Abstract
This article presents state-of-the-art and future perspectives of quantum computing and communication. Timeline of relevant findings in quantum informatics, such as quantum algorithms, quantum cryptography protocols, and quantum computing models, is summarized. Mathematics of information representation with quantum states is presented. The quantum circuit and adiabatic models of quantum computation are outlined. The functionality, limitations, and security of the quantum key distribution (QKD) protocol is presented. Current implementations of quantum computers and principles of quantum programming are shortly described.
TopBackground
Feynman’s (1982) observation, that a classical computer cannot efficiently simulate the stochastic parallelism of quantum states, started research on using quantum mechanical effects for efficient information processing. Operating principles and implementation possibilities of quantum computing were outlined in Oxford University (Deutsch, 1985).
Bennett and Brassard (1984) proposed a quantum protocol, BB84, for perfectly secret information transfer. Efficient quantum algorithms, for example integer factorization (Shor, 1994) and unsorted search (Grover, 1996), were proposed. BB84 has been used for distribution of symmetric encryption/decryption keys (Quantum Key Distribution, QKD) in research networks (Elliot, Pearson, & Troxel, 2004; Quellette, 2004; Poppe, Momtchil, & Maurhart, 2008). Commercial QKD technology has been available over 10 years. Quantum digital signatures have been proposed and experimentally verified (Gottesman & Chuang, 2001; Lu & Feng, 2005; Clarke et al., 2012).
Small scale quantum circuit based computers have been built and successfully tested in research laboratories (Vandersypen et al., 2001; Monz et al., 2011). Since 2011 large scale commercial adiabatic quantum computers (Das & Chakrabarti, 2008) are available.
Achievements in quantum computing and quantum communication from 1970 till October 2015 are listed in (Timeline, 2015). Universities engaged in research and education on this topic are for example (qis.mit.edu, 2013; …from Quantum, 2011; mathQI, 2015; Quantum, 2015a; Quantum, 2015b)
TopTwo quantum states, for which a state transition exists, can represent a bit called a quantum bit or qubit, if the energy level of both states is measurable. However, quantum states are probabilistic. A measured energy level is one of several possibilities. Each possible outcome has a probability. The sum of all possible outcome probabilities is of course 1.
The No Cloning qubit property is the impossibility to clone an unknown quantum state. However, Teleportation, which changes the original qubit state, can transfer also an unknown quantum state.
Key Terms in this Chapter
Photon: A discrete packet of electromagnetic energy.
Qubit: A two-state quantum-mechanical system which can be a concurrent superposition of both states.
Avalanche Photodiode: The semiconductor analog to a photomultiplier.
AQC: Adiabatic quantum computation, calculation by quantum annealing.
Enlanglement: A multiple qubits state which cannot be created by combining single qubit states.
Polarization: The propagation plane of an electromagnetic wave.
Teleportation: Transfer of a qubit state.
Quantum Gate: A device changing the state of one or multiple qubits.
Decoherence Time: The time a qubit state prevails before collapse.