Quantum control and process tomography of a semiconductor quantum dot hybrid qubit

Abstract

A simply prepared quantum bit that is a hybrid of spin and charge enables full control on the Bloch sphere with π-rotation times of less than 100 picoseconds in two orthogonal directions; the speed arises from the charge-like characteristics, and the spin-like features result in increased quantum coherence. In many quantum bit or qubit systems, speed and coherence have opposing roles, with specific devices optimized for one or the other. Here Mark Eriksson and colleagues describe a hybrid qubit combining high speed, arising from its charge-like characteristics, with quantum coherence, arising from its spin-like features. This new device, made up of three electrons in two dots, is simple to prepare using standard silicon fabrication technology and exhibits gate fidelities between 85% and 95%, the highest so far reported in an electrically gated semiconductor quantum dot qubit. The similarities between gated quantum dots and the transistors in modern microelectronics1,2—in fabrication methods, physical structure and voltage scales for manipulation—have led to great interest in the development of quantum bits (qubits) in semiconductor quantum dots3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18. Although quantum dot spin qubits have demonstrated long coherence times, their manipulation is often slower than desired for important future applications, such as factoring19. Furthermore, scalability and manufacturability are enhanced when qubits are as simple as possible. Previous work has increased the speed of spin qubit rotations by making use of integrated micromagnets11, dynamic pumping of nuclear spins12 or the addition of a third quantum dot17. Here we demonstrate a qubit that is a hybrid of spin and charge. It is simple, requiring neither nuclear-state preparation nor micromagnets. Unlike previous double-dot qubits, the hybrid qubit enables fast rotations about two axes of the Bloch sphere. We demonstrate full control on the Bloch sphere with π-rotation times of less than 100 picoseconds in two orthogonal directions, which is more than an order of magnitude faster than any other double-dot qubit. The speed arises from the qubit’s charge-like characteristics, and its spin-like features result in resistance to decoherence over a wide range of gate voltages. We achieve full process tomography in our electrically controlled semiconductor quantum dot qubit, extracting high fidelities of 85 per cent for X rotations (transitions between qubit states) and 94 per cent for Z rotations (phase accumulation between qubit states).