Quantum key distribution is a vital component of quantum secure communications that ensures communication security with the fundamentals of quantum mechanics.
Quantum Key Distribution (QKD)
The increasing reliance on electronic information systems to store, process and transfer valuable data has made security one of the most important aspects of system design. Further data security is provided by cryptography with computational complexitybased algorithms that require the distribution of cryptographic keys using symmetric or public key techniques. Computationally sophisticated algorithms security guarantees is that when a competitor is able to complete the task of decrypting the encrypted message, the information will no longer be usable.
Although predictions can be made based on technological trends such as Moore's Law, history has taught us that it is difficult to predict fundamental innovations in technology or mathematics that can challenge these security assumptions. For example, Ron Rivest, one of the inventors of the RSA Public Key Distribution Algorithm, published a security challenge in 1977: Factor a 129bit number to decode a coded message. At the time, Rivest was convinced that the factoring issue would take millions of years, even on the most powerful computers expected to be available in the future. But, surprisingly, that number was factored in 1994 by a team of researchers at MIT, Iowa State University and Oxford University. In fact, advances in factoring techniques have allowed researchers to decode the encrypted message much earlier than predicted. Other stunning computing advances can also challenge today's security assumptions in the future. For example, the realization of a largescale quantum computer to practice Peter Shor's quantum factoring algorithm has a detrimental effect on the security of the public key infrastructure used in most secure communications today.
A different cryptographic method that is not sensitive to advances in mathematics or computing is the VernamMauborgne's onetime pad developed in 1917. Shannon proved in 1949 that a completely random onetime pad was impenetrable to known cryptographic attacks. While this is great, the onetime pad has several executive challenges: the length of the key must be the same as the message and be delivered securely (for example by using a secure courier) to the communicating parties. This seems like a daunting task, given the large amount of data that is encrypted daily and that each of these keys must be physically transmitted over long distances.
However, a new approach to the key distribution problem can offer a potential solution and a final realization of unconditional security. Unlike computational complexitybased cryptography, the principles of quantum key distribution security are rooted in the laws of quantum physics. QKD has the potential to continuously distribute secure keys over communications networks such as fiber optic and free space, without compromising key security or relying on unstructured mathematical and computational assumptions. Security in QKD is achieved by encrypting random data in the quantum states of individual photons, transferring photons onto a quantum channel and performing a procedure, such as BB84, to obtain a shared and final key. One of the important principles of BB84 security is the detection of eavesdropping. In BB84, the transmitter chooses one of two conjugate bases to encode its qubits. The nonorthogonality of these quantum states makes it impossible to measure (and with full fidelity) qubit values for an eavesdropper. When an eavesdropper measures the qubit which is in a superposition of states, the quantum system is reduced to one of two possible states by the eavesdropper detector. This disorder can be used to detect the presence of an eavesdropper (who is trying to actively measure and redistribute the quantum bit sequence) on a quantum channel. Applying eavesdropper on the channel will cause the error and the awareness of the communicating parties about his presence (by increasing the error rate of the quantum channel bit). QKD has several unique security advantages:
 The quantum bits used in the key cannot be registered by Eve. Photons that are passively extracted and measured by Eve are not received by Bob and thus not part of the key. Eve's attempt to actively measure and resend bits to Bob will increase Bob's error rate and expose his manipulation efforts.
 More powerful computing technologies do not help Eve to guess the key. QKD does not use the mathematical complexity to protect the information exchanged between Alice and Bob. As a result, more powerful computing techniques, including quantum computing do not help Eve to obtain the key.
 Alice and Bob can exchange a secure key in the presence of Eve. Postprocessing steps, such as enhancing privacy in QKD methods, attribute quantum channel errors to Eve manipulation. Alice and Bob use theoretical information estimates to determine the size of their final key to ensure its security.
 Security is based on the wellknown laws of physics. QKD security is based on the fundamental assumptions of quantum mechanics including superposition and the nocloning theorem. To make a successful, undetectable attack on the ideal QKD system, Eve must prove that these assumptions can be violated.
 Analyzing threats to the current network security infrastructure, comparing with traditional cryptographic approaches, and realizing practical and scalable multiuser quantum networks will eventually lead to widespread adoption of QKD systems. Otherwise, QKD is a valuable technique that can be used to achieve secure key distribution without any restriction.
