This article dives into a breakthrough: producing ultracold rubidium atoms in microgravity, all with light. Researchers managed to cool atoms below 100 nanokelvin using only optical methods.
They pulled this off during parabolic flights by ditching magnetic traps in favor of a crossed optical dipole trap built from 1550 nm telecom laser beams. The team used a time-averaged, or “painted,” potential to start with a big trapping volume, then compressed it to raise density and collision rates.
This method could open doors for space-ready quantum sensors and fundamental-physics experiments.
All-optical cooling in microgravity
The team took a different route here, skipping over the usual atom-chip and magnetic trapping tricks. Instead, they went with a completely optical approach.
Two crossed 1550 nm beams from fibered telecom lasers did the heavy lifting, creating the optical dipole trap to hold and cool rubidium atoms. An acousto-optic modulator painted a time-averaged potential by shifting the beam positions, which formed a big capture region at first. Later, they squeezed this region down to ramp up density and collisions—key for efficient evaporative cooling.
How the optical trap is formed
The trap relies on three main ingredients:
- Crossed optical beams at 1550 nm create a deep, conservative potential, trapping atoms without any magnetic fields.
- A time-averaged painted potential comes from quickly modulating beam positions using an acousto-optic modulator. This lets them shape the trap however they want, on the fly.
- They start with a large capture volume, then adiabatically compress it to boost density and collision rates. That’s what makes fast cooling in microgravity possible.
Experimental sequence and atom numbers
The experiment kicks off with a strong two-dimensional MOT, loading about 1.5 × 10^8 rubidium atoms. After that, they run through optical molasses cooling steps: red molasses brings the temperature to 4.5 µK, and grey molasses fine-tunes things before loading the dipole trap.
All in all, they grab around 6 × 10^6 atoms in the dipole trap within a quick 150 ms window. That’s thanks to tight optical control and fast timing.
Step-by-step timeline
- 2D MOT loading: 1.5 × 10^8 atoms
- Red optical molasses: cools to 4.5 µK
- Grey molasses: more cooling and prep
- Dipole-trap loading: ~6 × 10^6 atoms in 150 ms
Stability in microgravity: adaptive optics and alignment
Parabolic flights throw all sorts of accelerations at the setup, which would normally knock laser beams out of alignment and mess up the trap. To get around this, the researchers added real-time beam-position compensation that tracks and fixes misalignments as they happen.
This keeps the crossed-beam trap steady throughout each sequence. That adaptive trick is what lets them cool reliably in the wild microgravity swings of parabolic flights.
Maintaining a stable trap during accelerations
- Real-time feedback nudges beam positions back in line when acceleration knocks them off.
- This keeps the trap geometry and coherence intact during all cooling steps.
- Stability matters—without it, you can’t repeatably compress or evaporate the cloud.
From painting to ultracold gas: compression and evaporation
After loading, they compress the painted potential over about 250 ms. Trap depth jumps about threefold, and phase-space density rises by two orders of magnitude.
Evaporative cooling then runs through three linear power ramps, with a final laser-power tweak tuned for microgravity. In less than four seconds, they end up with roughly 2.5 × 10^4 atoms at temperatures below 100 nK. Not bad for a microgravity setup.
Evaporation in a microgravity environment
- Adiabatic compression cranks up trap depth and phase-space density.
- Three linear ramps steadily bleed energy from the atom cloud.
- One last, gravity-aware power drop finishes the journey to <100 nK.
Validation and implications
To check their work, they used time-of-flight expansion measurements up to 100 ms. These confirmed ultracold temperatures and showed the cooling sequence really worked.
By skipping magnetic fields, this all-optical approach avoids interference and the tight optical access limits of atom chips. That makes it a compelling option for quantum sensors and physics experiments in space.
What this means for space science
- No magnetic traps—so less chance of interference in precision measurements.
- It’s a scalable, optical-path method that fits the constraints of space setups.
- This could power future geodesy and precision tests with ultracold atoms in orbit or on other space platforms.
Outlook: challenges and opportunities
The results look compelling, but scaling up the atom number isn’t straightforward. Extending experiment duration beyond parabolic flights also presents a real hurdle.
Researchers need to tackle these issues to create deployable, space-qualified ultracold-atom platforms. These platforms could support geodesy, fundamental physics, and quantum sensing in space.
The team’s all-optical, microgravity-compatible method points the way toward robust, compact ultracold-atom systems. These systems might eventually operate in orbit, on long-duration flights, or in other microgravity environments.
Here is the source article for this story: Lasers Cool Atoms To Below 100 NanoKelvin In Space