Reading notes on “Coherent population trapping resonances with linearly polarized light for all-optical miniature atomic clocks”

Published on: Fri Jan 07 2011

Link to the Paper:

Coherent population trapping resonances with linearly polarized light for all-optical miniature atomic clocks

This paper is about how to make miniature atomic clocks, small enough to fit on computer chips. In particular it is talking about a CPT (coherent population trapping) interaction scheme. CPT is done by using two resonant optical fields and a single diode laser.

Atoms are placed into a superposition of two hyperfine ground states. Once the atoms are in the superposition they are very sensitive and the CPT effect can be used to sense atomic frequencies. Hyperfine defines an atomic energy state resulting from the interactions of the nucleus with internally generated electric and magnetic fields.

CPT resonance provides the best results when the light fields are of the same polarization. Types of polarization are circular, linear and elliptical. The desired angular momentum for the strongest resonance are in states where m = 0.

This paper is an experimental and theoretical study about a particular CPT interaction scheme using alkali metal atoms, and two linearly polarized optical fields. This optical set up is called lin|lin CPT. Alkali metals are: Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr). I think using Alkali metal’s lets the CPT resonance be magneto-insensitive.

The first portion of the paper is a theoretical analysis of the idea. It explains that the hyperfine transition is the change in omega, the angular frequency. This is called the hyperfine splitting.

Usually CPT schemes use circularly polarized optical fields, but this causes a lower density of interacting atoms, because many of the atoms become trapped. Using linear/elliptical polarization permits greater density of interacting atoms. This means a higher CPT resonance. The two formula’s showing the dark state are nearly the same except the non-circular formula has m = +- 1 instead of zero.

Then there is an experimental section. The goal was to produce a resonant bichromatic field, and to do this the output of an external cavity diode laser was externally phase-modulated by and electro-optical modulator. An electro-optical modulator lets the user control the phase of light exiting a crystal based on the electric field the crystal is exposed to, because the refractive index of the crystal changes depending on the strength of the electric field. There are also types that let you control frequency or amplitude.

Once the laser was at a particular frequency it was stabled to a saturation spectroscopy resonance.  The saturation spectroscopy lets the hyperfine transitions within an atom be found. Usually there is a Doppler effect, which makes it difficult to pinpoint the exact transition points. To stabilize the laser a vacuum reference cell was used. I think vacuum reference cells have a drive field and a probe field. What is a vacuum reference cell and what is it doing?

After the light was regulated, it passed through a Rb vapor cell. I think experimental measurements where gathered here, although I’m not sure if measurements where taken right after using a photo detector.  The Rb vapor cell was protected with magnetic shielding and held at a constant temperature.

Throughout the paper there are references to a “Lambda system” (Uppercase) I don’t know what this system means.