What is Quantum Technology?
Quantum is short for quantum mechanics. In principle, any object has a quantum behavior, but only at microscopic scales are these effects easily observable. Quantum technology is an emerging field of science and engineering, which is about creating practical applications of these physical phenomena, particularly quantum entanglement, quantum superposition, and quantum tunneling. These novel technologies include quantum computing, quantum sensors, quantum cryptography, quantum networks, quantum simulation, quantum metrology, and quantum imaging.
Towards Unconditionally Secure Communications
Quantum technologies have the potential to revolutionize the field of secure communications. Currently available classical cryptographic protocols rely on the relative complexity of the encryption and decryption algorithms and are therefore only computationally secure. Quantum communication allows the transfer of sensitive information, such as cryptography keys, in a way that protects its confidentiality.
Quantum Key Distribution
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.
Because single-photons cannot be subdivided and copied an eavesdropper cannot divide the emitted photon into two or make a copy to be kept and another to be sent to the receiver. Either the receiver receives the photon, or the eavesdropper gets it – but both of them can not receive it at the same time. Because the transmitted photons cannot be intercepted without being destroyed, the act of interception tips off the sender and receiver whom only need to keep the signal received by the receiver, generating an encryption key others cannot obtain. This provides the means for parties to exchange with absolute security an enciphering key. This key can then be used to encrypt classical information for transmission over a conventional, non-secure telecommunication communication channel.
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 100miles, hindering the possibility of a true global quantum network. Where transmission over a longer distance is required, measures must be taken to counteract the unavoidable losses of the transmitted signal. The missing ingredient is the capacity to amplify quantum signals.
Traditionally a signal repeater receives, amplifies, and then forwards the message. The direct implementation of an amplifier in a quantum network is not straight-forward as quantum information cannot be amplified. For quantum systems, the act of measuring 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 losses. The entanglement distribution range is thus extended by concatenating QRs over successive fiber-optic links, where entanglement is created independently for each link and extended by swapping.
Quantum memories and entanglement sources are both vital innovations for quantum communications to grow beyond small point-to-point links. A quantum memory allows for on-demand storage and retrieval of broadband pulses of classical/quantum light (quantum states), in a way that preserves the overall state. For example, in the case of an entangled-photon pair, quantum memories need to store the two photons for some amount of time while keeping their entanglement property intact, i.e. without having the photons be disturbed in a way that constitutes a measurement of their properties in the computational basis.