Towards Unconditionally Secure Communications
Quantum technologies have the potential to revolutionize the field of secure communications completely. Currently available classical cryptographic protocols rely on the relative complexity of the encryption and decryption algorithms and are therefore only computationally secure. Quantum information technology deals with the storage, transmission, and processing of information using quantum-mechanical systems. Quantum communication also allows the transfer of sensitive information, such as cryptography keys, in a way that protects its confidentiality. In quantum cryptography, quantum information is exchanged in the form of qubits encoded by the sender in the quantum state of single photons. The receiver decodes the information by performing a quantum measurement on the state. Any attempt to measure the transmitted state by an eavesdropper will produce errors detectable by the receiver. This provides the means for parties to exchange with unconditional security an enciphering key. This key can then be used to encrypt classical information for transmission over a conventional, non-secure telecommunication communication channel.
Quantum Key Distribution
Digital technology has revolutionized the communication industry through high-speed, ultra-broadband and secure networks. It has been proven that breaking cipher protocols will be efficient and fast using quantum computers. Although such devices do not exist yet, hacking schemes such as harvest and decrypt are based on the anticipation of quantum computing. Furthermore, as soon as the first quantum computers become available today's protected information will no longer be secure without using quantum methods for encryption, such as Quantum Key Distribution, currently considered to be the most powerful data encryption scheme ever developed. In QKD two parties use single photons that are randomly polarized to states representing ones and zeroes to transmit a series of random sequences that are used as cryptographic keys. This sequence of numbers becomes a quantum key to lock/unlock encrypted messages. Because the transmitted photons cannot be intercepted without being destroyed, the act of interception tips off the message receiver.
Distance Limitations and Quantum Repeaters
To establish a worldwide quantum communication network the main challenge consists in extending the length of the communication channels. Due to losses introduced during transmission, direct quantum communication in fiber-optics is limited to distances of about 100 miles, hindering the possibility of a true global quantum network. A missing ingredient is the capacity to amplify quantum signals. Where transmission over a longer distance is required, measures must be taken to counteract the unavoidable losses of the transmitted signal. Traditionally a signal repeater receives, amplifies, and then forwards the message. The direct implementation of an amplifier in a quantum network is however not straight-forward as quantum information cannot be detected or amplified. For quantum systems, a measurement changes the quantum-state of the information carrying photons, with the result that the same fundamental principle which protects quantum communication from eavesdroppers also prevents the traditional amplification of the signal. Instead, the role of amplifiers is fulfilled by quantum repeaters which work with entangled photon pairs, to achieve preservation of the quantum properties. These repeaters enable quantum state distribution and storage, overcoming problems of loss. The entanglement distribution range is thus extended by concatenating these QRs over successive fiber-optic links, where entanglement is created independently for each link and extended by swapping.