Editing Quantum key distribution (section)

=== Experimental ===
In 1991, [[John Rarity]], [[Paul Tapster]] and [[Artur Ekert]], researchers from the UK Defence Research Agency in Malvern and Oxford University, demonstrated quantum key distribution protected by the violation of the Bell inequalities.

In 2008, exchange of secure keys at 1&nbsp;Mbit/s (over 20&nbsp;km of optical fibre) and 10&nbsp;kbit/s (over 100&nbsp;km of fibre), was achieved by a collaboration between the [[University of Cambridge]] and [[Toshiba]] using the BB84 protocol with [[decoy state]] pulses.<ref>{{Cite journal |arxiv = 0810.1069|last1 = Dixon|first1 = A.R.|title = Gigahertz decoy quantum key distribution with 1 Mbit/s secure key rate|journal = Optics Express|volume = 16|issue = 23|pages = 18790–7|author2 = Z.L. Yuan|last3 = Dynes|first3 = J.F.|last4 = Sharpe|first4 = A. W.|last5 = Shields|first5 = A. J.|year = 2008|doi = 10.1364/OE.16.018790|pmid = 19581967|bibcode = 2008OExpr..1618790D|s2cid = 17141431}}</ref>

In 2007, [[Los Alamos National Laboratory]]/[[NIST]] achieved quantum key distribution over a 148.7&nbsp;km of optic fibre using the BB84 protocol.<ref>{{cite journal | last1=Hiskett | first1=P A | last2=Rosenberg | first2=D | last3=Peterson | first3=C G | last4=Hughes | first4=R J | last5=Nam | first5=S | last6=Lita | first6=A E | last7=Miller | first7=A J | last8=Nordholt | first8=J E |author8-link=Beth Nordholt| title=Long-distance quantum key distribution in optical fibre | journal=New Journal of Physics | publisher=IOP Publishing | volume=8 | issue=9 | date=2006-09-14 | issn=1367-2630 | doi=10.1088/1367-2630/8/9/193 | pages=193|doi-access=free| bibcode=2006NJPh....8..193H | arxiv=quant-ph/0607177 }}</ref> Significantly, this distance is long enough for almost all the spans found in today's fibre networks. A European collaboration achieved free space QKD over 144&nbsp;km between two of the [[Canary Islands]] using entangled photons (the Ekert scheme) in 2006,<ref>{{Cite journal |arxiv = quant-ph/0607182|last1 = Ursin|first1 = Rupert|title = Entanglement-based quantum communication over 144 km &nbsp;km|journal = Nature Physics|volume = 3|issue = 7|pages = 481–486|first2 = Felix|last2 = Tiefenbacher|last3 = Schmitt-Manderbach|first3 = T.|last4 = Weier|first4 = H.|last5 = Scheidl|first5 = T.|last6 = Lindenthal|first6 = M.|last7 = Blauensteiner|first7 = B.|last8 = Jennewein|first8 = T.|last9 = Perdigues|first9 = J.|last10 = Trojek|first10 = P.|last11 = Ömer|first11 = B.|last12 = Fürst|first12 = M.|last13 = Meyenburg|first13 = M.|last14 = Rarity|first14 = J.|last15 = Sodnik|first15 = Z.|last16 = Barbieri|first16 = C.|last17 = Weinfurter|first17 = H.|last18 = Zeilinger|first18 = A.|year = 2006|doi = 10.1038/nphys629|bibcode = 2006quant.ph..7182U| s2cid=108284907 }}</ref> and using BB84 enhanced with [[decoy states]]<ref name="HwangDecoy">{{cite journal | last=Hwang | first=Won-Young | title=Quantum Key Distribution with High Loss: Toward Global Secure Communication | journal=Physical Review Letters | volume=91 | issue=5 | date=2003-08-01 | issn=0031-9007 | doi=10.1103/physrevlett.91.057901 | pmid=12906634 | page=057901| arxiv=quant-ph/0211153 | bibcode=2003PhRvL..91e7901H | s2cid=19225674 }}</ref><ref name="VWDecoy">H.-K. Lo, in Proceedings of 2004 IEEE ISIT (IEEE Press, New York, 2004), p. 137</ref><ref name="wangDecoy">{{cite journal | last=Wang | first=Xiang-Bin | title=Beating the Photon-Number-Splitting Attack in Practical Quantum Cryptography | journal=Physical Review Letters | volume=94 | issue=23 | date=2005-06-16 | issn=0031-9007 | doi=10.1103/physrevlett.94.230503 | pmid=16090451 | page=230503|arxiv=quant-ph/0410075| bibcode=2005PhRvL..94w0503W | s2cid=2651690 }}</ref><ref name="LoDecoy">H.-K. Lo, X. Ma, K. Chen, [https://archive.today/20120712103031/http://prl.aps.org/abstract/PRL/v94/i23/e230504 "Decoy State Quantum Key Distribution"], Physical Review Letters, 94, 230504 (2005)</ref><ref name="PracticalDecoy">{{Cite journal | doi=10.1103/PhysRevA.72.012326|title = Practical decoy state for quantum key distribution| journal=Physical Review A| volume=72|issue = 1|pages = 012326|year = 2005|last1 = Ma|first1 = Xiongfeng| last2=Qi| first2=Bing| last3=Zhao| first3=Yi| last4=Lo| first4=Hoi-Kwong| arxiv=quant-ph/0503005|bibcode = 2005PhRvA..72a2326M|s2cid = 836096}}</ref> in 2007.<ref>{{cite journal | last1=Schmitt-Manderbach | first1=Tobias | last2=Weier | first2=Henning | last3=Fürst | first3=Martin | last4=Ursin | first4=Rupert | last5=Tiefenbacher | first5=Felix | last6=Scheidl | first6=Thomas | last7=Perdigues | first7=Josep | last8=Sodnik | first8=Zoran | last9=Kurtsiefer | first9=Christian | last10=Rarity | first10=John G. | last11=Zeilinger | first11=Anton | last12=Weinfurter | first12=Harald |display-authors=5| title=Experimental Demonstration of Free-Space Decoy-State Quantum Key Distribution over 144&nbsp;km | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=98 | issue=1 | date=2007-01-05 | issn=0031-9007 | doi=10.1103/physrevlett.98.010504 | pmid=17358463 | page=010504|url=http://xqp.physik.lmu.de/publications/files/articles_2007/prl_98_010504.pdf| bibcode=2007PhRvL..98a0504S | s2cid=15102161 }}</ref>

