On a
daily 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 and
the way these items work is somewhat taken for granted. Lasers being used to
heat 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 main
character is nearly cut in half by a high-powered laser. However, laser cooling
comes as more of a surprise to people. Laser cooling is a term which refers to
the cooling of atoms using techniques involving lasers and produces ultracold
atoms. Ultracold atoms are atoms that are maintained at temperatures close to
absolute zero, typically below some tenths of microkelvins (mK). There are many various
techniques available to produce these systems and there is currently a lot of
research being done in this field. Work in this field has historically been
very successful as it led to the creation of the Bose-Einstein condensate and
to the development of modern atomic clocks. At least two Nobel prizes have been
awarded to physicists working in this field.

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Lasers:

 

A
laser is a device that emits light through a process of optical amplification
based on the stimulated emissions of electromagnetic radiation and its
invention has launched a multi-billion dollar industry. Laser is an acronym for
Light Amplification by Stimulated Emission of Radiation and it was in 1917 that
Einstein proposed the process that makes lasers works, his “stimulated
emission” theory. He theorized that, besides absorbing and emitting light
spontaneously, electrons could be stimulated to emit light of a particular
wavelength. Einstein’s theory would not be put into use into use until the
1950s, nearly 40 years later, when the first maser was produced. It was in 1954
when Charles Hard Townes demonstrated the ammonia maser, the first device based
on Einstein’s predictions. The maser obtains the first amplification and
generation of electromagnetic waves by stimulated emission. However, this
technology lacked the continuous output of today’s masers and lasers due to a
lack of knowledge about population inversion of multiple energy levels, which
is essential in the operation of any continuously emitting lasers (James). It
would therefore not be until the 1960s – after this was realized, that
continuous lasers were produced, relatable to the current devices. (james).

 

 

The
first laser was produced by Theodore H. Maiman in 1960. Lasers emit light that
is spatially and temporally coherent. Spatial coherence is the feature of laser
light which allows it travel over large distances without diverging and to be
focused on a very small area 1. High temporal
coherence allows lasers to emit light with a very narrow spectrum, i.e. a
single colour of light. Lasers have many applications such as in printers,
scanners, rangefinders and skin treatments. The applications of lasers are vast
and differ greatly and it was in the 1970s and 80s when learned how to use
lasers to cool atoms to temperatures just barely above absolute zero.

 

 

 

 

 

Laser
Cooling:

 

Laser
cooling techniques rely on the fact that when an atom absorbs and re-emits a
photon its momentum changes. A Zeeman Slower (or Zeeman Decelerator) is a piece
of scientific apparatus that is commonly used in quantum optics to cool a beam
of atoms from room temperature or above to a few kelvins. This apparatus
consists of a cylinder, through which a beam of atoms travels, a pump laser
that is shone on the beam in the direction opposite to the beam’s motion, and a
magnetic field. This magnetic field is commonly produced by a solenoid like
coil. A Zeeman Slower takes advantage of the atomic interaction of light in the
same manner as a Doppler cooler, which is based on the Doppler effect and will
be discussed later in this report. Photons fired at the atom near resonant
frequency are absorbed, which slows the atom. According to the principles of
Doppler cooling, an atom modelled as a two-level atom can be cooled using a
laser. If an atom moves in a specific direction and encounters a
counter-propagating laser beam resonant with its transition, it is very likely
to absorb a photon. The atoms travelling fastest relative to the propagating
light will absorb photons, which due to conservation of momentum will slow the
atoms down. If an atom is travelling with velocity n and absorbs a
photon with momentum ?k=h/l
the atom is slowed by ?k/m 2.

However, as the atom begins to slow from these interactions, the atom will
cease 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 relative
frequency shifts 3. The Zeeman slower uses the
fact that a magnetic field can change the resonant frequency of an atom using
the Zeeman effect to tackle this problem. The spatially varying Zeeman shift of
the resonant frequency enables lower and lower velocity classes to be resonant
with the laser, as the atomic beam propagates along the slower, hence slowing
the beam. It was William D. Phillips who first developed this technique and in
1982, along with Harold Metcalf, he published a paper on laser cooling of
neutral atoms. This was the first paper to feature the cooling of neutral
atoms, previously it had only been ions which had been cooled via laser
cooling. In their experiment, they sent a beam of sodium atoms through a Zeeman
slower which had a large magnetic field at the entrance but got smaller over a
distance of 60 centimeters 4. The Zeeman
slower allowed them to slow the atoms to 40 percent of their initial velocity
and has become a standard way of decelerating an atomic beam. Laser cooling
techniques were improved and in 1985 in the Bell Labs by Chu et al. temperatures
of 240 mK were ahcived, which were
thought to be the lowest possible temperatures 5.

However, three years later in 1988, a group led by Phillips discovered that the
technique used by Chu and colleagues to shatter the Doppler limit. Using
several new temperature measurement techniques, their atoms were recorded at
roughly 43 microKelvin. In 1988, Claude Phillips was awarded the Nobel Prize
for his discovery in 1997 together with Chu and Claude Cohen-Tannoudji “for
development of methods to cool and trap atoms with laser light” 6.

 

 

Doppler Cooling:

 

Doppler cooling is another
mechanism that can be used to trap and slow the motion of atoms to cool a
substance. It is the first investigated method and is still the most common
used. It was proposed in 1975 by two groups. Doppler cooling, like a Zeeman slower,
involves light with frequency tuned slightly below and electronic transition in
an atom and again relies on the conservation of momentum when an atom absorbs a
photon to cool the system. In a Zeeman slower, the atoms are travelling in a
beam which is being met with a counter propagating laser beam. However, in a
system, atoms are usually travelling in random directions, single frequency
lasers can be placed at multiple angles and axes to slow down more atoms.

