[Paper Review] Non-local temporal interferometry for highly resilient free-space quantum communication

Lukas Bulla, Matej Pivoluska+10 more|arXiv (Cornell University)|Apr 15, 2022

Quantum Information and Cryptography4 citations

TL;DR

This paper demonstrates a 10.2 km free-space quantum link using non-local temporal interferometry to distribute hyper-entangled, high-dimensional energy-time photons. By stabilizing a Franson interferometer across the link and employing adaptive dimensionality in quantum key distribution, the system achieves a positive asymptotic key rate even under strong atmospheric noise and daylight conditions, proving robustness for real-world quantum communication.

ABSTRACT

Entanglement distribution via photons over long distances enables many applications, including quantum key distribution (QKD), which provides unprecedented privacy. The inevitable degradation of entanglement through noise accumulated over long distances remains one of the key challenges in this area. Exploiting the potential of higher-dimensional entangled photons promises to address this challenge, but poses extreme demands on the experimental implementation. Here, we present an interstate free-space quantum link, distributing hyper-entanglement over $10.2\,$km with flexible dimensionality of encoding by deploying a phase-stable non-local Franson interferometer. With this distribution of multidimensional energy-time entangled photons, we analyse the achievable key rate in a dimensionally-adaptive QKD protocol that can be optimized with respect to any environmental noise conditions. Our approach enables and emphasises the power of high-dimensional entanglement for quantum communication, yielding a positive asymptotic key rate well into the dawn of the day.

Motivation & Objective

• To enable long-distance, free-space quantum communication resilient to atmospheric noise and background radiation.
• To overcome the challenge of maintaining phase stability in non-local interferometers over 10.2 km of free space.
• To implement a dimensionally-adaptive quantum key distribution protocol that optimizes key rate under variable environmental noise.
• To demonstrate that high-dimensional entanglement (beyond qubits) is essential for maintaining a positive key rate in noisy, real-world channels.
• To validate the feasibility of using hyper-entangled photons for practical quantum communication under harsh conditions, including daylight and turbulence.

Proposed method

• Deployed a phase-stable, non-local Franson interferometer using a shared pump laser reference between sender (Alice) and receiver (Bob) across a 10.2 km free-space link.
• Implemented two 4f-lens systems in Bob’s Mach-Zehnder interferometer to compensate for wave-front distortions caused by atmospheric turbulence.
• Achieved sub-20 ps clock synchronization between Alice and Bob using a two-step locking procedure based on laser-stabilized optical references.
• Used polarization beam splitters and post-selection-free detection to realize Franson interference in the time-energy degree of freedom without requiring classical post-selection.
• Employed a hyper-entangled photon source based on periodically poled KTP (ppKTP) crystals, generating entangled photons at 808.9 nm via spontaneous parametric down-conversion.
• Implemented a dimensionally-adaptive QKD protocol that dynamically adjusts the state dimension to balance noise resilience and information capacity, with all optimizations performed in post-processing.

Experimental results

Research questions

• RQ1 Can non-local temporal interferometry be stabilized over a 10.2 km free-space link to enable high-fidelity quantum interference?
• RQ2 To what extent does high-dimensional energy-time entanglement improve resilience to atmospheric noise and background radiation in free-space QKD?
• RQ3 What is the minimum state dimension required to achieve a positive asymptotic key rate in a noisy, turbulent channel with strong solar background?
• RQ4 Can a dimensionally-adaptive QKD protocol maintain secure key generation without prior knowledge of noise characteristics?
• RQ5 Is entanglement still certifiable and usable for key distribution long after sunrise, when background radiation is high?

Key findings

• The system achieved stable non-local Franson interference over 10.2 km of free space, demonstrating phase coherence across a long-distance atmospheric channel.
• A positive asymptotic key rate was maintained even during daylight, under high solar background radiation, and in the presence of strong atmospheric turbulence.
• A state dimension of at least 4 was required to achieve a positive key rate, with optimal performance observed at higher dimensions, confirming the advantage of high-dimensional encoding.
• The dimensionally-adaptive QKD protocol enabled optimization of key rate based on real-time channel conditions without modifying the hardware or requiring prior noise knowledge.
• Entanglement remained certifiable and distillable well into the day, even when no key could be generated, indicating robustness for future protocols.
• The implementation demonstrated feasibility for space-based quantum communication, as the channel conditions resemble those of low Earth orbit (LEO) satellite links in terms of loss and turbulence.