{{As of|2015|8}} the longest distance for optical fiber (307&nbsp;km)<ref name="Korzh14">
{{cite journal
|last1=Korzh
|first1=Boris
|last2=Lim
|first2=Charles Ci Wen
|last3=Houlmann
|first3=Raphael
|last4=Gisin
|first4=Nicolas
|last5=Li
|first5=Ming Jun
|last6=Nolan
|first6=Daniel
|last7=Sanguinetti
|first7=Bruno
|last8=Thew
|first8=Rob
|last9=Zbinden
|first9=Hugo
|arxiv=1407.7427
|title=Provably Secure and Practical Quantum Key Distribution over 307&nbsp;km of Optical Fibre
|year=2015
|doi=10.1038/nphoton.2014.327
|volume=9
|issue=3
|journal=Nature Photonics
|pages=163–168
|bibcode=2015NaPho...9..163K
|s2cid=59028718
}}
</ref> was achieved by [[University of Geneva]] and [[Corning Inc.]] In the same experiment, a secret key rate of 12.7&nbsp;kbit/s was generated, making it the highest bit rate system over distances of 100&nbsp;km. In 2016 a team from Corning and various institutions in China achieved a distance of 404&nbsp;km, but at a bit rate too slow to be practical.<ref>{{cite journal
|title=Satellite-based entanglement distribution over 1200 kilometers
| volume=356
| doi=10.1126/science.aan3211
| pmid=28619937
| number=6343
| journal=Science
| last1=Yin
| first1=Juan
|last2=Cao
|first2= Yuan 
|last3= Li
|first3= Yu-Huai
|last4= Liao
|first4= Sheng-Kai
|last5= Zhang
|first5= Liang
|last6= Ren
|first6= Ji-Gang 
|last7= Cai
|first7= Wen-Qi
|last8=Liu
|first8= Wei-Yue
|last9= Li
|first9= Bo 
|last10= Dai
|first10=Hui
|display-authors=etal
| year=2017
| pages=1140–1144| arxiv=1707.01339
| bibcode=2017arXiv170701339Y
| s2cid=5206894
}}</ref>

