# Temporal-mode measurement tomography of a quantum pulse gate

## TL;DR

This paper presents a comprehensive characterization and calibration of a quantum pulse gate (QPG) for high-dimensional quantum information processing using temporal modes, achieving high fidelity in state tomography.

## Contribution

It provides the first full modal characterization of a QPG with weak coherent states and demonstrates high-fidelity high-dimensional TM state tomography.

## Key findings

- Average fidelity of 0.85 to ideal device in 7-dimensional space
- QPG can perform high-dimensional TM state tomography with 0.99 fidelity
- Full measurement operators enable precise device calibration

## Abstract

Encoding quantum information in the photon temporal mode (TM) offers a robust platform for high-dimensional quantum protocols. The main practical challenge, however, is to design a device that operates on single photons in specific TMs and all coherent superpositions. The quantum pulse gate (QPG) is a mode-selective sum-frequency generation designed for this task. Here, we perform a full modal characterisation of a QPG using weak coherent states in well-defined TMs. We reconstruct a full set of measurement operators, which show an average fidelity of 0.85 to a theoretically ideal device when operating on a 7-dimensional space. Then we use these characterised measurement operators of the QPG to calibrate the device. Using the calibrated device and a tomographically complete set of measurements, we show that the QPG can perform high-dimensional TM state tomography with 0.99 fidelity.