  The implementation of circuit quantum electrodynamics allows coupling distant
qubits by microwave photons hosted in on-chip superconducting resonators.
Typically, the qubit-photon interaction is realized by coupling the photons to
the electric dipole moment of the qubit. A recent proposal suggests storing the
quantum information in the electric quadrupole moment of an electron in a
triple quantum dot. The qubit is expected to have improved coherence since it
is insensitive to dipolar noise produced by distant voltage fluctuators. Here
we experimentally realize a quadrupole qubit in a linear array of three quantum
dots in a GaAs/AlGaAs heterostructure. A high impedance microwave resonator
coupled to the middle dot interacts with the qubit quadrupole moment. We
demonstrate strong quadrupole qubit--photon coupling and observe improved
coherence properties when operating the qubit in the parameter space where the
dipole coupling vanishes.
