Quantum communication

Quantum communication applies quantum principles in order to communicate via the transmission of quantum states. Quantum communication is a field of applied quantum physics, closely related to quantum information processing and quantum teleportation, with the primary application of protecting information channels from bad actors through quantum cryptography.

Quantum communication technologies encode data using quantum states and transmit it at macroscopic distances, enabling transformative applications in cryptography. The most technologically mature application in quantum communication is quantum key distribution (QKD), which allows remote users to securely share a private encryption key.

The first protocol describing QKD was proposed by Charles H. Bennett and Gilles Brassard in 1984. The protocol, known as BB84, describes encoding digital information using polarized photons where it is theoretically impossible to intercept the information without a high probability of disturbing the transmission in a detectable way

Another approach to quantum communication involves transferring information through a process known as quantum teleportation that makes use of the phenomenon entanglement. Many see the field of quantum communication as building towards a new quantum internet where the underlying communication network is built upon quantum protocols. A quantum internet could facilitate greater data security and offer an idealized infrastructure for connecting quantum computers.

Superposition is a property exhibited by quantum particles, which allows them to take on the properties of waves with no well-defined position or state.

Entanglement is a property of quantum mechanics whereby a set of particles are generated or interact, in such a way that the quantum state of individual particles within the group cannot be described independently of the state of the others. This property holds even when the particles are separated by a large distance.

QKD is a mechanism that relies on the properties of quantum mechanics to agree on encryption keys between remote parties. Theoretically, QKD networks are ultra-secure with the ability to ensure keys are not observed or tampered with in transit by an adversary without alerting the original parties.

While encrypted data is sent as classical bits in QKD, the keys to decrypt the information are encoded and transmitted in quantum states using qubits. Due to superposition, qubits can simultaneously represent multiple combinations of binary data (0 and 1). If a hacker attempts to observe the physical state of a qubit, its quantum state collapses to either 0 or 1, and their activity is revealed.

With the advancement of quantum computers threatening the security of public-key cryptography (asymmetric) used for cryptocurrencies, many see post-quantum cryptography (non-quantum-based systems resistant to quantum computing attack) as a key development to maintain the future of blockchain technologies. QKD on the other hand is a technology based on the secure distribution of symmetric encryption keys.

QKD can be broken into different types based on protocol, variable used (discrete or continuous), and whether the communication is facilitated by fiber optics or satellite.

QKD can be implemented using several different protocols, all requiring a public quantum channel (ability to transfer qubits without destroying the information they hold) and an authenticated public classic channel. The first QKD protocol was proposed in 1984, by Bennet and Brassard, and is known as the BB84 protocol. The following are other prominent QKD protocols:

• E91 (proposed by Artur Ekert in 1991)
• B92 (proposed by Bennett in 1992)
• Continuous variable protocols
• SARG04
• Coherent one-way (COW)

The two major types of QKD protocol are prepare and measure (P&M) and entanglement-based (EB). P&M protocols, such as BB84, are based on individual qubits and require the sender to encode classical information into a set of quantum states before sending them via an insecure quantum channel. The receiver then measures the quantum states such that classical data is produced via quantum processes.

EB protocols, such as E91, require entangled qubits such that both sender and receiver perform mutually unbiased measurements to obtain correlated outcomes that are completely random. If a separate source prepares and distributes the entangled qubits they cannot be correlated by an eavesdropper implying secrecy of the key.

There are three distinct phases within QKD protocols along with the option to discard the key at any stage:

• Raw key exchange
• Key sifting
• Key distillation

Flowchart demonstrating QKD protocol procedures (double lines indicate processes where classical authentication is required).

Two or more remote users exchange keys, encoded into the quantum states (qubits). Multiple technologies can be used for the quantum channel, as long as the encoding and transfer of the key prevent interception from eavesdroppers without the users being alerted. The raw key exchange is the only phase of QKD using the quantum channel; all subsequent exchanges utilize a secure classical channel, known as "classical post-processing."

Remote users decide which of the measurements (of qubits) to select for use as their encryption key. The rules defining this decision vary depending on the protocol in use.

Key distillation was first proposed by Bennett et al in 1992 after reviewing experimental results and in previous work on the use of an authenticated public channel for repairing information losses from an imperfect private channel. There are three key steps of key distillation:

• Error correction—determining the error rate of the transmission, if it is more than a pre-determined value the key is discarded (lost information possibly accessed by an eavesdropper is too great to ensure secrecy of the key).
• Privacy amplification—counteracting any knowledge about the raw key potentially obtained by an eavesdropper through compression, the level of which is determined by the error rate.
• Authentication—strong classical authentication to prevent man-in-the-middle attacks. QKD can improve authentication by subsequent keys using part of the initial quantum exchange key.

