# Local Modular Hamiltonians from the Quantum Null Energy Condition

## TL;DR

This paper derives a generalized formula for the modular Hamiltonian of half-spaces bounded by null cuts in quantum field theories, linking it to the quantum null energy condition and extending the Rindler case.

## Contribution

It provides a new explicit expression for the modular Hamiltonian for regions bounded by null cuts, generalizing the Rindler wedge case using the quantum null energy condition.

## Key findings

- Derived a formula for the second derivative of the modular Hamiltonian as the stress tensor component.
- Integrated the formula to obtain a simple expression for the modular Hamiltonian.
- Validated assumptions in free and holographic theories to all orders in 1/N.

## Abstract

The vacuum modular Hamiltonian $K$ of the Rindler wedge in any relativistic quantum field theory is given by the boost generator. Here we investigate the modular Hamiltoninan for more general half-spaces which are bounded by an arbitrary smooth cut of a null plane. We derive a formula for the second derivative of the modular Hamiltonian with respect to the coordinates of the cut which schematically reads $K" = T_{vv}$. This formula can be integrated twice to obtain a simple expression for the modular Hamiltonian. The result naturally generalizes the standard expression for the Rindler modular Hamiltonian to this larger class of regions. Our primary assumptions are the quantum null energy condition --- an inequality between the second derivative of the von Neumann entropy of a region and the stress tensor --- and its saturation in the vacuum for these regions. We discuss the validity of these assumptions in free theories and holographic theories to all orders in $1/N$.