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QuNET initia­tive: One step clo­ser to highly secure quan­tum communication
Quan­tum key suc­cessfully dis­tri­bu­ted bet­ween two points with com­bi­na­tion of free-space and fiber links

Rese­ar­chers from Jena, Ber­lin, Erlan­gen-Nurem­berg and Wess­ling have suc­cessfully dis­tri­bu­ted quan­tum keys bet­ween two points using a com­bi­na­tion of free-space and fiber links under ever­y­day con­di­ti­ons. On a hete­ro­ge­neous test bed in Jena, they achie­ved key rates in the kilo­bit range per second in day­light. The expe­ri­ment was imple­men­ted as part of the QuNET initia­tive, a pilot pro­ject fun­ded by the Ger­man Fede­ral Minis­try of Edu­ca­tion and Rese­arch (BMBF) to deve­lop highly secure com­mu­ni­ca­tion sys­tems based on quan­tum technologies.

The com­mu­ni­ca­tion of the future is to become more secure with the help of light par­tic­les. This is the goal of the QuNET initia­tive by the BMBF. The initiative’s part­ners – the Max Planck Insti­tute for the Phy­sics of Light, Fried­rich Alex­an­der Uni­ver­sity Erlan­gen-Nurem­berg, the DLR Insti­tute of Com­mu­ni­ca­ti­ons and Navi­ga­tion, and the two Fraun­ho­fer Insti­tu­tes for Applied Optics and Pre­cis­ion Engi­nee­ring IOF and the Hein­rich Hertz Insti­tute HHI – have now taken an important step toward quan­tum-safe net­works: With a key expe­ri­ment, they have shown how mul­ti­ple quan­tum-secu­red point-to-point links can be rea­li­zed and com­bi­ned for future sca­lable quan­tum-safe net­works. They not only com­bi­ned trans­mis­si­ons of quan­tum keys via free-space and fiber links, but also achie­ved trans­mis­sion rates in the kilo­bit range per second in daylight.

»One goal of the key expe­ri­ment was to demons­trate the dis­tri­bu­tion of quan­tum keys in hete­ro­ge­neous ad-hoc links in day­light,« explains Dr. Thors­ten Goe­bel, coor­di­na­tor in the QuNET Office at the Fraun­ho­fer Insti­tute for Applied Optics and Pre­cis­ion Engi­nee­ring IOF. »Hete­ro­ge­neous in this case means that we dis­tri­bute quan­tum keys bet­ween two points with a com­bi­na­tion of free-space and fiber links to bridge fiber gaps. And all of this with an ad-hoc cha­rac­ter, i.e., estab­li­shing the con­nec­tion as quickly as possible.«

Test bed over two kilo­me­ters in the urban area of Jena

In this spe­ci­fic case, the rese­ar­chers estab­lished a quan­tum-secu­red con­nec­tion on a nearly two-kilo­me­ter-long test track in Jena. The quan­tum key dis­tri­bu­tion was imple­men­ted in two stages: The jour­ney began on the roof of the Jena public uti­lity com­pany. There is a green con­tai­ner with a telescope for sen­ding quan­tum keys in its belly. From here, light par­tic­les, which form the basis for gene­ra­ting a highly secure quan­tum key, first fly over 1.7 kilo­me­ters as the crow flies to the Beu­ten­berg Cam­pus Jena. There, they are picked up by a recei­ving sta­tion in ano­ther con­tai­ner on the grounds of the Fraun­ho­fer Insti­tute. From this inter­me­diate node, the signal is fed into a fiber link and for­warded via 300 meters of fiber to a buil­ding adja­cent to the insti­tute. There, a quan­tum key is finally gene­ra­ted from the mea­su­re­ments on the light particles.

Even in the case of lon­ger trans­mis­sion distances i.e., when a direct dis­tri­bu­tion of quan­tum keys is not pos­si­ble, the rese­ar­chers have taken pre­cau­ti­ons: By sui­ta­bly com­bi­ning keys at trus­ted inter­me­diate sta­ti­ons along these lon­ger distances, the key dis­tri­bu­tion beco­mes pos­si­ble over even grea­ter distances.