In June 2017, physicists led by [[Thomas Jennewein]] at the [[Institute for Quantum Computing]] and the [[University of Waterloo]] in [[Waterloo, Ontario|Waterloo, Canada]] achieved the first demonstration of quantum key distribution from a ground transmitter to a moving aircraft. They reported optical links with distances between 3–10&nbsp;km and generated secure keys up to 868 kilobytes in length.<ref>{{cite journal | last1 = Pugh | first1 = C. J. | last2 = Kaiser | first2 = S. | last3 = Bourgoin | first3 = J.- P. | last4 = Jin | first4 = J. | last5 = Sultana | first5 = N. | last6 = Agne | first6 = S. | last7 = Anisimova | first7 = E. | last8 = Makarov | first8 = V. | last9 = Choi | first9 = E. | last10 = Higgins | first10 = B. L. | last11 = Jennewein | first11 = T. | year = 2017 | title = Airborne demonstration of a quantum key distribution receiver payload | journal = Quantum Science and Technology | volume = 2 | issue = 2| page = 024009 | doi = 10.1088/2058-9565/aa701f | arxiv = 1612.06396 | bibcode = 2017QS&T....2b4009P | s2cid = 21279135 }}</ref>

Also in June 2017, as part of the [[Quantum Experiments at Space Scale]] project, Chinese physicists led by [[Pan Jianwei]] at the [[University of Science and Technology of China]] measured entangled photons over a distance of 1203&nbsp;km between two ground stations, laying the groundwork for future intercontinental quantum key distribution experiments.<ref>{{cite web|url=https://www.science.org/content/article/china-s-quantum-satellite-achieves-spooky-action-record-distance |title=China's quantum satellite achieves 'spooky action' at a record distance |date=2017-06-15 |access-date=2017-06-15}}</ref> Photons were sent from one ground station to the satellite they had named ''[[Micius (satellite)|Micius]]'' and back down to another ground station, where they "observed a survival of two-photon entanglement and a violation of Bell inequality by 2.37 ± 0.09 under strict Einstein locality conditions" along a "summed length varying from 1600 to 2400 kilometers."<ref>{{cite journal | last1 = Yin | first1 = J. | author-link33 = J.- W. Pan | last2 = Cao | first2 = Y. | last3 = Li | first3 = Y.- H. | last4 = Liao | first4 = S.- K. | last5 = Zhang | first5 = L. | last6 = Ren | first6 = J.- G. | last7 = Cai | first7 = W.- Q. | last8 = Liu | first8 = W.- Y. | last9 = Li | first9 = B. | last10 = Dai | first10 = H. | last11 = Li | first11 = G.- B. | last12 = Lu | first12 = Q.- M. | last13 = Gong | first13 = Y.- H. | last14 = Xu | first14 = Y. | last15 = Li | first15 = S.- L. | last16 = Li | first16 = F.- Z. | last17 = Yin | first17 = Y.- Y. | last18 = Jiang | first18 = Z.- Q. | last19 = Li | first19 = M. | last20 = Jia | first20 = J.- J. | last21 = Ren | first21 = G. | last22 = He | first22 = D. | last23 = Zhou | first23 = Y.- L. | last24 = Zhang | first24 = X.- X. | last25 = Wang | first25 = N. | last26 = Chang | first26 = X. | last27 = Zhu | first27 = Z.- C. | last28 = Liu | first28 = N.- L. | last29 = Lu | first29 = C.- Y. | last30 = Shu | first30 = R. | last31 = Peng | first31 = C.- Z. | last32 = Wang | first32 = J.- Y. | last33 = Pan | first33 = J.- W. | year = 2017| title = Satellite-based entanglement distribution over 1200 kilometers | journal = Science | volume = 356 | issue = 6343| pages = 1140–4 | doi = 10.1126/science.aan3211 | pmid = 28619937 | arxiv = 1707.01339 | doi-access = free }}</ref> Later that year BB84 was successfully implemented over satellite links from ''Micius'' to ground stations in China and Austria. The keys were combined and the result was used to transmit images and video between Beijing, China, and Vienna, Austria.<ref>
{{cite journal
|  title = Satellite-Relayed Intercontinental Quantum Network
|  volume = 120
|  doi = 10.1103/PhysRevLett.120.030501
|  pmid = 29400544
|  number =3 
|  journal =	Physical Review Letters
|  last1 =Liao
|first1 = Sheng-Kai
|last2= Cai
|first2= Wen-Qi
|last3= Handsteiner|
first3 =   Johannes|
last4= Liu|
first4= Bo |
last5 =Yin|
first5= Juan|
last6= Zhang|
first6= Liang|
last7= Rauch|
first7= Dominik |
last8 = Fink|
first8= Matthias |
last9 = Ren|
first9= Ji-Gang |
last10= Liu|
first10= Wei-Yue
|display-authors=etal|
  year =2018|
  pages =030501
|arxiv=1801.04418|bibcode=2018PhRvL.120c0501L| s2cid = 206306725
}}</ref>

