Ultracold Atoms; A hotline for physicists

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Abstract

Until recently, both the properties of quantum and exotic matter have been difficult to observe. However, in the last few decades, a promising discovery has arisen – ultracold atoms. The ensuing article will unravel and unlock the vast potential held by this field. The article builds on prior knowledge of ultracold atoms, including other key apparatus that is needed to synthesise them, such as the magneto-optical trap (MOT), evaporative cooling, and other methods. It also accumulates information regarding the applications of ultracold atoms, and therefore what they hold for the future.

Introduction

Molecules are unpredictable – a chaos of high-velocity movement, with gas molecules moving at 500ms-1 on average – that is faster than commercial aeroplanes. However, researchers have managed to cool these high-flying atoms into ultracold states, allowing scientists to uncover the underlying principles pinning the quantum world, and to apply this discovery to superfluid systems, quantum data storage, and more. Paul Julienne, a theoretical physicist at the National Institute of Standards and Technology in Gaithersburg says, “there is an awful lot of detail and rich physics to explore” [1] with these newly discovered particles. Indeed, scientists have a long way to go before these ultracold atoms are in common use. Nevertheless, physicists are currently developing technologies that utilise these atoms in many ways, which may lead to a whole new subfield of science.

Synthesising ultracold atoms

The cooling of atoms to temperatures close to absolute zero proves highly difficult, and the following article will review the methods to overcome this problem.

Laser cooling

The basic underlying principle in cooling atoms is by slowing them down. An atomic beam’s (a stream of atoms) velocity is dampened when a counter-propagating laser (pulse of light moving in the opposite direction of an atomic beam) is fired.

An atomic beam with velocity v is irradiated by a laser and for each photon that a ground state atom may absorb (the lowest energy state of an atom), the photon’s momentum is also absorbed, which is equal to ħk. This value will remain constant for each experiment. Almost immediately after excitation, de-excitation occurs, as the atom spontaneously re-emits and scatters its absorbed photon (a packet of energy). Photons are emitted in all random directions and the atom will be slower than its initial state. [2]

[2]

It is important to note that the decrease in energy caused by the emission of a single photon is relatively minuscule compared to the kinetic energy of the atoms, e.g., for a stream of sodium atoms, it takes 30,000 photons to dampen its velocity into an ultracold state [3]. However, atoms can absorb and re-emit photons within 30 nanoseconds, this means in a course over just a millisecond, atoms can be substantially decelerated.

Optical molasses and Doppler cooling

The key issue with laser cooling is that atoms must propagate in the opposite direction of the firing laser beam to experience a damping force. Since the inherent motion of atoms is random, laser cooling can only dampen the motion of atoms to such an extent.

However, the issue may be resolved by utilising optical molasses. Optical molasses are composed of a single-frequency source of light, which uses Doppler cooling to dampen the motion of atoms, and hence reduce its temperature even more. Taking a one-dimensional plane for simplicity, two counterpropagating beams are fired at the stream of atoms, which have already been slowed down by classical laser cooling beforehand. The laser’s frequency is tuned partially lower than the atomic absorption resonance (the ideal frequency for an atom to absorb for excitation). This is done, since atoms moving towards the laser will experience an apparent increase in the frequency of the counterpropagating beam, due to the Doppler effect and blue shift. The doppler effect is the change in frequency of a wave when an observer moves relative to a wave source. In the case of doppler cooling, the atoms (the “observer”) move towards the wave source (laser), the waves become “bunched” together and therefore the atoms experience a higher frequency. This also leads to “blueshift”, which is when there is a displacement of the light spectrum to shorter wavelengths (higher frequencies), due to the doppler effect.

The increase in the frequency will match the atomic absorption resonance, and laser cooling will occur.

In a 3d format, the optical molasses are composed of 6 crisscrossing counterpropagating beams, where the atoms are placed into the region of the three overlapping beams (called “optical molasses”, in a typical experiment, this would be 1 cubic centimetre, containing 10 million atoms moving on average of 10cms-1, five-thousandths of their speed at room temperature before they undergo further cooling in the molasses[4] ).

