Mott Insulator Transition in a Quantum Fluid of Light

Project Information

MISTiQ-Light

Grant agreement ID: 101087533

DOI

10.3030/101087533

EC signature date 26 May 2023

Start date 1 October 2023

End date 30 September 2028

Funded under

• European Research Council (ERC)

Total cost

€ 2 000 000,00

EU contribution

€ 2 000 000,00

Coordinated by

SORBONNE UNIVERSITE
France

Periodic Reporting for period 1 - MISTiQ-Light (Mott Insulator Transition in a Quantum Fluid of Light)

Reporting period: 2023-10-01 to 2026-03-31

Summary of the context and overall objectives of the project

Light is best known as a carrier of information, used in imaging, telecommunications, and sensing. In recent years, however, physicists have discovered that under suitable conditions light can also behave like a fluid, exhibiting collective effects similar to those observed in liquids and quantum gases. When photons interact with matter, they can influence one another and form organized states governed by the laws of many-body physics.

The MISTiQ-Light project explores this emerging concept of a quantum fluid of light, with the long-term objective of creating a strongly correlated state known as a Mott insulator. In such a state, particles are no longer free to move independently but become localized in a regular structure due to strong interactions. While Mott insulators are well known in solid-state physics and ultracold atomic gases, realizing an equivalent state with light would represent a major conceptual and technological advance.

The project addresses a key limitation of current photonic technologies: scalability. Many quantum optical systems rely on weakly interacting photons, which restricts their ability to generate large, controllable quantum states. By engineering strong photon–photon interactions and loading light into an artificial lattice in a controlled way, MISTiQ-Light aims to create a new form of light capable of producing large arrays of identical single photons or other non-classical states in parallel.

Beyond its long-term technological potential, the project tackles fundamental questions about how quantum many-body phenomena can emerge in systems that are intrinsically driven and out of equilibrium. By combining advanced optical techniques with concepts originally developed for quantum gases, MISTiQ-Light seeks to open a new route for quantum simulation and to deepen our understanding of collective quantum behaviour in novel physical platforms.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During the reporting period, the project focused on establishing the experimental and conceptual foundations required to realize a strongly interacting quantum fluid of light. This involved both the development of experimental platforms and the investigation of fundamental physical mechanisms governing collective and quantum behaviour in photonic systems.

A first line of activity relied on mature photonic platforms, in particular exciton–polariton systems and atomic vapour setups, which enabled rapid progress on the physics of quantum fluids of light. Using these systems, the project achieved precise control and measurement of collective excitations, hydrodynamic regimes, and non-equilibrium dynamics. These studies demonstrated that light can support well-defined collective modes and quantum fluctuations analogous to those found in condensed-matter and ultracold-atom systems, thereby validating the core physical concepts underpinning the project.

In parallel, significant effort was devoted to the development of advanced measurement techniques capable of probing quantum properties of light at the many-body level. New spectroscopic and correlation-based approaches were implemented to access excitation spectra, fluctuations, and mode coupling with high resolution. These tools are essential for identifying the transition from weakly interacting regimes to strongly correlated states.

A complementary activity addressed the role of dimensionality and propagation effects. Reduced-dimensional photonic systems were investigated to clarify how light propagation can be used as an effective temporal dimension, providing insight into adiabatic loading strategies and dynamical control. This work directly informed the design choices for the final experimental platform.

The construction and commissioning of the core cold-atom platform, required to reach strong and local photon–photon interactions, progressed throughout the period. An initial system was used to validate key technical steps, followed by the deployment of a new experimental apparatus optimized for high optical depth. This milestone marks the transition toward the phase of the project where strongly correlated photonic states can be explored.

Overall, the project has already demonstrated that quantum fluids of light constitute a robust and versatile platform for studying collective quantum phenomena, while putting in place the infrastructure and methodologies needed to reach the strongly interacting regime targeted in the later stages of the action.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The results obtained so far go beyond the previous state of the art by demonstrating that light can be controlled as a genuine quantum many-body system, rather than only as a weakly interacting information carrier. The project has shown that fluids of light can support well-defined collective excitations, complex hydrodynamic behaviour, and quantum fluctuations that are measurable with high precision. This establishes photonic systems as a serious alternative to more traditional platforms, such as ultracold atoms or solid-state materials, for exploring collective quantum phenomena.

A key advance lies in the level of experimental control achieved. The project has developed techniques that allow quantum properties of light to be probed at the level of collective modes and correlations, not just at the level of individual photons. This opens access to regimes that were previously inaccessible in optical systems and provides new tools for studying non-equilibrium quantum physics in a controlled environment.

Another important result is the clarification of how strong interactions between photons can be engineered and diagnosed. By exploring different physical platforms and interaction mechanisms, the project has identified realistic pathways toward regimes where light becomes strongly correlated. This significantly reduces the risk associated with creating novel photonic quantum states and informs the design of future experiments.

In terms of potential impact, these results lay the groundwork for new approaches to quantum simulation, where complex quantum models can be emulated using light. In the longer term, they may enable scalable sources of non-classical light, such as large arrays of identical single photons, which are essential for advanced quantum technologies.

Further progress toward practical applications will require continued research to reach and stabilize the strongly interacting regime, followed by experimental demonstrations of large-scale operation. Ensuring uptake will depend on access to advanced experimental infrastructure, continued methodological development, and close interaction between fundamental research and emerging quantum-technology ecosystems.