controlled along non-Abelian “braiding” path
Source: Adapted from McMahon (2018).
3.3.7.2Diamond defects (nitrogen-vacancy defect centers)
An interesting approach, although one that may have scalability challenges for commercial deployment, is diamond center defects. Imperfections in the crystal lattice within diamonds are commonplace and have been exploited for a variety of uses from crystallography to the development of novel quantum devices. Defects may be the result of natural lattice irregularities or artificially introduced impurities. For quantum computing, impurities are introduced by implanting ions to make nitrogen-vacancy photonic centers. A nitrogen vacancy can be created in a diamond crystal by knocking out a carbon atom and replacing it with a nitrogen atom and also by knocking out a neighboring carbon atom so that there is a vacant spot. The nitrogen vacancy produces the so-called Farbe center (color center), which is a defect in a crystal lattice that is occupied by an unpaired electron. The unpaired electron creates an effective spin which can be manipulated as a qubit. The nitrogen-vacancy defect center is attractive for quantum computing because it produces a robust quantum state that can be initialized, manipulated, and measured with high fidelity at room temperature (Haque & Sumaiya, 2017).
3.3.7.3Quantum dots
Another early-stage approach, in the form of a semiconductor concept, is quantum dots (quantum dots are nanoparticles of semiconducting material) (Loss & DiVincenzo, 1998). In this method, electrically controlled quantum dots that can be used as qubits are created from electron spins trapped in a semiconductor nanostructure, and then electrical pulses are used to control them for computation. A semiconductor-based structure is fabricated that is similar to that of classical processors. Metal electrodes are patterned on the semiconductor layer so that electrostatic fields can be made from the wires to trap single electrons. The spin degrees of freedom of the electrons are used as qubits. Within the semiconductor nanostructure, there are small silicon chambers that keep the electron in place long enough to hybridize its charge and spin and manipulate the electron spin–orbit interactions for computation (Petta et al., 2005). The coherence interactions typically last longer in silicon than in other materials, but can be difficult to control. There has been some improvement in controlling qubit decoherence in quantum dot computing models (Kloeffel & Loss, 2013).
3.3.7.4Nuclear magnetic resonance
Nuclear magnetic resonance (NMR) is one of the first approaches to quantum computing, but is seen as being difficult to scale for commercial purposes. NMR uses the same technology that is used in medical imaging. The physics principle is that since atoms have spin and electrical charge, they may be controlled through the application of an external magnetic field. In 2001, IBM demonstrated the first experimental realization of quantum computing, using NMR (Vandersypen et al., 2001). A 7-qubit circuit performed the simplest instance of Shor’s factoring algorithm by factoring the number 15 (into its prime factors of 3 and 5).
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