Kicked rotor in Wigner phase space - The University of Texas at Austin

Fortschr. Phys. 51, No. 4–5 (2003) 477
We conclude this subsection by discussing the action of this operator on a momentum eigenstate. For
this purposewecalculatetherepresentation of Û in the momentum basis by inserting the completeness
relation 1l = dp | p 〉〈 p | twice. When we recall the formula
wearriveat
e −ilˆx | p 〉 = | p − lk - 〉 (10)
Û =
∞
−∞
dp
∞
l=−∞
Sl(κ) exp −i p2
2k-
| p − lk- 〉〈 p |. (11)
Hence, the Floquet operator Û couples only momentum eigenstates separated by integer multiples of k- .
3.2 Wigner function
Wenow analyzethedynamics of thekicked particlefrom thepoint of view of phasespace. In particular, we
discuss the time evolution of the corresponding Wigner function. Two possibilities offer themselves: (i) We
can solvethequantum Liouvilleequation [12] of thekicked particle, or (ii) wecan usethemapping of the
state vectors derived in the preceding section to obtain a mapping of the corresponding Wigner functions.
In thepresent section wepursuethesecond approach.
For this purposewerecall thedefinition
Wn+1(x, p) ≡ 1
2πk- ∞
−∞
dξ e −ipξ/k- 1 〈x + 2ξ|ψn+1〉〈ψn+1|x − 1
2
ξ〉 (12)
of theWigner function of thestate| ψn+1 〉.
When wesubstitutethemapping, Eq. (6), into theright-hand sideof Eq. (12), wearriveat
Wn+1(x, p) = 1
2πk- ∞
dξ e −ipξ/k-
Ukick(x + ξ/2)U ∗ †
kick(x − ξ/2)〈x + ξ/2| Ûfree|ψn〉〈ψn| Û free |x − ξ/2〉.
−∞
Herewehaveused theproperty Ûkick(ˆx)| x 〉 = Ukick(x)| x 〉.
Sincethepotential V (x) is periodic, the bilinear form
Ukick(x + ξ/2)U ∗ kick(x − ξ/2)=e −iκ[V (x+ξ/2)−V (x−ξ/2)] ≡ e −iκV(x,ξ/2)
with the generalized potential
V(x, y) ≡ V (x + y) − V (x − y) (14)
is also periodic in ξ/2 with period 2π. We can therefore expand
e −iκV(x,ξ/2) =
∞
l=−∞
Sl(κ; x)e −ilξ/2
into a Fourier series where the expansion coefficients
Sl(κ; x) ≡ 1
π
d
2π
−π
ξ
e
2
ilξ/2 e −iκV(x,ξ/2)
still depend on the position x and are periodic with a period of 2π.
(13)
(15)
(16)