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Berkeley Scientific Journal

The Science of Cryogenics - eScholarship

Nick Catrini

Published Jan 1, 2014• 0 citations
OpenAlex

Abstract

T he S cience of C ryogenics B S J Nick Catrini Have you ever wondered how NASA launched their monumental space shuttles? Or perhaps you have heard of the scientists who can slow the speed of light to almost zero? Or about Walt Disney’s attempt at cryogenic stasis, where he waits to be awoken when a cure for lung cancer is developed? Unfortunately, Walt Disney being put into cryogenic stasis is a rumor and after his death he was cremated-but all three of these ideas have their roots in the field of cryogenics. Cryogenics is the art of achieving extremely low temperatures and observing phenomena that don’t normally occur in the chaotic, energy rich universe we experience from day to day. People have been freezing things for ages, but the age has come where there’s a demand for colder and colder temperatures, to the point where even the vacuum of outer space is a blazing inferno by comparison to what’s needed in the lab today. Simply put, the colder you get to absolute zero, the easier it is to observe the true quantum nature of atoms, where generalizations and models made for atoms at room temperature begin to break down, and what’s actually going on takes a different route. The ability to find and exploit these differences could lead to breakthrough technologies such as room temperature superconductors, quantum computers, improvements on existing technologies, and it could also answer questions about the origin and nature of the universe itself. Liquid nitrogen is cold, space is colder, but what scientists need is in nanokelvin, billionths of a degree. One historic study that used ultracold temperatures focused on slowing the speed of light to seventeen meters per second in “If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.” Niels Bohr an ultracold atomic gas. The method of getting of the gas to be ultracold was the focal point of the study, since the colder it would get, the slower light would travel through it. The scientists reached a whopping 435 nanokelvin, but in a surprising way to the unfamiliar layman (Hau, 1999). In order to slow light down to seventeen meters per second, lasers were used to cool the atoms down to 50 microkelvin, and then a magnetic field was applied to the cloud of cold atoms to filter out any atoms that do not have magnetic dipoles against the field, creating an effect similar to a coffee cup cooling off, where the improperly aligned atoms steamed off and out of the magnetic field, or cup, and the remaining aligned ones were the only ones left (Marangos, 1999). Now, nanokelvin temperatures are great, but in some cases even nanokelvin is too hot for simulating some phenomena (Campbell, 2011). Again, atomic gases are cooled with lasers put into arrays called optical lattices that “trap” atoms and prevent them from moving, lowering the temperature. Now, why are scientists studying this again? As Niels Bohr once said, “If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.” For one, the mere attempt to use advances in science and technology to tackle hard problems can lead to various discoveries in the process, such as the Bose-Einstein condensate or the slowing of light to 17 meters per second. But now, is the cost worth it? Slowing light promotes stronger light-matter interaction, which can benefit devices that clean up distortion and noise over long distance data transmission through optical fibers (Krauss, 2008). Last but not least, one of the most important discoveries in all of cryogenics’ time is nuclear magnetic resonance imaging over scanning. That handful of words is better known as an MRI or PET scan you’d get at a hospital. MRI is revolutionary since it performs non-invasive imaging of anatomical structures with no known adverse biological effects, it can determine data not available by other techniques, and it increases the specificity of a diagnosis (Partain, 1984). Research in these cooler temperatures today covers areas such as quantum magnetism and high temperature superconductivity, and the working memory needed for a quantum computer. The words “quantum computer” may sound very enticing, like some sort of science fiction mystery magic machine capable of solving any problem and running day to day tasks exponentially faster than even the most expensive supercomputer, but the reality is a stark contrast. In fact, the quantum computer got its roots in NMR-based technology (Ladd, 2010). A quantum computer is special because unlike a classical computer that has “bits” of information, a 1 or a 0, a quantum computer uses “qubits” which can be 1, 0, or anything in between. Qubits are particularly useful for a select set of problems, usually anything dealing with many 6 • B erkeley S cientific J ournal • E xtremes • F all 2014 • V olume 19 • I ssue 1