In the BB84 protocol, two remote users (Alice and Bob) encode their quantum key into polarised photons using conjugate bases, either rectilinear basis (vertical/horizontal polarisations) or a diagonal basis (45^o and 135^o polarisations). Assigning agreed upon values (0 or 1) to the different polarisations. Alice can produce photons with four different polarizations (0 and 1 for the two different bases) and using a random number generator she sends a stream of randomly polarized photons to Bob for measurement (style described as a prepare and measure protocol).

Possible basis and polarisation of BB84 qubits.

The measurements are performed by passing the photons through different filters (that potentially change the original polarization state of the photons) and recording the results. Bob does not know the basis (rectilinear or diagonal) Alice used for each photon and can only set his receiving bases randomly. However, if he chooses correctly, the polarisation is recorded correctly. If he chooses incorrectly, the information about the initial photon polarization is lost. This is the raw key exchange stage.

Raw key exchange during the BB84 protocol.

Key sifting is performed over a public classical channel with Alice and Bob exchanging information on the basis used for each photon. No information regarding the final key can be intercepted as they are only broadcasting the basis used not the polarization of the photons. Any photons processed using non-matching keys are discarded from the raw key material. On average, key sifting should leave half the exchanged qubits available for use in the final encryption key.

Key sifting during the BB84 protocol.

If an eavesdropper (Eve) were to intercept the polarized photons sent by Alice and resend them to Bob, she could only perform the same measurement as Bob and on average discover 50% of the information about the key. However, when Bob receives the resent qubits, he only gets a correct result 50% of the time when he and Alice are using the same base. This is because Eve has changed the polarization of 50% of the photons. This change will result in a high error rate and will be highlighted during the key sifting stage with the key discarded.

As well as protecting against intercept/resend attacks, the BB84 protocol has methods for protecting against Photon Number Splitting (PNS) attacks.

QKD protocols do not provide authentication and are potentially vulnerable to physical man-in-the-middle attacks, whereby an adversary can agree individual shared secret keys with two parties who believe they are communicating with each other.

Channel authentication in QKD is also challenging, due to the requirement of random keys that are as long as the message itself and only used once.

Classical networks use repeaters to amplify the signal in fiber optic cables and compensate for photon absorption. QKD networks require a similar solution, to boost the signal of the qubits in transit. At these trusted nodes or "quantum repeaters," quantum keys are decrypted into bits before being re-encrypted into fresh quantum states. This offers a point of weakness whereby the key could be accessed by potential hackers or adversarial governments operating the nodes on the network.

To avoid this decryption into classical bits while still allowing the transfer of qubits over long distances requires the use of quantum processors that can amplify data while it remains in a quantum form. These types of processors have been demonstrated in principle, but not to the extent a working quantum repeater prototype with quantum processors has been produced.

QKD is possible using both fiber optic and satellite to transfer the quantum states. Optical losses limit the distance possible for fiber QKD until quantum repeater technology matures, with simulations suggesting capping practical applications to 500km and below. Satellite QKD is restricted by key generation only possible with an uninterrupted line of sight to the ground station and complications of detecting single photons in the presence of sunlight.

The most advanced satellite QKD demonstrations involve Chinese satellite Micius, with other projects underway in Europe (QKDSat, Arqit, and RefQ).

Quantum teleportation is a process by which the quantum state of a particle at one location can be inferred from another location without the particle being transmitted. The process utilizes the properties of quantum entanglement for two distant particles to determine the state of a third particle.

First theorized in 1993 and experimentally performed in 1997, quantum teleportation has now been demonstrated experimentally using a variety of systems:

• single photons
• coherent light fields
• nuclear spins
• trapped ions

Scaling quantum teleportation technologies to produce commercial communication networks involves significant scientific and engineering challenges. Including reliable and scalable methods of producing entangled particles and maintaining their entangled states over large distances.

There are a number of other quantum-based cryptographic approaches that have been experimentally demonstrated:

• Quantum signatures
• Position-based quantum cryptography
• Bit commitment
• Quantum coin flipping
• Oblivious transfer

A quantum internet would be a network that allows quantum devices to exchange information utilizing the properties of quantum mechanics. The quantum internet has the potential to offer advantages in terms of data security and offer the ideal infrastructure for connecting and transferring information between quantum computers.

Quantum communication is a significant area of interest within the defense sector. Large European security and defense companies, such as Thales, Airbus, Selex, and Qinetiq, are pursuing quantum technologies related to communication and are actively involved in the quantum flagship coordinated by the European Union.

