A device-independent quantum key distribution system for distant users

TLDR

Device‑independent quantum key distribution (DIQKD) enables secret key generation over untrusted channels using uncharacterized devices, with security certified by Bell‑inequality tests that are robust against implementation loopholes but are difficult to realize over long distances due to the need for high‑quality entanglement. The authors present an experimental system that enables DIQKD between two users separated by 400 m. The system generates and analyzes event‑ready entanglement between two independently trapped single rubidium atoms located in separate buildings 400 m apart. With an entanglement fidelity of ≥0.892, a Bell‑inequality violation of S = 2.578, a quantum‑bit‑error rate of 0.078, and a secret‑key rate of 0.07 bits per entanglement event, the experiment demonstrates secure key exchange with potentially untrusted devices.

Abstract

Abstract Device-independent quantum key distribution (DIQKD) enables the generation of secret keys over an untrusted channel using uncharacterized and potentially untrusted devices 1–9 . The proper and secure functioning of the devices can be certified by a statistical test using a Bell inequality 10–12 . This test originates from the foundations of quantum physics and also ensures robustness against implementation loopholes 13 , thereby leaving only the integrity of the users’ locations to be guaranteed by other means. The realization of DIQKD, however, is extremely challenging—mainly because it is difficult to establish high-quality entangled states between two remote locations with high detection efficiency. Here we present an experimental system that enables for DIQKD between two distant users. The experiment is based on the generation and analysis of event-ready entanglement between two independently trapped single rubidium atoms located in buildings 400 metre apart 14 . By achieving an entanglement fidelity of $$ {\mathcal F} \,\ge 0.892(23)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>ℱ</mml:mi> <mml:mspace/> <mml:mo>≥</mml:mo> <mml:mn>0.892</mml:mn> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mn>23</mml:mn> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> and implementing a DIQKD protocol with random key basis 15 , we observe a significant violation of a Bell inequality of S = 2.578(75)—above the classical limit of 2—and a quantum bit error rate of only 0.078(9). For the protocol, this results in a secret key rate of 0.07 bits per entanglement generation event in the asymptotic limit, and thus demonstrates the system’s capability to generate secret keys. Our results of secure key exchange with potentially untrusted devices pave the way to the ultimate form of quantum secure communications in future quantum networks.