A quantum computer is a device that performs operations according to the rules of quantum theory. There are various types of quantum computers of which nowadays the two most important ones considered for practical realization are the circuit-model quantum computer and the quantum annealer. A circuit-model quantum computer is the quantum equivalent of the classical circuit-model computer. It works with qubits, the quantum equivalent of bits, on which sequences of quantum circuit operations are performed. These so-called quantum algorithms can in principle be designed to solve a variety of problems, including optimization problems. Quantum annealing is a quantum-mechanical paradigm to solve combinatorial optimization problems by a process using quantum fluctuations. Its main potential use is in solving those problems featuring a discrete search space with many local minima. Both types of quantum computer models come with their own implementation challenges. Practical realizations of circuit-model quantum computers are currently limited in size to about ten qubits while quantum annealers with more than 2000 qubits are commercially available. Advantages and limitations of quantum annealers will be discussed in the context of applications. |
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