Strong Microwave Photon Coupling to the Electron Quadrupole Moment

The implementation of circuit quantum electrodynamics (cQED) allows coupling of distant qubits by microwave photons hosted in on-chip resonators. Typically, the qubit-photon interaction is realized by coupling the photons to the electrical dipole moment of the qubit. A recent proposal [1] suggests storing the quantum information in the quadrupole moment of an electron in a triple quantum dot. This type of qubit is expected to have an improved coherence since the qubit does not have a dipole moment and is consequently better protected from electric noise. We report the experimental realization of such a quadrupole qubit hosted in a triple quantum dot in a GaAs/AlGaAs heterostructure. A high-impedance microwave resonator is capacitively coupled to the middle of the triple dot to realize interaction with the qubit quadrupole moment. We demonstrate strong quadrupole qubit-photon coupling with a qubit-photon coupling strength of g / 2π ≈ 130 MHz and a qubit decoherence rate of γ₂ / 2π ≈ 30 MHz. Furthermore, we observe improved coherence properties of the qubit when operating in the parameter space where the dipole coupling vanishes.

[1] M. Friesen et al., Nature Comm. 8, 15923 (2017)