r15 r16 r0 r1 r2 Simplified principle of Doppler laser cooling:

Doppler cooling is a mechanism that can be used to trap and slow the motion of atoms to cool a substance. The term is sometimes used synonymously with laser cooling, though laser cooling includes other techniques.

Contents 1 History 2 Brief explanation 3 Detailed explanation 4 Limitations 4.1 Minimum temperature 4.2 Maximum concentration 4.3 Atomic structure 5 Configurations 6 Applications 7 Further reading 8 References 9 See also


Doppler cooling was simultaneously proposed by two groups in 1975, the first being David J. Wineland and Hans Georg Dehmelt and the second being Theodor W. Hänsch and Arthur Leonard Schawlow. It was first demonstrated by Wineland, Drullinger, and Walls in 1978 and shortly afterwards by Neuhauser, Hohenstatt, Toschek and Dehmelt. One conceptually simple form of Doppler cooling is referred to as optical molasses, since the dissipative optical force resembles the viscous drag on a body moving through molasses. Steven Chu, Claude Cohen-Tannoudji and William D. Phillips were awarded the 1997 Nobel Prize in Physics for their work in laser cooling and atom trapping. Brief explanation

Doppler cooling involves light whose frequency is tuned slightly below an electronic transition in an atom. Because the light is detuned to the "red" (i.e. at lower frequency) of the transition, the atoms will absorb more photons if they move towards the light source, due to the Doppler effect. Thus if one applies light from two opposite directions, the atoms will always absorb more photons from the laser beam pointing opposite to their direction of motion. In each absorption event, the atom loses a momentum equal to the momentum of the photon. If the atom, which is now in the excited state, emits a photon spontaneously, it will be kicked by the same amount of momentum but in a random direction. The result of the absorption and emission process is a reduced speed of the atom, provided its initial speed is larger than the recoil velocity from scattering a single photon. If the absorption and emission are repeated many times, the mean velocity, and therefore the kinetic energy of the atom will be reduced. Since the temperature of an ensemble of atoms is a measure of the random internal kinetic energy, this is equivalent to cooling the atoms.

The Doppler cooling limit is the minimum temperature achievable with Doppler cooling. Detailed explanation

The vast majority of photons that come anywhere near a particular atom are almost completely unaffected by that atom. The atom is almost completely transparent to most frequencies (colors) of photons.

A few photons happen to "resonate" with the atom, in a few very narrow bands of frequencies (a single color rather than a mixture like white light). When one of those photons comes close to the atom, the atom typically absorbs that photon (absorption spectrum) for a brief period of time, then emits an identical photon (emission spectrum) in some random, unpredictable direction. (Other sorts of interactions between atoms and photons exist, but are not relevant to this article.)

The popular idea that lasers increase the thermal energy of matter is not the case when examining individual atoms. If a given atom is practically motionless (a "cold" atom), and the frequency of a laser focused upon it can be controlled, most frequencies do not affect the atom — it is invisible at those frequencies. There are only a few points of electromagnetic frequency that have any effect on that atom. At those frequencies, the atom can absorb a photon from the laser, while transitioning to an excited electronic state, and pick up the momentum of that photon. Since the atom now has the photon's momentum, the atom must begin to drift in the direction the photon was traveling. A short time later, the atom will spontaneously emit a photon in a random direction, as it relaxes to a lower electronic state. If that photon is emitted in the direction of the original photon, the atom will give up its momentum to the photon and will become motionless again. If the photon is emitted in the opposite direction, the atom will have to provide momentum in that opposite direction, which means the atom will pick up even more momentum in the direction of the original photon (to conserve momentum), with double its original velocity. But usually the photon speeds away in some other direction, giving the atom at least some sideways thrust.

Another way of changing frequencies is to change the positioning of the laser. For example, using a monochromatic (single-color) laser that has a frequency that is a little below one of the "resonant" frequencies of this atom (at which frequency the laser will not directly affect the atom's state). If the laser were to be positioned so that it was moving towards the observed atoms, then the doppler effect would raise its frequency. At one specific velocity, the frequency would be precisely correct for said atoms to begin absorbing photons.

