Atom-molecule dark states in a Bose-Einstein condensate

Rated as a AIP top story 2005

Our team has created a dark quantum superposition state of atoms and  molecules. The observation of this dark state demonstrates that we have produced a quantum degenerate gas of molecules and that atom-molecule coherence is established.

In our experiments we use a Rubidium Bose-Einstein condensate which is illuminated by two laser beams. The wavelengths of these laser beams are chosen such that they fuse together two free Rb atoms into one Rb2 molecule, This process is called photoassociation. If the laser beams are right on resonance, the dark state appears, resulting in a striking suppression of photoassociation.

Phys. Rev. Lett. 95, 063202 (2005), cond-mat/0505732

Photo of our BEC apparatus
We work with a Rubidium Bose-Einstein condensate (BEC) with up to one million atoms. The Bose condensed atoms occupy the lowest quantum mechanical state and form a "giant" superfluid matter wave.

What is a dark state?

Normally, when people talk about dark states, they mean a coherent superposition state of two stable levels ‘g’ and ‘a’ within an atom. These levels are resonantly coupled to an excited atomic level with two lasers. Although both lasers are on resonance, no photons are scattered and the excited level b is not populated. This is due to destructive interference of the two matterwave amplitudes in level 'b' which are generated by the lasers 1 and 2.

If Ω1 and Ω2 denote the coupling strengths of the respective transitions, then the dark state looks like this:

Dark states are useful tools and have already found numerous applications. For example:

  • ultrasensitive magnetometers
  • subrecoil laser cooling
  • coherent transfer of population between two long-lived states (STIRAP)
  • lasing without inversion
  • electromagnetically induced transparency

Atom - molecule dark states:

The atom - molecule dark states which we have observed, are similar to the dark states in an atom. The atomic level scheme (above) is simply replaced by a level scheme involving free atoms and molecules. The figures below explain how we have observed the atom-molecule dark states.

In order to understand what is going on, we first start out with photo-association: We create excited molecules (b) from atoms (a) by shining in a single laser.

Atomic loss signal in one-color photoassociation as a function of the laser detuning d1 from the electronically excited molecular line b. The laser fuses together two free atoms from the atomic BEC (level ‚a‘) to form an excited molecule. Since this molecule can spontaneously decay losses are introduced which are maximal on resonance.

When we shine in a second laser the dark state appears!

When we apply a second laser (fixed frequency) which resonantly couples the excited molecular state b to a long lived molecular ground state g, the losses are strongly suppressed at δ1 = 0. The dark state, a coherent superposition of atoms and molecules, has formed. Depending on the intensity of laser 2, this dark resonance can get very narrow.

In our experiments, the dark states serve to analyze the atom-molecule system and to detect atom-molecule coherences. The dark states can be used in the future to optimize the conversion of atomic condensates into molecular BECs.