  Electron spins in silicon quantum dots are excellent qubits due to their long
coherence times, scalability, and compatibility with advanced semiconductor
technology. Although high gate fidelities can be achieved with spin qubits,
charge noise in the semiconductor environment still hinders further
improvements. Despite the importance of charge noise, key questions about the
specific nature of the fluctuators that cause charge noise remain unanswered.
Here, we probe individual two-level fluctuators (TLFs) in Si/SiGe quantum dots
through simple quantum-dot transport measurement and analyses based on the
Allan variance and factorial hidden Markov modeling. We find that the TLF
switching times depend sensitively on gate voltages, decrease with temperature,
and depend on the current through a nearby quantum dot. A model for the data of
the primary TLF we study indicates that it may be a bistable charge dipole near
the plunger gate electrode, heated by current through the sensor dot, and
experiencing state transitions driven not by direct electron-phonon coupling
but through some other mechanism such as coupling to electrons passing through
the sensor dot.
