Quantum Polaritonics

An international research partnership of state-of-the-art experimental laboratories and theoretical groups between the University of Cambridge, MIT-Skoltech, and the University of Southampton.

In simple words:

There are problems in science and engineering that require computing time exceeding the age of the universe (14 billion years) or that are by nature incalculable and therefore neither a "classical" nor a "quantum" computer can solve. Some of these challenging problems can be addressed by simulators, also known as optimisers, which we describe below.

The Quantum Polaritonics international research partnership is driven by our vision to develop an in-house simulator that would allow us to address some of the most fascinating problems of modern science and engineering. To that end, we put together an international team of physical scientists and engineers who share our vision as well as our passion for scientific discovery. There is never ending excitement to unravel the exotic physics underlying quantum phenomena and put them to action in achieving our goal.

The search for an optimal solution is similar to looking for the lowest point in a mountainous terrain with many valleys, trenches, and drops. A hiker may go downhill and think that has reached the lowest point of the entire landscape, but there may be a deeper drop just behind the next mountain. Such a search may seem daunting in natural terrain, but imagine its complexity in a high-dimensional space! This is exactly the problem to tackle when the objective to find the minimum of a system represents a real-life problem with many unknowns, parameters, and constraints.

Modern supercomputers can only deal with a small subset of such problems when the dimensions of the systems to be minimised are relatively small or when the underlying structure of the system offers a shortcut to the global minimum. Even a hypothetical quantum computer, when realised, would offer at best a quadratic speed-up for the “brute-force” search for the global minimum.

Now imagine that one hovers far above the mountains in the direction of the sun, looking for the deepest point from the sky. We would expect that it is possible to measure the deepest point from the air; alas, for the type of problems here the surface of the land is pitch black not allowing for aerial measurements. To find the global minimum of the complex landscape problem above, a problem that in many ways resembles the physical systems we are tackling in our labs, we engineered water into the landscape. Imagine that we start raising the level of water underneath the landscape, while looking for the first glimpse of sun light scattering from the surface of the water. With the water raising from the bottom up, the coordinates where we observe the first reflection from the sun correspond to the deepest point of our complex landscape.

In our case, instead of water we use a quantum fluid that is created by shining a laser on a semiconductor device. The device consists of stacked layers of atoms such as gallium, arsenic, indium, and aluminium; the atoms being deposited with single atomic layer precision. The electrons in these layers absorb and emit light of a specific colour. The admixture of electrons and light leads to the formation of a new type of particle, called polariton, that is ten thousand times lighter than electrons allowing it to achieve sufficient densities to form an exotic state of matter known as a Bose-Einstein condensate. In a Bose-Einstein condensate the quantum phases of polaritons synchronise and create a quantum fluid that can be observed by detecting the light that is emitted from it.

But how to create a potential landscape that corresponds to the function to be minimised and to force polaritons to condense at its lowest point? To do this, we focus on a particular type of optimisation problem, but a type that is general enough so that any other hard problem can be related to it, namely minimisation of the XY model, which is one of the most fundamental models of statistical mechanics. We have shown that we can create polaritons at vertices of an arbitrary graph: as polaritons condense, the quantum phases of polaritons arrange themselves in a configuration that corresponds to the absolute minimum of the objective function.

The XY Model is a universal classical spin model alongside other universal spin models such as the Ising and Heisenberg models. They are characterised by the given degrees of freedom, "spins", by their interactions, "couplings," and by the associated cost function, "Hamiltonian". Various physical platforms have been proposed to simulate such models using superconducting qubits, optical lattices, coupled lasers etc. We introduced polariton graphs as a new platform for finding the global minimum of classical XY Hamiltonians in a variety of geometries and coupling strengths. This system is based on well-established semiconductor and optical control technologies and benefit from flexible tunability and easy readability. Polariton condensates can be imprinted into any two-dimensional graph by spatial modulation of the pumping laser offering straightforward scalability. Polariton simulators have the potential to reach the global minimum of the XY Hamiltonian in a bottom-up approach by gradually increasing excitation density to threshold. This is an advantage over classical or quantum annealing techniques, where the global ground state is reached through transitions over metastable excited states with an increase of the cost of the search with the size of the system.

