# Lorentz-invariant mass and entanglement of biphoton states

## TL;DR

This paper explores how the Lorentz-invariant mass of biphoton states, generated via spontaneous parametric down conversion, correlates with their entanglement degree, providing insights into relativistic quantum properties.

## Contribution

It demonstrates the applicability of Lorentz-invariant mass to biphoton states and links it directly to the entanglement measure known as the Schmidt parameter.

## Key findings

- Lorentz-invariant mass applies to biphoton states.
- Mass is proportional to the Schmidt parameter for highly entangled states.
- Provides conditions relating mass and entanglement in biphoton systems.

## Abstract

The concept of the Lorentz-invariant mass of a group of particles is shown to be applicable to biphoton states formed in the process of spontaneous parametric down conversion. The conditions are found when the Lorentz-invariant mass is related directly with (proportional to) the Schmidt parameter $K\gg 1$ determining a high degree of entanglement of a biphoton state with respect to transverse wave vectors of emitted photons.