Abstract (English)
The main objective of this thesis was the investigation of components for gate-defined Si/SiGe spin qubit devices, amid the currently ongoing scaling from demonstrator spin qubit devices to large-scale fault-tolerant quantum computing. We focused in particular on the sensor of such a spin qubit device which is a pivotal component of a quantum processor. Not only is fast and high fidelity ...
Abstract (English)
The main objective of this thesis was the investigation of components for gate-defined Si/SiGe spin qubit devices, amid the currently ongoing scaling from demonstrator spin qubit devices to large-scale fault-tolerant quantum computing. We focused in particular on the sensor of such a spin qubit device which is a pivotal component of a quantum processor. Not only is fast and high fidelity single-shot readout of the quantum mechanical two-level system the foundation for eventual quantum error correction schemes, but a low readout error rate just as well is necessary on a research level to develop a deeper physics understanding in the still vastly and dynamically advancing field of quantum computing. As one aspect in that regard we introduced a new sensor dot design, aiming to improve the readout signal of a capacitively coupled qubit dot by an order of magnitude compared to a standard SET-based readout. We employed a simulation based ansatz to develop a gate design that allowed a larger physical separation of the drain reservoir, reducing the capacity to the reservoir, while preserving the tunnel coupling by the introduction of a 1000 nm long potential slide. The asymmetric design of the capacitances reflects in the measurement of tilted Coulomb diamonds. By operating this new sensor not with a constant voltage but with a constant current bias, we used the tilting of the steep Coulomb diamond edge to significantly increase the sensor output signal.
As another aspect of this PhD project, we focused on the operation of a qubit device. Notably, the qubits were intended to be directly loaded from the sensor dots at both ends of the device, so the sensor dots filled the role of the electron reservoirs. During the tuning of the compact qubit device, we observed a fizzling of the N=1 charge-sensed loading transition, using the left sensor dot of the device as electron reservoir. Although the qubit was energetically tuned into the N=1 regime, we measured the occurrence of multiple tunneling events, despite expecting an once loaded electron to remain confined and to not tunnel off the qubit again. The attempt of spin-to-charge conversion, by spin dependent tunneling of this last electron to the sensor dot, failed, as we could not separate the real spin signal from the detection of such additional tunneling events in a single-shot readout window, leading to a large false-counting rate. We could significantly reduce both, the fizzling of the transition line and the false-counting error rate by accumulating electrons in the whole right half of the device, forming thereby an about 100 nm wide and 500 nm long channel, which we then used as electron reservoir. We successfully recorded single-shot spin readout and determined a spin relaxation constant exceeding 25 ms. However, we still faced a large detection error rate. We estimate that from a spin up detection of about 12 %, more than half the signal was falsely identified as spin up signal. We interpreted the result as sign that the discrete DOS of an electron reservoir is problematic, completely hindering a Elzerman-type spin readout when using the fully-discrete sensor dot as reservoir and resulting in a large error rate when using the 1D-like channel for the reservoir.
As a further usage of the sensor dot, we employed the current measurement of the SET sensor in an accumulation-type device for a spectral analysis of the charge noise in chapter 7. We measured state-of-the-art power spectral densities of the sensor current, that featured a white noise floor for frequencies exceeding 103 Hz and a f−1 charge noise course for lower frequencies. As we set up a new dilution cryostat setup we used the PSD measurement as a tool to classify the noise floor in this setup and technically optimized the amplification and filtering strategy, yielding a state-of-the art noise amplitude in the new setup, measuring S0(1 Hz) = 0.64 µeV/ √Hz. We observed that an initial f−1 power law dependence was modified into a f−2 branch with increasing gate voltages, which we interpreted as an evidence that 2-level fluctuators may dominate the noise amplitude.
Translation of the abstract (German)
Das Hauptziel dieser Arbeit war die Untersuchung von Komponenten für gatter-definierte Si/SiGe-Spin-Qubit-Bauelemente inmitten der derzeit stattfindenden Skalierung von Prototypen-Spin-Qubit-Bauelementen zu groß angelegten fehlertoleranten Quantencomputern. Wir konzentrierten uns insbesondere auf den Sensor eines solchen Spin-Qubit-Bauelements, der eine zentrale Komponente eines Quantenprozessors ...
Translation of the abstract (German)
Das Hauptziel dieser Arbeit war die Untersuchung von Komponenten für gatter-definierte Si/SiGe-Spin-Qubit-Bauelemente inmitten der derzeit stattfindenden Skalierung von Prototypen-Spin-Qubit-Bauelementen zu groß angelegten fehlertoleranten Quantencomputern. Wir konzentrierten uns insbesondere auf den Sensor eines solchen Spin-Qubit-Bauelements, der eine zentrale Komponente eines Quantenprozessors ist. Ein schnelles und zuverlässiges Single-Shot-Auslesen des quantenmechanischen Zwei-Level-Systems ist nicht nur die Grundlage für eventuelle Quantenfehlerkorrekturverfahren, sondern eine niedrige Auslesefehlerrate ist auch auf Forschungsebene notwendig, um ein tieferes physikalisches Verständnis auf dem sich immer noch dynamisch entwickelnden Gebiet des Quantencomputers zu entwickeln.
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