4 ms·

A simple way of thinking it is consider laser as waves, they act like harmonic potentials (like sine curves) with crests and troughs. Atoms that are inside the

by Xpol 5y ago

A simple way of thinking it is consider laser as waves, they act like harmonic potentials (like sine curves) with crests and troughs. Atoms that are inside the potentials are trapped, like a ball in a deep well. Atoms trapped in optical lattices have a lifetime with a time constant which the population of trapped atoms keeps decreasing, but the timescale is like tens of seconds, so the atom loss is not significant when the clock interrogation is in milliseconds. In this paper the lattice is a 1D vertical lattice, so you can think of atoms being trapped in a huge stack of pancakes.

dmix 5y ago
Thanks for the explanation. Do you know how they get the strontium atoms in place to be hit by the lasers? That’s the part that throws me given the scale.
- Xpol 5y ago
  After strontium atoms coming from an oven (usually at 400-ish degrees) reaches the main chamber, they're trapped via both optically (via lasers) and magnetically (there's coils surrounding the chamber), which we called magneto-optical trap, or MOT. For strontium, there're two stages of MOT, one is a blue (461nm) MOT which cools and traps hotter atoms into temperature of ~mK, and the second stage is the red (689nm) MOT that further cools the atoms into ~uK range. A MOT typically consists of 3 laser beams at different directions, and retroreflected on the other side after hitting the atoms (so 6 beams effectively). Some MOT can be done with less lasers. All 3 pairs of lasers (and 2 MOT, so 6 pairs) have to align perfectly so that they all hit the same point of intersection within the chamber. To align these lasers, one method is to shine another 461nm pulse to the chamber and check for fluorescence signal via a photodiode or camera located on the other side of the chamber. Since the beams are generally much larger than the atomic cloud itself, as long as you hit something, it is easy to optimize the signal. For a blue MOT with very high atom number, you can even see a small blue bulb suspending in the mid-air inside the chamber, which could serve as a rough reference to start with. So now atoms are trapped into the red MOT, you then turn off the red lasers while having the lattice laser (at 813nm) on, so the atoms are loaded into the optical lattice. Using the same method, you take a fluorescence image at the end of the experimental sequence to check if there's any signal. Note that the position of the red MOT depends on both the laser alignment and magnetic fields, so one can either (1) align the lattice beam, in this case, a vertical beam from top to bottom and retroreflected or (2) adjust the magnetic fields to fine-tune the red MOT position. It's an iteration of fine adjustments and looking at images. Another way to align the lattice to the atoms is instead of shining lattice beams to the atoms, you first send red light through the same fiber. When they hit the atoms in the red MOT, the red light will excite the atoms so you won't see anything now with the fluorescence imaging (as they no longer in the ground state that responds to the blue transition). We call this "the killing beam" as the name suggest. Once you know the rough alignment, switch it back to lattice laser and do the optimizations until you see atoms loaded into the lattice (the distribution and spread of the atom ensemble are different when viewed with camera for atoms that are still in red MOT vs loaded into lattice).

by Xpol 5y ago

dmix 5y ago

Thanks for the explanation. Do you know how they get the strontium atoms in place to be hit by the lasers? That’s the part that throws me given the scale.

Xpol 5y ago
After strontium atoms coming from an oven (usually at 400-ish degrees) reaches the main chamber, they're trapped via both optically (via lasers) and magnetically (there's coils surrounding the chamber), which we called magneto-optical trap, or MOT. For strontium, there're two stages of MOT, one is a blue (461nm) MOT which cools and traps hotter atoms into temperature of ~mK, and the second stage is the red (689nm) MOT that further cools the atoms into ~uK range. A MOT typically consists of 3 laser beams at different directions, and retroreflected on the other side after hitting the atoms (so 6 beams effectively). Some MOT can be done with less lasers. All 3 pairs of lasers (and 2 MOT, so 6 pairs) have to align perfectly so that they all hit the same point of intersection within the chamber. To align these lasers, one method is to shine another 461nm pulse to the chamber and check for fluorescence signal via a photodiode or camera located on the other side of the chamber. Since the beams are generally much larger than the atomic cloud itself, as long as you hit something, it is easy to optimize the signal. For a blue MOT with very high atom number, you can even see a small blue bulb suspending in the mid-air inside the chamber, which could serve as a rough reference to start with. So now atoms are trapped into the red MOT, you then turn off the red lasers while having the lattice laser (at 813nm) on, so the atoms are loaded into the optical lattice. Using the same method, you take a fluorescence image at the end of the experimental sequence to check if there's any signal. Note that the position of the red MOT depends on both the laser alignment and magnetic fields, so one can either (1) align the lattice beam, in this case, a vertical beam from top to bottom and retroreflected or (2) adjust the magnetic fields to fine-tune the red MOT position. It's an iteration of fine adjustments and looking at images. Another way to align the lattice to the atoms is instead of shining lattice beams to the atoms, you first send red light through the same fiber. When they hit the atoms in the red MOT, the red light will excite the atoms so you won't see anything now with the fluorescence imaging (as they no longer in the ground state that responds to the blue transition). We call this "the killing beam" as the name suggest. Once you know the rough alignment, switch it back to lattice laser and do the optimizations until you see atoms loaded into the lattice (the distribution and spread of the atom ensemble are different when viewed with camera for atoms that are still in red MOT vs loaded into lattice).