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Interfacing quantum states in carbon nanotube devices

Final Report Summary - QUANTUMCANDI (Interfacing quantum states in carbon nanotube devices)

The main objective of the project was to interface photons with excitons, spins and phonons in atomically thin semiconductors with the goal to understand fundamentals of photophysical phenomena in nanoscale materials, and to utilize them for applications in quantum technologies. Initially, the project focused on single-walled semiconductor carbon nanotubes as the key nanoscale material. It has been extended in due course to layered semiconductors in the form of transition metal dichalcogenides. For both nanoscale materials at the atomically thin limit, the project has been successful in advancing the fundamental understanding of light-matter interactions.

Specifically, the role of exciton localization in carbon nanotubes with respect to light absorption and emission characteristics, such as single-photon emission statistics in the photoluminescence from localized nanotube excitons, has been explored. It has been demonstrated that crystalline and environmental disorder lead to ubiquitous exciton localization in cryogenic nanotubes by virtue of symmetry breaking, which in turn gives rise to the emergence of a permanent electrostatic exciton dipole moment as an asset for environmental nanoscale electrometry with the sensitivity down to the elementary charge and immediate proximity to the object of interest. The capability of all-optical detection of environmental electric fields with nanotube excitons has been demonstrated in a proof-of-principle study of fluctuating charge traps in dielectric surfaces, establishing carbon nanotubes as dipolar probes for quantum sensing applications.

Moreover, the sensitivity of nanotube excitons to their immediate environment has been shown to result in peculiar interactions with vibronic degrees of freedom of the micellar surfactant and substantial modifications of the cryogenic photoluminescence spectra of localized nanotube excitons. The extensive understanding of the role of exciton localization with beneficial properties for applications as solid-state single-photon sources for quantum communication has advanced the developments of intentional creation of exciton-localizing defects by covalent nanotube side-wall chemistry. Studies of covalently functionalized nanotubes identified quantum light emission from neutral and charged excitons localized in deep defect traps with emission wavelengths in the technologically relevant telecommunication band.

Studies of layered transition metal dichalcogenides such as MoS2 or WSe2 with cryogenic spectroscopy have successfully utilized the spin-valley-selective light-matter interface provided by optical transitions in the monolayer limit to pioneer opto-valleytronic imaging of monolayer semiconductors. Although developed specifically on monolayer MoS2, the method can be generalized to other transition metal dichalcogenides and their respective van der Waals heterostructures for efficient all-optical screening of crystal quality. It is sensitive to crystalline disorder and doping, as demonstrated for defect-localized emission from charged excitons pinned at local surface adsorbates of monolayer MoS2. For tungsten-based dichalcogenide monolayers and bilayers, as well as for MoSe2-WSe2 van der Waals heterostructures, extensive spectroscopy studies established the role of momentum-dark excitons as long-lived reservoirs of exciton population and spin-valley polarization. The notion of such valley-dark excitons with spectral signatures in the form of phonon sidebands at cryogenic temperatures substantially complements the understanding of photoexcitation processes in transition metal dichalcogenide semiconductors.

Finally, a novel technology to enhance light-matter interactions of both nanoscale materials with optical micro-cavities has been developed and applied at ambient conditions as well as in cryogenic environments. Fiber-based micro-cavities have been successfully utilized to enhance Raman scattering by individual nanotubes, to demonstrate Purcell enhancement of interlayer exciton transitions in MoSe2-WSe2 heterostructures, and to realize strong coupling of extended WS2 monolayers. The results establish cavity-enhanced microscopy and spectroscopy as a valuable enabling and analytic tool in material screening and quantum optics and demonstrate the potential of creating and studying quantum gases and condensates of exciton-polaritons in coupled cavity - transition metal dichalcogenide systems.