Active locking and entanglement in type II optical parametric oscillators

TL;DR

This paper proposes and analyzes a new method using laser injection at half the pump frequency to achieve phase locking and entanglement in type II optical parametric oscillators, offering advantages over traditional internal wave-plate techniques.

Contribution

It introduces a less invasive injection-based locking mechanism that maintains high entanglement levels and enables better experimental control in type II optical parametric oscillators.

Findings

Laser injection achieves phase locking of the output beams.

High levels of entanglement are preserved with injection locking.

A regime of highly squeezed vacuum states is identified at large injections.

Abstract

Type II optical parametric oscillators are amongst the highest-quality sources of quantum-correlated light. In particular, when pumped above threshold, such devices generate a pair of bright orthogonally-polarized beams with strong continuous-variable entanglement. However, these sources are of limited practical use, because the entangled beams emerge with different frequencies and a diffusing phase-difference. It has been proven that the use of an internal wave-plate coupling the modes with orthogonal polarization is capable of locking the frequencies of the emerging beams to half the pump frequency, as well as reducing the phase-difference diffusion, at the expense of reducing the entanglement levels. In this work we characterize theoretically an alternative locking mechanism: the injection of a laser at half the pump frequency. Apart from being less invasive, this method should allow for an easier real-time experimental control. We show that such an injection is capable of generating the desired phase locking between the emerging beams, while still allowing for large levels of entanglement. Moreover, we find an additional region of the parameter space (at relatively large injections) where a mode with well defined polarization is in a highly squeezed vacuum state.