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T he S cience of C ryogenics B S J Nick Catrini Have you ever wondered how NASA launched their monumental space shuttles? Or perhaps you have heard of the scientists who can slow the speed of light to almost zero? Or about Walt Disney’s attempt at cryogenic stasis, where he waits to be awoken when a cure for lung cancer is developed? Unfortunately, Walt Disney being put into cryogenic stasis is a rumor and after his death he was cremated-but all three of these ideas have their roots in the field of cryogenics. Cryogenics is the art of achieving extremely low temperatures and observing phenomena that don’t normally occur in the chaotic, energy rich universe we experience from day to day. People have been freezing things for ages, but the age has come where there’s a demand for colder and colder temperatures, to the point where even the vacuum of outer space is a blazing inferno by comparison to what’s needed in the lab today. Simply put, the colder you get to absolute zero, the easier it is to observe the true quantum nature of atoms, where generalizations and models made for atoms at room temperature begin to break down, and what’s actually going on takes a different route. The ability to find and exploit these differences could lead to breakthrough technologies such as room temperature superconductors, quantum computers, improvements on existing technologies, and it could also answer questions about the origin and nature of the universe itself. Liquid nitrogen is cold, space is colder, but what scientists need is in nanokelvin, billionths of a degree. One historic study that used ultracold temperatures focused on slowing the speed of light to seventeen meters per second in “If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.” Niels Bohr an ultracold atomic gas. The method of getting of the gas to be ultracold was the focal point of the study, since the colder it would get, the slower light would travel through it. The scientists reached a whopping 435 nanokelvin, but in a surprising way to the unfamiliar layman (Hau, 1999). In order to slow light down to seventeen meters per second, lasers were used to cool the atoms down to 50 microkelvin, and then a magnetic field was applied to the cloud of cold atoms to filter out any atoms that do not have magnetic dipoles against the field, creating an effect similar to a coffee cup cooling off, where the improperly aligned atoms steamed off and out of the magnetic field, or cup, and the remaining aligned ones were the only ones left (Marangos, 1999). Now, nanokelvin temperatures are great, but in some cases even nanokelvin is too hot for simulating some phenomena (Campbell, 2011). Again, atomic gases are cooled with lasers put into arrays called optical lattices that “trap” atoms and prevent them from moving, lowering the temperature. Now, why are scientists studying this again? As Niels Bohr once said, “If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.” For one, the mere attempt to use advances in science and technology to tackle hard problems can lead to various discoveries in the process, such as the Bose-Einstein condensate or the slowing of light to 17 meters per second. But now, is the cost worth it? Slowing light promotes stronger light-matter interaction, which can benefit devices that clean up distortion and noise over long distance data transmission through optical fibers (Krauss, 2008). Last but not least, one of the most important discoveries in all of cryogenics’ time is nuclear magnetic resonance imaging over scanning. That handful of words is better known as an MRI or PET scan you’d get at a hospital. MRI is revolutionary since it performs non-invasive imaging of anatomical structures with no known adverse biological effects, it can determine data not available by other techniques, and it increases the specificity of a diagnosis (Partain, 1984). Research in these cooler temperatures today covers areas such as quantum magnetism and high temperature superconductivity, and the working memory needed for a quantum computer. The words “quantum computer” may sound very enticing, like some sort of science fiction mystery magic machine capable of solving any problem and running day to day tasks exponentially faster than even the most expensive supercomputer, but the reality is a stark contrast. In fact, the quantum computer got its roots in NMR-based technology (Ladd, 2010). A quantum computer is special because unlike a classical computer that has “bits” of information, a 1 or a 0, a quantum computer uses “qubits” which can be 1, 0, or anything in between. Qubits are particularly useful for a select set of problems, usually anything dealing with many 6 • B erkeley S cientific J ournal • E xtremes • F all 2014 • V olume 19 • I ssue 1
Introduction This chapter describes remarkable new experiments that have been made possible through recent advances in laser cooling of atoms. Cooling of atoms in a gas by laser light is in itself surprising and this technique is important in a range of applications. In particular the experiments described here take atoms that have been laser cooled and then use other methods to reach even lower temperatures, at which wonderful and fascinating quantum effects occur. As well as revealing new phenomena, these experiments explore the deep analogies between the roles played by light waves in lasers and matter waves. These techniques are widely accessible and enable skilled researchers in laboratories all over the world to explore exciting new physics. The key physics and techniques of laser cooling are described in depth in Chapter 6. What is “temperature”? We use the words “cold” and “hot” every day; for example to refer to the temperature of the air outside. But what do “cold” and “hot” mean for the air’s molecules? Broadly speaking, we can say that atoms in a hot gas move faster than those in a cold gas and that cooling, by taking energy out of the gas, slows the atoms down. In this description of the gas as a collection of atoms behaving like billiard balls, or little hard spheres, the lowest possible temperature occurs when the atoms stop moving but, as we shall see, this picture does not give an accurate description at very low energies, for which quantum effects become important. In this chapter we will look at the fascinating properties of atomic gases cooled to temperatures within one millionth of a degree Kelvin from absolute zero. We shall use the term “ultra-cold” to refer to such extremely low temperatures that have only recently been achieved in experiments on atomic gases. The techniques used to cool liquids and solids have a longer history and have made available a different set of possibilities (see Chapter 8). Note that most of this chapter refers to gases in which the individual particles are atoms, but the general statements about temperature scales apply equally well to gases of molecules, e.g. air, whose major constituents, oxygen and nitrogen, exist as diatomic molecules.
