# time evolution operator is unitary

This is typically taken as an axiom or basic postulate of quantum mechanics, while generalizations of or departures from unitarity are part of speculations about theories that may go beyond quantum mechanics.

[ "article:topic", "showtoc:no", "Time-Evolution Operator", "authorname:atokmakoff", "Exponential Operators", "time-reversal operator", "propagators", "Baker\u2013Hausdorff relationship", "license:ccbyncsa" ], 1.5: Numerically Solving the Schrödinger Equation. generates displacements in $$y$$ and $$\hat { D_z }$$ in $$z$$. Adopted or used LibreTexts for your course? \begin{array} { r l } { \mathrm { e } ^ { i \hat { G } \lambda } \hat { A } \mathrm { e } ^ { - i \hat { G } \lambda } = \hat { A } + i \lambda [ \hat { G } , \hat { A } ] + \left( \dfrac { i ^ { 2 } \lambda ^ { 2 } } { 2 ! }

A function of an operator is defined through its expansion in a Taylor series, for instance, $\hat { T } = e ^ { - \hat { i } \hat { A } } = \sum _ { n = 0 } ^ { \infty } \dfrac { ( - i \hat { A } ) ^ { n } } { n ! } = Time evolution is the change of state brought about by the passage of time, applicable to systems with internal state (also called stateful systems).In this formulation, time is not required to be a continuous parameter, but may be discrete or even finite.In classical physics, time evolution of a collection of rigid bodies is governed by the principles of classical mechanics. This is typically taken as an axiom or basic postulate of quantum mechanics, while generalizations of or departures from unitarity are part of speculations about theories that may go beyond quantum mechanics.  If the Hamiltonian itself has an intrinsic time dependence, as occurs when interaction strengths or other parameters vary over time, then computing the family of unitary operators becomes more complicated (see Dyson series). If we rotate first about $$x$$, the operation, \[e ^ { - i \dfrac { \pi } { 2 } L / h } e ^ { - i \dfrac { \pi } { 2 } L _ { x } / h } | z _ { 0 } \rangle \rightarrow | - y \rangle \label{122}$, leads to the particle on the –y axis, whereas the reverse order, $e ^ { - i \dfrac { \pi } { 2 } L _ { x } / \hbar } e ^ { - i \dfrac { \pi } { 2 } L _ { y } / \hbar } | z _ { 0 } \rangle \rightarrow | + x \rangle \label{123}$, leads to the particle on the +x axis. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Since rotations about different axes do not commute, we expect the angular momentum operators not to commute. 1. If the operator $$\hat { A }$$ is Hermitian, then, $\hat { T } ^ { \dagger } = \hat { T } ^ { - 1 }.$. from the statement that time evol… t We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. The final state of these two rotations taken in opposite order differ by a rotation about the z axis. This is also referred as Rᴢ(θ) which is rotation about the Z axis by an angle θ . Im(M). Time evolution 5.1 The Schro¨dinger and Heisenberg pictures 5.2 Interaction Picture 5.2.1 Dyson Time-ordering operator 5.2.2 Some useful approximate formulas 5.3 Spin-1 precession 2 5.4 Examples: Resonance of a Two-Level System 5.4.1 Dressed states and AC Stark shift 5.5 The wave-function dictates.

 A unitarity bound is any inequality that follows from the unitarity of the evolution operator, i.e. For instance, $\hat { R } _ { x } ( \phi ) = e ^ { - i \phi L _ { x } / h }$, gives a rotation by angle $$\phi$$ about the $$x$$ axis. Unitary. If $$\hat{A}$$ and $$\hat{B}$$ do not commute, but $$[ \hat { A } , \hat { B } ]$$ commutes with $$\hat{A}$$ and $$\hat{B}$$, then, $e ^ { \hat { A } + \hat { B } } = e ^ { \hat { A } } e ^ { \hat { B } } e ^ { - \dfrac { 1 } { 2 } [ \hat { A } , \hat { B } ] } \label{124}$, $e ^ { \hat { A } } e ^ { \hat { B } } = e ^ { \hat { B } } e ^ { \hat { A } } e ^ { - [ \hat { B } , \hat { A } ] } \label{125}$. − The evolution is given by the time-dependent Schr¨ odinger . Indeed, we know that, $\left[ L _ { x } , L _ { y } \right] = i \hbar L _ { z }$. Just as $$\hat { D } _ { x } ( \lambda )$$ is the time-evolution operator that displaces the wavefunction in time, $\hat { D } _ { x } = e ^ { - i \hat { p } _ { x } x / h }$, is the spatial displacement operator that moves $$\psi$$ along the $$x$$ coordinate. The optical theorem in particular implies that unphysical particles must not appear as virtual particles in intermediate states.

from the statement that time evolution preserves inner products in Hilbert space.

Similar to the time-propagator $$\boldsymbol { U }$$, the displacement operator $$\boldsymbol { D }$$ must be unitary, since the action of $$\hat { D } ^ { \dagger } \hat { D }$$ must leave the system unchanged.

More generally, if $$\hat{A}$$ and $$\hat{B}$$ do not commute, $e ^ { \hat { A } } e ^ { \hat { B } } = { \mathrm { exp } } \left[ \hat { A } + \hat { B } + \dfrac { 1 } { 2 } [ \hat { A } , \hat { B } ] + \dfrac { 1 } { 12 } ( [ \hat { A } , [ \hat { A } , \hat { B } ] ] + [ \hat { A } , [ \hat { B } , \hat { B } ] ] ) + \cdots \right] \label{126}$, \[\left. Time evolution described by a time-independent Hamiltonian is represented by a one-parameter family of unitary operators, for which the Hamiltonian is a generator: This, in turn, will depend on whether the Hamiltonians at two points in time commute. Now the action of two rotations $$\hat { R } _ { x }$$ and $$\hat { R } _ { y }$$ by an angle of $$\phi = \pi / 2$$ on this particle differs depending on the order of operation, as illustrated in Figure 8. We will see that these exponential operators act on a wavefunction to move it in time and space, and are therefore also referred to as propagators.