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Why does quantum analogue of current density operator $mathbf j = rho mathbf v$ involve an anti-commutator?

Physics Asked on January 24, 2021

I am reading through Luttinger’s "Theory of Thermal Transport Coefficients". In equation (1.14), the charge-density operator is defined as
$$rho(mathbf r) = esum_j delta(mathbf r-mathbf r_j).$$ In equation (1.16), the current-density operator is subsequently defined as $$mathbf j(mathbf r) = frac{e}{2}sum_j left( mathbf v_jdelta(mathbf r-mathbf r_j) + delta(mathbf r-mathbf r_j) mathbf v_j right)$$
where $mathbf v_j$ is the velocity operator of the $j^text{th}$ particle. My question pertains to why the current-density is defined in this way, with the anti-commutator

$$mathbf j(mathbf r) = esum_j frac{1}{2} {mathbf v_j,delta(mathbf r-mathbf r_j) }. $$ It is clear that the role of the anti-commutator is to make the current-density Hermitian, as the charge-density and velocity operators don’t commute themselves, but there are other ways to construct a Hermitian operator. For example, taking
$$mathbf j(mathbf r) = esum_j frac{i}{2} [mathbf v_j,delta(mathbf r-mathbf r_j) ] $$
where $[,]$ denotes the commutator is also Hermitian. Why is the anti-commutator considered the "correct" way to construct a Hermitian operator?

2 Answers

In general, operator ordering is a subtle issue. But in this simple case where the classical operator is just a product of two terms, it's straightforward to see why the anticommutator is preferred over the commutator. We want the quantum theory to limit to the classical one, and in the classical limit the commutator is small (since it's order $hbar$). So in this limit, the anticommutator would have the expected classical behavior, while the commutator would just go to zero.

Correct answer by knzhou on January 24, 2021

The symmetrization postulate for operators (see page 2 of this document) states that for $p$ and $f(x)$ the quantum analogue of $pf(x)$ is $frac{1}{2}(hat p f(hat x)+f(hat x)hat p)$

Answered by ZeroTheHero on January 24, 2021

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