Scientists from CNR Nanotec in Lecce and the University of Warsaw used a new generation of semi­conductor photonic gratings to optically tailor complexes of quantum droplets of light that became bound together into macro­scopic coherent states. The research underpins a new method to simulate and explore inter­actions between artificial atoms in a highly recon­figurable manner, using optics. 

Condensed matter systems and photonic technologies are  regularly used by researchers to create microscale platforms that can simulate the complex dynamics of many interacting quantum particles in a more accessible setting. Some examples include ultracold atomic ensembles in optical lattices, super­conducting arrays, and photonic crystals and waveguides. In 2006 a new platform emerged with the demonstration of macro­scopically coherent quantum fluids of exciton-polaritons to explore many-body quantum phenomena through optical techniques.

When a piece semiconductor is placed between two mirrors – an optical microresonator – the electronic excitations within can become strongly influenced by photons trapped between the mirrors. The resulting new bosonic quantum particles – exciton-polaritons – can under the right circum­stances undergo a phase transition into a non­equilibrium Bose-Einstein condensate and form a macroscopic quantum fluid or a droplet of light. Quantum fluids of polaritons have many salient properties, one being that they are optically con­figurable and readable, permitting easy measure­ments of the polariton dynamics. This is what makes them so advantageous to simulate many-body physics.

Polariton condensates must be continuously optically pumped with external lasers to replenish particles, otherwise the condensate dissipates within picoseconds. However, the harder you pump the condensate the more energetic it becomes due to repulsive inter­particle forces, leading to particles escaping the condensate and subsequent decay of spatial correlations. This is a funda­mental problem for optically programmable polariton simulators. Scientists needed to come up with a way to make the condensate more stable and long lived, while still being optically pumped.

Now, the scientists achieved this goal using a new generation of semiconductor photonic gratings. They used subwavelength properties of the photonic grating to imbue polaritons with new properties. First, the polaritons could be driven to condense into an ultralong lifetime state known as bound state in the continuum (BIC). The fascinating thing about BICs is that they are mostly non-radiative due to symmetry enforced protection from the outside continuum of photonic modes. Second, the polaritons obtained a negative effective mass due to the dispersion relation coming from the grating. This meant that the pumped polaritons could not escape so easily through normal decay channels anymore. Now, the researchers possessed polariton fluids that were both extremely long lived and safely confined using only optical techniques.

Combined, these mechanisms allowed Antonio Gianfrate and Danielle Sanvitto to optically pump multiple polariton droplets that could interact and hybridize into macroscopic complexes. They could tailor and reversibly configure molecular arrange­ments and chains using this new form of artificial atoms: condensates of negative-mass BIC polaritons. The BIC property provided polaritons with much longer lifetimes whereas the negative mass property caused them to become optically trapped. The findings were supported by a BIC Dirac-polariton theory developed between Helgi Sigurdsson from the University of Warsaw, Hai Chau Nguyen from the University of Siegen, Germany, and Hai Son Nguyen from University Lyon, France.

The ultimate advantage of the platform is that the artificial quantum complexes can be all-optically programmed yet they retain very high lifetimes because of their protection from the continuum. This could lead to a new venture into optically programmable large-scale quantum fluids defined by unpre­cedented coherence scales and stability for structured nonlinear lasing and polariton-based simulation of complex systems.

“There are still several interesting ways to explore in this artificial polaritonic Dirac system. As an example, the coupling mechanism between polariton droplets along and perpendicular to the grating direction is very different. Along the waveguide, polaritons are effectively negative mass particles strongly bound to their pump spot. Perpen­dicular to the waveguide they move as positive mass particles undergoing ballistic transport. The mixture of these two mechanisms opens a new window to look at emergent behaviours of synchrony and pattern formation in structured polariton quantum fluids” concludes Helgi Sigurðsson from the University of Warsaw. (Source: FUW)

Reference: A. Gianfrate et al.: Reconfigurable quantum fluid molecules of bound states in the continuum, Nat. Phys. 20, 61 (2024); DOI: 10.1038/s41567-023-02281-3

Link: CNR Nanotec, Institute of Nanotechnology, Lecce, Italy • Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Warsaw, Poland