# Fully tunable and switchable coupler for photonic routing in quantum detection and modulation

## TL;DR

This paper introduces a fully tunable and switchable 2x2 photonic coupler with high bandwidth and stability, enabling advanced quantum photonic routing, switching, and state preparation with sub-nanosecond control.

## Contribution

The work presents a novel Mach-Zehnder interferometer-based coupler with real-time phase locking, tunable splitting ratios, and demonstrated applications in quantum information processing.

## Key findings

- Achieved switchable splitting ratios from 0:100 to 100:0 with 26 dB extinction
- Demonstrated sub-nanosecond switching between coupling regimes
- Enabled stable photon-number-resolving and temporal qudit state preparation

## Abstract

Photonic routing is a key building block of many optical applications challenging its development. We report a 2$\times$2 photonic coupler with splitting ratio switchable by a low-voltage electronic signal with 10~GHz bandwidth and tens of nanoseconds latency. The coupler can operate at any splitting ratio ranging from 0:100 to 100:0 with the extinction ratio of 26 dB in optical bandwidth of 1.3 THz. We show sub-nanosecond switching between arbitrary coupling regimes including balanced 50:50 beam splitter, 0:100 switch, and a photonic tap. The core of the device is based on Mach-Zehnder interferometer in a dual-wavelength configuration allowing real-time phase lock with long-term sub-degree stability at single-photon level. Using the reported coupler, we demonstrate for the first time the perfectly balanced time-multiplexed device for photon-number-resolving detectors and also the active preparation of a photonic temporal qudit state up to four time bins. Verified long-term stable operation of the coupler at the single photon level makes it suitable for wide application range in quantum information processing and quantum optics in general.