[Paper Review] Free-space quantum key distribution to a moving receiver

Jean‐Philippe Bourgoin, Brendon L. Higgins+6 more|arXiv (Cornell University)|May 1, 2015

Quantum Information and Cryptography39 references44 citations

TL;DR

This paper demonstrates the first free-space quantum key distribution (QKD) from a stationary transmitter to a moving receiver, simulating a low-Earth-orbit satellite. Using real-time beam pointing, polarization, and time-of-flight compensation, the system achieves an asymptotic secure key rate of 40 bits per second over a dynamic link, validating the feasibility of satellite-based QKD with mobile platforms.

ABSTRACT

Technological realities limit terrestrial quantum key distribution (QKD) to single-link distances of a few hundred kilometers. One promising avenue for global-scale quantum communication networks is to use low-Earth-orbit satellites. Here we report the first demonstration of QKD from a stationary transmitter to a receiver platform traveling at an angular speed equivalent to a 600 km altitude satellite, located on a moving truck. We overcome the challenges of actively correcting beam pointing, photon polarization and time-of-flight. Our system generates an asymptotic secure key at 40 bits/s.

Motivation & Objective

• To demonstrate the feasibility of quantum key distribution (QKD) between a stationary source and a moving receiver platform.
• To overcome the challenges of dynamic beam pointing, polarization drift, and time-of-flight variation in free-space QKD.
• To validate the viability of ground-to-satellite QKD using a mobile receiver simulating low-Earth-orbit satellite motion.
• To achieve asymptotically secure key generation under realistic dynamic conditions with high channel loss.

Proposed method

• A stationary BB84 decoy-state QKD system was used with a 1550 nm single-photon source and free-space optical link.
• Real-time active compensation was implemented for beam pointing, polarization state, and time-of-flight using feedback from a polarization tracking system.
• A custom pointing system maintained optical alignment with a truck moving at angular speeds equivalent to a 600 km altitude satellite.
• Post-processing included error correction via low-density parity check codes and privacy amplification using reduced-Toeplitz-matrix two-universal hashes.
• The system used a 0.16 ns coincidence window to improve signal-to-noise ratio and reduce QBER.
• Modeling of source imperfections identified inefficiencies in spontaneous four-wave mixing crystals and phase deviations in modulators as key contributors to intrinsic QBER.

Experimental results

Research questions

• RQ1 Can QKD be successfully established with a receiver platform moving at angular speeds consistent with a low-Earth-orbit satellite?
• RQ2 What level of real-time compensation is required to maintain secure key generation under dynamic pointing, polarization, and timing variations?
• RQ3 How does intrinsic QBER in the source affect secure key rate and finite-size key extraction in mobile QKD systems?
• RQ4 To what extent can system imperfections such as crystal efficiency imbalance and modulator phase errors degrade QKD performance?

Key findings

• The system achieved an asymptotic secure key rate of 40 bits per second over a 4-second link duration with a measured QBER of 6.55%.
• Post-processing of 11,477 raw key bits yielded 5,844 sifted key bits and 160 secure key bits, excluding finite-size effects.
• Modeling revealed that intrinsic QBER was primarily due to 91% state purity and 94% fidelity, with a 2:1 efficiency imbalance between two SFG crystals.
• The measured system loss was 30.6 dB, and the single-photon QBER upper bound was 5.85%.
• The study confirmed that finite-size statistics would require approximately 13,210 seconds of data to extract a secure key at ten standard deviations.
• Improvements such as reducing source QBER and adding a second-stage fine-pointing mechanism could enable higher key rates and longer-distance QKD with mobile platforms.