Ladder Operators

you have a pair of ladders, a+ and a- then:

a+- = x +- iy
thus a+- a-+ = (xx + yy) +- i[x,y]
thus 1/2 [a+,a-] = i[x,y] = (a+a-) - 1/2{a+,a-}
thus 1/2 {a+,a-} = (xx + yy) = (a+a-) - 1/2[a+,a-]
[H,[x,y]] = 0
[H,xx + yy] = 0
[H,x] not = 0
[H,y] not = 0
thus [H,a+-] not = 0
thus [H,a+-a-+] = 0
[[a+,a-],a+-] = ka+- where [k,H] = 0

The result : if Y is an eigenfunction of {a+,a-} then so is (a+ Y) and so is (a- Y), each with different eigenvalues, shifted by the eigenvalue of [a+,a-].

This theory is invariant under [a+,a-] <-> {a+,a-}

The result of this theory (quantitative) is:

HY = EY
{a+,a-} Y = n Y
[a+,a-] Y = m Y
[[a+,a-],a+-] = l+- a+-
n,l,m commute with a+ and a- and H
a+-a-= = +-(1/2[a+,a-] +- 1/2{a+,a-} )
		= +- 1/2 ( m +- n )
{a+,a-}(a+ Y) = (n - 2m - l+)(a+ Y)
{a+,a-}(a- Y) = (n + 2m + l-)(a- Y)

Two common cases are:

[x,y] = 1
{a+,a-} = N
this is a Harmonic Oscillator (N+1/2=H), or creation operator
causes a linear step of operator N

[x,[x,y]] = y
this is a spin or angular momentum. This system is also characterized by [x,y]=z and xy = z/2 (where z is defined as [x,y])
This is also a representation of the spherical symmetry group.

Finally : the hard problem:

Given N, find an x and y which create raising and lowering operators for N.
i.e. N = 2(xx + yy) , and we have the various constraints above on x and y.

If anyone knows the answer to this problem, let me know.


Charles Bloom / cb at my domain
Send Me Email


Back to the Index

The free web counter says you are visitor number