Advances Quantum Measurement of Time with High-Dimensional Superpositions
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Scientists are tackling a significant hurdle in quantum technology , the precise measurement of light’s temporal properties within complex environments. Dylan Danese, Vatshal Srivastav, and Will McCutcheon, alongside Saroch Leedumrongwatthanakun, Mehul Malik et al from the Institute of Photonics and Quantum Sciences at Heriot-Watt University, demonstrate a novel method for programming quantum measurements of time using a single multi-mode fibre. Their research overcomes limitations of traditional techniques, which struggle with scaling and are restricted to specific measurement types, by harnessing the interplay between spatial and temporal information. This breakthrough enables high-quality measurements of complex time-bin superpositions , up to dimension 11 , and paves the way for more scalable and efficient quantum architectures and key distribution systems. This ingenious method circumvents the challenges associated with traditional Franson-type interferometers, which require multiple cascaded devices and active phase control, and are limited to restricted phase-only measurements. Experiments show that the single fiber acts as a common-path interferometer, simplifying experimental setups and easing the overheads typically associated with time-bin qudit measurements.
This research establishes a new paradigm for manipulating and measuring quantum states encoded in the time-domain, offering a significant advantage for applications like scalable quantum computing architectures and record key-rates in quantum key distribution. The work opens up possibilities for creating high-capacity quantum networks and enhancing the noise tolerance of quantum information processing, with recent demonstrations in platforms ranging from photons to superconducting circuits. Furthermore, the scientists validated the quality of their measurements through quantum tomography, employing a tunable Franson interferometer to prepare arbitrary time-bin qubits and assess the accuracy of the fiber-based measurement scheme across all two-dimensional subspaces. Multi-mode fibre characterisation for time-bin qudit interferometry . Measurements confirm the ability to program generalized measurements of three time-bins, represented as |tin⟩= c0|t0⟩+ c1|t1⟩+ c2|t2⟩, using a carefully designed interferometer. The researchers constructed an interferometer with three unbalanced paths, each containing a tunable phase-shifter and a fixed delay, resulting in five time-delayed peaks after recombination. The central peak’s amplitude, proportional to f0. c0+f1. c1+f2. c2, allows for projective measurements of any phase-only time-bin superposition state |ψ⟩= 1 √ 3(f0|t0⟩+ f1|t1⟩+ f2|t2⟩). To achieve this in higher dimensions, the team utilized a digital micromirror device (DMD) to shape the spatial mode of the time-bin superposition, inputting it into a 40-meter-long multi-mode fiber. Tests prove that by creating a coherent superposition of three MMF τ-modes, they engineered an equivalent three-path unbalanced interferometer within the fiber, enabling complex amplitude and phase measurements. A detailed measurement of the multi-mode fiber’s transmission matrix (MSTM) using a DMD and In-GaAs camera revealed 125 circularly polarized (CP) modes, with 421 wavelength steps of 3.8pm each, resulting in a matrix containing 421 × 125 × 125 complex values. Subsequent Fourier transformation yielded a time-resolved transmission matrix (TRTM) with a resolution of 5ps over a temporal range of approximately 1.6ns. Singular value decomposition (SVD) of the TRTM identified 70 resolvable τ-modes, from which 11 were selected with a time-separation of approximately 160ps to serve as the time-bin basis.
Fibre Interferometry Enables High-Dimensional Qubit Measurement with unprecedented By carefully selecting and superposing specific spatial modes, the team effectively engineered a scalable, common-path interferometer capable of performing high-quality measurements on time-bin qudits, quantum bits utilising time as the information carrier, in dimensions up to 11. These delays, when coherently combined, mimic the behaviour of large, unbalanced interferometers, but within the confines of a single fibre. Experimental results, including fidelity measurements for states up to dimension 11, confirm the ability to accurately measure arbitrary time-bin superpositions. The authors acknowledge that the overall experimental efficiency is currently limited by the mode generation and projection stages, specifically the digital micromirror device (DMD) and spatial light modulator (SLM) used in the setup. Future work could focus on integrating these components or exploring alternative technologies, such as utilising an SLM for both mode generation and measurement, to improve performance. While acknowledging limitations in current efficiencies, the demonstrated principle offers a promising avenue for future development and practical implementation of time-bin qudits in quantum communication and computation. 👉 More information 🗞 Programming Quantum Measurements of Time inside a Complex Medium 🧠 ArXiv: https://arxiv.org/abs/2601.14565 Tags: