QUANTUM COMPUTING: AN OVERVIEW

Abstract

Quantum computing and quantum information processing are emerging disciplines in which the principles of quantum physics are employed to store and process information. We use the classical digital technology at almost every moment in our lives: computers, mobile phones, mp3 players, just to name a few. Even though quantum mechanics is used in the design of devices such as LSI, the logic is purely classical. This means that an AND circuit, for example, produces definitely 1 when the inputs are 1 and 1. One of the most remarkable aspects of the principles of quantum physics is the superposition principle by which a quantum system can take several different states simultaneously. The input for a quantum computing device may be a superposition of many possible inputs, and accordingly the output is also a superposition of the corresponding output states. Another aspect of quantum physics, which is far beyond the classical description, is entanglement. Given several objects in a classical world, they can be described by specifying each object separately. In a quantum world, however, only a very tiny fraction of all possible states can be described by such separate specifications. In other words, most quantum states cannot be described by such individual specifications, thereby being called “entangled”. Why and how these two features give rise to the enormous computational power in quantum computing ane quantum information processing will be explained in this contribution. A part of this lecture note is based on our book [1]. General references are [2–4].