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Quantum devices in topological matter: carbon nanotubes, graphene, and novel superfluids

Periodic Reporting for period 4 - QuDeT (Quantum devices in topological matter: carbon nanotubes, graphene, and novel superfluids)

Reporting period: 2020-07-01 to 2021-12-31

The project addresses quantum devices in hybrid systems formed using carbon nanotubes, graphene, and superfluid helium. The aim is to develop and utilise ultra sensitive nano electromechanical resonators to investigate fundamental physics questions, to understand properties of these non-trivial materials, and to discover and create new quantum systems.

The project provides novel approach for the development of future quantum technologies, as well as analogies with other phenomena and systems of interest. While the 21st century is expected to be the era of quantum technology, a thorough understanding and control of quantum mechanical properties of matter will pave the way for these emerging technologies. The potential of harnessing quantum phenomena will open vast and unexpected industrial and economic opportunities for decades to come. New applications and advances with broad technological and societal impact are expected in quantum sensors, simulators, communication, and computing.
Several different types of ultra sensitive sensors have been constructed in the project. Graphene and carbon nanotube mechanical resonators make sensitive sensors due to their small mass and nearly ideal structure. At low temperatures, high quality factors can be reached, which facilitates detection of tiny frequency shifts or small variation in the resonance amplitude. The frequency shifts can be employed for mass or force detection while amplitude changes indicate altered dissipation.

We have employed the developed ultra sensitive sensors for generating new information on quantum systems with specific topological order. Among them, as an example, are emergent composite fermions in graphene in a large magnetic field. Force sensing using a superconducting carbon nanotube has been employed to obtain first evidence of the Josephson force, which is a result of the interplay between superconducting phase dynamics and mechanical motion. Novel findings using nanotube sensors have been made on exotic 2D topological phases and quantum phase transitions in 3He.

Our experiments demonstrated the usefulness of Josephson inductance of superconductor-graphene-superconductor junctions for various sensing applications. The temperature dependence of this Josephson inductance offers a high resolution thermometer, which could be employed for sensitive bolometry at microwave frequencies. These techniques have improved the rate of detection of GHz microwave photons to a level where various calorimetric schemes become realistic, for example single photon detection with high quantum efficiency.

Besides sensing, the suspended graphene platforms have acted as excellent model systems for 1/f noise, which is still open problem though its first observation was made in 1925. Our experiments on 1/f noise point towards clustering of impurities, scattering centers, in generation of the noise. Moreover, our results indicate that the clustering of impurities leads to mobility fluctuations; such noise was shown to be largely suppressed by magnetic field.

The gained understanding on quantum tunneling in the project could be utilized in generation of entanglement, which is one of the central concepts in quantum technology. Our results demonstrate entanglement generation via splitting of Cooper pairs, which can be driven even by thermal gradient - a new control knob for experiments. The achieved Cooper pair splitting into graphene quantum dots provides a route for high-flux production of entangled particles. We also pioneered entanglement generation among microwave photons using wide-band traveling wave parametric amplifier and could demonstrate transfer rates of over Giga entangled bits per second. These results can be exploited in future quantum technology, possibly for secure telecommunications and distributed quantum computing.
List of a few main topics where the QuDeT work has gone beyond the state of art:

- Novel findings made using nanotube sensors on exotic 2D topological phases and quantum phase transitions in 3He.
- First observation of Josephson force using a superconducting carbon nanotube.
- High resolution SGS thermometer employed for sensitive bolometry at microwave frequencies; record fast response, about 100 faster than regular metallic systems with the same sensitivity.
- New frontier in 1/f noise studies based on suspended graphene platforms with added mobile impurities; observation of clustering of impurities in the generation of noise.
- Novel schemes to generate entanglement via splitting of Cooper pairs.
- High-gain, wide-band traveling wave parametric amplifiers for microwave photon entanglement generation.
- Entanglement transfer rates of over Giga entangled bits per second.
- Novel correlated states in graphene in large magnetic fields.
Carbon nanotube device used in the measurements
Graphene device used in the measurements