Side-Channel-Free Quantum Key Distribution Source using a KTP Polarization Modulator and a Broadband Laser

We discuss progress toward demonstrating a side-channel-free decoy-state quantum key distribution (QKD) source based on a polarization-modulator and a wavelength-stable attenuated pulsed laser. By modulating the polarization of the quantum state, a QKD encoding with three states is attained. The polarization-modulator-based QKD source enhances security by eliminating side-channel attacks based on spectral, spatial, or temporal distinguishability when multiple sources are used to generate the different QKD states. Herein, we describe our QKD source design and evaluate critical subsystems metrics, including the quantum bit error rate (QBER), quantum state tomography, and achievable key rates. The QKD source is designed to function with minimum size, weight, and power (SWaP). In our system, we use a commercial KTP waveguide modulator (AdvR, Inc.). Our QKD source operates with both narrow-band and broad-band lasers, and we show characterization results using a He-Ne laser and a diode laser, respectively. The polarization-modulator-based QKD source has potential in future mobile quantum networks, such as unmanned aerial vehicles (UAV) and autonomous vehicles, in addition to fixed fiber-based quantum networks.