Nonlocal Temporal Interferometry for Highly Resilient Free-Space Quantum Communication

Abstract

Entanglement distribution via photons over long distances enables many applications, including quantum key distribution, 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 a long-range free-space quantum link, distributing entanglement over 10.2 km with flexible dimensionality of encoding by deploying a phase-stable nonlocal Franson interferometer. With this distribution of multidimensional energy-time entangled photons, we analyze the achievable key rate in a dimensionally adaptive quantum key distribution protocol that can be optimized with respect to any environmental noise conditions. Our approach enables and emphasizes the power of high-dimensional entanglement for quantum communication, yielding a positive asymptotic key rate well into the dawn of the day.

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Plain Language Summary

Quantum communication promises secure communication based on the laws of physics, instead of computational assumptions. To achieve the highest level of possible security, entangled particles need to be exchanged between communicating parties: Any eavesdropper would reveal their presence by diminishing the entanglement. But sending entangled photons over real channels introduces loss and noise. The latter is hard to overcome for free-space quantum communication, which should ideally work irrespective of weather conditions. Here, we overcome this challenge and demonstrate a robust quantum communications link across more than 10 km of free space in an urban environment.

So far, most work on generating and sending entanglement has focused on using 2 degrees of freedom of photons, such as polarization. This offers two dimensions to the encoding. But by measuring additional properties of the photons, one has access to a greater number of dimensions, and that creates entanglement that is much more robust against noise. The pristine quantum correlations in high-dimensional spaces are more easily distinguishable from a random background. By entangling photons in their time of arrival, we harness this property and build a free-space link over 10.2 km of urban environment.

Using a nonlocally stabilized and synchronized interferometer, a telescope, and a novel quantum key distribution scheme, we demonstrate the ability to obtain a critical key bit rate well within daylight conditions, paving the way for future quantum communication based on high-dimensional photonics.