# Homework Help: TDSE, time evolution between states (Check my working please?)

1. Jun 9, 2013

### unscientific

1. The problem statement, all variables and given/known data
Hi guys, I've recently taken up quantum, so it's all very new to me, it would be greatly appreciated if someone could check my working!

Let ψ1(x) and ψ2(x) be two orthonormal solutions of the TISE with corresponding
energy eigenvalues E1 and E2. At time t = 0, the particle is prepared in the symmetric
superposition state:

and subsequently allowed to evolve in time. What is the average energy of the system
as a function of time? What is the minimum time ¿ for which the system must evolve
in order to return to its original state (up to an overall phase factor), when it starts in
the state ψ(+) (x)

Determine the probability to ¯nd the system in the antisymmetric superposition
state

as a function of time when it starts in the state ψ(+) (x)

At time t1 the particle is found in the antisymmetric superposition state. What is
the probability to ¯nd the particle in the symmetric superposition state at time t1 + T,
where T is the time found above?

2. Relevant equations

3. The attempt at a solution

Not sure if the last part is right, as it suggests that the probability of finding the particle in the left half of the box is independent of time! Then again, in an infinite square well the potential doesn't depend on time, so TDSE is reduced to TISE?

Last edited: Jun 9, 2013
2. Jun 9, 2013

### Staff: Mentor

I think you should express $\Delta \omega$ in terms of the energies.

For the last part, why do you think those mixed terms are zero if you integrate them from 0 to L/2?
The problem statement itself is bad here, I think. The answer depends on the phases used to define the first two states, and those phases are arbitrary.

3. Jun 9, 2013

### unscientific

Because ψ1 and ψ2 are orthogonal states? And to get the answer in the previous parts they were zero...Is the reason why they are non-zero here because the limits of integration are not from -∞ to ∞?

4. Jun 9, 2013

### Staff: Mentor

Right, orthogonality just means that the integral over the whole space is zero.