Quan­tum Key Dis­tri­bu­ti­on rea­li­zed over Long Distances

Thanks to its immu­ni­ty to cyber­at­tacks, Quan­tum Com­mu­ni­ca­ti­on offers uni­que secu­ri­ty advan­ta­ges com­pared to tra­di­tio­nal tele­com­mu­ni­ca­ti­ons solu­ti­ons and is con­side­red a key tech­no­lo­gy for secu­re data trans­mis­si­on in the future. Recent­ly, a team of rese­ar­chers suc­cee­ded in imple­men­ting Quan­tum Key Dis­tri­bu­ti­on (QKD) under real-world con­di­ti­ons – within the exis­ting fiber optic net­work of Deut­sche Tele­kom – suc­cessful­ly over an unpre­ce­den­ted distance of 254 km bet­ween Frankfurt/Main and Kehl. The results of the expe­ri­ment have now been published in a paper.

Quan­tum Com­mu­ni­ca­ti­on had to be ope­ra­ted along­side tra­di­tio­nal com­mu­ni­ca­ti­on ser­vices – a chall­enge, as signal dis­tur­ban­ces, tem­pe­ra­tu­re fluc­tua­tions, and pha­se drift can signi­fi­cant­ly impact sta­bi­li­ty. To over­co­me the­se dif­fi­cul­ties, the rese­ar­chers reli­ed on a com­bi­na­ti­on of inno­va­ti­ve pha­se syn­chro­niza­ti­on, tem­po­ral sta­bi­liza­ti­on, and adap­ti­ve cali­bra­ti­on algo­rith­ms. Detec­tors that ope­ra­te at room tem­pe­ra­tu­re were used, eli­mi­na­ting the need for cos­t­ly cryo­ge­nic coo­ling. Ins­tead of the com­mon­ly used SNSPDs (Super­con­duc­ting Nano­wire Sin­gle-Pho­ton Detec­tors), Ava­lan­che Pho­to­di­odes were employ­ed – semi­con­duc­tor com­pon­ents that, while less effi­ci­ent, are far more prac­ti­cal for ever­y­day use. The achie­ved key rate was an avera­ge of 110 bits per second – though see­mingly mode­st, this repres­ents a signi­fi­cant tech­no­lo­gi­cal breakth­rough for quan­tum-secu­red trans­mis­si­on over more than 250 km.

For the pro­ject, the rese­ar­chers used Twin-Field-QKD (TF-QKD), a method that allows for effi­ci­ent sca­ling, par­ti­cu­lar­ly over long distances. In this pro­cess, two light pul­ses sent by the com­mu­ni­ca­ti­on part­ners meet in the midd­le – here in Darm­stadt – and inter­fe­re with each other. This quan­tum mecha­ni­cal inter­fe­rence allows for con­clu­si­ons to be drawn about the trans­mit­ted sta­tes wit­hout nee­ding to read them direct­ly. While clas­si­cal pro­to­cols like BB84 lose signi­fi­cant per­for­mance bey­ond appro­xi­m­ate­ly 100 kilo­me­ters, TF-QKD remains sta­ble even over grea­ter distances. Ano­ther key fea­ture of the imple­men­ta­ti­on in Frank­furt was the use of the Mea­su­re­ment-Device-Inde­pen­dent approach: even if the cen­tral mea­su­re­ment node is hacked or mani­pu­la­ted by a third par­ty, the secu­ri­ty of the key trans­mis­si­on remains int­act. This archi­tec­tu­re is well-sui­ted for lar­ge net­works, whe­re not every inter­me­dia­te node can be ful­ly con­trol­led.

In addi­ti­on to its tech­ni­cal suc­cess, the pro­ject also has a stra­te­gic dimen­si­on. Euro­pe – and par­ti­cu­lar­ly Ger­ma­ny – can take a lea­ding role in the area of digi­tal sove­reig­n­ty. While simi­lar efforts exist world­wi­de, inte­gra­ting QKD into exis­ting ter­restri­al infra­struc­tures pres­ents an addi­tio­nal lay­er of com­ple­xi­ty that has been scar­ce­ly addres­sed until now. Over­all, it is clear that the tran­si­ti­on from rese­arch to prac­ti­cal appli­ca­ti­on has now been suc­cessful­ly achie­ved.