Periodic Reporting for period 3 - 2DQP (Two-dimensional quantum photonics)
Periodo di rendicontazione: 2021-01-01 al 2022-06-30
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.
In the first 30 months of the project, we have made both fundamental and technological breakthroughs. A first highlight is the demonstration of Coulomb blockade, the phenomenon whereby electrons or holes can be loaded one-by-one into a quantum dot, with a quantum dot in a 2D heterostructure for the first time [Nature Nanotechnology 14, 442 (2019)]. Gate-tunable quantum-mechanical tunnelling of particles between a quantum confined state and a nearby Fermi reservoir of delocalized states has underpinned many advances in spintronics and solid-state quantum optics. Thanks to a tunable Fermi reservoir, we demonstrate deterministic loading either a single electron or a single hole into the quantum dot. We observe hybrid excitons, composed of localized quantum dot states and delocalized continuum states, arising from ultra-strong spin-conserving tunnel coupling through the atomically thin tunnel barrier. Our results establish a foundation for engineering next-generation devices to investigate either novel regimes of Kondo physics or isolated quantum bits in a vdW heterostructure platform.
A second topic is controlling the dielectric environment of 2D quantum dots for increased light-matter interaction efficiency [Applied Physics Letters 112, 191105 (2018); Nanophotonics 7, 253 (2018); Nature Communications 10, 1 (2019); arXiv:1905.10181 (2019)] and incorporation into photonic chips [Optical Materials Express 9, 441 (2019); arXiv:2002.07657 (2020)] in a scalable fashion [arXiv:2005.05361 (2020)]. These results pave the way for future engineering of scalable quantum photonic chips, provided that coherent single photons can successfully be generated.
We have vigorously pursued the generation of indistinguishable single photons in the two-dimensional platform. Indistinguishability among two photons results in their coalescence into a single output of a beamsplitter when they arrive simultaneously (so called- Hong-Ou-Mandel interference). This is highly challenging to realize: indistinguishability requires the identical wavepackets in every way. While we have demonstrated so far a very limited two-photon interference visibility, it is likely limited by inhomogeneous broadening and dephasing mechansims in the 2D quantum dots. We are actively pursuing coupling the 2D quantum dots to photonic cavities to realize a strong Purcell enhancement which can mask or mitigate the dephasing mechanisms. This will be a significant target in the remaining 30 months of 2DQP.
A second topic is controlling the dielectric environment of 2D quantum dots for increased light-matter interaction efficiency [Applied Physics Letters 112, 191105 (2018); Nanophotonics 7, 253 (2018); Nature Communications 10, 1 (2019); arXiv:1905.10181 (2019)] and incorporation into photonic chips [Optical Materials Express 9, 441 (2019); arXiv:2002.07657 (2020)] in a scalable fashion [arXiv:2005.05361 (2020)]. These results pave the way for future engineering of scalable quantum photonic chips, provided that coherent single photons can successfully be generated.
We have vigorously pursued the generation of indistinguishable single photons in the two-dimensional platform. Indistinguishability among two photons results in their coalescence into a single output of a beamsplitter when they arrive simultaneously (so called- Hong-Ou-Mandel interference). This is highly challenging to realize: indistinguishability requires the identical wavepackets in every way. While we have demonstrated so far a very limited two-photon interference visibility, it is likely limited by inhomogeneous broadening and dephasing mechansims in the 2D quantum dots. We are actively pursuing coupling the 2D quantum dots to photonic cavities to realize a strong Purcell enhancement which can mask or mitigate the dephasing mechanisms. This will be a significant target in the remaining 30 months of 2DQP.
The ability to stack unlimited combinations of atomic layers with arbitrary crystal angle (θ) has opened a new paradigm in quantum material design. For example, easily tunable Bloch minibands emergent in moiré lateral superlattices have enabled remarkable observations with graphene heterostructures, such as nearly flat bands with narrow bandwidths at specific θ that can lead to superconductivity and correlated insulator states. This new paradigm was not anticipated when 2DQP was conceived in 2016 and began in 2018. We have built on this idea to realize highly tunable quantum light sources. We have demonstrated that the small lattice mismatch or relative twist in a heterobilayer can create moiré trapping potentials for excitons and these can create arrays of quantum emitters [arxiv:2001.04305 accepted at Science Advances]. In addition, we have shown the trapping potentials exhibit the underlying crystal symmetry and spin-layer-valley locking [Nature Materials 19, 630 (2020)]. Further, the exciton has a large out of plane permanent dipole, enabling a huge energy tuning via the Stark effect. This is an exciting development as it is now possible to couple the trapped interlayer excitons to cavities via the easy energy tunability. We are actively pursuing this now. Finally, we are also actively pursuing the realiziation of quantum simulators based on the Hubbard model in this platform.