Quantum nonlinear optics with 2D materials

When two beams from light torches cross, they do not clash like sabres from "Star Wars", but simply continue each its own way. This follows from the fact that free photons do not interact. However, when placed in an appropriate medium, photons can effectively feel the presence of each other, making the response of the optical system dependent on the number of photons. In this case, we say it has an optical nonlinearity provided by the medium. Typically the larger the volume, the stronger the nonlinearity, and the goal is to achieve prominent nonlinearity at the smallest possible scale. Together with the "sabre-effect", nonlinearity can ultimately provide the efficient manipulation of quantum states for photons. Thus with high level of nonlinearity single photon states can be prepared and used in quantum information processing. This would result in ultrafast quantum computing and communication platforms, serving as the basis for quantum applications that include secure communication networks, increased computational power and sensing at a level impossible to reach without quantum technologies.

When light is confined in an optical cavity (for instance, set by two mirrors), its interaction with the medium is greatly enhanced. If the average number of roundtrips made by photons becomes large, they can hybridize with excitations in the medium, leading to half-light half-matter quasiparticles - polaritons. The hybridization makes confined light and the resulting polaritons able to interact. This ability stays behind the progress in numerous applications of classical nonlinear optics, including optical solitons for fast broadband communication. However, the task of finding an optimal system, where large nonlinearity for polaritons is achieved in the limit of few quanta, remains an open question.

In the project, I will discover ways to increase optical nonlinearity at the minuscule scale. This will become possible by studying strong light-matter coupling in two-dimensional (2D) materials, where monolayer thickness can be smaller than a nanometer. Considering combinations of a few layers, I will show that the nonlinear response for polaritons can be elevated to the level where single photon processes become observable. The research will thus enable these easy-to-produce miniature systems for quantum optical processing to function as a platform for affordable quantum technologies.

The planned outcome of the "2D-for-quantum" project corresponds to establishing two-dimensional (2D) materials strongly coupled to light as a versatile platform for observing quantum effects. This urge is motivated by the following arguments: 1) relative simplicity of the planned 2D material sample fabrication; 2) fast operation for optical devices; 3) operation at relatively high temperatures, where strong coupling is routinely observed even at room temperature; 4) potential for producing cost-efficient quantum devices based on semiconductor monolayers and heterostructures.

The main beneficiaries from the project are thus:
1) experimentalists working on 2D material-based devices, who will get access to the quantum operation mode;
2) emergent start-ups in quantum communication that will receive a new platform for single photon generation;
3) companies in computing and healthcare, the combination of lowered cost for quantum-enabled devices and fast operation times will boost application in optical information processing and sensing;
4) general public and students who will benefit from technological developments, as well as the educational side of the project.

The proposal lays the plan for the theoretical investigation and qualitative transformation of 2D material optics at strong coupling, required to harness its nonlinear properties and reach the quantum operation regime. However, while being a theoretical proposal, it specifically targets optimal sample configurations, putting an emphasis on realization in experiments. This will directly impact the field of optical nonlinear and quantum components, setting the path towards commercialization.

The development of the 2D material polaritonics will further impact the landscape of start-ups and companies working on the single photon based solutions. For instance, emergent businesses in quantum communication, with the example of Nu Quantum (Cambridge, UK), will benefit from proposals for novel ways to generate single photons using 2D materials. Other companies that expressed interest include KETS Quantum Security in Bristol and Nordic Quantum Computing Group in Oslo, opening further prospects for future prototyping of quantum devices.

Finally, two-level impact of society is expected. First, in the short term this will correspond to outreach activities and engagement in public discussions on the use of cases of quantum technologies. I will make sure that benefits from the 2D materials platform and quantum optical solutions are thoroughly communicated. Second, future impact shall become visible at the commercialization stage, where cost-efficient optical components will bring quantum technologies closer to an end-user, and benefit areas of medicine, chemistry, and sensing.