Light offers a vast potential in the development of modern quantum technologies due to its intrinsic resilience to decoherence effects and its capacity to convey a huge amount of information. The many modes of light, would they be spatial modes or spectral modes, are as many quantum harmonic oscillators, leading to a largely unexplored Hilbert space[1]. One avenue for employing light to process quantum information focuses on the continuous variable regime, where the observables of interest are the quadratures of the electric field. They have proven their worth as a platform for creating huge entangled states in a deterministic fashion, easily manipulatable with standard techniques in optics.
In this presentation we will first review the basic principles of multimode quantum light in the continuous variable regime, and illustrate them in quantum metrology experiments. We will show how it allows for an intuitive understanding of the sensitivity limits in high precision measurement, and experimentally reach the fundamental limits impose by the vacuum fluctuations in simple problems, as for instance estimating the separation between two incoherent sources[2].
We will then consider quantum information processing with multimode light. We will first demonstrate how to generate large entangled states using time/frequency modes[3]. However to reach a quantum advantage, and perform a task that cannot be efficiently simulated with a classical device, we require more than just entanglement. The additional ingredient is non-Gaussian statistics in the outcomes of the quadrature measurements. We will demonstrate how photon subtraction, a well know non-gaussian operation, can be rendered mode-dependent and allow for the generation of non-Gaussian multimode state of lights, required for quantum information processing[4].
[1] C. Fabre and N. Treps, Modes and States in Quantum Optics, Rev. Mod. Phys. 92, 035005 (2020).
[2] P. Boucher, C. Fabre, G. Labroille, and N. Treps, Spatial Optical Mode Demultiplexing as a Practical Tool for Optimal Transverse Distance Estimation, Optica, 7, 1621 (2020).
[3] J. Roslund, R. M. de Araújo, S. Jiang, C. Fabre, and N. Treps, Wavelength-Multiplexed Quantum Networks with Ultrafast Frequency Combs, Nature Photonics 8, 109 (2014).
[4] Y.-S. Ra, A. Dufour, M. Walschaers, C. Jacquard, T. Michel, C. Fabre, and N. Treps, Non-Gaussian Quantum States of a Multimode Light Field, Nature Physics 11, 1 (2019).