Mobile quan­tum link allows bridging of fiber gaps

»Our key expe­ri­ment thus demons­tra­tes how the com­bi­na­tion of mul­ti­ple links can suc­ceed in bridging fiber gaps, i.e., distances where the lack of lines makes fully fiber-based trans­mis­sion impos­si­ble,« Dr. Goe­bel con­ti­nues. »An often-cited exam­ple here would be a sum­mit in rural regi­ons with patchy fiber infra­struc­ture.« But natu­ral boun­da­ries, such as bridging a river, are also a con­ceiva­ble appli­ca­tion sce­na­rio for a point-to-point con­nec­tion bet­ween trans­mit­ter and recei­ver that can be estab­lished for a short time.

Ano­ther important aspect of the expe­ri­ment is its mobile cha­rac­ter. The two quan­tum con­tai­ners used by the rese­ar­chers, also cal­led QuBU­Ses, are basi­cally trans­por­ta­ble. They could, for exam­ple, be taken to any loca­tion by a vehicle and could estab­lish a quan­tum-secu­red con­nec­tion there as nee­ded. In this way, quan­tum com­mu­ni­ca­tion can be imple­men­ted at the most diverse locations.

Rese­ar­chers achieve key gene­ra­tion rates in the kilo­bit range

With their expe­ri­ment, the rese­ar­chers also achie­ved key gene­ra­tion rates in the kilo­bit range per second, even in direct mid­day sun­light. The rese­ar­chers thus also achie­ved an important cri­ter­ion for prac­ti­cal use, because intense solar radia­tion usually impairs the exch­ange of quan­tum-based keys. In many expe­ri­ments, the quan­tum keys were the­r­e­fore dis­tri­bu­ted at night and pre-stored for day­time com­mu­ni­ca­tion. With the deve­lo­p­ment of spe­cial fil­ters, day­time key gene­ra­tion is now possible.

In addi­tion, one aspect of this expe­ri­ment was the demons­tra­tion of hybrid quan­tum key dis­tri­bu­tion. This invol­ves the simul­ta­neous imple­men­ta­tion of dif­fe­rent pro­to­cols for key dis­tri­bu­tion, which was thus able to demons­trate the agi­lity of the deve­lo­ped infra­struc­ture, in par­ti­cu­lar the QuBUS plat­form, in terms of the pro­to­cols used. This is important, since the deve­lo­p­ment of quan­tum key dis­tri­bu­tion still offers many pos­si­bi­li­ties for expan­sion, and these should not be rest­ric­ted by the infra­struc­ture used. Thus, the deve­lo­ped infra­struc­ture is also future-proof and can be used for any pro­to­cols of quan­tum key dis­tri­bu­tion wit­hout major adaptations.

For this pur­pose, pro­to­cols for quan­tum key dis­tri­bu­tion were also tes­ted, which mea­sure the elec­tric fields ins­tead of indi­vi­dual light par­tic­les. The rese­ar­chers were able to show that this approach, which is very close to the tech­no­logy used in clas­si­cal tele­com­mu­ni­ca­ti­ons, is sui­ta­ble for quan­tum key dis­tri­bu­tion wit­hout addi­tio­nal fil­ters even in the case of fluc­tua­ting trans­mis­sion chan­nels of the free-space link in daylight.

First quan­tum-secu­red video­con­fe­rence alre­ady rea­li­zed in 2021

The expe­ri­ment in Jena is the second public demons­tra­tion of tech­no­logy deve­lo­p­ment in the QuNET initia­tive: QuNET rese­ar­chers had alre­ady suc­cessfully imple­men­ted a quan­tum-secu­red video­con­fe­rence bet­ween two fede­ral agen­cies in August 2021. At that time, a con­nec­tion bet­ween the Fede­ral Minis­try of Edu­ca­tion and Rese­arch and the Fede­ral Office for Infor­ma­tion Secu­rity (BSI) was implemented.

Under the fol­lo­wing link you will find the com­plete press release (incl. FAQ on the QuNET initia­tive) as well as press images: