Decoherence, backaction and noise in two-electron spin qubits in GaAs quantum dots
Bethke, Patrick; Bluhm, Jörg (Thesis advisor); Hassler, Fabian (Thesis advisor)
Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2023
Underlying many open technical and scientific questions in the field of qubits is the concept of decoherence. In semiconductor spin qubits, the dominant decoherence channels are magnetic spin noise from nearby nuclear spins, and charge noise in the host crystal that couples to the qubit via modification of the exchange interaction. In this thesis, we examine aspects of these different facets of noise and decoherence using a qubit encoded in the singlet and triplet state of two electron spins in a double quantum dot in GaAs. Decoherence of a quantum system or qubit arises from its interaction with the environment. Environments with a large number of weakly coupled degrees of freedom, such as an electron spin surrounded by a large number of nuclear spins, are commonly approximated by a classical, fluctuating field whose dynamics are unaffected by the qubit state. A full quantum description still implies some backaction from the qubit on the environment. We present experimental evidence for such a backaction for an electron-spin qubit in GaAs coupled to a mesoscopic environment of order $10^6$ nuclear spins. Employing a correlation measurement technique, we detect the backaction of a single qubit-environment interaction whose duration is comparable to the qubit's coherence time. We repeatedly let the qubit interact with the spin bath and measure the bath's state. Between these cycles, the qubit is reinitialized to different states. The correlations of the measurement outcomes are strongly affected by the intermediate qubit state indicating the action of a single electron spin on the nuclear spins. The exchange interaction is vital in quantum control of electron spin qubits. For high-fidelity gate operations, a deep understanding of its behavior is crucial in two regards: First, an accurate model of the response to a change in control voltage, such as the detuning of a double dot, is necessary to develop accurate control pulses, second, it allows to accurately model the sensitivity of spin qubits to charge noise. Using a combination of experimental techniques we characterize the exchange interaction in a double quantum dot over three orders of magnitude and a large range of detuning, and develop an extended Hubbard model that qualitatively reproduces the observed behavior. Qubit dephasing to due charge noise mediated by fluctuations in the exchange interaction strength is a limiting factor in the implementation of high-fidelity gates for qubit control not just in GaAs, but also silicon-based systems. The spectral properties of charge noise in a system can provide valuable insight into its physical origin. Up to the low MHz range, the spectrum can be characterized up by direct measurement or inference from qubit decoherence under dynamical decoupling sequences. The spectrum of charge noise at higher frequencies is an open question. Adapting two methods for spectroscopy from NMR, we employ the qubits as a spectrometer for charge noise by looking at decoherence due to longitudinal relaxation in the lab or rotating frame. Thereby, a new frequency range given by the qubit energy splitting on the order of 100's of MHz becomes available for spectrometry. We present estimates for an adapted inversion recovery scheme for the $S$-$T_0$ qubit in GaAs and first exploratory experimental results.
- Department of Physics 
- Chair of Experimental Physics and Institute of Physics II