Quantum Communication

Quantum communication is the application of quantum theory to communication networks. The most investigated aspect of this field is quantum key distribution (QKD), which involves securely establishing a secret string of random numbers with which a message may be encoded. By utilising the laws of quantum mechanics, two parties can theoretically communicate without any risk of eavesdropping. The field was established in 1984 and has rapidly expanded with a veritable zoo of protocols being developed. Physical realisations have been demonstrated which allow QKD to be performed with enough speed and ease that it is now used commercially. Current work explores increasing the speed of QKD, proving the security of QKD protocols and performing it during the day and between satellites in low earth orbit.

1984: BB84 Protocol

Bennett and Brassard developed the first QKD scheme which is provably secure. The scheme involves preparing qubits in one of four states before sending them to another party. Due to the no cloning theorem the states may not be read by an eavesdropper without disturbing them. The two parties may detect such an eavesdropper by the disturbance of the states. Subsequently the two parties share a random secure key from which they may encode their data using a one-time pad protocol over a public classical channel. BB84 is the basis of the most common QKD protocol.

Charles and Bennett propose BB84 protocol, the first Quantum Key Distribution Protocol. DOI:10.1016/j.tcs.2011.08.039

This short video explains the working principle of BB84 protocol.

1989: First QKD Implementation

The first physical implementation of QKD was achieved by Bennett and Brassard using the BB84 scheme. A 403-bit string was shared over a distance of 30 cm, which after privacy amplification - a distillation stage to protect against eavesdropping - was reduced to only 175 bits of secret key.

This paper discusses the first experimental prototype of implementing BB84. https://doi.org/10.1145/74074.74087

This online QKD simulator helps understand the effects of different components and channel effects of the experiments on the protocol.

1991: E91 Protocol

Ekert introduced a QKD protocol with its security derived from the properties of entanglement. A source shares a pair of entangled particles between two parties, with each party performing a measurement on their particle in a random basis from a given set. When the measurements bases coincide, a secure shared key can be created between the parties.

Reference: https://doi.org/10.1103%2FPhysRevLett.67.661

Aspect, who experimentally tested Bell’s inequalities, talks about the workings of the Ekert protocol.

1997: First Entanglement Distribution

Entanglement distribution is a critical component for quantum networks, allowing for distributed quantum computation as well as quantum communication protocols. This was first experimentally demonstrated by Bouwmeester et al. in free-space using polarisation entangled photons.

Reference: https://www.nature.com/articles/37539.pdf

This short video explains the idea of entanglement-based Quantum Key distribution.

2000: First Entanglement Based QKD Implementation

Using polarisation entangled photon pairs, secret keys of up to 800 bits per second were generated between two users 360m apart. This was a direct implementation of E91, and a 43 kbit image was subsequently transmitted between the users using the secret key as the seed for a hashing function used to extend the key from 0.8 to 43 kbits.

This paper demonstrates QKD using entangled photon pairs https://doi.org/10.1103/PhysRevLett.84.4729.

This video series talks about various QKD protocols that leverage entanglement.

2003: BB84 Decoy State Protocol

The decoy state protocol was proposed to overcome photon number-splitting attacks against BB84. These occur when an eavesdropper measures some of the signal when more than one photon is present, compromising security. This protocol is highly favoured due to true single photon sources being unavailable.

Reference: https://link.aps.org/doi/10.1103/PhysRevLett.91.057901

This article briefly talks about the weaknesses in current QKD implementations and how decoy states can be used to overcome QKD attacks. This review paper explains several practical challenges of QKD and protocols to overcome them.

2007: Entanglement-Based Quantum Communication over 144km

QKD was performed over 144 km between the islands of Tenerife and La Palma by Ursin and Rarity et al. using polarisation entangled photons. This was an order of magnitude improvement over existing free-space quantum communication experiments, demonstrating an essential step in realising long range free-space quantum communication.

Reference: https://link.aps.org/doi/10.1103/PhysRevLett.91.057901

This video explains about different entanglement sources that can be used for quantum key distribution.

2010: QKD in Daylight

BB84 QKD was performed for the first time in daylight conditions using telecom wavelengths.

Reference: https://doi.org/10.1080/09500340008244059

This paper discusses a prototype of daylight QKD in integrated photonics platforms.

2012: Measurement-Device-Independent QKD

Measurement-device-independent QKD was proposed by Lo et al. By changing how the measurements are performed, measurement devices no longer need to be trusted, and can even be controlled by an adversary. Adversaries will not be able to gain information about the key established by the two users even when in control of the detectors.

Reference: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.108.130503

This animation explains the theory behind MDI QKD. A recent demonstration of free space MDI-QKD over 200km.

2013: QKD to a moving aircraft

QKD was demonstrated to a moving platform (300kph) for the first time.

Reference: https://doi.org/10.1038/nphoton.2013.46

This article briefly introduces the optical setup for airborne QKD. This video talks about extending the application of QKD to drones.

2014: Real World Implementation of QKD

This black paper examined the reality of QKD security and questioned the claims of “security guaranteed by the laws of physics”. QKD security is theoretically absolute, however, in reality it is much more subtle and open to side channel attacks. These attacks exploit flaws in hardware to sidestep the protocol security. For QKD to become viable, it is vital that any side channel which leak information must be closed to maintain security. Many papers claiming security have ignored these issues and as such have been subsequently refuted.

Reference: https://doi.org/10.1016/j.tcs.2014.09.015

This nature paper explains practical challenges in implementing various QKD schemes.

2017: QKD to orbiting satellite

The Chinese built Micius satellite performed QKD between itself and the earth. By propagating in space this overcame the distance restrictions imposed by fibre loss. The satellite later mediated a QKD connection between two ground-based parties. Satellite QKD is the only currently known scalable route to a truly global QKD network.

Reference: https://doi.org/10.1038/nature23655

This nature paper talks about different techniques to achieve high key rates in satellite-based communication. This paper discusses all satellite based QKD missions and their challenges.

2020: 8-user quantum network

The University of Bristol developed an 8 user QKD network with all possible user combinations having connectivity.

Reference: https://doi.org/10.1126/sciadv.aba0959

This animated video explains the concept of a Multi-node Quantum Network. A TedX talk on Quantum Internet. This video talks about the Quantum Network initiatives in the Quantum Communications Hub.

2020: 4600km quantum network

By using two satellite-to-ground links along with 700 fibre based QKD links, a 4600km QKD network with full connectivity was demonstrated in China.

Reference: https://doi.org/10.1038%2Fs41586-020-03093-8

This short animated video talks about UK’s first quantum internet. This video talks about University of Bristol’s progress in achieving a large-scale quantum network.