Title: Singlet-singlet qubit with two spins in a triple quantum dot*

Abstract:

Quantum dots act as artificial atoms with tunable properties, making them attractive building blocks for qubit architectures. Recent advancements in spin qubits hosted in quantum dot systems have enabled the development of devices comprising approximately ten spin qubits, achieving high-fidelity quantum gate operations. An alternative approach to single-spin qubits is encoding the qubit in a multi-spin system—such as singlet-triplet or exchange-only qubits—offering advantages in qubit control, as most operations rely primarily on electrical tuning of system parameters. In this work, we propose a new type of encoded qubit based on two spins confined in a triple quantum dot configuration. This design leverages both charge and spin degrees of freedom to realize a qubit whose logical states are both spin singlets. As a result, the encoded qubit is intrinsically insensitive to magnetic noise, offering a potential robust advantage over conventional designs. We identify the optimal operating conditions, or “sweet spot,” for this singlet-singlet qubit and describe how to implement high-fidelity gate operations. Our results introduce a novel qubit architecture that may offer improved scalability for semiconductor-based quantum dot platforms.