A dressed singlet-triplet qubit in germanium

Abstract

Abstract In semiconductor hole spin qubits, low magnetic field ( B ) operation extends the coherence time ( $${T}_{2}^{*}$$ T 2 * ) 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 $${T}_{2}^{*}=1.9\,\mu {{{\rm{s}}}}$$ T 2 * = 1.9 μ s . Moreover, by applying the resonant drive continuously, we realize a dressed ST qubit with a tenfold increase in coherence time ( $${T}_{2\rho }^{*}=20.3\,\mu {{{\rm{s}}}}$$ T 2 ρ * = 20.3 μ s ). 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.