# Interaction picture

Interaction picture

In quantum mechanics, the Interaction picture (or Dirac picture) is an intermediate between the Schrödinger picture and the Heisenberg picture. Whereas in the other two pictures either the state vector or the operators carry time dependence, in the interaction picture both carry part of the time dependence of observables.

Equations that include operators acting at different times, which hold in the interaction picture, don't necessarily hold in the Schrödinger or the Heisenberg picture. This is because time-dependent unitary transformations relate operators in one picture to the analogous operators in the others. Not all textbooks and articles make explicit which picture each operator comes from, which can lead to confusion and mistakes.

## Definition

Operators and state vectors in the interaction picture are related by a change of basis (unitary transformation) to those same operators and state vectors in the Schrödinger picture.

To switch into the interaction picture, we divide the Schrödinger picture Hamiltonian into two parts, HS = H0,S + H1,S. (Any possible choice of parts will yield a valid interaction picture; but in order for the interaction picture to be useful in simplifying the analysis of a problem, the parts will typically be chosen so that H0,S is well understood and exactly solvable, and H1,S contains some harder-to-analyze perturbation to this system.)

If the Hamiltonian has explicit time-dependence (for example, if the quantum system interacts with an applied external electric field that varies in time), it will usually be advantageous to include the explicitly time-dependent terms with H1,S, leaving H0,S time-independent. We will proceed assuming that this is the case. (If there is a context in which it makes sense to have H0,S be time-dependent, then one can proceed by replacing $e^{\pm i H_{0,S} t/\hbar}$ by the corresponding time-evolution operator in the definitions below.)

### State vectors

A state vector in the interaction picture is defined as $| \psi_{I}(t) \rang = e^{i H_{0, S} t / \hbar} | \psi_{S}(t) \rang$

(where $| \psi_{S}(t) \rang$ is the same state vector in the Schrödinger picture.)

### Operators

An operator in the interaction picture is defined as $A_{I}(t) = e^{i H_{0,S} t / \hbar} A_{S}(t) e^{-i H_{0,S} t / \hbar}.$

(Note that the AS(t) will typically not depend on t, and can be rewritten as just AS. It only depends on t if the operator has "explicit time dependence", for example due to its dependence on an applied, external, time-varying electric field.)

#### Hamiltonian operator

For the operator H0 itself, the interaction picture and Schrödinger picture are the same: $H_{0,I}(t) = e^{i H_{0,S} t / \hbar} H_{0,S} e^{-i H_{0,S} t / \hbar} = H_{0,S}$

(this can be proved using the fact that operators commute with differentiable functions of themselves.) This particular operator can thus be called H0 with no ambiguity.

For the perturbation Hamiltonian H1,I, we have: $H_{1,I}(t) = e^{i H_{0,S} t / \hbar} H_{1,S} e^{-i H_{0,S} t / \hbar}$

where the interaction picture perturbation Hamiltonian becomes a time-dependent Hamiltonian (unless [H1,s,H0,s] = 0).

It is possible to obtain the interaction picture for a time-dependent Hamiltonian H0,s(t) as well but the exponentials need to be replaced by the unitary propagator for the evolution due to H0,s(t) or more explicitly with a time-ordered exponential integral.

#### Density matrix

The density matrix can be shown to transform to the interaction picture in the same way as any other operator. In particular, let ρI and ρS be the density matrix in the interaction picture and the Schrödinger picture, respectively. If there is probability pn to be in the physical state $|\psi_n\rang$, then $\rho_I(t) = \sum_n p_n(t) |\psi_{n,I}(t)\rang \lang \psi_{n,I}(t)| = \sum_n p_n(t) e^{i H_{0, S} t / \hbar}|\psi_{n,S}(t)\rang \lang \psi_{n,S}(t)|e^{-i H_{0, S} t / \hbar} = e^{i H_{0, S} t / \hbar} \rho_S(t) e^{-i H_{0, S} t / \hbar}$

## Time-evolution equations in the interaction picture

### Time-evolution of states

Transforming the Schrödinger equation into the interaction picture gives: $i \hbar \frac{d}{dt} | \psi_{I} (t) \rang = H_{1, I}(t) | \psi_{I} (t) \rang.$

This equation is referred to as the Schwinger-Tomonaga equation.

### Time-evolution of operators

If the operator AS is time independent (i.e., does not have "explicit time dependence"; see above), then the corresponding time evolution for AI(t) is given by: $i\hbar\frac{d}{dt}A_I(t)=\left[A_I(t),H_0\right].\;$

In the interaction picture the operators evolve in time like the operators in the Heisenberg picture with the Hamiltonian H' = H0.

### Time-evolution of the density matrix

Transforming the Schwinger-Tomonaga equation into the language of the density matrix (or equivalently, transforming the von Neumann equation into the interaction picture) gives: $i\hbar \frac{d}{dt} \rho_I(t) = \left[ H_{1,I}(t), \rho_I(t)\right].$

## Use of interaction picture

The purpose of the interaction picture is to shunt all the time dependence due to H0 onto the operators, leaving only H1, I affecting the time-dependence of the state vectors.

The interaction picture is convenient when considering the effect of a small interaction term, H1, S, being added to the Hamiltonian of a solved system, H0, S. By switching into the interaction picture, you can use time-dependent perturbation theory to find the effect of H1, I.

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