Quantum communications for other applications
Although QKD is the first practical application of quantum information science, many new concepts are emerging that can infiltrate photonic communication technologies, for example quantum communications will require efficient network quantum computers to facilitate distributed quantum computing over a few meters (for photonic qubits communication) to hundreds of kilometers (for geographically separate computing nodes). Photonic qubits transmitted by cables and optical fiber interconnections can serve as a quantum channel for these applications. Quantum communication techniques can also be used to distribute entanglement over a distance. Geographical distribution of quantum entangled states can improve clock synchronization. Finally, other security applications for quantum communications, such as entanglementbased voting, have been proposed to improve the security and privacy of elections. Quantum communication networks will be needed for the aforementioned applications and other potential applications, and to enable the sharing and exchange of quantum information.
Quantum Communications Networks
Since the realization of QKD in 1990s, many research groups around the world have been conducting experimental investigations to demonstrate the use of quantum communications over intervals that are attractive for securing networks. Both entangled qubits and weak coherent qubits have been found to propagate within hundreds of kilometers for dedicated fiber optic and free space links. In 2002, a team from Los Alamos achieved a QKD of 10 kilometers of free space on earth surface and daylight, demonstrating the ease of QKD from a ground station to a rotating satellite in the low Earth orbit. So far the longest free space link, which has been proven to distribute keys, was more than 144 kilometers at night using an entangledbased QKD scheme. Fiber QKD also made significant progress. In 2003, a team from Mitsubishi reported a fiber distance of over 87 kilometers. That record was quickly broken in 2004 by a group of Toshiba that reached a record 122km. The world record in the fiber QKD is owned by the Los Alamos Group for a distance of 185 kilometers.
Practical constraints on the development of the QKD system, such as light loss in the transmission environment and resilience to component accuracy, appear to limit the current production of QKD systems to approximately 200 km. Amazing advances in singlephoton sources, noisefree singlephoton detectors and a variety of new optical fibers can improve the accessibility of these systems in the future. New methods can also be used to extend the transmission distance. By combining these innovations, QKD systems can go beyond intercity distances in the near future.
Although pointtopoint quantum communications are important, multiuser quantum networking will need that Alice can route her quantum communications to other parties. Many of the approaches used in conventional optical networks can also route quantum channels. For example, microelectromechanical optical systems (MEMS) can build an optical router for quantum communications between nodes connected to a transparent optical network. If Alice's QKD transmitter uses a tunable laser, wavelength routing architectures are also a possibility. Most lowloss optical switch devices are compatible with quantum networks, provided the optical path is almost free of inband noise and does not require a quantum channel to pass through an amplifier environment.
Most realized QKD systems to support the quantum channel between Alice and Bob have focused on freespace line sight paths or dark optical fiber links. However, such pointtopoint links are not scalable or costefficient for QKD to deploy in multiuser quantum networks. Access to dedicated dark fiber can be costly, especially if all purposes are required for quantum service. In addition, Alice and Bob still need conventional network communication to exchange classical information in order to complete the QKD method and transmit their encrypted information. For these reasons, the compatibility of quantum communications with conventional optical channels over the same fiber infrastructure, and the technical challenges of upgrading an existing optical network to support quantum communication services, must be addressed.
Advanced single photon detection technologies are being developed. For these systems, detectors themselves can be a useful sensing tool for detecting network noise levels. In spite of these challenges, research shows that the coexistence of conventional and quantum channels on the same network is possible.
Outlook
Over the past decade, great progress has been made in expanding access to QKD systems and demonstrating their compatibility with DWDM optical networks to provide secure communications. QKD is an important first step towards the distribution and sharing of quantum information over geographical distances using photonic qubits. Although there are some challenges in defining a lownoise quantum channel design, today there is the technology needed to run QKD services over large cities' fiber networks. By precisely analyzing the noise generated by conventional optical channels in singlephoton level and understanding the mechanisms of interaction between conventional signals, networks can be enhanced to support quantum communication services. Automated QKD systems have been developed to operate on conventional reconfigurable networks on Earth, without the need for fundamental physics knowledge or engineering principles of quantum communication systems. Integrating components into a QKD system on a chip is the next crucial step towards making lowcost, compact, and productive systems possible. Integrated quantum devices are important driving forces for the development of quantum communication services and the networks that will support those services in the future.
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