On adaily basis, the average person uses many pieces of technology involving lasers- barcode scanners, DVD players, games consoles, laser printers and the likes.It can be said that these items have become common place in recent years andthe way these items work is somewhat taken for granted. Lasers being used toheat things is also common knowledge with the popularity of laser eye surgery,laser cutting and a scene in a popular British spy film, where the maincharacter is nearly cut in half by a high-powered laser. However, laser coolingcomes as more of a surprise to people.
Laser cooling is a term which refers tothe cooling of atoms using techniques involving lasers and produces ultracoldatoms. Ultracold atoms are atoms that are maintained at temperatures close toabsolute zero, typically below some tenths of microkelvins (mK). There are many varioustechniques available to produce these systems and there is currently a lot ofresearch being done in this field. Work in this field has historically beenvery successful as it led to the creation of the Bose-Einstein condensate andto the development of modern atomic clocks. At least two Nobel prizes have beenawarded to physicists working in this field. Lasers: Alaser is a device that emits light through a process of optical amplificationbased on the stimulated emissions of electromagnetic radiation and itsinvention has launched a multi-billion dollar industry. Laser is an acronym forLight Amplification by Stimulated Emission of Radiation and it was in 1917 thatEinstein proposed the process that makes lasers works, his “stimulatedemission” theory. He theorized that, besides absorbing and emitting lightspontaneously, electrons could be stimulated to emit light of a particularwavelength.
Einstein’s theory would not be put into use into use until the1950s, nearly 40 years later, when the first maser was produced. It was in 1954when Charles Hard Townes demonstrated the ammonia maser, the first device basedon Einstein’s predictions. The maser obtains the first amplification andgeneration of electromagnetic waves by stimulated emission.
However, thistechnology lacked the continuous output of today’s masers and lasers due to alack of knowledge about population inversion of multiple energy levels, whichis essential in the operation of any continuously emitting lasers (James). Itwould therefore not be until the 1960s – after this was realized, thatcontinuous lasers were produced, relatable to the current devices. (james). Thefirst laser was produced by Theodore H.
Maiman in 1960. Lasers emit light thatis spatially and temporally coherent. Spatial coherence is the feature of laserlight which allows it travel over large distances without diverging and to befocused on a very small area 1. High temporalcoherence allows lasers to emit light with a very narrow spectrum, i.
e. asingle colour of light. Lasers have many applications such as in printers,scanners, rangefinders and skin treatments.
The applications of lasers are vastand differ greatly and it was in the 1970s and 80s when learned how to uselasers to cool atoms to temperatures just barely above absolute zero. LaserCooling: Lasercooling techniques rely on the fact that when an atom absorbs and re-emits aphoton its momentum changes. A Zeeman Slower (or Zeeman Decelerator) is a pieceof scientific apparatus that is commonly used in quantum optics to cool a beamof atoms from room temperature or above to a few kelvins. This apparatusconsists of a cylinder, through which a beam of atoms travels, a pump laserthat is shone on the beam in the direction opposite to the beam’s motion, and amagnetic field. This magnetic field is commonly produced by a solenoid likecoil. A Zeeman Slower takes advantage of the atomic interaction of light in thesame manner as a Doppler cooler, which is based on the Doppler effect and willbe discussed later in this report. Photons fired at the atom near resonantfrequency are absorbed, which slows the atom.
According to the principles ofDoppler cooling, an atom modelled as a two-level atom can be cooled using alaser. If an atom moves in a specific direction and encounters acounter-propagating laser beam resonant with its transition, it is very likelyto absorb a photon. The atoms travelling fastest relative to the propagatinglight will absorb photons, which due to conservation of momentum will slow theatoms down. If an atom is travelling with velocity n and absorbs aphoton with momentum ?k=h/lthe atom is slowed by ?k/m 2.However, as the atom begins to slow from these interactions, the atom willcease to be in resonance with the beam of light, and so the slowing will stop.This is due to the Doppler effect; as the velocity decreases the relativefrequency shifts 3. The Zeeman slower uses thefact that a magnetic field can change the resonant frequency of an atom usingthe Zeeman effect to tackle this problem.
