Editing Quantum key distribution (section)

=== Twin fields quantum key distribution ===
Twin fields quantum key distribution (TFQKD) was introduced in 2018, and is a version of DIQKD designed to overcome the fundamental rate-distance limit of traditional quantum key distribution.<ref name=":3">{{Cite journal |last1=Lucamarini |first1=M. |last2=Yuan |first2=Z. L. |last3=Dynes |first3=J. F. |last4=Shields |first4=A. J. |date=May 2018 |title=Overcoming the rate–distance limit of quantum key distribution without quantum repeaters |url=https://www.nature.com/articles/s41586-018-0066-6 |journal=Nature |language=en |volume=557 |issue=7705 |pages=400–403 |doi=10.1038/s41586-018-0066-6 |pmid=29720656 |issn=1476-4687|arxiv=1811.06826 |bibcode=2018Natur.557..400L |s2cid=256768464 }}</ref> The rate-distance limit, also known as the rate-loss trade off, describes how as distance increases between Alice and Bob, the rate of key generation decreases exponentially.<ref name=":4">{{Cite journal |last1=Takeoka |first1=Masahiro |last2=Guha |first2=Saikat |last3=Wilde |first3=Mark M. |date=2014-10-24 |title=Fundamental rate-loss tradeoff for optical quantum key distribution |journal=Nature Communications |language=en |volume=5 |issue=1 |pages=5235 |doi=10.1038/ncomms6235 |pmid=25341406 |arxiv=1504.06390 |bibcode=2014NatCo...5.5235T |issn=2041-1723|doi-access=free }}</ref> In traditional QKD protocols, this decay has been eliminated via the addition of physically secured relay nodes, which can be placed along the quantum link with the intention of dividing it up into several low-loss sections. Researchers have also recommended the use of quantum repeaters, which when added to the relay nodes make it so that they no longer need to be physically secured.<ref name=":4" /> Quantum repeaters, however, are difficult to create and have yet to be implemented on a useful scale.<ref name=":3" /> TFQKD aims to bypass the rate-distance limit without the use of quantum repeaters or relay nodes, creating manageable levels of noise and a process that can be repeated much more easily with today's existing technology.<ref name=":3" />

The original protocol for TFQKD is as follows: Alice and Bob each have a light source and one arm on an interferometer in their laboratories. The light sources create two dim optical pulses with a randomly phase ''p''<sub>a</sub> or ''p''<sub>b</sub> in the interval {{nowrap|[0, 2π)}} and an encoding phase ''γ''<sub>a</sub> or ''γ''<sub>b</sub>. The pulses are sent along a quantum to Charlie, a third party who can be malicious or not. Charlie uses a beam splitter to overlap the two pulses and perform a measurement. He has two detectors in his own lab, one of which will light up if the bits are equal (00) or (11), and the other when they are different (10, 01). Charlie will announce to Alice and Bob which of the detectors lit up, at which point they publicly reveal the phases ''p'' and ''γ''.<ref name=":3" /> This is different from traditional QKD, in which the phases used are never revealed.<ref name=":5">{{Cite journal |last1=Wang |first1=Shuang |last2=Yin |first2=Zhen-Qiang |last3=He |first3=De-Yong |last4=Chen |first4=Wei |last5=Wang |first5=Rui-Qiang |last6=Ye |first6=Peng |last7=Zhou |first7=Yao |last8=Fan-Yuan |first8=Guan-Jie |last9=Wang |first9=Fang-Xiang |last10=Chen |first10=Wei |last11=Zhu |first11=Yong-Gang |last12=Morozov |first12=Pavel V. |last13=Divochiy |first13=Alexander V. |last14=Zhou |first14=Zheng |last15=Guo |first15=Guang-Can |date=February 2022 |title=Twin-field quantum key distribution over 830-km fibre |url=https://www.nature.com/articles/s41566-021-00928-2 |journal=Nature Photonics |language=en |volume=16 |issue=2 |pages=154–161 |doi=10.1038/s41566-021-00928-2 |bibcode=2022NaPho..16..154W |s2cid=117167883 |issn=1749-4893}}</ref>