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Quantum information processing is a modern research topic in which many different disciplines within and beyond physics interact. This chapter first makes contact with basic concepts of classical information theory, which is often not familiar to physicists. It then introduces Shannon's entropy and related entropic quantities, revisits the sampling theorem, discusses the relation between thermodynamics, measurement, and information theory, and looks into Landauer's principle. The chapter looks into the very basics of quantum information, the description of qubits, and operations of qubits. Finally, qubit implementations are discussed based on charge and spin qubits in quantum dots. Experimental results on single and two-qubit operations, and issues of decoherence are discussed.

This chapter presents a review of quantum dots in two-dimensional electron gases. The spin qubits realized in these systems are strongly affected by the surrounding nuclear spins. An elegant demonstration is given that the nuclear spin bath is not as irreversible as one might think. Echo sequences can efficiently undo the dephasing of the qubitbrought about by the motion of nuclear spins.

A common theme in the implementation of quantum technologies involves addressing the seemingly contradictory needs for controllability and isolation from external effects. Undesirable effects of the environment must be minimized, while at the same time techniques and tools must be developed that enable interaction with the system in a controllable and well-defined manner. This chapter addresses several aspects of this theme with regard to a particularly promising candidate for developing applications in both metrology and quantum information, namely the nitrogen-vacancy (NV) centre in diamond. The chapter describes how the quantum states of NV centres can be manipulated, probed, and efficiently coupled with optical photons. It also discusses ways of tackling the challenges of controlling the optical properties of these emitters inside a complex solid state environment.

Quantum dots are small islands connected to typically metallic electrodes. Their small size shows up in the large spacing of the single-electron energy levels inside these islands. This chapter explains the main transport phenomena taking place in quantum dots, and different theoretical approaches constructed to describe them. The approaches include the self-consistent mean field theory relevant in the weakly interacting limit and the theory of Coulomb blockade in the strongly interacting limit. Between these limits one can find the Kondo effect, which is phenomenologically explained in the chapter. The discussion also covers the properties of double quantum dots, and some of their properties, such as the spin states, Pauli spin blockade, and usage as quantum bits.