On use into use until the 1950s,

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. 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. These early
attempts did not have a continuous output which is an essential feature of modern
lasers and maser. This was due to a lack of understanding of population
inversion. 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.

Lasers emitting light of a single colour is due to temporal coherence. 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 physicists learned how to use lasers to cool atoms to
temperatures close to 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 is a piece of scientific apparatus
that is commonly used to cool a beam of atoms to temperatures of a few kelvin.

In this apparatus, a beam of atoms travels through a cylinder which has an
applied magnetic field, a pump laser is shone on the beam in the opposite direction
to the beam’s motion. 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. The principles of
Doppler cooling state that a two-level atom can be laser cooled. If this atom
encounters a laser beam propagating in the opposite direction, and in resonance
with the atoms transition, it can absorb a photon. If it is a beam of such atoms,
photons will be absorbed by the atoms travelling fastest relative to the laser
beam. This will cause the atom to slow due to conservation of momentum. 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. This problem can be counteracted
using the Zeeman effect, which is utilized by a Zeeman slower. The resonant
frequency of an atom can be changed by an applied magnetic field and this allows
slower atoms to be in resonance with the beam and for the system of atoms to be
cooled to lower temperatures.

 

 

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 decreased over a distance of 60 centimeters 4.

The Zeeman slower allowed them to slow the sodium 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 achieved, 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. As with a Zeeman slower, Doppler cooling
involves light with frequency tuned slightly below an atom’s transition and relies
on the conservation of momentum when a photon is absorbed by an atom. 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 (or the Doppler cooling limit) and it is the lowest
achievable with the Doppler cooling technique – 240 microkelvin for sodium
atoms 7. This limit is the result of the fact
that when a photon is spontaneously emitted from an atom, it is emitted in a
random direction. These emissions, therefore, do not affect the overall mean
velocity of the system but they do cause a transfer of heat to the atom,
resulting in an equilibrium point. This is the point, at which the cooling rate
is equal to the heat transfer from the spontaneous emissions 8. This point is the Doppler cooling limit and in
order to cool the atoms further, other methods had to be invented.

 

One such
method is Sisyphus cooling, which relies on the principles of Doppler cooling but
it involves two incident lasers which have orthogonal polarization relative to
each other. The interference of these two lasers creates what is known as a polarization
gradient which the atoms move through. This method manages to excite atom into
a higher energy state, using optical pumping, and remove the energy from the
atom when it decays back to the ground state. By removing this energy, the atom
can never heat back up, like in Doppler cooling, and so temperatures below the
Doppler limit can be reached ?.

 

 

This explanation for lower temperatures was the one used by Steven
Chu and Claude Cohen-Tannoudji, and is mentioned in the earlier timeline. Their
explanation won the Nobel prize for physics in 1997. Although Sisyphus cooling
allows for much lower temperatures to be accessed than normal doppler cooling,
it also has its own limit, known as the recoil limit. This is also similar to
the doppler limit, but instead depends on the velocity induced by a single
photon scatter from spontaneous emission. Temperatures below those
corresponding to a single photon recoil therefore cannot be reached.

     Just as sub doppler limit
techniques were discovered, sub recoil ones were too, again courtesy of Claude
Cohen-Tannoudji et. al. His method involves the use of a 3-level electronic
configuration, where the general idea is to trap cooled atoms in the lower
electronic state where they no longer interact with the lasers. Optical pumping
is used to pump specific atoms, depending on their altered momentum after the
laser interaction, into a non-absorbing state .

Atoms then remain trapped in this zero-velocity state where they cannot
interact with the laser, allowing for temperatures one thousandth of the recoil
limit 14.

 

 

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 9. A MOT combines laser
cooling and magneto-optical trapping to produce these sources. 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. For magneto-optical trapping, the atoms involved need to have a
certain atomic structure known as a closed optical loop. An atom with this
structure always returns to its original state after the excitation and
spontaneous emission which occurs when a photon is absorbed and emitted by the
atom. This structure is essential as the cooling of an atom involves a large
number of these cycles, which could be in the range of many thousand times. This
is due to the fact that an atom at room temperature has a much greater momentum
than a single photon and the atom’s momentum will decrease, each cycle, by up
to ?k.

 

 

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 10. 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 atoms reach such low temperatures, they begin
to “clump together”. When they do so, the atoms enter the same energy state and
become physically identical to each other. This is due to the fact there is
almost no free energy in the atoms, so they are hardly moving with respect to
the rest of the system and it behaves as though it were a single atom 11. If a Bose-Einstein condensate has the slightest
interaction with an external environment, it could be enough to heat the system
to temperatures above which condensates cannot form 12.

This is due to fact that Bose-Einstein condensates are very fragile compared to
more common states of matter. 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 13. 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.

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 14. 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.

 

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 15. 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.

 

 

 

Limitations
and Advances:

 

As previously
discussed, in-order for a system to be laser cooled, the atoms must have certain
atomic structure known as a closed optical loop. This is due to the fact that
with each absorption-emission cycle, an atoms momentum can only decrease by up to
?k and as the atoms momentum is much larger than
the momentum of a single photon, this process will have to be repeated many
times to significantly decrease the temperature. Therefore,
there was a period of time where laser
cooling had 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 beam is no longer in
resonance with the atom’s transition and the cooling will stop. 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 1617
and is based on coherent scattering. Other laser cooling methods are based on
the spontaneous emission from and excited state in an atom whereas coherent
scattering is independent of an atom’s structure. It
describes the emission of radiation by an oscillating atomic dipole that is
driven by an external electric field. This new technique is not dependant on the
detuning relative to atomic transitions and would therefore be applicable to
any sort of material.

 

 

 

Many
questions of fundamental physics concepts have been explored with the aid of
Bose-Einstein condensates and its discovery has led to an increase in experimental
and theoretical activity. Examples include experiments that have demonstrated
interference between condensates due to wave-particle duality 18. Another current research interest is the
creation of Bose-Einstein condensates in microgravity in order to use its
properties for high precision atom interferometry. In 2008, a Bose-Einstein
condensate was demonstrated in a weightlessness environment for the first time,
using a drop tower in Bremen, Germany, by a research team led by Ernst M. Rasel
19. In 2017, a Bose-Einstein condensate was
created in space by the same team 20 and this
is the subject of two experiments on the International Space Station, which
will take place in the near future. Cold Atom Laboratory is a piece of
experimental equipment that is set to launch in 2018 to the International Space
Station (ISS). It will be a facility to study systems of ultracold atoms in the
weightless environment of the ISS and will enable research which cannot be done
on earth. Due to the temperature regime and force free environment that will be
experience by the atoms, temperatures will be achievable as low as 1 picokelvin
21.

 

 

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