This content introduces Stimulated Raman Adiabatic Passage (SERP) as a technique to achieve near-perfect population transfer between atomic states, overcoming limitations of direct excitation by avoiding a short-lived intermediate state.
Key Points
Imagine an atom. It's resting quietly in
its ground state like a pendulum at the
bottom of its swing. But we don't want
it to stay there. Whether we're building
lasers, performing quantum logic, or
controlling molecules, we often need to
move the atom into an excited state.
This process, population inversion, is a
building block of modern atomic and
molecular physics. But here's the
challenge. How do we do this coherently?
That is, how do we transfer population
without loss from decay or dependence on
exact pulse areas or resonance
conditions? This becomes especially
important when the necessary
intermediate state is short-lived and
subject to spontaneous
emission. This is where stirrup or
stimulated ramon adobatic passage comes
in. It's a technique that allows
nearperfect population transfer between
two states without ever significantly
occupying the intermediate lossy
state. Before introducing SERP, it's
instructive to consider why two-level
systems fail to enable robust and
complete population transfer in many
settings. When driven by broadband
light, population dynamics are governed
by the Einstein rate equations. Even
under strong illumination, detailed
balance enforces symmetric transition
rates. The excited population saturates
at 50% and inversion cannot be
sustained. Under monochromatic driving,
the system exhibits RAI oscillations. A
pulse with area pi can in principle
transfer all population to the excited
state. However, the process is highly
sensitive to experimental imperfections.
Errors in intensity, d-tuning or timing
distort the pulse area and spontaneous
emission during evolution through a
short-lived excited state introduces
decoherence. Averaged over trials,
coherent exitation leads to significant
loss in fidelity. We will see later how
adeticity allows for more efficient
inversion. To circumvent these
limitations, a three-level system is
introduced with a loss intermediate
state two and two long live states one
and three. The system is driven with two
coherent fields, a pump field coupling
one and two and a stokes field coupling
two and
three. This setup is necessary because
in many atomic and molecular systems the
initial and target states one and three
such as two metastable or highlighting
levels are often not directly dipole
coupled. That is the dipole matrix
element vanishes due to par or angular
momentum selection rules. To bridge this
gap, we use a real intermediate state 2
that is dipole allowed and hence
strongly coupled to both one and three.
However, this strength comes at a cost.
Such states couple efficiently to the
electromagnetic vacuum and decay rapidly
via spontaneous emission making them highly
highly
lossy. Let's set the stage with a simple
three-le atom. We'll call the states 1,
2, and three. The key idea is that state
2 is shortlived. It decays rapidly via
spontaneous emission. So, while we'll
need it to mediate transitions, we'd
like to avoid spending too much time there.
there.
To control the system, we introduce two
laser fields. A pump laser couples one
to two and a Stokes laser couples two to
three. These fields oscillate with the
respective carrier frequencies omega p
for the pump and omega s for the stokes.
And we'll describe them with time
dependent rabi
frequencies. Now under the rotating wave
approximation which drops the rapidly
oscillating terms, we can write the
Hamiltonian in a much simpler form.
It's a 3x3 matrix with time dependent
couplings between the states and the
detunings delta P and delta S that
account for how far off resonance the
lasers are. But if we tune the lasers
just right so that their detunings are
equal, the Hamiltonian simplifies even
further. This is the form we'll use to
understand how population moves through
the system and how we can cleverly guide
it along a path that avoids decay
entirely. Now that we have our
Hamiltonian, let's see what it tells us
about how the system evolves. When we
diagonalize it and find its igen states,
we get a special state that lives
entirely in one and three with no
component in the lossy state too. This
is the dark state, a coherent superp
position. The mixing angle depends on
the relative strength of the pump and stokes
stokes
couplings. As the laser pulse evolves in
time, this dark state smoothly rotates
guiding population from one to three
while entirely avoiding two.
As long as the system follows the state
adobatically and the pulses are applied
in the correct time order, the dark
state remains an igen state with zero
value. Once we've identified the dark
state, we can now ask how do we move the
path. The ideabatic theorem tells us
that if the Hamiltonian changes slowly
compared to the energy gap between the
igen states, the system will stay in the
same state.
To find the condition in terms of the
mixing angle, we start with the time
dependent shortening equation in this instantaneous