The following diagram illustrates 3d molasses [6];

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Therefore, an atom moving to the left, absorbs a higher frequency of light coming from the ‘left’ source, Conversely, the photon will absorb a lower frequency of light propagating from the ‘right’ beam. This causes a damping force (as explained before). Having three pairs of beams, all the same intensity, any atom moving in any direction will, therefore, feel a damping force – hence why optical molasses are even more efficient at cooling atoms to an ultracold state. Experiments can cool Na to a temperature of 240 microkelvins, fractions above absolute zero [5]

 

Applications of ultracold atoms

An atomic fountain; a gravity yardstick

Physicists have long quantified and calculated forces; however, gravity – a fundamental force – still has a long way to go to be measured precisely. Precise calculations of gravity are important to enable more valid tests in Einstein’s theory of general relativity. Achim Peter’s group report (at Stanford University) in the 26 August issue of Nature have measured the acceleration induced by gravity to a staggering accuracy of three parts in a billion – equivalent to measuring the width of Ireland from east to west with an accuracy of one millimetre [7]. They have done this by using an ultracold pool of caesium atoms (there was no particular reason caesium was used, but it is used in atomic clocks and is the definition of a second in physics, and therefore, is often used in standard experiments). If the atoms were warmer and jiggling about, the crucial, precise measurements of gravity would be washed away. Trapped in their optical molasses, the atoms were then propelled by another laser beam from their equilibrium and left to fall under the force of gravity.

At such a small scale, atoms exhibit quantum-mechanical wave properties. In warm conditions, these are forgotten, but when cooled to an ultracold state, the quantum world begins to unravel itself. Wave-particle duality is a phenomenon that describes that all matter may behave as a wave, including yourself. However, this is only apparent in matter on the nanoscale, such as atoms. Caesium atoms behave like an ‘atomic wave’, which can interfere with the downward travelling gravitational wave; this interference pattern, and therefore the strength of gravity, can then be measured. Peter’s group even managed to observe their measurements rise and fall according to the coastal tides, which is due to the fluctuation in the gravitational field due to the motion of the moon. Therefore, not only do ultracold atoms serve as a gravity measuring device, they exhibit observable quantum properties that allow them to do so.

Freezing light and Bose-Einstein condensates

Many mediums can slow down light, water reduces the speed of light by 75% of its speed in a vacuum, and diamond, which has one of the highest refractive indexes of a transparent material, slows down light by a factor of 2.4 [7]. However, light travels at 3 x 108 ms-1 and thus will continue to propagate at high velocities, despite a reduction in velocity. Although up until now it has been impossible to stop light, ultracold atoms now hold the key to this mystery.

Lene Hau and her group at Harvard University created a cloud of ultracold atoms, which managed to completely freeze yellow light for 1.5 seconds [8]. Ultracold atoms – cooled in optical molasses – are still not cold enough for this to be pulled off, so how did Lene Hau solve this?

After Lene Hau had accumulated 10 billion ultracold atoms in the optical molasses, her group turned off the lasers and the electromagnets holding the atoms in place, and utilised evaporative cooling; a process involving kicking out hotter atoms, and leaving the cooler ones behind, for 38 seconds [9]. Hotter atoms in the molasses have more kinetic energy than average atoms, and so will be able to escape the electromagnets holding the sample together, resulting in an overall decrease in the average energy of the system.

At this point, the cloud is 500 billionths of a degree, forming a special state of matter known as Bose-Einstein condensate (BEC), in which only millions of atoms are left behind after evaporative cooling.

In a BEC, atoms behave in a completely synchronised fashion and have the ability to trap light.

This occurs because as photons from a beam fly into the BEC, it leaves an ‘imprint’, which is stored in a quantum property known as ‘spin’ in the cloud of the BEC. This deteriorates in a matter of milliseconds. To overcome this, Hau strengthened the magnetic field applied to the atom cloud, to protect the imprint from any interfering neighbouring atoms.

According to Hau, eventually the ‘interactions between the imprint and the rest of the atoms becomes repulsive, and the imprint separates from the cloud like a drop of oil in water’. Then her team turned off the magnetic field and the light beam emerged from the cloud 1.5 seconds later.

