Sub-Doppler cooling in a compact conical hollow mirror integrated with anti-Helmholtz coils

To understand the work of these researchers, one must first appreciate the delicate art of slowing down atoms. Atoms are the fundamental building blocks of matter, but when they are hot, they zip around at incredible speeds, colliding and scattering in chaotic ways. To study them with extreme precision or to use them for advanced technologies like atomic clocks, scientists must first cool them down to temperatures near absolute zero. This is typically done using laser light. Although light usually pushes things away, scientists have figured out how to tune lasers so that atoms moving toward a beam absorb photons and receive a gentle "kick" that slows them down. By arranging lasers from all directions, they can create a trap where atoms are held in place and cooled to a state of near-motionlessness. This technique, known as a magneto-optical trap, is the standard starting point for many modern quantum experiments, but the equipment required is often bulky and complex, needing multiple powerful lasers and large magnetic coils to function.

A team of physicists at the University of Tokyo and collaborating institutions has now demonstrated a way to achieve this cooling with far less equipment and energy. They built a compact device that uses a single beam of laser light and a specially shaped mirror to trap and cool rubidium atoms. Instead of the usual setup that requires six separate laser beams pointing at the atoms from different angles, their design uses a hollow cone made of aluminum. When a single laser beam is shot down the center of this cone, the inner walls reflect the light back, effectively splitting it into multiple beams that surround the atoms. The shape of the cone also focuses the light toward the center, making the trap much more efficient. This allows the researchers to capture a large number of atoms using significantly less laser power than traditional methods.

The researchers integrated this conical mirror with magnetic coils into a single, small mechanical unit. They shot a laser beam into the cone and used magnetic fields to hold the rubidium atoms in place. They found that the number of atoms they could trap reached its peak when the laser intensity was set to a specific, relatively low level of 6.5 milliwatts per square centimeter. This is roughly four times less power than what is typically needed for standard traps that use six separate beams. Under these optimal conditions, they successfully trapped about ten million atoms. This efficiency is a direct result of the cone's geometry, which concentrates the light exactly where the atoms are, rather than spreading it out over a larger area.

Once the atoms were trapped, the team wanted to see how cold they could get them. They applied a technique called polarization gradient cooling, which uses the subtle properties of light to remove even more energy from the atoms. The results showed that the atoms became incredibly cold, reaching temperatures well below the standard limit for this type of cooling. However, the cooling was not the same in every direction. The atoms cooled down to about 13.4 microkelvin in the direction moving sideways across the cone, but they remained warmer, at about 106 microkelvin, in the direction moving up and down the cone's axis. This difference happens because the pattern of light created by the cone is different in these two directions; the light is arranged in a way that is very effective at stopping sideways motion but less effective at stopping motion along the cone's length.

The team also measured how hot the atoms were right after they were first trapped, before the extra cooling step was applied. They found that the atoms moving sideways were about 1.2 millikelvin, which is roughly seven times hotter than what is typically seen in standard six-beam traps with similar settings. This confirms earlier computer simulations that suggested the cone's shape creates a specific type of force that is less efficient at cooling atoms moving in circles around the cone's center. While this radial temperature is higher than ideal, the ability to trap such a large number of atoms with a single, compact, and power-efficient device represents a significant step forward. The researchers have proven that this simple, integrated mirror system can serve as a practical and effective source for cold atoms, paving the way for smaller, more portable quantum sensors and atomic clocks that could be used outside of large laboratories.