How Quantum Key Distribution Works (BB84 & E91)

Discussion about how quantum key distribution methods based on measuring the polarization of photons can be used to keep communications secure. In particular, the methods of Bennett and Brassard (BB84) and Ekert (E91) are explained.

Contents
00:00 - Introduction
00:25 - One-time pad
02:17 - Public key cryptography
03:22 - Photon polarization
04:46 - BB84
08:31 - No-cloning theorem
09:06 - Quantum networks
09:55 - E91
11:50 - Closing remarks

References
[1] Black Marble 2016, NASA's Goddard Space Flight Center
[2] IQM Quantum Computer in Espoo Finland by Ragsxl,
[3] C. H. Bennett and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing”, Proceedings of the International Conference on Computers, Systems and Signal Processing 175 (1984).
[4] C. H. Bennett and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing”, Theoretical Computer Science 560, 7 (2014).
[5] S.-K. Liao et al., “Satellite-Relayed Intercontinental Quantum Network”, Physical Review Letters 120, 030501 (2018).
[6] A. K. Ekert, “Quantum Cryptography Based on Bell’s Theorem”, Physical Review Letters 67, 661 (1991).
[7] R. Ursin et al., “Entanglement-based quantum communication over 144 km”, Nature Physics 3, 481 (2007).
[8] 3D cut of the LHC dipole , CERN, OPEN-PHO-ACCEL-2014-003-8
[9] NASA's Laser Communications: To the Space Station and Beyond, NASA's Goddard Space Flight Center
[10] David J. Griffiths and Darrell F. Schroeter, “Introduction to Quantum Mechanics Third Edition”, (CUP 2018). ISBN: 978-1-107-18963-8

[skyvoux]

Very good explanation of QKD! The use of photon polarization in these protocols is called a Discrete Variable QKD, it has some downsides like requiring very good single-photon detectors, so nowadays there's a lot of research on Continuous Variable QKD (CV-QKD), which can use the very Electric Field of the signal as the quantum variable, the best part is that it's been shown to work on regular telecommunication devices, which is much more attractive for companies.
I did some research in this field and it does seem like a complete CV-QKD system is almost achieved, the major problem is the speed, which is much slower than conventional communication due to the high complexity of the reconciliation protocols.

[EspHack]

the kind of video I have to watch again over a few weeks/months, I've missed that so thank you sir.

[pizzacrusher]

hooray! another IM video! I’ve been eagerly awaiting it.

[mikedrop]

He has returned to drop another nugget of knowledge upon us!

[chrisanders]

This video is so powerful. Keep up the good work!

[forloop]

I have a couple of quesitons:
1.Is this type of encryption theoretically unbreakable?
2.When the satellite recorded the signal sent to it by China, shouldn't that signal be stored in the satellite's memory, a memory that can be read.
Thanks!

[theondono]

Wouldn’t this mean that you’re limited to the distance of a single fiber optic? If all the security comes from not being able to clone the photons, no repeaters are possible, the best you can hope for is connecting to an intermediate node and form a sort of “chain” of nodes that go from one end to the other.

But now every single one of these nodes is a critical part of your security. Given that the threat to current technology (Quantum Computers) are nowhere near what they’d need to be an actual threat, this looks like a great source for academic papers with very little practical benefit.

[CrackheadMagnate]

Hey @imporable matter, will u do a sparc video in the future ? I saw it had a Q of 11 but I'm not a fusion scientist so I came here

[Danji_Coppersmoke]

BB84 (7:00) basis exchange ONLY AFTER information bit HAD BEEN sent... even if both knew their basis ahead of transmission. ( just my 2c.. no expert.. I first confused with thought what if middle man got all those basis and measure it... but then there is nothing to measure since all photons had been sent... maybe I am just slow learner.. lol) .. great explanation... I knew nothing before .. now a little.. lol

[andrewnelson]

Could we get a future video about public key encryption?

[chengong]

Wouldn’t the atmosphere or molecules in a fibre optic cable mess up the superposition?

[leilanicatrose]

Sorry but this is quite confusing at 8:15, why on earth will bob receive a diagonal to right photon, if the one sent by the eavesdropper is vertical, surely he should receive a vertical photon sent by the eavesdropper?

[vikoct]

Method is very flimsy for practical use, a eavesdropper can make the network unusable by just reading the bits unlike in classical networks where eavesdropper can read data but it will remain untouched, so the communication can work even if there is passive snooping going on which is the most way of eavesdropping, yes active snopping will have same effect as passive snooping in quantum secured network but its very rare as you need more privileges to do it.

[Pentagrammar]

One thing i don't understand is to how change our internet etc into photon based

[ianthehunter]

Looks like someone is a fan of cartoon shows 😄1:58

[BubblesTheAlphaWhale]

This might seem a silly idea for a video, but I was too young to understand the craze that happened surrounding the concept of Cold Fusion. Could you explain what they hell it was, and how it would have supposedly worked. I don't personally believe such a thing could exist, but trying to read into it and understand why it received so much backlash is a nightmare to unpack that I just can't make any sense of.

[thjulybd]

So when they got them wrong but nobody was listening, wuold they think someone was listening?

[charlesdrury]

Fiber optic would not affect Because if it did the Internet we shut down

[WatomaSjdob-bo]

Wilson Eric Johnson Lisa Rodriguez Melissa

[FindingCousinn-wi]

Williams Eric Lewis Laura Robinson Nancy