Periodic Reporting for period 4 - CNT-QUBIT (Carbon Nanotube Quantum Circuits)
Reporting period: 2020-03-01 to 2021-04-30
Any quantum computer requires entanglement. One route to achieve entanglement between electron spin qubits in quantum dots is to use the direct interaction of neighbouring qubits due to their electron wavefunction overlap. This approach, however, becomes rapidly impractical for any large scale quantum processor, as distant qubits can only be entangled through the use of qubits in between. Here I propose an alternative strategy which makes use of an intriguing quantum mechanical effect by which two spatially separated spin qubits coupled to a single electrical resonator become entangled if a measurement cannot tell them apart. The quantum information encoded in the entangled electron spin qubits will be transferred to carbon-13 nuclear spins which are used as a quantum memory with coherence times that exceed seconds. Entanglement with further qubits then proceeds again via projective measurements of the electron spin qubits without risk of losing the existing entanglement. When entanglement of the electron spin qubits is heralded – which might take several attempts – the quantum information is transferred again to the nuclear spin states. This allows for the coupling of large numbers of physically separated qubits, building up so-called graph or cluster states in an all-electrical and scalable solid-state architecture.
Outcome: The project developed charge and spin qubits in double quantum dots in ultraclean freestanding carbon nanotubes. The devices were embedded in resonant LC circuits, achieving state-of-the art capacitance sensitivities for high-fidelity (>99.9%) qubit readout in sub-microsecond timescales and so-called video mode imaging of the charge stability diagrams. The record capacitance sensitivity (for any material platform) was achieved by developing novel quantum paraelectric strontium titanate varactors for impedance matching and frequency tuning at mK temperatures, as well as the incorporation of novel ultra-low noise sub-GHz Josephson parametric amplifiers in collaboration with VTT Finland. During the project we furthermore developed devices that incorporated micromagnets for electric - rather than magnetic - control of the quantum dot spin states, as well as the ability of carbon nanotube double quantum dots readout using a proximal single quantum dot carbon nanotube detector. We could furthermore 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 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. This will allow us to herald entanglement on sub-microsecond timescales - that is, well within spin relaxation and coherence times - if two or more devices are measured simultaneously. This last step has not yet been demonstrated but the capabilities developed during the project has laid the groundwork to make this possible.
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.