Abstract
Electron spins in semiconductors are promising qubits because their long coherence times enable nearly 109 coherent quantum gate operations. However, developing a scalable high-fidelity two-qubit gate remains challenging. Here, we demonstrate an entangling gate between two double-quantum-dot spin qubits in GaAs by using a magnetic field gradient between the two dots in each qubit to suppress decoherence due to charge noise. When the magnetic gradient dominates the voltage-controlled exchange interaction between electrons, qubit coherence times increase by an order of magnitude. Using randomized benchmarking, we measure single-qubit gate fidelities of ~ 99%, and through self-consistent quantum measurement, state, and process tomography, we measure an entangling gate fidelity of 90%. In the future, operating double quantum dot spin qubits with large gradients in nuclear-spin-free materials, such as Si, should enable a two-qubit gate fidelity surpassing the threshold for fault-tolerant quantum information processing.
Highlights
The quantum phase coherence of isolated spins in semiconductors[1,2,3,4,5,6,7] can persist for long times, reaching tens of milliseconds for electron spins[8] and tens of minutes for nuclear spins.[9]. Such long coherence times enable single-qubit gate fidelities exceeding the threshold for fault-tolerant quantum computing[8] and make spins promising qubits
We present a technique to suppress decoherence caused by charge noise
We show that when the magnetic gradient in a GaAs singlet-triplet qubit dominates the electrically controlled exchange interaction, coherence times increase by an order of magnitude
Summary
The quantum phase coherence of isolated spins in semiconductors[1,2,3,4,5,6,7] can persist for long times, reaching tens of milliseconds for electron spins[8] and tens of minutes for nuclear spins.[9]. The key idea is to apply a large transverse qubit energy splitting that does not depend on electric fields and, suppresses the effects of charge fluctuations We implement this scheme with two singlet-triplet qubits, each of which consists of two electrons in a double-quantum-dot.[2] In each qubit, the voltage-controlled exchange interaction J(ε), where ε represpenffiffits the gate j#"iÞ= 2 and triplet jTvo0ilta1⁄4geðj,"#sipþlitsj#"tihÞe=psffi2iffingstleattesjSiin1⁄4eðnje"r#giyÀ,[2] where the left (right) arrow indicates the spin of the left (right) electron. Ωrot j þ δΩ2 2j is first-order insensitive to fluctuations in the magnetic gradient
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