A two-state system has two possible states, and if they are not degenerate energy levels the system can become "excited" when it absorbs a quantum of energy.

A Java applet that visualizes Rabi Cycles of two-state systems (laser driven).

A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops.

In quantum mechanics, a two-state system (also known as a two-level system or TLS) is a system which has two possible quantum states.

More formally, the Hilbert space of a two-state system has two degrees of freedom, so a complete basis spanning the space must consist of two independent states.

The physics of a quantum mechanical two-state system is trivial if both states are degenerate, that is, if the states have the same energy.

In fact, in nature, it is difficult to identify any true two-state systems; merely systems where the energetics of the circumstances isolate two particular states.

A qubit is a two-state quantum-mechanical system such as the polarization of a single photon: here the two states are vertical polarization and horizontal polarization.

In particular, for a two-state system:

In a two-state system, folding and unfolding rates dominate the observed relaxation rates below and above the denaturation midpoint (Cm).