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Complex Light and Optical Forces

Programming quantum measurements of time inside a multimode fibre

Mehul Malik, Dylan Danese, Vatshal Srivastav, Saroch Leedumrongwatthanakun, Will McCutcheon

Published Mar 5, 2026• 0 citations
Publisher

Abstract

The temporal degree-of-freedom of light is incredibly exciting for many quantum technologies, ranging from photonic quantum computing to quantum key distribution. The generalized measurement of large and complex superpositions of the time-of-arrival of a photon presents a unique experimental challenge. Here we show how the coupling of spatial and temporal information inside a multi-mode fiber (MMF) can be harnessed to program generalized measurements for large photonic time-bin qudits. By exciting superpositions of special sets of spatial modes that are temporally unscattered through the fiber, we engineer arbitrary measurements of photon time-bins in dimensions up to 11. The single fiber serves 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.

DOI
10.1117/12.3088470

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The temporal degree-of-freedom of light is incredibly exciting for many quantum technologies, ranging from photonic quantum computing to quantum key distribution. The generalized measurement of large and complex superpositions of the time-of-arrival of a photon presents a unique experimental challenge. Here we show how the coupling of spatial and temporal information inside a multi-mode fiber (MMF) can be harnessed to program generalized measurements for large photonic time-bin qudits. By exciting superpositions of special sets of spatial modes that are temporally unscattered through the fiber, we engineer arbitrary measurements of photon time-bins in dimensions up to 11. The single fiber serves 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.
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.
The field of high-dimensional quantum photonics involves the use of multimode photonic degrees-of-freedom such as the spatial, temporal, or spectral structure of light to encode multi-level quantum states. Recent years have seen rapid progress in the development of methods to generate, manipulate, and distribute such quantum states of light and their use in a range of quantum technology applications that offer practical advantages over conventional qubit-based approaches. High-dimensional quantum states of light encoded in photonic time-bins, frequency-bins, transverse-spatial modes, waveguide paths, and temporal modes have enabled noise-robust fundamental tests of quantum mechanics, error-resilient and high-capacity quantum communication protocols, andas well as efficient approaches for quantum information processing, to name just a few examples. However, research in this field has progressed fairly independently, with little exchange across different photonic degrees-of-freedom or between experiment and theory and no comprehensive comparison between degrees-of-freedom. This roadmap aims to bridge this gap by surveying progress in each area and identifying shared challenges and opportunities that cut across two or more photonic degrees-of-freedoms. We review early work and state-of-the-art experimental techniques under development for high-dimensional quantum states encoded in single and entangled photons, as well as theoretical tools for their measurement and certification. We outline the main outstanding challenges for theory and each experimental degree-of-freedom, identifying promising future directions of research that may enable these to be overcome. We end by discussing interconnections and shared challenges centered around their distribution, measurement, and manipulation, with a view towards their integration into next-generation quantum technology platforms and applications.
We demonstrate two approaches for unbalanced interferometers as time-bin qubit analyzers for quantum communication, robust against mode distortions and polarization effects as expected from free-space quantum communication systems including wavefront deformations, path fluctuations, pointing errors, and optical elements. Despite strong spatial and temporal distortions of the optical mode of a time-bin qubit, entangled with a separate polarization qubit, we verify entanglement using the Negative Partial Transpose, with the measured visibility of up to $0.85\ifmmode\pm\else\textpm\fi{}0.01$. The robustness of the analyzers is further demonstrated for various angles of incidence up to $0.{2}^{\ensuremath{\circ}}$. The output of the interferometers is coupled into multimode fiber yielding a high system throughput of 0.74. Therefore, these analyzers are suitable and efficient for quantum communication over multimode optical channels.
The photonic temporal degree of freedom is one of the most promising platforms for quantum communication over fiber networks and free-space channels.In particular, time-bin states of photons are robust to environmental disturbances, support high-rate communication, and can be used in high-dimensional schemes.However, the detection of photonic time-bin states remains a challenging task, particularly for the case of photons that are in a superposition of different time bins.Here, we experimentally demonstrate the feasibility of picosecond time-bin states of light, known as ultrafast time bins, for applications in quantum communications.With the ability to measure time-bin superpositions with excellent phase stability, we enable the use of temporal states in efficient quantum key distribution protocols such as the BB84 protocol.