Finance is a sector involved in quantum cryptography. The first field application of QKD in Europe was in 2004, involving a bank transaction in Vienna. Some Swiss banks have tested the use of ID Quantiqu's QKD equipment. Gazprombank, in Moscow, has tested equipment from the Russian Quantum Center. Australian Westpac Banking Corp has invested in the QKD startup QuintessenceLab and AXA is funding a chair on quantum cryptography at the Barcelone Institute of Photonic Sciences (ICFO).

QKD applications are found in the protection of critically important infrastructure in sectors such as energy, transport, banking, health, water, and satellites. The use of QKD networks to protect energy grids is being studied both in China (State Grid Corp) and in the USA (Los Alamos National Labs and Department of Energy). In transport, EuroContorl funded a project to evaluate the enhancement of air-ground telecommunication security by using quantum cryptography technology. A QKD network to protect the transmission of genome analysis data is being tested by Toshiba in Tokyo since August 2015.

Quantum communication patent analysis based on publicly available data hosted by Derwent Innovation show activity is primarily focused on QKD technologies with the China National Intellectual Property Administration awarding the largest number of patents, followed by the US Patent and Trademark Office and the Japan Patent Office.

Quantum communication patents by country between 2011-2020.

All quantum communication patents by year from 2011 to 2020, top organizations shown.

Top 10 organizations awarded quantum communication patents between 2011-2020.

China leads the way in QKD networks with the longest QKD network in the world—a 1,263 mile (2,032km) ground link between Beijing and Shanghai, used by banks and other financial companies.

The Chinese Academy of Sciences has demonstrated entanglement–enhanced QKD with the Micius satellite. Launched in 2016, it facilitated space–to–ground quantum communication across distances of over 7500 km between China and Austria. This technology is in its infancy, offering secure key transmission rates of only 0.43 bits per second.

The first commercial QKD network installed in the USA was by Batelle in 2013, connecting their headquarters in Columbus, Ohio, to a satellite office in Dublin, Ohio. The network uses fiber-optic lines and hardware (Cerberis model) from Swiss company ID Quantique, along with their Centauris encryptor, providing a 1Gbps link with Layer2 encryption.

Batelle is working with ID Quantique to develop a QKD trusted node (quantum repeater) to expand the distance of the Batelle Quantum Network (BQN). The company plans to expand from Columbus to their offices near Washington DC a total distance of 420 miles (700km).

Startup Quantum Xchange has a deal to access 500 miles (805 kilometers) of fiber-optic cable running along the East Coast to build a QKD network. The first leg will link Manhattan with New Jersey, locations where banks have large data centers. The network is known as Phio.

In 2010, Siemens deployed a QKD secured link in the Netherlands between data centers in The Hague and Zoetermeer. The telecom provider KPN is also planning a field deployment of QKD systems.

In the UK, the Quantum Communications Hub is developing a large-scale quantum network test-bed, which involves shorter–scale metropolitan networks in Cambridge and Bristol as well as a long–haul network connecting Cambridge–London–Bristol, with the scope to extend this. The Cambridge metropolitan network has already been developed, using already existing fiber links, demonstrating successful QKD in channels with simultaneous data traffic of 100 Gbps.

The OpenQKD initiative in Europe brings together 38 partners (across academia, industry, and startups) from thirteen EU countries to advance quantum communication capabilities on the continent. This includes facilitating cooperation and deploying testbed sites across Europe accessible for field trials to external stakeholders.

A QKD network in Tokyo has been tested by Toshiba since August 2015. The network protects the transmission of genome data produced at the Toshiba Life Science Analysis Centre as it is sent to Tohoku Medical Megabank Organization, a distance of 7km.

A significant number of companies around the world are currently working on quantum communication, primarily based in Japan, China, the EU and Switzerland, and the USA.

Research entities studying the fundamental limits and possibilities of quantum communication often carry this work out in groups focusing on more general research programs in quantum information theory. These fields utilize the same quantum principles and apply them to applications across communication, computation, cryptography, as well as foundational research. Experimental work on quantum communication is also typically carried out in research groups developing elements for general quantum technologies, which transfer to more industrially-oriented organizations once the technology is mature enough for commercialization.

Research entities developing technology specifically focused on quantum communication real-world applications include the following:

• National Institute of Information and Communications Technology (NICT)
• Korea Institute of Science and Technology
• Austrian Institute of Technology
• Oak Ridge National Laboratory
• National Institute for Standard and Technology (NIST)
• DARPA
• Los Alamos National Laboratory

Companies with commercially available proven QKD solutions include:

• ID Quantique
• MagiQ Technologies
• QNu Labs
• Quintessence Labs
• QRate
• QuantumCTek
• Toshiba