News:

02/10/2020:

The LHO group at Southampton University received support from the new Horizon 2020 "Polariton Logic" (POLLOC). In collaboration with ETH, IBM Zurich, CNRS and AMO GmbH, the project will aim to engineer solutions for all-optical computing using organic polaritonics platform. As a natural continuation of our recent article demonstrating a room-temperature organic polariton transistor (A. Zasedatelev et al., Nature Photonics, 13, 378–383(2019)), we are excited to see how far the partners will be able to push the state of the art.

15/09/2020:

In collaboration with Lancaster University, we demonstrate a new optical method to synthesize artificial solid-state crystal structures for cavity-polaritons using only laser light, with a publication in Nature Communications. The results could lead to the realization of field-programmable polariton circuitry and to new strategies to create guided light and robust confinement of coherent light sources. Read more on Skoltech website, Science Codex and Phys.org.

18/02/020:

Prof. Natalia Berloff, Skoltech/University of Cambridge, and collaborators proposes an alternative theoretical framework for simulating spin Hamiltonians with a network of spatially localised polariton condensates that do not interact with one another geometrically, in a report published in Nanophotonics.

22/05/2020:

Our paper on Optical Control of Couplings in Polariton Condensate Lattices is featured on the cover of Physical Review Letters. The article is a collaboration between Skoltech and Southampton University. By allowing precise control of the interaction between nodes, this devellopment brings polariton simulators one step closer to reality. Read more at Phys.org.

13/02/2020:

The work of Prof. Natalia Berloff (Skoltech & University of Cambridge) is featured in the inside back cover of Advanced Quantum Technologies. There is a growing interest in investigating new states of matter using out‐of‐equilibrium lattice spin models in two dimensions. However, a control of pairwise interactions in such systems has been elusive as due to their nonequilibrium nature they maintain nontrivial particle fluxes even at the steady state. In this work, we suggest how to overcome this problem and formulate a method for engineering reconfigurable networks of nonequilibrium condensates with control of individual pairwise interactions.

10/01/2020:

Prof. Pavlos Lagoudakis (Skoltech & Southampton University) has become the first foreign academic to win a research grant from the Russian Foundation for Basic Research. The grant awarded are to support our research on organic polaritonics, and specifically on “Hybrid polariton condensates and transistors based on organic and inorganic semiconductors” (in collaboration with the German DFG and Wurzburg University) and on “Vibron-polariton collective states and laser generation at room temperature” (a follow up of our liquid crystal research recently highlighted in Science, in collaboration with Polish colleagues). Read more on Skoltech Website.

14/11/2019:

In collaboration with Polish colleagues, we demonstrated in Science the creation of spin-orbit synthetic Hamiltonians in liquid-crystal optical cavities. As the properties of the cavity were modified by an external voltage, the photons behaved like massive quasiparticles endowed with a magnetic moment, called “spin”, under the influence of an artificial magnetic field. Read more at SciTechDaily, EurekAlert, EENews.

29/09/2019:

The Hybrid Photonic Labs at Skoltech hosted today Dr. Maria Kandyla (National Hellenic Research Foundation, Athens, Greece), who gave a talk on "Development and applications of laser processed hybrid nanomaterials". She discussed in details how combining silicon micro/nanostructures with thin semiconducting films results in electronic heterojunctions with large active surface area and improved optoelectronic performance. We are thankful to the speaker for making the trip from Greece and look forward to a fruitful collaboration.

01/07/2019:

The first Polaritonics Day took place today at the new Skoltech campus near Moscow. Lectures by leading academics were attended throughout the day by representatives of the academic community and relevant industrial partners. The topics covered the full scope of modern polaritonics, from the more traditional foundations of the field to new exotic physics. Attendees were afterwards given the opportunity to visit the Skoltech Hybrid Photonics Labs where the lab technologies were demonstrated. An event dinner was organized in the evening, during which attendees could discuss the state of the field and plan future endeavours.