Something very similar happens in a laser cooling apparatus, except such devices start with a warm cloud of atoms moving in numerous directions at variable velocity. Starting with a laser frequency well below the resonant frequency, photons from any one laser pass right through the majority of atoms. However, atoms moving rapidly towards a particular laser catch the photons for that laser, slowing those atoms down until they become transparent again. (Atoms rapidly moving away from that laser are transparent to that laser's photons — but they are rapidly moving towards the laser directly opposite it). This utilization of a specific velocity to induce absorption is also seen in Mössbauer spectroscopy.

On a graph of atom velocities (atoms moving rapidly to the right correspond with stationary dots far to the right, atoms moving rapidly to the left correspond with stationary dots far to the left), there is a narrow band on the left edge corresponding to the speed those atoms start absorbing photons from the left laser. Atoms in that band are the only ones that interact with the left laser. When a photon from the left laser slams into one of those atoms, it suddenly slows down an amount corresponding to the momentum of that photon (the dot would be redrawn some fixed "quantum" distance further to the right). If the atom releases the photon directly to the right, then the dot is redrawn that same distance to the left, putting it back in the narrow band of interaction. But usually the atom releases the photon in some other random direction, and the dot is redrawn that quantum distance in the opposite direction.

Such an apparatus would be constructed with many lasers, corresponding to many boundary lines that completely surround that cloud of dots.

As the laser frequency is increased, the boundary contracts, pushing all the dots on that graph towards zero velocity, the given definition of "cold". Limitations Minimum temperature

The atom performs a random walk in momentum space with steps equal to the photon momentum due to spontaneous emission and photon absorption. This constitutes a heating effect, which counteracts the cooling process and imposes a limit on the amount by which the atom can be cooled. Moreover, the optical transition used for cooling in reality must have a finite frequency width, which limits the velocity discrimination (i.e. the likelihood that an atom will scatter light from the "correct" beam, as described above), and therefore the temperature. This temperature is called the Doppler temperature. Lower temperatures, down to the recoil temperature, may be obtained by sub-Doppler cooling such as Raman Cooling. Beyond that, evaporative cooling is used to further cool the ultracold atoms. Maximum concentration

The concentration must be minimal to prevent the absorption of the photons into the gas in the form of heat. This absorption happens when two atoms collide with each other while one of them has an excited electron. There is then a possibility of the excited electron dropping back to the ground state with its extra energy liberated in additional kinetic energy to the colliding atoms — which heats the atoms. This works against the cooling process and therefore limits the maximum concentration of gas that can be cooled using this method. Atomic structure

Only certain atoms and ions have optical transitions amenable to laser cooling, since it is extremely difficult to generate the amounts of laser power needed at wavelengths much shorter than 300 nm. Furthermore, the more hyperfine structure an atom has, the more ways there are for it to emit a photon from the upper state and not return to its original state, putting it in a dark state and removing it from the cooling process. It is possible to use other lasers to optically pump those atoms back into the excited state and try again, but the more complex the hyperfine structure is, the more (narrow-band, frequency locked) lasers are required. Since frequency-locked lasers are both complex and expensive, atoms which need more than one extra repump laser are rarely cooled; the common rubidium Magneto-optical trap, for example, requires one repump laser. This is also the reason why, molecules are in general difficult to laser cool: in addition to hyperfine structure, molecules also have rovibronic couplings and so can also decay into excited rotational or vibrational states. However, laser cooling of molecules has been demonstrated first to work for SrF molecules, and subsequently other diatomics such as CaF and YO as welll. Configurations

Counter-propagating sets of laser beams in all three Cartesian dimensions may be used to cool the three motional degrees of freedom of the atom. Common laser-cooling configurations include optical molasses, the magneto-optical trap, and the Zeeman slower.

