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Physics LibreTexts

1.1: Language of Quantum Mechanics

  • Page ID
    10503
    1. Quantum Mechanics is the description of matter and radiation in all its details and in particular of the happenings on the atomic and subatomic scales. Quantum mechanics deals with states and observables of physical systems. Built into the machinery of quantum mechanics is the notion that a measurement of an observable of a physical system in general disturbs the system and changes its state.
    2. A physical syster¿ is a system of physical objects which can be isolated from the ¡est of the physical world. A state of a physical system is the result of a series of physical manipulations on the system. These manipulations are said to prepare a state of a system or to prepare a system in a state. States are either pure or mixed.
    3. A pure state of a physical system is characterized by the existence of an experiment which gives a result p¡edictable with certainty when performed 5. on the system in that state. We generally use lower case Greek letters I ty' >, I 4, >, I X >, etc., to denote pure states.
    4. A mixed state of a physical system is characterized by the absence of any experiment which gives a result predictable with certainty when performed on the system in that state. Mixed states are generally easier to prepare than pure states. We will nevertheless not deal with mixed states in most of plB; unless stated otherwise all states refer¡ed to are pure states.l
    5. An observable of a physical system is an aspect of the system which is measu¡able. A measurement consists of the manipulation of apparatuses ard the eventual reading or recording of a sca.le. We generally use upper case Roman letters A, B, C, ¿/c., to denote observables. Observables A an