In August 2017, a group at Shanghai Jiaotong University experimentally demonstrate that polarization quantum states including general qubits of single photon and entangled states can survive well after travelling through seawater,<ref>{{Cite journal |doi=10.1364/OE.25.019795 |title=Towards quantum communications in free-space seawater |year=2017 |last1=Ji |first1=Ling |last2=Gao |first2=Jun |last3=Yang |first3=Ai-Lin |last4=Feng |first4=Zhen |last5=Lin |first5=Xiao-Feng |last6=Li |first6=Zhong-Gen |last7=Jin |first7=Xian-Min |journal=Optics Express |volume=25 |issue=17 |pages=19795–19806 |pmid=29041667 |s2cid=46757097 |arxiv=1602.05047 |bibcode=2017OExpr..2519795J }}</ref> representing the first step towards underwater quantum communication.

In May 2019 a group led by Hong Guo at Peking University and Beijing University of Posts and Telecommunications reported field tests of a continuous-variable QKD system through commercial fiber networks in Xi'an and Guangzhou over distances of 30.02&nbsp;km (12.48&nbsp;dB) and 49.85&nbsp;km (11.62&nbsp;dB) respectively.<ref>{{cite journal|last1=Zhang|first1=Yichen|last2=Li|first2=Zhengyu|last3=Chen|first3=Ziyang|last4=Weedbrook|first4=Christian|last5=Zhao|first5=Yijia|last6=Wang|first6=Xiangyu|last7=Huang|first7=Yundi|last8=Xu|first8=Chunchao|last9=Zhang|first9=Xiaoxiong|last10=Wang|first10=Zhenya|last11=Li|first11=Mei|last12=Zhang|first12=Xueying|last13=Zheng|first13=Ziyong|last14=Chu|first14=Binjie|last15=Gao|first15=Xinyu|last16=Meng|first16=Nan|last17=Cai|first17=Weiwen|last18=Wang|first18=Zheng|last19=Wang|first19=Gan|last20=Yu|first20=Song|last21=Guo|first21=Hong|title=Continuous-variable QKD over 50&nbsp;km commercial fiber|journal=Quantum Science and Technology|date=2019|volume=4|issue=3|pages= 035006|doi=10.1088/2058-9565/ab19d1|arxiv=1709.04618|bibcode=2019QS&T....4c5006Z|s2cid=116403328}}</ref>

In December 2020, Indian [[Defence Research and Development Organisation]] tested a QKD between two of its laboratories in Hyderabad facility. The setup also demonstrated the validation of detection of a third party trying to gain knowledge of the communication. Quantum based security against eavesdropping was validated for the deployed system at over {{cvt|12|km}} range and 10&nbsp;dB attenuation over fibre optic channel. A [[continuous wave]] laser source was used to generate photons without depolarization effect and timing accuracy employed in the setup was of the order of picoseconds. The [[Single-photon avalanche diode|Single photon avalanche detector]] (SPAD) recorded arrival of photons and key rate was achieved in the range of kbps with low Quantum bit error rate.<ref>{{Cite news|last=Ministry of Defence|date=2020-12-09|title=Quantum Communication between two DRDO Laboratories|url=https://pib.gov.in/PressReleasePage.aspx?PRID=1679349|work=Press Information Bureau|access-date=2021-03-22}}</ref>

In March 2021, [[Indian Space Research Organisation]] also demonstrated a free-space Quantum Communication over a distance of 300 meters. A free-space QKD was demonstrated at [[Space Applications Centre]] (SAC), Ahmedabad, between two line-of-sight buildings within the campus for video conferencing by quantum-key encrypted signals. The experiment utilised a [[NAVIC]] receiver for time synchronization between the transmitter and receiver modules. Later in January 2022, Indian scientists were able to successfully create an atmospheric channel for exchange of crypted messages and images. After demonstrating quantum communication between two ground stations, India has plans to develop Satellite Based Quantum Communication (SBQC).<ref>{{Cite news|date=2021-03-22|title=ISRO makes breakthrough demonstration of free-space Quantum Key Distribution (QKD) over 300 m|url=https://www.isro.gov.in/update/22-mar-2021/isro-makes-breakthrough-demonstration-of-free-space-quantum-key-distribution-qkd|work=Indian Space Research Organisation|access-date=2021-03-22|archive-date=22 March 2021|archive-url=https://web.archive.org/web/20210322164134/https://www.isro.gov.in/update/22-mar-2021/isro-makes-breakthrough-demonstration-of-free-space-quantum-key-distribution-qkd|url-status=dead}}</ref><ref>{{Cite news|date=2022-01-31|title=Department of Space demonstrates entanglement based quantum communication over 300m free space along with real time cryptographic applications|url=https://www.isro.gov.in/update/31-jan-2022/department-of-space-demonstrates-entanglement-based-quantum-communication-over|work=Indian Space Research Organisation|access-date=2022-02-01|archive-date=1 February 2022|archive-url=https://web.archive.org/web/20220201151914/https://www.isro.gov.in/update/31-jan-2022/department-of-space-demonstrates-entanglement-based-quantum-communication-over|url-status=dead}}</ref>

