Dephasing of a charge qubit is usually credited to charge noise in the
environment. Here we show that charge noise may not be the limiting factor for
the qubit coherence. To this end, we study coherence properties of a
crystal-phase defined semiconductor nanowire double quantum dot (DQD) charge
qubit strongly coupled to a high-impedance resonator using radio-frequency (RF)
reflectometry. Response of this hybrid system is measured both at a charge
noise sensitive operation point (with finite DQD detuning) and at an
insensitive point (so-called sweet spot with zero detuning). A theoretical
model based on Jaynes-Cummings Hamiltonian matches the experimental results
well and yields only a 10 % difference in dephasing rates between the two
cases, despite that the sensitivity to detuning charge noise differs by a
factor of 5. Therefore the charge noise is not the limiting factor for the
coherence in this type of semiconducting nanowire qubits.Comment: 7 pages, 3 figure