Freezing light has large applications, such as quantum communication. Imprints of stored photons can be manipulated to send and receive quantum messages. Another application of freezing light is quantum computers, which if built, can solve problems at such an advanced rate compared to the classical computers we own today.

Quantum computers use qubits, which are composed of rapidly moving photons, but they are difficult to manipulate in a way for quantum computers to function. According to Hau, building a slow-light system to control these photons may be essential in the future for building industrial-scale quantum computers.

Atomtronic devices

Ultracold atoms do not limit themselves to the quantum world; they can also be utilised in electronics. Physicists are developing circuits that are filled with ultracold gaseous atoms, which can flow as a controlled current. Atomtronics, a sub-field of ultracold atom physics involving the study of electronics using atoms – usually Bose-Einstein condensates, is a new and emerging study that provides an alternative to the mainstream use of electrons for running devices.

Atomtronic devices require hundreds of thousands of atoms to work in sync with one another – hence why the Bose-Einstein condensate comes into play. At the Joint Quantum Institute in Gaithersburg, Md., graduate student Anand Ramanathan and his colleagues made atomtronic sensors [10], by chilling sodium atoms into a Bose-Einstein condensate. Cleverly, only four counterpropagating beams were used to shape the optical molasses into a flattened ring shape with a radius of 20 micrometres. Then the second pair of lasers transferred energy to the BEC to initiate its rotation, which is possible since as discussed before, atoms within a BEC especially behave in a synchronised fashion. It also happens that such a structure is frictionless, and therefore able to theoretically maintain its rotation forever. Of course, due to our technology, BEC’s lifetime is limited to less than a minute, but a step towards developing these technologies may lead to the rise of further atomtronic devices.

One of Ramanathan’s colleagues, Gretchen Campbell, added a barrier to the flow of condensate around the loop, synthesised by a blue laser – which can speed up or slow down a current, depending on the speed of rotation. Theoretically, if the condensate were kept still when the barrier is attached to it, there would be a sudden jump in current proportional to the rotation speed, thus acting as an extremely sensitive rotation sensor. Atomtronics have a long way to go, but their high sensitivity and lack of friction make them a highly attractive market for the future.

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Sympathetic cooling systems using ultracold reservoirs

Physicists in Germany and Denmark have managed to store up to 38 million rubidium atoms at a temperature of 102 micro kelvins. Often, gaseous ultracold atoms are short-lived systems, in which the isolated atoms are usually utilised until their numbers are deteriorated (such as the BEC discussed before), however, sometimes it is more useful for a constant supply of ultracold atoms.

Jan Mahnke and colleagues at Leibniz Universität Hannover and Aarhus Universitet have managed to synthesise an L-shaped copper device, which acts as a pumped reservoir of ultracold atoms.[11]. The cooling process begins in the 3d laser cooling system, and then pulses of cold atoms are propagated through a magnetic guide towards a reservoir trap. Each pulse contains about 84 million atoms, and one pulse is produced every second. Once in the trap, the atoms can collide with each other. Some will lose and others will gain energy. Atoms whose energy is greater than the average energy within the reservoir (and therefore are hotter than their ultracold neighbours), can escape from the trap, and the ultracold atoms will remain. The effect has an overall reduction in the average energy of particles left behind, similar to evaporation. After 50 seconds, the number of atoms lost by evaporation is equal to atoms gained by it, and it reaches an equilibrium value of a constant stream of 38 million atoms, at 102 microkelvins. Importantly, the atoms must travel 20cm around a U-shaped bend, and this ensures no stray light affects the reservoir trap.

The trap has interesting applications; the atoms in the trap are highly sensitive to motion, which involves passing these atoms through an interferometer and observing changes caused by acceleration changes in the system. This could become a crucial tool in navigation devices, offering an emergency alternative when GPS systems are down.

Another application includes sympathetic cooling (a process in which particles of one type cool particles of another type) of molecules and tiny objects, which is achieved by putting the relevant object in direct contact with the reservoir: an advantage of using this system is that hot atoms escape the reservoir automatically, and will be replaced by colder ones, thus leading to an enhanced cooling system.

