Strong photon coupling to the quadrupole moment of an electron in a triple quantum dot*

We experimentally couple the photonic excitations of a superconducting microwave resonator to a single electron hosted in a quantum dot charge qubit. The two qubit states with a 4.3 GHz separation arise in a four-electron triple quantum dot formed by locally gating a two-dimensional electron gas in GaAs. We demonstrate strong electron-photon coupling via the quadrupole moment in a parameter regime with negligible dipole coupling. The quadrupolar qubit-photon coupling strength is estimated from the vacuum Rabi mode splitting to be g₀/2π = 150 MHz. The qubit can also be tuned into a regime where it operates as a conventional double quantum dot charge qubit diplole coupled to the resonator. The experiment is motivated by a recent proposal [1,2] which aims at avoiding decoherence by distant charge fluctuations. Using spectroscopy measurements we determine the decoherence rate of the quadrupolar qubit to be γ₂/2π = 32 MHz. Comparing the coupling of charge noise to the conventional dipolar qubit and the quadrupolar qubit, we find that the coherence of the system is limited by short-range charge noise originating from noise sources residing near the triple quantum dot.

[1] M. Friesen, J. Gosh, M.A. Eriksson, and S.N. Coppersmith, Nature Comm. 8, 15923 (2017).
[2] J. Gosh, S.N. Coppersmith, and M. Friesen, Phys. Rev. B 95, 241307 (2017).