quantum bit error rate (QBER) and the security parameter. PDL skews the Jones amplitudes, breaks these state relations, inflates QBER and can drive the secret key rate to low values at moderate channel loss. To counter this impairment, post-selection methods non-optically compensating PDL have been proposed. In this work, we introduce an optical PDL compensation which is a passive optical compensator consisting of a matched PDL element followed by a half-wave plate that swaps the orthogonal polarization components. The combined transfer matrix is effectively polarization-isotropic apart from a uniform attenuation so both the security parameter and the secret key rate are preserved, at the expense of additional insertion loss. Experiments with a free-space RFI QKD link confirm the scheme’s effectiveness: while uncompensated PDL quickly degrades secret key raete, the compensator restores state and sustains secret key rate across the entire operating range explored, validating the effectiveness of the compensator and demonstrating a practical path toward PDL tolerant, PIC-based QKD systems.