Atomic ions, trapped in an ion trap, can be cooled with a single laser beam as long as that beam has a component along all three motional degrees of freedom. This is in contrast to the six beams required to trap neutral atoms. The original laser cooling experiments were performed on ions in ion traps. (In theory, neutral atoms could be cooled with a single beam if they could be trapped in a deep trap, but in practice neutral traps are much shallower than ion traps and a single recoil event can be enough to kick a neutral atom out of the trap.)

Billy Kenny (footballer born 1973) and Doppler cooling

William Aidan "Billy" Kenny (born 19 September 1973) is an English former professional footballer who played as a midfielder for Everton and Oldham Athletic, making a total of 21 appearances in the Football League. Kenny retired from professional football at the age of just 21.

Kenny is the son of former Everton midfielder Billy Kenny Snr.

Contents 1 Club career 2 After football 3 Post-career reputation 4 References

Club career

Kenny was a product of Everton's youth system and made his debut in 1992 in a 1–1 home draw with Coventry City. In total Kenny made 17 league appearances for Everton, plus another 4 appearances in cup competitions, scoring one goal. He also made a single appearance for England under 21s in a Under 21 Championship qualifier against Turkey under 21s in the Altay Alsancak Stadium in March 1993.

At the start of his career Kenny showed great promise and was thought to have the potential to become a key player for both Everton and England, with former Everton player Peter Beardsley hailing Kenny as the "Goodison Gazza".

His reputation was further enhanced when he was named man of the match in the Premier League's first ever Merseyside derby. However, Kenny's fledgling career was blighted by injury at first, and later cocaine and alcohol use. These issues came to a head when Kenny gave blood and urine samples that showed the extent of his substance abuse. After a spell in a drying-out clinic failed to get Kenny's Everton career back on track he was released from the club for 'gross misbehaviour' by manager Mike Walker.

Kenny then joined Oldham Athletic who were being managed by former Everton striker Graeme Sharp. Kenny again struggled at Oldham, scoring an own goal in a 3–1 loss to Port Vale and was sacked after making just four league appearances for the team. Following this spell at Oldham Kenny retired from the professional game at the age of 21, although he did make further appearances in non-league football for Barrow AFC, and also played amateur football for Royal Seaforth in the Liverpool County Combination League After football

In an article in the Daily Mail published in 1998, Kenny reflected on the premature end to his professional career. He described how his problems began when faced a six-month lay-off from football due to shin splints and, no longer needing to maintain strict training and fitness routines, Kenny began drinking and using drugs. In the article Kenny spoke frankly of how his problems developed saying: "I suddenly found myself out of the game for six months with nothing to do. I couldn't train and I was bored all the time. That's when I started going out drinking with people I thought were mates but who were just using me because I was a famous Everton player."

Kenny went on to explain the devastating effect drug use had on his career: "I was completely hooked on coke. I needed it just to get by. I was an addict and a complete mess. It got so bad that I wasn't even interested in whether Everton were winning or losing … Some Monday mornings, I got home at four or five o'clock, had a couple of lines of coke, slept for an hour or so, then got a cab to the training ground. I looked a real mess." However, Kenny took full responsibility for the behaviour which led to the failure of his professional career saying: "I could blame a million people for it, but it was all down to me." Kenny also urged young footballers to stay away from clubs and bars and instead concentrate on their football. Post-career reputation

Because Kenny never came close to fulfilling his potential, and ended his professional career at such a young age, he is often seen as a "lost talent" of English football. Rob Smyth of the Guardian placed Kenny at number one in a list of Football’s Lost Talents, while Football365.com named Kenny in seventh place in their list of Football's Wasted Talents. In a 2009 article in the Guardian Kenny's short-lived career was described as "one of the biggest wastes of talent in modern times". Despite the low number of appearances he made for Everton, and the early age at which he called time on his career, Kenny is still held in high regard by Everton fans.