How Can Lasers Be Used to Trap Atoms?

By Cyril Robinson Azariah JJul 23 2018

Image Credit: donatas1205/Shutterstock.com

Spectacular progress has been made in atomic physics, which has propelled the discipline into a new era of using a laser cooling technique to trap atoms. In 1997, the Nobel Prize in Physics was awarded to Chu, Cohen-Tannoudji, and Phillips for building up the strategies to cool and trap atoms with light. Through exchanges of energy and momentum between atoms and laser light at the resonant or quasi-resonant condition, tests of atoms at temperatures in the nanokelvin and picokelvin range could now be acquired, i.e. with speeds extending in the cm/s to mm/s.

Figure 1. Logarithmic temperature scale in physics

1. Atom Traps

The devices which are used for confining and supporting isolated atoms are called atom traps. The freezing cold atoms cannot be confined by the material walls, because the lowest temperature walls might be hot compared to the temperature of the atoms. Instead, the atoms are trapped by force fields.

Atoms are always in motion. The atoms are stopped and allowed to cool down by agitations of radiation pressure from laser light, before being trapped in a bottle whose boundary is defined by the magnetic fields. Hence, the cooled atoms are perfect for discovering the basic questions of physics.

2. Magnetic Traps

The Magnetic traps rest on the inherent magnetism that many atoms have. When an atom obtains a magnetic moment it then acts as a simple magnet and its energy is converted into magnetic energy due to the magnetic field. This change in energy was first discovered by probing the spectra of atoms in magnetic fields. This effect is termed as the Zeeman effect after its inventor Pieter Zeeman, a physicist from the Netherlands.

Figure 2. Schematic diagram of Atom Traps

3. Laser Cooling Technique

Laser cooling of gas occurs when a laser beam is tuned to a frequency lower than the intrinsic frequency emitted and absorbed by the atoms. Such atoms moving toward the laser beam will then have the appropriate Doppler shift to absorb and reradiate the light, and they will slow down. Atoms moving with the laser beam, on the other hand, see the frequency of the light shifted still lower, and they are less likely to absorb the light. They will speed up very little. Accordingly, on the whole, the atoms slow down.

4. Laser Cooling vs Doppler Effect

Laser cooling in such a system is based on the Doppler’s effect. The fundamental thinking is to red-shift the recurrence of the light to underneath the reverberation recurrence of the atoms. All things considered, the atoms pushing toward the laser are blue-shifted into reverberation. The more strongly absorb photons than the atoms which move away from the laser and are red-shifted further away from resonance.

While the Doppler cooling technique works for atoms in dilute gases it is not suitable for cooling of macroscopic objects like solids. Indeed, in solids translational motion is absent and the thermal energy is contained in vibrational modes known as phonons. Most luminescent materials emanate photons with lower vitality than assimilated ones. The related vitality contrast is the Stokes shift, named after the physicist George G. Stokes. The Stokes shift depends on electron-phonon coupling, which permits light to collaborate with the vibrational methods of solids. This procedure is joined by heat generation in the system.

5. Few of the global research works on Laser Cooling

Across the globe, various researchers working on trapping atoms by laser cooling. The new interdisciplinary field of quantum-atom optics (QAO) has encouraged the convergence of nuclear, atomic, and optical material science, condensed matter material science, and computational material science. The field is driven by an extraordinary level of experimental control of degenerate Bose and Fermi systems that have an all-around characterized hypothetical premise in quantum many-body hypotheses. This prompts trials of hypotheses that were once inaccessible including the following topics:

1. Atom lasers and development of quantum deteriorate gases
    
2. Coherent controls of matter waves
    
3. Quantum measurements and matching connections in ultracold Bose and Fermi systems
    
4. Macroscopic relationships, correlations, entanglement, and basic tests
    
5. Computational material science and hypothetical strategies for Bose and Fermi systems

Professor Tilman Esslinger's Quantum Optics Group research team at ETH Zurich, Switzerland used ultracold atoms to synthetically create key models in quantum many-body physics. The properties of the trapped quantum gases are administered by the transaction between nuclear movement and a connection between the particles. This calculated effortlessness is one of a kind in experimental physics and gives an immediate connection between the test and the model depicting the framework. It allows them to shine new light on wide range of fundamental phenomena and address open problems.

Tilman and his team are working continuously on this research, especially with the physics of quantum phase transitions and crossovers, low-dimensional systems and non-equilibrium dynamics, thereby establishing the basis for quantum simulation of many-body Hamiltonians. They loaded a quantum degenerate gas of potassium atoms into the periodic potential of an optical lattice to visualize Hubbard models with atoms and access superfluid, metallic and Mott-insulating phases.

A many-body system with infinitely long-range interactions is formed by trapping a Bose-Einstein condensate inside an optical cavity, which has allowed them to observe the Dicke quantum phase transition from a normal to a superradiant phase. They are also working on broadening the ideas of quantum simulations to device-like structures connected to atomic reservoirs, using a combination of high-resolution microscopy and transport measurements.

Sources and Further References

• William D. Phillips and Harold J. Metcalf (1986). Cooling and Trapping Atoms. Scientific American INC.  http://physics.gu.se/~tfkhj/CoolingTrapping.pdf
• Bardou, F., et al. "how rare Events Bring Atoms to Rest." (2002).
• Nemova, Galina. Laser Cooling: Fundamental Properties and Applications. Pan Stanford, 2016.

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