On behalf of the organizing committee, we would like to thank all speakers and attendees for a very successful and productive day!

20/06/2019:

Our teams at Skoltech and the University of Southampton, along with partners at IBM Zurich and University of Wuppertal, are proud to report on the front cover of Nature Photonics the first ambient-temperature, cascadable, all-optical transistor capable using polaritoncs in an organic microcavity with ladder-type polymer gain medium. We demonstrate a net gain of ~10 dB μm−1, sub-picosecond switching time, cascaded amplification and all-optical logic operation at ambient conditions. Read more on IBM Website, Pro-Physik (DE), Elektronik Praxis (DE), RIA Novosti (RU), 3DNews (RU).


07/06/2019:

The Hybrid Photonics Labs at Skoltech, along with other partners of the Quantum Polaritonics Partnership, are glad to invite all interested parties to attend the first Skoltech Polaritonics Day on July the 1st. The event will highlight the latest cutting edge research in the field of strong light matter coupling phenomena, and event will bring together a global network of scientists from industry and academia to discuss their latest research in the field.

Confirmed guest speakers include Prof. Vladimir Agranovich (Troitsk, Russia) Prof. Atac Imamoglu (ETH, Switzerland), Franko Nori (Riken, Japan), Dr. Rainer Mahrt (IBM Zurich, Switzerland), Prof Sven Hoefling (University of Wurzburg, Germany).

More details on the event website.

21/03/2019:

Prof. Berloff (Skoltech & University of Cambridge) presented today the results of her research using polariton platforms for solving hard optimization problems "mappable" into the XY model, at an Open Colloquium organized by the Russian Quantum Center at the National University of Science and Technology MISIS. In her talk, she discussed the range of optimization problems that can be efficiently solved by polariton graphs and focused on elucidating the relationship between the energy spectrum of the XY Hamiltonian and the total number of condensed polariton particles. This theory underpinned the recent Nature Materials publication "Realizing the classical XY Hamiltonian in polariton simulators", a fruit of the Quantum Polaritonics Partnership.

10/02/2019:

Skoltech researchers presented on Russian national news (TV channel Rossiya 1 )their work on the polariton simulator and discussed how Skoltech is providing the right international environment to foster such developments.

See here for the full interview (in Russian).

22/10/2018:

Skoltech senior researcher Dr. Anton Zasedetalev gave an interview to the online science TV channel Шаг России, where he disccuses the Hybrid Photonics Labs recent work on polariton simulation and how he believes polariton physics can make an impact in the world of high performance computing.

See here for the full interview (in Russian).

10/10/2018:

The work of Prof. Lagoudakis (Skoltech & University of Southampton) is highlighted on the cover of Light Science & Application. The result of fruitful collaboration with Polish colleagues at the University of Warsaw and at the Institute of Applied Physics of the Military University of Technology, the article, "Tunable optical spin Hall effect in a liquid crystal microcavity", demonstrates for the first time external control of spin currents by modulating the splitting between transverse electric and magnetic fields in liquid crystals integrated in microcavities.

04/07/2018:

We are glad to see our work highlighted on the back cover of Advanced Optical Materials. A collaboration of researchers of the Quantum Polaritonics Partnership with colleagues at the University of Sheffield, the article "A Yellow Polariton Condensate in a Dye Filled Microcavity" demonstrates polariton condensation in the yellow part of the visible spectrum from a planar organic semiconductor microcavity containing the molecular dye bromine‐substituted boron‐dipyrromethene. Such structures maybe be of interest in the development of new devices, including high-efficiency lasers and light-sources.

Read more on Advanced Science News,

25/09/2017:

The researchers of the Partnership are proud to see their work published today in the prestigious journal Nature Materials. The article, Realizing the classical XY Hamiltonian in polariton simulators, is the culmination of several years of intense collaboration between Skoltech, the University of Southampton and the University of Cambridge. By showing how polaritonics platforms can be used to map computationally complex problems into a classical spin system, such as the XY model, this is a very exciting development for the whole field of polaritonics. Big congratulations to all researchers involved!

Read more on Science Alert, New Atlas, Futurism, Nanowerk.