Summary
Discrete Variable Quantum Key Distribution (DV-QKD) uses quantum mechanics as a method for distribution of encryption keys for secured communications. QKD can provide improved resistance to in-path or MITM attacks, wherein an eavesdropper intercepts communications as they travel between the sender and reciever. End To End Encryption (E2EE) methods such as PGP can also provide protection against eavesdroppers but may potentially be crackable in the future such as via Quantum Computers. Free Space Optical Communication (FSO) is inherently well suited as a complimentary technology to QKD, as communication photon emitters are already present.
QKD takes advantage of the polarization of light photons. Photons can be polarized or vibrate on either their horizontal or vertical axis, which can then be measured. Because the polarization is a quantum property, it is in the super position of being both horizontal and vertically polarized until it is measured. An eavesdropper measuring these photons causes their quantum state to collapse and it to be either horizontally or vertically polarized, which can then be observed by the communicating parties.
Polarizing filters can be used to only allow horizontal or vertically polarized photons to pass through. For example a pair of polarized sunglasses only allow through vertically polarized light, essentially “measuring” it.
DV-QKD requires single photon detectors, prompting research into Continuous Variable QKD (CV-QKD) which uses the field as it’s signal versus the polarization of individual photons.
BB84 Protocol
BB84 is named after its creators Charles H. Bennett and Gilles Brassard, and the year of its publication. [1] In BB84 A horizontal photon can be considered a 0, and a vertical photon a 1. Photon emitters by default have a + shape, and emit or receive horizontally or vertically polarized photons. In order to generate photons in a superposition of horizontal and vertical, the emitter is turned 45 degrees to make an x shape. If the receiver also rotates their receiver, they will accurately measure the received photons. the + position is referred to as the rectilinear basis, and the x position the Diagonal basis.
To exchange keys, the sender picks the rotation of their polarizer or it’s bases and bits to send randomly, and the receiver rotates randomly as well. The sender and receiver then share with each other over an unsecured channel or publicly which bases they used and compare their measurements when they were the same. Measurements taken using different bases, for example the sender used an x basis and the receiver a + basis, are not used. The remaining 1 and 0 values can be used as a mutually agreed upon key that is known only to them.
An eavesdropper does not know when polarizers are rotated or not, so if they measure and retransmit a photon they have a 50% chance of modifying its polarization and producing a 1 when a 0 is expected or vice versa. This can then be detected by the sender and receiver comparing sample values from their key publicly.
This approach has been successfully demonstrated using FSO satellites. [2] [3]
E91 Protocol
Ekert 91 (E91)[4] is a more straightforward way of distributing quantum keys, where quantum-entangled photons are used. When one entangled photon is measured, it effects the other. For example if the sender measures a horizontal for their side of the entangled photons, the receiver will always measure a vertical.
Similarly to BB84, the sender and receiver randomly choose a polarization for their emitter and receiver, share their bases publicly, and create a key from the resulting 1s and 0s. If an eavesdropper is present they will break the entangled property of the photon which can be deduced by comparing the expected and observed measurements when the receivers were not aligned.
References
[1]https://arxiv.org/abs/2003.06557
[2] https://arxiv.org/ftp/arxiv/papers/1707/1707.00542.pdf
[3]https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.030501
[4] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.67.661
https://ieeexplore.ieee.org/document/8597918
https://link.springer.com/article/10.1007/s11276-023-03289-6
https://mpl.mpg.de/fileadmin/user_upload/Chekhova_Research_Group/Lecture_4_12.pdf
https://www.exploratorium.edu/snacks/polarized-sunglasses#:~:text=Polarizing%20sunglasses%20absorb%20this%20horizontally,making%20the%20surface%20look%20brighter.