In July 2022, researchers published their work experimentally implementing a device-independent quantum key distribution (DIQKD) protocol that uses quantum entanglement (as suggested by Ekert)<ref name=":0" /> to insure resistance to quantum hacking attacks.<ref name=":1" /> They were able to create two ions, about two meters apart that were in a high quality entangled state using the following process: Alice and Bob each have ion trap nodes with an <sup>88</sup>Sr<sup>+</sup> qubit inside. Initially, they excite the ions to an electronic state, which creates an entangled state. This process also creates two photons, which are then captured and transported using an optical fiber, at which point a Bell-basis measurement is performed and the ions are projected to a highly entangled state. Finally the qubits are returned to new locations in the ion traps disconnected from the optical link so that no information can be leaked. This is repeated many times before the key distribution proceeds.<ref name=":1" />

A separate experiment published in July 2022 demonstrated implementation of DIQKD that also uses a Bell inequality test to ensure that the quantum device is functioning, this time at a much larger distance of about 400m, using an optical fiber 700m long.<ref name=":2" /> The set up for the experiment was similar to the one in the paragraph above, with some key differences. Entanglement was generated in a quantum network link (QNL) between two <sup>87</sup>Rb atoms in separate laboratories located 400m apart, connected by the 700m channel. The atoms are entangled by electronic excitation, at which point two photons are generated and collected, to be sent to the bell state measurement (BSM) setup. The photons are projected onto a |ψ<sup>+</sup> state, indicating maximum entanglement. The rest of the key exchange protocol used is similar to the original QKD protocol, with the only difference being that keys are generated with two measurement settings instead of one.<ref name=":2" />

Since the proposal of Twin Field Quantum Key Distribution in 2018, a myriad of experiments have been performed with the goal of increasing the distance in a QKD system. The most successful of which was able to distribute key information across a distance of 833.8&nbsp;km.<ref name=":5" />

In 2023, scientists at Indian Institute of Technology (IIT) Delhi have achieved a trusted-node-free quantum key distribution (QKD) up to ''380 km'' in standard telecom fiber with a very low quantum bit error rate (QBER).<ref>{{Cite web|url=https://www.educationtimes.com/article/campus-beat-college-life/99733828/emerging-technologies-iit-delhi-researchers-achieve-secure-quantum-communication-for-380-km-in-standard-telecom-fiber|title=EMERGING TECHNOLOGIES: IIT Delhi researchers achieve secure quantum communication for 380 km in standard telecom fiber - EducationTimes.com|website=www.educationtimes.com}}</ref>

In 2024 scientists in South Africa and China achieved quantum key distribution in the atmosphere with a record breaking distance of 12,900&nbsp;km,  using lasers and a [[Microsatellite (spaceflight)|microsatellite]] in [[low Earth orbit]]. They transferred over a million quantum-secure bits between South Africa and China during one orbit of the satellite.<ref>{{Cite journal |last=Li |first=Yang |last2=Cai |first2=Wen-Qi |last3=Ren |first3=Ji-Gang |last4=Wang |first4=Chao-Ze |last5=Yang |first5=Meng |last6=Zhang |first6=Liang |last7=Wu |first7=Hui-Ying |last8=Chang |first8=Liang |last9=Wu |first9=Jin-Cai |last10=Jin |first10=Biao |last11=Xue |first11=Hua-Jian |last12=Li |first12=Xue-Jiao |last13=Liu |first13=Hui |last14=Yu |first14=Guang-Wen |last15=Tao |first15=Xue-Ying |date=2025-03-19 |title=Microsatellite-based real-time quantum key distribution |url=https://www.nature.com/articles/s41586-025-08739-z |journal=Nature |language=en |pages=1–8 |doi=10.1038/s41586-025-08739-z |issn=1476-4687}}</ref>