Polarization correction towards satellite-based QKD without an active feedback

Abstract

Quantum key distribution (QKD) is a cryptographic protocol to enable two parties to share a secure key string, which can be used in one-time pad cryptosystem. There has been an ongoing surge of interest in implementing long-haul photonic-implementation of QKD protocols. However, the endeavour is challenging in many aspects. In particular, one of the major challenges is the polarization degree of freedom of single-photons getting affected while transmission through optical fibres, or atmospheric turbulence. Conventionally, an active feedback-based mechanism is employed to achieve real-time polarization tracking. In this work, we propose an alternative approach where we first perform a state tomography to reconstruct the output density matrix. We then evaluate the optimal measurement bases at Bob’s end that leads to the maximum (anti-)correlation in the measurement outcomes of both parties. As a proof-of-principle demonstration, we implement an in-lab BBM92 protocol — a particular variant of a QKD protocol using quantum entanglement as a resource — to exemplify the performance of our technique. We experimentally generate polarization-entangled photon pairs having 94% fidelity with | ψ 〉 1 = 1 / (cid:112) 2 ( | HV 〉 + | VH 〉 ) state and a concurrence of 0.92. By considering a representative 1 ns coincidence window span in our implementation involving a novel alternative to an active feedback-based mechanism, we are able to achieve a quantum-bit-error-rate (QBER) of ≈ 5%, and a key rate of ≈ 35 Kbps. The performance of our implemented protocol is independent of the local polarization rotations through optical fibres. We have also developed an algorithmic approach to optimize the trade-off between the key rate and QBER. Our approach obviates the need for active polarization tracking. Our method is also applicable to entanglement-based QKD demonstrations using partially mixed as well as non-maximally entangled states, and extends to single-photon implementations over fibre channels.