arXiv subjects

Kai Redeker

Publications and source records attributed to Kai Redeker.

5 recordsLinked to original sources

arXiv2021

Experimental device-independent quantum key distribution between distant users

Device-independent quantum key distribution (DIQKD) is the art of using untrusted devices to establish secret keys over an untrusted channel. So far, the real-world implementation of DIQKD remains a major challenge, as it requires the demonstration of a loophole-free Bell test across two remote locations with very high quality entanglement to ensure secure key exchange. Here, we demonstrate for the first time the distribution of a secure key -- based on asymptotic security estimates -- in a fully device-independent way between two users separated by 400 metres. The experiment is based on heralded entanglement between two independently trapped single Rubidium 87 atoms. The implementation of a robust DIQKD protocol indicates an expected secret key rate of r=0.07 per entanglement generation event and r>0 with a probability error of 3%. Furthermore, we analyse the experiment's capability to distribute a secret key with finite-size security against collective attacks.

quant-ph↗

arXiv2021

Self-testing with finite statistics enabling the certification of a quantum network link

Self-testing is a method to certify devices from the result of a Bell test. Although examples of noise tolerant self-testing are known, it is not clear how to deal efficiently with a finite number of experimental trials to certify the average quality of a device without assuming that it behaves identically at each run. As a result, existing self-testing results with finite statistics have been limited to guarantee the proper working of a device in just one of all experimental trials, thereby limiting their practical applicability. We here derive a method to certify through self-testing that a device produces states on average close to a Bell state without assumption on the actual state at each run. Thus the method is free of the I.I.D. (independent and identically distributed) assumption. Applying this new analysis on the data from a recent loophole-free Bell experiment, we demonstrate the successful distribution of Bell states over 398 meters with an average fidelity of $\geq$55.50% at a confidence level of 99%. Being based on a Bell test free of detection and locality loopholes, our certification is evidently device-independent, that is, it does not rely on trust in the devices or knowledge of how the devices work. This guarantees that our link can be integrated in a quantum network for performing long-distance quantum communications with security guarantees that are independent of the details of the actual implementation.

quant-ph↗

arXiv2021

Certified Randomness from Remote State Preparation Dimension Witness

Randomness in Bell test data can be device-independently certified by Bell's theorem without placing assumptions about the experimental devices. The device-independent randomness has very demanding requirement about the experimental devices and relatively lower output randomness. With the same Bell test data we can extract substantially more randomness without using Bell's theorem. To achieve this goal, we introduce a remote state preparation dimension witness and a semi-device-independent randomness certification model which is based on it. This is one important step towards practical use of Bell test in randomness generation.

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arXiv2019

Long-distance distribution of atom-photon entanglement at telecom wavelength

Entanglement between stationary quantum memories and photonic channels is the essential resource for future quantum networks. Together with entanglement distillation it will enable for efficient distribution of quantum states. Here we report on the generation and observation of entanglement between a Rb-87 atom and a photon at telecom wavelength over 20 km optical fiber. For this purpose, we use polarization-preserving quantum frequency conversion to transform the wavelength of a photon entangled with the atomic spin state from 780 nm to the telecom S-band at 1522 nm. We achieve an unprecedented external device conversion efficiency of 57% and observe an entanglement fidelity between the atom and telecom photon of $\geqslant$78.5$\pm$0.9% over 20 km optical fiber, mainly limited by decoherence of the atomic state. This result is an important milestone on the road to distribute quantum information on a large scale.

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arXiv2017

Event-Ready Bell Test Using Entangled Atoms Simultaneously Closing Detection and Locality Loopholes

An experimental test of Bell's inequality allows ruling out any local-realistic description of nature by measuring correlations between distant systems. While such tests are conceptually simple, there are strict requirements concerning the detection efficiency of the involved measurements, as well as the enforcement of spacelike separation between the measurement events. Only very recently could both loopholes be closed simultaneously. Here we present a statistically significant, event-ready Bell test based on combining heralded entanglement of atoms separated by $398\,\mathrm{m}$ with fast and efficient measurements of the atomic spin states closing essential loopholes. We obtain a violation with $S=2.221\pm0.033$ (compared to the maximal value of 2 achievable with models based on local hidden variables) which allows us to refute the hypothesis of local-realism with a significance level $P<2.57\cdot10^{-9}$.

quant-ph↗