 

To
cool atoms to such low temperatures, atoms are usually trapped and pre-cooled
via laser cooling in a magneto-optical trap (MOT). The development of the first
MOT by Raab et al. was a crucial step towards the creation of sources of
ultracold atoms 7. A MOT combines laser
cooling and magneto-optical trapping to produce these sources. For
magneto-optical trapping, the atoms involved need to have a certain atomic
structure. As a thermal atom at room temperature has many thousands of times the
momentum of a single photon, the cooling of an atom must involve many
absorption-spontaneous emission cycles, with the atom losing up to ?k of
momenta each cycle. Because of this, if an atom is to be laser cooled, it must
possess a specific energy level structure known as a closed optical loop, where
following an excitation-spontaneous emission event, the atom is always returned
to its original state. A Grating Magneto-Optical Trap (GMOT), instead of using four or
more appropriately polarized beams, uses a diffraction grating to create a MOT
from a single input beam 8. This makes a GMOT
advantageous over a four-beam MOT as it requires much less optical access and
the single circularly polarized input beam requires no further optics. This
leads to the implementation and alignment of a GMOT being a simple process.

 

A
magneto-optical trap is usually the first step to achieving a Bose-Einstein
condensate which is a state of matter of a gas of bosons cooled to temperatures
very close to absolute zero. When they reach this temperature, the atoms are
hardly 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 energy
states. They become identical, from a physical point of view, and the whole
group starts behaving as though it were a single atom 9.

Compared to more commonly encountered states of matter, Bose-Einstein
condensates are extremely fragile. The slightest interaction with the external
environment 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 were
predicted by Einstein in the 1920s and was first produced in 1995, 70 years
after Einstein’s prediction. It was observed in a gas of rubidium atoms cooled
to 170 nanokelvins (nK) by Eric Cornell and Carl Wieman at the University of
Colorado 11. Four months later, Ketterle et
al. managed to demonstrate important properties of these condensates and
for their achievements, Cornell, Wiemann and Ketterle were awarded the Nobel
prize in 2001.

 

 

 

The
unique quantum properties and the great experimental control available in such
systems means that ultracold atoms are central to modern precision measurements
12. Slower
atoms lead to longer interaction times which is easier to study and achieve
more precise measurements. Laser cooled atoms are essential for research involving atomic
clocks which play a crucial role in timekeeping, communications, and navigation
systems 12. Atomic clocks are the most
accurate time and frequency standards known, and are used as primary standards
for controlling the frequency of television broadcasts. The accuracy of these
clocks depends on the temperature of the atoms in the system used. The clock
probes these atoms and therefore as colder atoms move much more slowly, they
can be probed for longer and hence gaining more precise measurements.

 

 

 

Limitations
and Advances:

 

By
the late 1980s, researchers had achieved what they thought were the lowest
possible temperatures, according to Doppler cooling theory. This temperature is
known as the Doppler temperature and it is the lowest achievable with the
Doppler cooling technique – 240 microkelvin for sodium atoms 13. This limitation exists as the “kick” associated
with each photon absorption event is much smaller than the momentum of a
thermal atom, a larger number of absorption-emission events (on the order of
thousand or more) is required to significantly change the atom’s velocity.

Therefore, laser cooling has only been demonstrated with atoms that can be
optically cycled many times back to their initial ground state. However, most
atoms (and all molecules) have multiple ground states to which the excited
state can decay. Once the atom reaches a different ground state, the laser no
longer has the correct detuning relative to the atomic transition, and the
cooling stops. In particular, molecules have many vibrational and rotational
levels, and consequently no laser cooling of molecules has been demonstrated.

At the Centre for Ultracold atoms and Research Laboratory of Electronics in
MIT, a group led by Vladan Vuletic are proposing new laser cooling methods for
atoms, ions and molecules. The method is called cavity cooling 1415 and is based on coherent scattering, rather
than on spontaneous emission from and excited state.

 

 

 

 

 

 

Advances
have been made in producing portable apparatus that benefits from the
advantages of atoms in the microkelvin regime. Atom chips have been developed
that enables laser cooling and trapping into a compact system, which was a
previous obstacle as ultrahigh vacuum chambers and cooling lasers were already
available in small packages. Systems of this type have been developed now to
deliver as many atoms as a conventional six beam MOT of the same volume 16. Optical lattices are a valuable technique in
atomic clocks and quantum simulators and GMOTs have opened possibilities of
introducing lattices to atom chips in a simple way.

 

Cold
Atom Laboratory is a piece of experimental equipment that will facilitate the
study of ultracold quantum gases in the microgravity environment of the
International Space Station (ISS). It is set to launch in 2018 and it will
enable research in a temperature regime and force free environment that is
inaccessible to laboratories on earth. Temperatures as low as 1 picokelvin will
be achievable.

 

Bose-Einstein
condensates have proven useful in exploring a wide range of questions in
fundamental physics, and the years since the initial discoveries by the JILA
and MIT groups have seen an increase in experimental and theoretical activity.

Examples include experiments that have demonstrated interference between
condensates due to wave-particle duality 17.

 

Another
current research interest is the creation of Bose-Einstein condensates in
microgravity in order to use its properties for high precision atom
interferometry. The first demonstration of a BEC in weightlessness was achieved
in 2008 at a drop tower in Bremen, Germany by a consortium of researchers led
by Ernst M. Rasel 18. The same team
demonstrated in 2017 the first creation of a Bose-Einstein condensate in space 19 and it is also the subject of two upcoming
experiments on the International Space Station 2021.

 

Researchers
in the new field of atomtronics use the properties of Bose-Einstein condensates
when manipulating groups of identical cold atoms using lasers 40 22. 

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