Why combination of ground and first excited states of harmonic oscillator show a change in probability density with time?
The combination of the ground and first excited states of a harmonic oscillator forms a wave function that is not time-independent. This means that the probability density of finding the oscillator at a particular location changes with time.

To understand why this is the case, we can consider the time-evolution of the wave function. The wave function for a harmonic oscillator can be written as a sum of the ground state and the first excited state, where the ground state is the lowest energy state and the first excited state has slightly higher energy. The ground state has a probability density that is symmetrically distributed around the equilibrium position of the oscillator, while the first excited state has a probability density that is shifted slightly to one side.

As time passes, the wave function undergoes periodic oscillations, with the amplitude of the oscillations increasing over time. This is because the ground state and the first excited state have slightly different frequencies, and as they interfere with each other, the amplitude of the resulting wave function changes with time. This change in amplitude leads to a corresponding change in the probability density of finding the oscillator at a particular location.

In particular, the regions where the ground state and the first excited state have the same phase will experience constructive interference, leading to an increase in the probability density in those regions. Conversely, the regions where the ground state and the first excited state have opposite phase will experience destructive interference, leading to a decrease in the probability density in those regions.

Overall, this means that the probability density of finding the oscillator at a particular location will change periodically with time as a result of the interference between the ground state and the first excited state. This phenomenon is known as quantum beats and is a general feature of quantum systems that are in a superposition of states with slightly different energies or frequencies.