Categories:
Optics,
Physics,
Quantum mechanics,
Two-level system.
Rotating wave approximation
Consider the following periodic perturbation H^1 to a quantum system,
which represents e.g. an electromagnetic wave
in the electric dipole approximation:
H^1(t)=V^cos(ωt)=2V^(eiωt+e−iωt)
Where V^ is some operator, and we assume that ω
is fairly close to a resonance frequency ω0
of the system that is getting perturbed by H^1.
As an example, consider a two-level system
consisting of states ∣g⟩ and ∣e⟩,
with a resonance frequency ω0=(Ee−Eg)/ℏ.
From the amplitude rate equations,
we know that the general superposition state
∣Ψ⟩=cg∣g⟩+ce∣e⟩ evolves as:
iℏdtdcgiℏdtdce=⟨g∣H^1(t)∣g⟩cg(t)+⟨g∣H^1(t)∣e⟩ce(t)e−iω0t=⟨e∣H^1(t)∣g⟩cg(t)eiω0t+⟨e∣H^1(t)∣e⟩ce(t)
Typically, V^ has odd spatial parity, in which case
Laporte’s selection rule
reduces this to:
dtdcgdtdce=iℏ1⟨g∣H^1∣e⟩cee−iω0t=iℏ1⟨e∣H^1∣g⟩cgeiω0t
We now insert the general H^1 defined above,
and define Veg≡⟨e∣V^∣g⟩ to get:
dtdcgdtdce=i2ℏVeg∗(ei(ω−ω0)t+e−i(ω+ω0)t)ce=i2ℏVeg(ei(ω+ω0)t+e−i(ω−ω0)t)cg
At last, here we make the rotating wave approximation:
since ω is assumed to be close to ω0,
we argue that ω+ω0 is so much larger than ω−ω0
that those oscillations turn out negligible
if the system is observed over a reasonable time interval.
Specifically, since both exponentials have the same weight,
the fast (ω+ω0) oscillations
have a tiny amplitude compared to the slow (ω−ω0) ones.
Furthermore, since they average out to zero over most realistic time intervals,
the fast terms can be dropped, leaving:
ei(ω−ω0)t+e−i(ω+ω0)tei(ω+ω0)t+e−i(ω−ω0)t≈ei(ω−ω0)t≈e−i(ω−ω0)t
Such that our example set of equations can be approximated as shown below,
and its analysis can continue
(see Rabi oscillation for more):
dtdcgdtdce=i2ℏVeg∗ceei(ω−ω0)t=i2ℏVegcge−i(ω−ω0)t
This approximation’s name is a bit confusing:
the idea is that going from the Schrödinger to
the interaction picture
has the effect of removing the exponentials of ω0 from the above equations,
i.e. multiplying them by eiω0t and e−iω0t
respectively, which can be regarded as a rotation.
Relative to this rotation, when we split the wave cos(ωt)
into two exponentials, one co-rotates, and the other counter-rotates.
We keep only the co-rotating waves, hence the name.
The rotating wave approximation is usually used in the context
of the two-level quantum system for light-matter interactions,
as in the above example.
However, it is not specific to that case,
and it more generally refers to any approximation
where fast-oscillating terms are neglected.