During the first part of the project our group has focussed on the installation of the low-temperature facilities and radio-frequency reflectometry techniques used for qubit readout. We demonstrated that the temperature of electrons in the carbon nanotube quantum dots that host the qubits is as low as 12 mK – much smaller than other relevant energy scales. We furthermore demonstrated significant improvements in the measurement readout of charge states of the quantum dots which – via spin-to-charge conversion – also allows fast spin-state readout. Fast readout is important as the measurement of the qubits has to be faster than the timescale on which entanglement is lost – estimated to be on the order of several tens of usec for realistic device parameters. We achieved this by introducing varactors (tunable capacitors) which allows us to impedance match the resonator-qubit circuits to the transmission lines that connect to them and to tune the resonator frequency. These varactors have been specifically designed and fabricated by our group such that they are compatible with the ultra-low temperatures at which the qubits are measured - in addition to being stable, low-loss and magnetic field insensitive. To further improve our measurement capabilities we have been collaborating with VTT Finland to incorporate frequency-tuneable ultra-low noise Josephson Parametric Amplifiers (JPAs) in our set-up. These JPA are made to operate in the frequency range of interested for our devices (tuneable around 600 MHz) and have sufficient magnetic shielding to apply magnetic fields up to 9 Tesla to our devices.
During the project, the quality of the devices were continuously improved. The carbon nanotubes quantum dots fabricated and measured at the final stages of the project were made completely freestanding and could be made free from contaminants and adsorbates, rendering them ultraclean, as confirmed by ultra-low device charge noise when measured at low temperatures. For electric spin control, rather than solely relying on spin-orbit coupling, we furthermore integrated micromagnets in our design. In addition, we could control the devices using microwave signals applied to the device gate electrodes. We obtained Landau-Zener-Stuckelberg interferometry patterns that allowed us to probe orbital and valley dynamics in the carbon nanotube double quantum dot devices. We furthermore developed code for the detection and coherent control of charge and spin states in the double quantum dots using FPGA based equipment, in collaboration with Zurich Instruments, Switzerland. Results were dissemination via research publications, patent applications, and workshops and conferences.
