User:Oliviazorrilla/Quantum key distribution

Device Independent Quantum Key Distribution
In traditional QKD, the quantum devices used must be perfectly calibrated, trustworthy, and working exactly as they are expected to. Deviations from expected measurements can be extremely hard to detect, which leaves the entire system vulnerable. A new protocol called Device Independent QKD (DIQKD) or Measurement Device Independent QKD (MDIQKD) allows for the use of uncharacterized or untrusted devices, and for deviations from expected measurements to be included in the overall system. These deviations will cause the protocol to abort when detected, rather than resulting in incorrect data.

DIQKD was first proposed by Mayers and Yao, building off of the BB84 protocol. They presented that in DIQKD, the quantum device, which they refer to as the photon source, be manufactured to come with tests that can be run by Alice and Bob to “self-check” if their device is working properly. Such a test would only need to consider the classical inputs and outputs in order to determine how much information is at risk of being intercepted by Eve. A self checking, or “ideal” source would not have to be characterized, and would therefore not be susceptible to implementation flaws.

Recent research has proposed using a Bell test to check that a device is working properly. Bell’s theorem ensures that a device can create two outcomes that are exclusively correlated, meaning that Eve could not intercept the results, without making any assumptions about said device. This requires highly entangled states, and a low quantum bit error rate. DIQKD presents difficulties in creating qubits that are in such high quality entangled states, which makes it a challenge to realize experimentally.

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. 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. 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. Quantum repeaters, however, are difficult to create and have yet to be implemented on a useful scale. 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.

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 pa or pb in the interval [0, 2π) and an encoding phase γa or γb. 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. This is different from traditional QKD, in which the phases used are never revealed.

Ultra-low loss optical fiber teleportation
Long distance transmission has always been the main application of quantum key distribution. As a quantum information transmission medium, optical fiber has the advantages of security and stability. In February 2022, Professor Han Zhengfu and his collaborators of Academician Guo Guangcan of the University of Science and Technology of China realized 833.8 km optical fiber quantum key distribution, which improved the safe transmission distance of quantum key distribution by more than 200 km from the world record, and took an important step towards realizing 1000 km ground-based quantum secret communication.

Ground-to satellite quantum teleportation
Continuous variable quantum key distribution is a key distribution method using continuous optical variables (such as light intensity and phase). It is a relatively new quantum key distribution technology, which can work in a wider spectral range, and has higher efficiency and security. Continuous variable quantum key distribution can be combined with traditional discrete variable quantum key distribution methods to provide more possibilities for quantum communication.

==== CV-QKD protocol ====


 * CV-QKD protocol is a key distribution protocol applied to satellite ground communication, information will first coded in continuous-variable properties, such as the amplitude and phase quadrature.


 * Alice prepares coherent states with a Gaussian modulation and sends them to the receiver, Bob, who measures either one of the quadratures with a homodyne detection system.


 * Random bits are encoded by modulation in CVQKD transmitter (Alice) and x or p quadrature are decoded by CVQKD receiver (Bob) in the process. Common phase reference will use a local oscillator in coherent communication.

Fundamental bounds for Satellite Communications
In basic theory, satellite communication still has some technical limitations including free-space diffraction, atmospheric extinction, background noise and fading. However, the research of QKD satellite communication still has a strong perspect.

Experimental
In July of  2022, researchers published their work experimentally implementing a device-independent quantum key distribution (DIQKD) protocol that uses quantum entanglement (as suggested by Ekter) to insure resistance to quantum hacking attacks. 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 88Sr+ 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.

A separate experiment published in July of 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. 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 87Rb 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+> 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.

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 833.8km.