Microwave-induced cooling in double quantum dots: Achieving millikelvin temperatures to reduce thermal noise around spin qubits

Spin qubits in gate-defined quantum dots are emerging as a leading technology due to their scalability and long coherence times. However, maintaining these qubits at ultralow temperatures typically requires complex cryogenic systems. This paper proposes a gate-defined double quantum dot (DQD) cooling system, where the DQDs act as refrigerants to reduce the local phonon environment around computational qubits. The cooling process occurs in two distinct stages: the first step involves microwave-induced state depopulation combined with fast cyclic detuning to transfer the DQD's population to the ground state, effectively lowering the DQD's temperature. In the second step, the cooled DQD interacts with and absorbs phonons resonant with the DQD spin energy, thereby filtering out these phonons that contribute to spin-lattice relaxation in the surrounding environment. This study focuses on the first step, presenting detailed calculations and numerical results that demonstrate the feasibility of achieving local DQD temperatures below $10\phantom{\rule{4pt}{0ex}}\mathrm{mK}$ at a bath temperature of $1\phantom{\rule{4pt}{0ex}}\mathrm{K}$. The sensitivity of the cooling performance to detuning energy, magnetic field strength, spin-orbit couplings, and diabatic return time is analyzed, while the phonon filtering in the second step will require further investigation.