Periodic Reporting for period 4 - 2DQP (Two-dimensional quantum photonics)
Período documentado: 2022-07-01 hasta 2023-12-31
Quantum technologies are primed to revolutionise the fields of communication, information processing, and simulation in the coming years. Similar to the contemporary technologies that underpin modern society, future quantum devices will likely build on a semiconductor heterostructure platform. However, conventional semiconductor heterostructures have fundamental limitations: they are based on combinations of epitaxially-grown and lattice-matched bulk materials, which constrains possible material combinations and prevents free engineering of the new quantum states required for future technologies. The emerging field of two-dimensional (2D) van der Waals (vdW) heterostructures offers an unprecedented opportunity to tackle these technological challenges. The remarkable flexibility in material combinations and tunability of their physical parameters positions them as a promising platform to realise novel quantum states with tailored properties.
The project Two-Dimensional Quantum Photonics (2DQP) aims to characterize, identify, engineer, and coherently manipulate localized quantum states in this platform. One spectacular example of a 2D quantum material is shown in the figure below. In this figure, two single sheets of atoms are stacked on top of each other but with a relative ‘twist’. The ‘twist’ creates a moire pattern, which is depicted in the ‘shadow’ beneath the two layers of atoms. This long-range periodic pattern fundamentally changes the electronic and optical properties of the new material. For instance, electrons can be trapped at specific sites in the pattern, and the distance between each site can be engineered by the choice of materials and twist angle. The distance between the electron traps determine how they can interact, and in certain conditions strong interactions emerge which fundamentally alters their behaviour – for instance the strong correlations can lead to magnetism or superconductivity. This opens unprecedented opportunities to engineer, probe, and exploit emergent quantum materials based on collective interactions. In the same quantum material, another opportunity arises: creating particles called excitons, which are an electron and a hole (an absence of an electron in the crystal) strongly coupled together, at the specific sites in the moire pattern. In this case, the exciton can ‘collapse’ to form a particle of light called a photon (the wavy line in the figure). As only one exciton can exist at a site at a time, this leads to an organized array of single photon sources. The properties of these photon sources are highly tunable, and may eventually be exploited as hardware for quantum communication technology or to investigate new types of collective interactions among the excitons based on topology that enable coherent transfer of quantum information on-chip.
The project Two-Dimensional Quantum Photonics (2DQP) aims to characterize, identify, engineer, and coherently manipulate localized quantum states in this platform. One spectacular example of a 2D quantum material is shown in the figure below. In this figure, two single sheets of atoms are stacked on top of each other but with a relative ‘twist’. The ‘twist’ creates a moire pattern, which is depicted in the ‘shadow’ beneath the two layers of atoms. This long-range periodic pattern fundamentally changes the electronic and optical properties of the new material. For instance, electrons can be trapped at specific sites in the pattern, and the distance between each site can be engineered by the choice of materials and twist angle. The distance between the electron traps determine how they can interact, and in certain conditions strong interactions emerge which fundamentally alters their behaviour – for instance the strong correlations can lead to magnetism or superconductivity. This opens unprecedented opportunities to engineer, probe, and exploit emergent quantum materials based on collective interactions. In the same quantum material, another opportunity arises: creating particles called excitons, which are an electron and a hole (an absence of an electron in the crystal) strongly coupled together, at the specific sites in the moire pattern. In this case, the exciton can ‘collapse’ to form a particle of light called a photon (the wavy line in the figure). As only one exciton can exist at a site at a time, this leads to an organized array of single photon sources. The properties of these photon sources are highly tunable, and may eventually be exploited as hardware for quantum communication technology or to investigate new types of collective interactions among the excitons based on topology that enable coherent transfer of quantum information on-chip.
The primary vision of the project was to engineer the ideal solid-state single quantum emitter in a two-dimensional (2D) semiconductor with electrical control and high extraction / coupling efficiency of on-demand indistinguishable single photons into well-defined optical modes. The project proceeded and the milestones central to the DoW were reached, although with varying levels of success. In addition, the unanticipated discovery of moire superlattices via ‘twisted’ 2D layers provided an inflection point in the project, both in how to realize the single photon emitters and in how interactions between carriers in the moire superlattice can be optically probed.
Building on our previous discovery that local strain fields assist in the creation of single photon emitters in 2D materials, we investigated the effect in different types of 2D materials and achieved scalable arrays of single photon emitters. The results on spatial positioning were crucially applied to create quantum emitters coupled to various types of photonic cavities and structures. Novel electrically tunable devices where Coulomb blockade was pioneered, offer a route to a coherent spin-photon interface. Coherent light-matter interaction, via resonance fluorescence, was explored in depth as a means to achieve robust two-photon interference between different photons and different quantum emitters. Unfortunately, due to severe spectral fluctuations in the photon emission (which persisted even in the best electrically controlled devices), only poor two-photon interference was achieved. Further, we discovered in the electrically tunable device work that the quantum emitters lacked a permanent in-plane dipole moment which could enable DC Stark tuning of the emission energy – this is an obstacle to scalable single photon arrays.