Conclusion

Ultracold atoms are a fascinating field of science, which gives scientists the toolkit required to delve into the realm of quantum physics. Physicists and engineers are constantly developing new technologies to synthesise ultracold atoms by improving laser cooling technology and optical molasses, improving the accessibility to use such unique materials. Applications for this topic of physics are unlimited and will only see the further spread of innovations and ideas stemming from this field. Gravity yardsticks, Atomtronic devices and the other topics mentioned are only snippets of what these atoms could achieve; much more is yet to be uncovered.

References

  1. Sanders, Laura. \”Physicists Hot for Ultracold.\” Science News 174, no. 13 (2008): 22-25. Accessed January 27, 2020. www.jstor.org/stable/20465782.
  2. Phillips, William D. \”Laser Cooling and Trapping of Neutral Atoms.\” Journal of the Washington Academy of Sciences 102, no. 4 (2016): 1-52. Accessed January 24, 2020. doi:10.2307/90013575.
  3. Peterson, I. \”Laser Cooling: Putting Atoms on Ice.\” Science News 127, no. 12 (1985): 183. Accessed January 24, 2020. www.jstor.org/stable/3969567.
  4. Peterson, I. \”Optical Molasses: Atoms in the Deep Freeze.\” Science News 134, no. 4 (1988): 52. Accessed January 24, 2020. www.jstor.org/stable/3972923.
  5. Chu, Steven. \”American Scientist Interviews: Steven Chu on Atom Traps and Optical Molasses.\” American Scientist 76, no. 1 (1988): 62-67. Accessed January 24, 2020. www.jstor.org/stable/27854965.
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  7. Philip, Ball. “Measuring gravity with an atomic fountain.” Nature (1999). Accessed January 27, 2020. https://www.nature.com/news/1999/990826/full/news990826-1.html
  8. Sanders, Laura. \”Trapped in Cloud of Ultracold Atoms, Light Stayed Frozen for 1.5 Seconds: Technique, If Improved, Could Lead to Light-Storage Devices.\” Science News 177, no. 2 (2010): 10. Accessed January 25, 2020. www.jstor.org/stable/27760056.
  9. HAU, LENE VESTERGAARD. \”Frozen LIGHT.\” Scientific American 285, no. 1 (2001): 66-73. Accessed January 25, 2020. www.jstor.org/stable/26059098.
  10. Sanders, Laura. \”Trapped in Cloud of Ultracold Atoms, Light Stayed Frozen for 1.5 Seconds: Technique, If Improved, Could Lead to Light-Storage Devices.\” Science News 177, no. 2 (2010): 10. Accessed January 25, 2020. www.jstor.org/stable/27760056.
  11. Powell, Devin. \”Ultracold Atoms on Spin Cycle Can Mimic Electricity.\” Science News 179, no. 6 (2011): 5-6. Accessed January 26, 2020. www.jstor.org/stable/41332210.
  12. Hamish, Johnston. “Reservoir of ultracold atoms is filled continuously.” Physicsworld (2015). Accessed January 27, 2020. https://physicsworld.com/a/reservoir-of-ultracold-atoms-is-filled-continuously/
  13. Wang, Brian. “First Atomtronic sensor from a bose-einstein condensate shaped as a doughnut.” nextbigfuture (2011). Accessed May 1, 2020. https://www.nextbigfuture.com/2011/02/first-atomtronic-sensor-from-bose.html
  14. HAU, LENE VESTERGAARD. \”Frozen LIGHT.\” Scientific American 285, no. 1 (2001): 66-73. Accessed January 25, 2020. www.jstor.org/stable/26059098.

About the Author

Cayman Osei-Bonsu is currently a student at the Royal Grammar School Newcastle UK and hopes to carry his passion to practise theoretical physics. He has currently completed a placement in partnership with Northumbria University to do research on magnetohydrodynamics. He will continue to publish scientific articles, hoping to bring his voice and discoveries to the wider public. 

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