Manipulation and coherence of a two-electron logical spin qubit using gallium arsenide double quantum dots

Gaurab Panda, Ryan Aridi, Haozhi Dong +2 more

Vertical spin-orbit coupling to produce quantum entanglement between electrons confined in quantum dots is investigated for a split-gate double layer heterostructure design. The quantum dots in each active layer may result from quantum point contact fabrication or be generated dynamically using a surface acoustic wave flying qubit approach. Optimization of spin-spin coupling versus all other couplings can set up competing requirements for heterostructure and device fabrication choices. In the present work, a series of active layers: indium antimonide, indium arsenide, indium gallium arsenide, and gallium arsenide, and device architectures is investigated. We find that device architectures that support spin entanglement are within achievable fabrication ranges for the active layer materials investigated.

Hendrik Bluhm, Michael Shulman, Oliver Dial +2 more

James Colless

There is currently a worldwide effort towards the realisation of large-scale quantum computers that exploit quantum phenomena for information processing. While these computing systems could potentially redefine the technological landscape, harnessing quantum effects is challenging due to their inherently fragile nature and the experimentally demanding environments in which they arise. In order for quantum computation to be viable it is first necessary to demonstrate the operation of two-level quantum systems (qubits) which have long coherence times, can be quickly read out, and can be controlled with high fidelity. Focusing on these key requirements, this thesis presents four experiments towards scalable solid state quantum computing using gate-defined quantum dot devices based on gallium arsenide (GaAs) heterostructures. The first experiment investigates a phonon emission process that limits the charge coherence in GaAs and potentially complicates the microwave control of multi-qubit devices. We show that this microwave analogy to Raman spectroscopy can provide a means of detecting the unique phonon spectral density created by a nanoscale device. Experimental results are compared to a theoretical model based on a non-Markovian master equation and approaches to suppressing electron-phonon coupling are discussed. The second experiment demonstrates a technique involving in-situ gate electrodes coupled to lumped-element resonators to provide high-bandwidth dispersive read-out of the state of a double quantum dot. We characterise the charge sensitivity of this method in the few-electron regime and benchmark its performance against quantum point contact charge sensors. The third experiment implements a low-loss, chip-level frequency multiplexing scheme for the readout of scaled-up spin qubit arrays. Dispersive gate-sensing is realised in combination with charge detection based on two radio frequency quantum point contacts to perform multiplexed readout of a double quantum dot in the few-electron regime. Demonstration of a 10-channel multiplexing device is achieved and limitations in scaling spin qubit readout to large numbers using multiplexed channels discussed. The final experiment ties previously presented results together by realising a micro-architecture for controlling and reading out qubits during the execution of a quantum algorithm. The basic principles of this architecture are demonstrated via the manipulation of a semiconductor qubit using control pulses that are cryogenically routed using a high-electron mobility transistor switching matrix controlled by a field programmable gate array. Finally, several technical results are also presented including the development of printed circuit board solutions to allow the high-frequency measurement of nanoscale devices at cryogenic temperatures and the design of on-chip interconnects used to suppress electromagnetic crosstalk in high-density spin qubit device architectures.

Brandur Thorgrimsson

Seigo Tarucha, Michihisa Yamamoto, A. Oiwa +2 more

Takashi Nakajima, Akito Noiri, Kento Kawasaki +11 more

The coherence of electron spin qubits in semiconductor quantum dots suffers mostly from low-frequency noise.During the past decade, efforts have been devoted to mitigate such noise by material engineering, leading to substantial enhancement of the spin dephasing time for an idling qubit.However, the role of the environmental noise during spin manipulation, which determines the control fidelity, is less understood.We demonstrate an electron spin qubit whose coherence in the driven evolution is limited by highfrequency charge noise rather than the quasistatic noise inherent to any semiconductor device.We employ a feedback-control technique to actively suppress the latter, demonstrating a π-flip gate fidelity as high as 99.04 Æ 0.23% in a gallium arsenide quantum dot.We show that the driven-evolution coherence is limited by the longitudinal noise at the Rabi frequency, whose spectrum resembles the 1=f noise observed in isotopically purified silicon qubits.