  We study quantum interference effects of a qubit whose energy levels are
continuously modulated. The qubit is formed by an impurity electron spin in a
silicon tunneling field-effect transistor, and it is read out by spin blockade
in a double-dot configuration. The qubit energy levels are modulated via its
gate-voltage-dependent g-factors, with either rectangular, sinusoidal, or ramp
radio-frequency waves. The energy-modulated qubit is probed by the electron
spin resonance. Our results demonstrate the potential of spin qubit
interferometry implemented in a silicon device and operated at a relatively
high temperature.
