A dressed singlet-triplet qubit in germanium.

Abstract

In semiconductor hole spin qubits, low magnetic field (B) operation extends the coherence time ( <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>T</mi></mrow> <mrow><mn>2</mn></mrow> <mrow><mo>*</mo></mrow> </msubsup> </math> ) but proportionally reduces the gate speed. In contrast, singlet-triplet (ST) qubits are primarily controlled by the exchange interaction ( J) and can thus maintain high gate speeds even at low B. However, a large J introduces a significant charge component to the qubit, rendering ST qubits more vulnerable to charge noise when driven. Here, we demonstrate a highly coherent ST hole spin qubit in germanium, operating at both low B and low J. By modulating J, we achieve resonant driving of the ST qubit, obtaining an average gate fidelity of 99.68% and a coherence time of <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>T</mi></mrow> <mrow><mn>2</mn></mrow> <mrow><mo>*</mo></mrow> </msubsup> <mo>=</mo> <mn>1.9</mn> <mspace></mspace> <mi>μ</mi> <mi>s</mi></math> . Moreover, by applying the resonant drive continuously, we realize a dressed ST qubit with a tenfold increase in coherence time ( <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>T</mi></mrow> <mrow><mn>2</mn> <mi>ρ</mi></mrow> <mrow><mo>*</mo></mrow> </msubsup> <mo>=</mo> <mn>20.3</mn> <mspace></mspace> <mi>μ</mi> <mi>s</mi></math> ). Frequency modulation of the driving signal enables universal control, with an average gate fidelity of 99.63%. Our results demonstrate the potential for extending coherence times while preserving high-fidelity control of germanium-based ST qubits, paving the way for more efficient operations in semiconductor-based quantum processors.