A device-independent quantum key distribution system for distant users

Abstract

Device-independent quantum key distribution (DIQKD) enables the generation of secret keys over an untrusted channel using uncharacterized and potentially untrusted devices1–9. The proper and secure functioning of the devices can be certified by a statistical test using a Bell inequality10–12. This test originates from the foundations of quantum physics and also ensures robustness against implementation loopholes13, 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 apart14. By achieving an entanglement fidelity of $$ {\mathcal F} \,\ge 0.892(23)$$ ℱ ≥ 0.892 ( 23 ) and implementing a DIQKD protocol with random key basis15, 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.

Simple and practical DIQKD security analysis via BB84-type uncertainty relations and Pauli correlation constraints

“…The resulting bound holds independently of the actual value of A 1 thanks to the monotonicity property discussed below: if we make in bound 3 the replacement | A 1 | → 0 we obtain a bound that remains valid 5. In the case of bounds (10), (11),(17), it also follows from the stronger property that the function fq(x) is monotonically increasing in x, as shown in Appendix B. of[14].…”

“…The monotonicity of the bound (13) is established in Appendix A from which the monotonicity of the other bounds follows 5 . This property will be important in Section 2.3 as it allows replacing in the entropy bounds the correlators on which they depend in the right-hand side by a lower bound on these correlators and in Section 2.4 where it allows the systematic computation of a convex envelope based on a discrete set of points.…”

“…Following its proposal fifteen years ago, realizing a working DIQKD protocol has long presented a significant challenge both to theorists, due to the mathematical difficulty of devising practical and rigorous security proofs, and to experimental researchers, due to the difficulty of distributing entangled quantum systems with low noise and high detection rates over long distances. Recent advances paved the way to three successful proof-of-principle experiments demonstrating the feasibility of this technology [4][5][6]. However, there is still a long way from these proof-of-principle experiments to practical Michele Masini: michele.masini@ulb.be Stefano Pironio: stefano.pironio@ulb.be Erik Woodhead: erik.woodhead@ulb.be DIQKD implementations, with the necessity to improve the distance and the rate at which the keys are distributed.…”

Hiding Secrets Using Quantum Entanglement

“…Credit: W. Z. Liu and C. Wu/University of Science and Technology of China the protocols specify. Now three research groups, one based in Germany, one in the UK, and one in China, have independently performed proof-of-principle experiments of a quantum encryption method that can secure information even if the devices used do not behave exactly as predicted [1][2][3]. The demonstrations are "a major breakthrough for cybersecurity," says Charles Lim of the National University of Singapore, who was involved in the Germany-based experiments.…”

Bell-state measurement exceeding 50% success probability with linear optics

“…In particular, in quantum communication, BSMs facilitate entanglement swapping and thereby the implementation of quantum repeaters. Furthermore, BSMs enable the realisation of measurement-device-independent quantum communication [9][10][11][12][13][14][15][16]. In quantum computing, BSMs have an important role in photonic quantum computing, in particular in measurement-based and fusion-based approaches [17][18][19][20], where, BSMs are an integral part of the generation of resource states for photonic quantum computing and in fusing small-scale units to large resource states for the realisation of quantum error correction [21,22].…”