At this time, moire materials had just been discovered which provide a novel means to spatially localize the electrons and holes and create arrays of quantum emitters with large permanent dipoles. We therefore pivoted away from the monolayer quantum emitter platform and pursued this new moire platform with vigour. We investigated the spin, valley, and layer degrees of freedom and discovered that each offers viable and unique pathways to a coherent spin-photon interface. We probed the interactions between quantum emitters in the moire lattice and other dipoles or localized charges and we investigated the single photon emission from the moire trapped quantum emitters, demonstrating a huge tunability in the emission energy. We also characterised the dipole orientation of the moire quantum emitters with an eye towards achieving efficient coupling between the quantum emitters and waveguide or cavity modes. At the end of the 2DQP project, the work on integrating moire quantum emitters to cavity modes such that Purcell enhancement can be achieved to enable indistinguishable photon generation is in progress with exciting preliminary results.
While not initially anticipated, an unforeseen opportunity that arose with the advent of moire materials was the to potential to engineer and widely tune particle interactions to realize correlated electronic states with new emergent properties. In the same devices we used for the moire quantum emitters, we were able to observe widely tunable correlated states using our optical spectroscopy techniques. We are currently building on this new platform and its wide tunability to investigate correlated quantum matter.
Finally, another unanticipated opportunity arose from our expertise in fabricating complex heterostructures with pristine interfaces. Our novel methodology here (patent pending) has led to IP generation, a successful ERC Proof-of-Concept award and also a spin-out company is being set up.
In summary, significant progress has been achieved and the research program largely proceeded to plan, although with some delays arising due to the COVID pandemic. The overall progress during the 6-year project has been fantastic: important milestones were completed and a strong knowledge base was discovered which will be the foundation for several more years of investigations and ultimately translate into practical (quantum) technologies.
Building on our previous discovery that local strain fields assist in the creation of single photon emitters in 2D materials, we investigated the effect in different types of 2D materials and achieved scalable arrays of single photon emitters. The results on spatial positioning were crucially applied to create quantum emitters coupled to various types of photonic cavities and structures. Novel electrically tunable devices where Coulomb blockade was pioneered, offer a route to a coherent spin-photon interface. Coherent light-matter interaction, via resonance fluorescence, was explored in depth as a means to achieve robust two-photon interference between different photons and different quantum emitters. Unfortunately, due to severe spectral fluctuations in the photon emission (which persisted even in the best electrically controlled devices), only poor two-photon interference was achieved. Further, we discovered in the electrically tunable device work that the quantum emitters lacked a permanent in-plane dipole moment which could enable DC Stark tuning of the emission energy – this is an obstacle to scalable single photon arrays.
At this time, moire materials had just been discovered which provide a novel means to spatially localize the electrons and holes and create arrays of quantum emitters with large permanent dipoles. We therefore pivoted away from the monolayer quantum emitter platform and pursued this new moire platform with vigour. We investigated the spin, valley, and layer degrees of freedom and discovered that each offers viable and unique pathways to a coherent spin-photon interface. We probed the interactions between quantum emitters in the moire lattice and other dipoles or localized charges and we investigated the single photon emission from the moire trapped quantum emitters, demonstrating a huge tunability in the emission energy. We also characterised the dipole orientation of the moire quantum emitters with an eye towards achieving efficient coupling between the quantum emitters and waveguide or cavity modes. At the end of the 2DQP project, the work on integrating moire quantum emitters to cavity modes such that Purcell enhancement can be achieved to enable indistinguishable photon generation is in progress with exciting preliminary results.
While not initially anticipated, an unforeseen opportunity that arose with the advent of moire materials was the to potential to engineer and widely tune particle interactions to realize correlated electronic states with new emergent properties. In the same devices we used for the moire quantum emitters, we were able to observe widely tunable correlated states using our optical spectroscopy techniques. We are currently building on this new platform and its wide tunability to investigate correlated quantum matter.
Finally, another unanticipated opportunity arose from our expertise in fabricating complex heterostructures with pristine interfaces. Our novel methodology here (patent pending) has led to IP generation, a successful ERC Proof-of-Concept award and also a spin-out company is being set up.
In summary, significant progress has been achieved and the research program largely proceeded to plan, although with some delays arising due to the COVID pandemic. The overall progress during the 6-year project has been fantastic: important milestones were completed and a strong knowledge base was discovered which will be the foundation for several more years of investigations and ultimately translate into practical (quantum) technologies.
The advent of moire materials provided a novel approach to realise quantum emiters, which was pursued with vigour. In addition, the novelty to realise strongly correlated materials in the moire platform offered a new route to investigate quantum materials in the future. Finally, the expertise we developed in heterostructure fabrication led to IP generation and the potential of a new spin-out company.