Cold Atoms

Cooling an atomic gas to near-absolute zero (-273.15 °C)

Cold atom research is essential for developing ultra-precise measurement capabilities, atomic clocks, and other technologies that aim to satisfy the growing demands of today’s expanding technology market. The aim of the research is to understand, measure, and control the unique quantum mechanical properties of atoms in this low-temperature regime.

There are several techniques used to cool atoms to near absolute zero and to confine them. They include “laser cooling” and “magneto-optical trapping”.

Laser cooling

Laser cooling is a technique that reduces the velocity of atoms by bombarding them with photons. The atom absorbs a Doppler shifted photon, then re-emits a photon of the same frequency as the Doppler shifted photon (but higher frequency than the original photon), so losing energy in the process.

The key to the technique is that only atoms travelling in the direction opposite to incident photons are affected. The frequency of the photons must be Doppler shifted for resonant absorption to occur and this frequency is chosen so that the condition is only fulfilled when atoms and photons are travelling in opposite directions.

Multiple orthogonal laser beams are typically used to produce this effect in three dimensions. 

Magneto-optical trapping

Magneto-optical trapping is a technique that combines laser cooling with a magnetic field to confine atoms to a specific volume. 

A magnetic field is applied to the laser cooled region. The magnetic field splits the energy levels of the atom into several sub-levels. This is known as the Zeeman effect.

This magnetic field is arranged to have a constantly increasing gradient away from the intersection of the orthogonal laser beams, defined as the centre of the trap. As the Zeeman effect is dependent on the magnitude of the magnetic field, the splitting of energy levels increases with radial distance from the centre of the trap. If an atom drifts away from the centre, the result is that the resonant frequency of the atom is shifted closer to the frequency of the laser. The atom is then more likely to absorb photons and experience a net force towards the centre of the trap.

The direction of the force that the atoms feel is dependent on the polarisation of the light, chosen to be either right-handed or left-handed circular polarised.  The polarisation is chosen so that photons moving towards the centre of the trap will be resonant with the correct shifted atomic energy level, forcing the atom towards the centre.

By confining cold atoms to a defined region, the systematic errors in the measurement of quantum properties such as spin are reduced.

Applications

Technologies that rely on cold atom research include atomic clocks and quantum gravity sensing devices.

Atomic clocks

Atomic clocks measure time based on the frequency of radiation emitted by atoms, typically rubidium or caesium, as the atoms transition between specific energy levels.

As cold atoms have lower velocity by definition, they are confined within the measurement apparatus for longer. As the energy transitions of the atoms can be probed over longer period of time, a more accurate measurement of frequency can be obtained.

Gravity sensing devices

Quantum gravity sensors are used to probe density, such as that beneath the surface of the Earth.

The acceleration due to gravity varies in response to changes in density. Quantum gravity sensors detect such variations beneath Earth’s surface by allowing a cloud of cold atoms to fall freely under the influence of gravity. An interferometer then measures the height of the atom cloud several times during free fall, calculating the acceleration due to gravity and, by inference, variations in density within the proximity of the sensor.

This technology benefits the energy and construction sectors by detecting sub-surface features with greater sensitivity than classical gravity sensors. Such features include:

  • pockets of natural oil and gas,
  • oil and gas pipelines, and
  • voids and sink holes.

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