**System state and quantum state** __System state:__ The state of a system is defined by the observables that can be experimentally measured.

In quantum mechanics, the state vector or quantum state of an object contains information about the way that object currently is. For example, the state vector Ψ of a spinless free particle moving on a line, also called particle's wavefunction, is represented by e

^{i(kx − ωt)}. In this formalism, the letter t denotes a moment in time, and the letter x denotes the particle's position at moment in time t. The parameters k, and ω, are related to the speed, and mass of the particle.

__Thus, a particle's state vector contains information about the position, mass, and speed of the particle.__ In general, one can always describe a quantum mechanical system, which could consist of more than one particle, by its state vector.

__Quantum state:__ A quantum state is a mathematical object that fully describes a quantum system.

Quantum states can be statistically mixed, corresponding to an experiment involving a random change of the parameters. States obtained in this way are called mixed states, as opposed to pure states which cannot be described as a mixture of others.

When performing a certain measurement on a quantum state, the result is in general described by a probability distribution, and the form that this distribution takes is completely determined by the quantum state and the observable describing the measurement.

Mathematically, a pure quantum state is typically represented by a vector in a Hilbert space. In physics, bra-ket notation is often used to denote such vectors. Linear combinations (superpositions) of vectors can describe interference phenomena. Mixed quantum states are described by density matrices.

__See also:__ Observable

http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1202369976/0#0 CSCO: Complete Set of Compatible Observables

http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1202417497/0#0 System State

http://en.wikipedia.org/wiki/State_%28physics%29 Quantum state

http://en.wikipedia.org/wiki/Quantum_state