# Control and enhancement of interferometric coupling between two photonic qubits

## TL;DR

This paper presents a method to control and enhance the interferometric coupling between two photonic qubits using local coupling and quantum filtering, verified through optical experiments with single photons.

## Contribution

The authors introduce a novel procedure combining local coupling and quantum filtering to control photonic qubit interactions, supported by experimental validation.

## Key findings

- Demonstrated tunable effective transmittance in photonic qubit coupling
- Achieved both enhancement and reduction of coupling strength
- Observed dependence of two-photon bunching on transmittance

## Abstract

We theoretically investigate and experimentally demonstrate a procedure for conditional control and enhancement of an interferometric coupling between two qubits encoded into states of bosonic particles. Our procedure combines local coupling of one of the particles to an auxiliary mode and single-qubit quantum filtering. We experimentally verify the proposed procedure using a linear optical setup where qubits are encoded into quantum states of single photons and coupled at a beam splitter with a fixed transmittance. With our protocol, we implement a range of different effective transmittances, demonstrate both enhancement and reduction of the coupling strength, and observe dependence of two-photon bunching on the effective transmittance. To make our analysis complete, we also theoretically investigate a more general scheme where each particle is coupled to a separate auxiliary mode and show that this latter scheme enables to achieve higher implementation probability. We show that our approach can be extended also to other kinds of qubit-qubit interactions.