Controlling the degree of entanglement in downconversion by targeted birth zone activation

TL;DR

This paper investigates how varying the pump beam waist in spontaneous parametric down-conversion affects entanglement, revealing a controllable transition between different entanglement regimes through targeted birth zone activation.

Contribution

It introduces a method to actively control entanglement by adjusting the pump waist, linking the number of biphoton birth zones to entanglement regimes in SPDC.

Findings

Demonstrated conservation of $V_m^2 + V_{12}^2$ in interference visibilities.

Identified three entanglement regimes based on the number of birth zones $N$.

Showed active control of entanglement degree by changing pump waist.

Abstract

We explore the consequences of varying the pump beam waist that illuminates a nonlinear crystal, realizing spontaneous parametric down-conversion (SPDC). The coherence is transferred from the marginal one-photon wavefunction to the two-photon wavefunction where it manifests into entanglement in the form of spatial correlation. We interpret this as a consequence of the number of independent emitters, called the biphoton birth zones, targeted by the pump beam on the crystal. The birth zone number $N$ characterises the number of such birth zones that fit along a diameter of the region illuminated by the pump waist. To experimentally observe the duality between the one- and two-photon interference, we employ a double slit and analyse their visibilities $V_m$ and $V_\text{12}$ respectively. We demonstrate the conservation of the quantity $V_m^2+V_\text{12}^2$. Finally, we identify three regimes of entanglement of the down-converted photons based on $N$. We show that changing the pump waist lets us actively control the degree of entanglement letting us access these regimes. We provide implications of each regime, and mention experimental use cases thereof.