Phase factor

For any complex number written in polar form (such as r e^iθ), the phase factor is the complex exponential factor (e^iθ). The phase factor is a unit complex number, i.e. a complex number of absolute value 1. It is commonly used in quantum mechanics. It is a special case of phasors, which may have arbitrary magnitude (i.e. not necessarily on the unit circle in the complex plane).

The variable θ appearing in such an expression is generally referred to as the phase. Multiplying the equation of a plane wave Ae^{i(k·r − ωt)} by a phase factor shifts the phase of the wave by θ:

$${\displaystyle e^{i\theta }A\,e^{i({\mathbf {k} \cdot \mathbf {r} -\omega t})}=A\,e^{i({\mathbf {k} \cdot \mathbf {r} -\omega t+\theta })}.}$$

In quantum mechanics, a phase factor is a complex coefficient e^iθ that multiplies a ket ${\displaystyle |\psi \rangle }$ or bra ${\displaystyle \langle \phi |}$. It does not, in itself, have any physical meaning, since the introduction of a phase factor does not change the expectation values of a Hermitian operator. That is, the values of ${\displaystyle \langle \phi |A|\phi \rangle }$ and ${\displaystyle \langle \phi |e^{-i\theta }Ae^{i\theta }|\phi \rangle }$ are the same.[1] However, differences in phase factors between two interacting quantum states can sometimes be measurable (such as in the Berry phase) and this can have important consequences.

In optics, the phase factor is an important quantity in the treatment of interference.