Programming Quantum Measurements of Time inside a Complex Medium

TL;DR

This paper introduces a novel method to perform high-dimensional quantum measurements of photon time-of-arrival using a single multi-mode fiber, simplifying experimental setups and enabling advanced quantum information processing.

Contribution

It demonstrates how spatial and temporal coupling in a multi-mode fiber can be used to program generalized, high-dimensional time-bin measurements in a scalable and efficient manner.

Findings

Achieved high-quality measurements of time-bin superpositions up to dimension 11.

Developed a fiber-based approach that replaces complex interferometers.

Enabled scalable and simplified quantum measurement of temporal states.

Abstract

The temporal degree-of-freedom of light is incredibly powerful for modern quantum technologies, enabling large-scale quantum computing architectures and record key-rates in quantum key distribution. However, the generalized measurement of large and complex quantum superpositions of the time-of-arrival of a photon remains a unique experimental challenge. Conventional methods based on unbalanced Franson-type interferometers scale poorly with dimension, requiring multiple cascaded devices and active phase stabilization. In addition, these are limited by construction to a restricted set of phase-only superposition measurements. Here we show how the coupling of spatial and temporal information inside a single multi-mode fiber can be harnessed to program completely generalized measurements for high-dimensional superpositions of photonic time-bin. Using the multi-spectral transmission matrix of the fiber, we find special sets of spatial modes that experience distinct dispersive delays through the fiber. By exciting coherent superpositions of these spatial modes, we engineer the equivalent of large, unbalanced multi-mode interferometers inside the fiber and use them to perform high-quality measurements of arbitrary time-bin superpositions in up to dimension 11. The single fiber functions as a scalable, common-path interferometer for time-bin qudits that significantly eases the experimental overheads of standard approaches based on unbalanced Franson-type interferometers, serving as an essential tool for quantum technologies that harness the temporal properties of light.