The spatially varying Zeeman shift ofthe resonant frequency enables lower and lower velocity classes to be resonantwith the laser, as the atomic beam propagates along the slower, hence slowingthe beam. It was William D. Phillips who first developed this technique and in1982, along with Harold Metcalf, he published a paper on laser cooling ofneutral atoms. This was the first paper to feature the cooling of neutralatoms, previously it had only been ions which had been cooled via lasercooling. In their experiment, they sent a beam of sodium atoms through a Zeemanslower which had a large magnetic field at the entrance but got smaller over adistance of 60 centimeters 4. The Zeemanslower allowed them to slow the atoms to 40 percent of their initial velocityand has become a standard way of decelerating an atomic beam.
Laser coolingtechniques were improved and in 1985 in the Bell Labs by Chu et al. temperaturesof 240 mK were ahcived, which werethought to be the lowest possible temperatures 5.However, three years later in 1988, a group led by Phillips discovered that thetechnique used by Chu and colleagues to shatter the Doppler limit. Usingseveral new temperature measurement techniques, their atoms were recorded atroughly 43 microKelvin. In 1988, Claude Phillips was awarded the Nobel Prizefor his discovery in 1997 together with Chu and Claude Cohen-Tannoudji “fordevelopment of methods to cool and trap atoms with laser light” 6. Doppler Cooling: Doppler cooling is anothermechanism that can be used to trap and slow the motion of atoms to cool asubstance. It is the first investigated method and is still the most commonused.
It was proposed in 1975 by two groups. Doppler cooling, like a Zeeman slower,involves light with frequency tuned slightly below and electronic transition inan atom and again relies on the conservation of momentum when an atom absorbs aphoton to cool the system. In a Zeeman slower, the atoms are travelling in abeam which is being met with a counter propagating laser beam.
However, in asystem, atoms are usually travelling in random directions, single frequencylasers can be placed at multiple angles and axes to slow down more atoms. Tocool atoms to such low temperatures, atoms are usually trapped and pre-cooledvia laser cooling in a magneto-optical trap (MOT). The development of the firstMOT by Raab et al. was a crucial step towards the creation of sources ofultracold atoms 7. A MOT combines lasercooling and magneto-optical trapping to produce these sources. Formagneto-optical trapping, the atoms involved need to have a certain atomicstructure. As a thermal atom at room temperature has many thousands of times themomentum of a single photon, the cooling of an atom must involve manyabsorption-spontaneous emission cycles, with the atom losing up to ?k ofmomenta each cycle. Because of this, if an atom is to be laser cooled, it mustpossess a specific energy level structure known as a closed optical loop, wherefollowing an excitation-spontaneous emission event, the atom is always returnedto its original state.
A Grating Magneto-Optical Trap (GMOT), instead of using four ormore appropriately polarized beams, uses a diffraction grating to create a MOTfrom a single input beam 8. This makes a GMOTadvantageous over a four-beam MOT as it requires much less optical access andthe single circularly polarized input beam requires no further optics. Thisleads to the implementation and alignment of a GMOT being a simple process. Amagneto-optical trap is usually the first step to achieving a Bose-Einsteincondensate which is a state of matter of a gas of bosons cooled to temperaturesvery close to absolute zero. When they reach this temperature, the atoms arehardly moving relative to each other; they have almost no free energy to do so.
At this point, the atoms begin to clump together, and enter the same energystates. They become identical, from a physical point of view, and the wholegroup starts behaving as though it were a single atom 9.Compared to more commonly encountered states of matter, Bose-Einsteincondensates are extremely fragile.
