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David Niegemann presents

 High Fidelity Spin Readout of Electron Spins in Silicon MOS Quantum Dots

Tuesday, 6th December 2022 at 2 pm

Seminar room – Building A – CNRS

The defence will be in English.



The discovery and demonstration of quantum algorithms that outperform all classical algorithms has given rise to the new research field of quantum information. Although today’s largest quantum processors consist of around 100 qubits, these are far from being perfect. Building a quantum processor with millions of qubits requires reliable technology with the potential to scale. In this context, spin qubits in semiconductor quantum dots provide an interesting qubit platform that could benefit from the large-scale fabrication techniques of the modern semiconductor industry. We use a device fabricated in a 300 mm FDSOI process, promising the scalability required by DiVincenzo’s first criterion: “A scalable physical system with a well-characterized qubit.” The device consists of a silicon nanowire connected to two reservoirs. Electrostatic gates, defined on top of the nanowire, allow the accumulation of quantum dots in the corners of the nanowire. In our device, we create a system of 2 × 2 quantum dots.

We use rf reflectometry to perform dispersive charge sensing, using one of the quantum dots as a sensor. Then, we use this device to realize a two-spin system in two quantum dots. We measure Pauli spin blockade using two methods: the singlet-triplet readout and the parity readout, which allows us to distinguish between the singlet S and the three triplet states 𝑇0, 𝑇− and 𝑇+ on one hand or between the unpolarized S and 𝑇0 spin states and the 𝑇− and 𝑇+ spin polarized states on the other hand. We demonstrate high fidelity for both types of readout. Fidelity of ST readout is >99% at 50kHz limited by relatively fast spin relaxation and parity readout exceeds 99.9% (99%) at 50kHz (250kHz). Thus, both readot methods meet DiVincenzo’s fifth condition: “a qubit-specific measurement capability.” Moreover, we perform these measurements at a temperature of 0.5 K, which shows the temperature robustness of this type of reading and allows for future cointegration with dissipative electronics. Finally using our readout method, we characterize the two-spin system using Landau-Zener and spin-funnel experiments and can also probe signatures of Wigner molecularization in our system.