### homework_3

```QUANTUM PHYSICS II
PROBLEM SET 3-NEW IMPROVED VERSION
due October 1st, before class
A.
Hermitian operators
i) Show that eigenkets of a hermitian operators corresponding to different eigenvalues are orthogonal.
ii) Show that the eigenvalues of a hermitian operators are real.
I’m not going to ask you to prove but it is also true that the set of eigenvectors form a complete set. So, by choosing
properly normalized eigenkets, one can form an orthonormal basis, called an eigenbasis of the given hermitian operator.
This result is sometimes called the “Spectral Theorem”.
B.
Bra-ket-ology
i) Use the Spectral Theorem to argue that any hermitian operator Aˆ can be written as
X
an |nihn|,
Aˆ =
(1)
n
where |ni are its eigenvectors, an the corresponding eigenvalues and the sum is over all eigenvectors.
P
P
iii) Let Aˆ = n an |nihn| be an hermitian operator. Show that its inverse is given by Aˆ−1 = n a1n |nihn|. What is
ˆ
the condition on Aˆ so the eigenvalues for the inverse to exist? Hint: what si the determinant of A?
C.
Harmonic oscillator
I hope you learned how to solve the harmonic oscillator using operator methods. If not, you can look it up on
Griffiths. The outcome of that discussion is that the eigenvalues/eigenvectors of
2
2
ˆ = pˆ + mω x
H
ˆ2
2m
2
(2)
are given by
1
ˆ
H|ni
= ~ω(n + )|ni,
2
1 n
|ni = √ a
ˆ+ |0i,
n!
(3)
where
a
ˆ± = √
1
(mωˆ
x ∓ iˆ
p),
2~mω
(4)
(the states |ni are already normalized) and the ground state |0i satisfies
a
ˆ− |0i = 0.
(5)
(The “0” on the right is actually the null ket. I don’t want to write it as |0i so it won’t be confused with the ground
state which is definitely NOT the null ket). Notice that (ˆ
a− )† = a
ˆ+ .
i) Show that
[ˆ
a− , a
ˆ+ ] = 1,
using the commutation relation [ˆ
x, pˆ] = i~.
ii) Compute h0|ˆ
x|0i.
iii) Compute h0|ˆ
x2 |0i.
iv) Compute the matrix element hn|ˆ
x|mi.
(6)
2
I.
SPIN 1/2 WITH BRAS & KETS
i) Find the matrix representing the operators Sˆx , Sˆy and Sˆz in the basis of eigenstates of Sˆz starting from the
relations:
Sˆ2 |smi = ~2 s(s + 1) |smi,
Sˆz |smi = m~ |smi,
p
Sˆ± |smi = ~ s(s + 1) − m(m ± 1) |sm ± 1i.
ii)Find now the matrix representing the operators Sˆx , Sˆy and Sˆz in the basis of eigenstates of Sˆx .
(7)
(8)
(9)
```