The slightest interaction with the externalenvironment can be enough to warm them past the condensation threshold,eliminating their interesting properties and forming a normal gas 10. The principles of Bose-Einstein condensates werepredicted by Einstein in the 1920s and was first produced in 1995, 70 yearsafter Einstein’s prediction. It was observed in a gas of rubidium atoms cooledto 170 nanokelvins (nK) by Eric Cornell and Carl Wieman at the University ofColorado 11. Four months later, Ketterle etal. managed to demonstrate important properties of these condensates andfor their achievements, Cornell, Wiemann and Ketterle were awarded the Nobelprize in 2001. Theunique quantum properties and the great experimental control available in suchsystems means that ultracold atoms are central to modern precision measurements12. Sloweratoms lead to longer interaction times which is easier to study and achievemore precise measurements. Laser cooled atoms are essential for research involving atomicclocks which play a crucial role in timekeeping, communications, and navigationsystems 12.
Atomic clocks are the mostaccurate time and frequency standards known, and are used as primary standardsfor controlling the frequency of television broadcasts. The accuracy of theseclocks depends on the temperature of the atoms in the system used. The clockprobes these atoms and therefore as colder atoms move much more slowly, theycan be probed for longer and hence gaining more precise measurements. Limitationsand Advances: Bythe late 1980s, researchers had achieved what they thought were the lowestpossible temperatures, according to Doppler cooling theory. This temperature isknown as the Doppler temperature and it is the lowest achievable with theDoppler cooling technique – 240 microkelvin for sodium atoms 13. This limitation exists as the “kick” associatedwith each photon absorption event is much smaller than the momentum of athermal atom, a larger number of absorption-emission events (on the order ofthousand or more) is required to significantly change the atom’s velocity.Therefore, laser cooling has only been demonstrated with atoms that can beoptically cycled many times back to their initial ground state. However, mostatoms (and all molecules) have multiple ground states to which the excitedstate can decay.
Once the atom reaches a different ground state, the laser nolonger has the correct detuning relative to the atomic transition, and thecooling stops. In particular, molecules have many vibrational and rotationallevels, and consequently no laser cooling of molecules has been demonstrated.At the Centre for Ultracold atoms and Research Laboratory of Electronics inMIT, a group led by Vladan Vuletic are proposing new laser cooling methods foratoms, ions and molecules. The method is called cavity cooling 1415 and is based on coherent scattering, ratherthan on spontaneous emission from and excited state. Advanceshave been made in producing portable apparatus that benefits from theadvantages of atoms in the microkelvin regime. Atom chips have been developedthat enables laser cooling and trapping into a compact system, which was aprevious obstacle as ultrahigh vacuum chambers and cooling lasers were alreadyavailable in small packages. Systems of this type have been developed now todeliver as many atoms as a conventional six beam MOT of the same volume 16.
Optical lattices are a valuable technique inatomic clocks and quantum simulators and GMOTs have opened possibilities ofintroducing lattices to atom chips in a simple way. ColdAtom Laboratory is a piece of experimental equipment that will facilitate thestudy of ultracold quantum gases in the microgravity environment of theInternational Space Station (ISS). It is set to launch in 2018 and it willenable research in a temperature regime and force free environment that isinaccessible to laboratories on earth. Temperatures as low as 1 picokelvin willbe achievable. Bose-Einsteincondensates have proven useful in exploring a wide range of questions infundamental physics, and the years since the initial discoveries by the JILAand MIT groups have seen an increase in experimental and theoretical activity.Examples include experiments that have demonstrated interference betweencondensates due to wave-particle duality 17.
Anothercurrent research interest is the creation of Bose-Einstein condensates inmicrogravity in order to use its properties for high precision atominterferometry. The first demonstration of a BEC in weightlessness was achievedin 2008 at a drop tower in Bremen, Germany by a consortium of researchers ledby Ernst M. Rasel 18. The same teamdemonstrated in 2017 the first creation of a Bose-Einstein condensate in space 19 and it is also the subject of two upcomingexperiments on the International Space Station 2021.
Researchersin the new field of atomtronics use the properties of Bose-Einstein condensateswhen manipulating groups of identical cold atoms using lasers 40 22.