[Paper Review] Noise suppression and long-range exchange coupling for gallium arsenide spin qubits

Filip K. Malinowski|arXiv (Cornell University)|Jun 12, 2017

Quantum and electron transport phenomena118 references3 citations

TL;DR

This thesis demonstrates noise suppression and long-range exchange coupling in gallium arsenide spin qubits using gate-defined quantum dots. By exploiting symmetric quantum dot configurations and dynamical decoupling, the authors achieve a sixfold improvement in singlet-triplet qubit coherence and extend electron spin coherence to 870 µs, while enabling tunable, long-range exchange coupling via a multielectron mediator dot.

ABSTRACT

This thesis presents the results of the experimental study performed on spin qubits realized in gate-defined gallium arsenide quantum dots, with the focus on noise suppression and long-distance coupling.

Motivation & Objective

• To reduce charge noise susceptibility in GaAs spin qubits through symmetric quantum dot configurations.
• To measure and suppress Overhauser field noise using tailored dynamical decoupling sequences.
• To demonstrate long-range exchange coupling between distant electrons mediated by a multielectron quantum dot.
• To identify ground state spins in multielectron quantum dots across different charge occupancies.
• To enable high-fidelity, long-range two-qubit gates by exploiting low-noise voltage regimes in multielectron mediators.

Proposed method

• Employed gate-defined GaAs quantum dots with tunable tunnel couplings and symmetric configurations to minimize gate voltage gradients.
• Applied dynamical decoupling sequences (XY8) to notch-filter narrowband Overhauser noise at megahertz frequencies.
• Used single-shot spin readout and quantum process tomography to characterize qubit coherence and exchange oscillations.
• Performed systematic measurements across nine charge occupancies of a multielectron quantum dot to map ground state spin configurations.
• Engineered gate voltage sweeps near charge transitions to tune exchange interaction sign and strength in even-occupied spin-1/2 dots.
• Demonstrated coherent exchange coupling between distant single-spin qubits mediated by a spin-0 multielectron dot with tunable coupling up to several GHz.

Experimental results

Research questions

• RQ1 How does symmetric quantum dot configuration reduce charge noise in singlet-triplet and resonant exchange qubits?
• RQ2 What is the spectral structure of Overhauser field noise in GaAs, and can it be effectively suppressed?
• RQ3 Can a multielectron quantum dot mediate long-range exchange coupling between distant electrons?
• RQ4 How does the exchange interaction between a single spin and a multielectron dot vary with gate voltage and charge occupancy?
• RQ5 Are there voltage regimes in multielectron dots that exhibit reduced susceptibility to charge noise, enabling high-fidelity two-qubit gates?

Key findings

• A sixfold improvement in the quality factor of double-dot singlet-triplet exchange oscillations was achieved via symmetric quantum dot operation.
• The electron spin coherence time in GaAs was extended to 870 µs, setting the highest lower bound to date.
• Overhauser field noise in the 1 mHz–1 kHz range follows classical spin diffusion, while MHz-scale noise is concentrated in three narrow bands due to Larmor precession of three nuclear isotopes.
• In even-occupied, spin-1/2 multielectron quantum dots, a few millivolt gate voltage sweep near charge transitions induces a sign change in the exchange interaction with a neighboring electron.
• Long-range exchange coupling between distant electrons was demonstrated via a spin-0 multielectron mediator dot, with coupling strengths tunable up to several gigahertz.
• Specific voltage regimes in the multielectron dot exhibit reduced charge noise susceptibility due to small level spacing and many-body effects, enabling high-fidelity two-qubit operations.