Singlet-triplet and exchange-only flopping-mode spin-qubits

Electron or hole spins in quantum dots coupled to a microwave cavity are an

established platform to realize qubits. In the first part of this work we combine

the singlet-triplet (ST0) qubit with the versatile flopping-mode spin qubit to achieve

tunable qubit-cavity coupling. We therefore introduce a spin qubit consisting of

two electrons in three quantum dots in a magnetic field gradient. Tunnel coupling

between the dots allows for an orbital degree of freedom. The system operates in the

ST0 regime near the |1, 0, 1〉 ↔ |0, 1, 1〉 charge transition. We analyze the three

relevant charge states and calculate the effective transversal and longitudinal

spin-photon coupling in this regime and investigate it by calculating the cavity

transmission near the dressed ST0 resonance. In the second part of this work these

calculations are extended to the exchange-only (EO) qubit, in a setup which comprises four

dots and three electrons. The EO qubit can be fully controlled by the electrically tunable

exchange parameters, and its coupling to a cavity holds promise for efficient realization

of two-qubit gates.