Laser radiations are used to slow down atoms by the process o f momentum transfer.This leads to reducing the temperature to microkclvin region.Gas phase atoms are trapped by using magnetic fields.The recent advances have led to the realization of the dream of physicists o f confining the atoms and reducing their velocities to the limit imposed by quantum mechanics.A number of new experiments arc possible with the cooled and trapped atoms and ions that would be useful to solve many problems of theoretical physics. 1 unher cooling by the process o f evaporative technique has led to the observation of Bose-Hinslcin Condensation predicted by Einstein and Rose nearly seventy-five years ago.A brief review of the method of laser cooling, magnetic trapping and evaporative cooling methods used for obtaining ultracold atoms are discussed It is possible to obtain temperature in the nanokelvin region without using cryogenic methods thus simplifying the experimental methods to a great extent.
Slowing, cooling, and trapping molecules One of the major achievements in physics during the last two decades of the twentieth century was the ability to cool and trap atoms [1]. Laser cooling can produce previously inconceivable atomic temperatures of less than 1 μK. Atoms with sufficiently low kinetic energy can be trapped in magnetic and optical traps and thus studied for a prolonged time, thereby improving the resolution of spectroscopy by many orders of magnitude as well as reducing Doppler broadening. Furthermore, besides the external degrees of freedom, the internal degrees of freedom of the atoms can also be manipulated, leading to full control of the atoms. One of the holy grails of physics, the achievement of Bose–Einstein condensation in a dilute gas, was achieved mainly because of laser cooling and trapping of atoms. One of the natural extensions of laser cooling and trapping of atoms is the application of the techniques to molecules. Molecules offer a much richer internal structure than atoms, and thus provide extended possibilities to experimentalists. However, atomic laser cooling is based on the repetitive scattering of light and requires a closed, or at least nearly closed, two-level system. Because of the complicated internal structure of molecules, a closed two-level system cannot usually be found because they have many different electronic, vibrational, and rotational states and in general can decay to many different states after absorption of light (see Chap. 16). This requires a complete rethinking of the schemes that are used to cool atoms. Still, because of the extended possibilities cold molecules can offer with respect to cold atoms, there have been a large number of techniques developed to slow, cool, and trap them. These techniques can be divided into two classes, namely direct and indirect cooling. In the first case, molecules at cryogenic or room temperature are directly slowed and cooled. Although many different slowing techniques are used, this chapter focuses on Stark slowing of molecules since it was the first technique developed. Apart from the Stark shift of the molecules, the Zeeman and light shift of molecules can also be used to slow them. The idea behind Stark slowing is to subject the moving molecules to a continuously increasing electric field so that their kinetic energy is converted into potential energy.
Forty years ago, twenty years after the discovery of the laser, physicists were developing laser cooling methods for ions trapped in electromagnetic fields.From the 1980s onwards, these techniques were refined and extended to atoms, thanks to the audacity and inventiveness of a generation of pioneering researchers.Actually, it was necessary to succeed simultaneously in trapping and cooling samples of atomic gases in a vacuum at a distance from any wall.Spectacular results followed and extraordinarily low temperatures were quickly reached, very close to absolute zero.The field of "cold atoms" was born, rewarded by successive Nobel prizes, first of which was awarded in 1997 to William D. Phillips, Steven Chu and Claude Cohen-Tannoudji, last in 2022 to Alain Aspect.Gaseous samples of a few thousand to a few billion atoms can be prepared at a few millionths of a degree above absolute zero, which means that the particles move at extremely low speed, of the order of centimetres per second.At these extreme temperatures, the behaviour of matter changes and its properties can only be described using quantum mechanics and the wave properties of particles.New physical phenomena have been discovered and innovations have followed the theoretical and experimental progress of the research.Initially imagined as a wonderful method for perfecting atomic physics, cold atoms have gradually proved to be powerful tools for research in cross-cutting fields of physics, such as condensed matter and even high-energy physics.These atoms are now referred to as 'quantum gases' at such low temperatures that their collective behaviour is modified by the laws of quantum mechanics.
Abstract The invention and development of laser methods for cooling and trapping atoms have led to the advent of the physics of ultracold matter. Low-temperature quantum physics has so far dealt with condensed media, wherein quantum phenomena (superfluidity and superconductivity) usually manifest themselves at temperatures of a few kelvin. Laser cooling and trapping of atoms have made it possible to successively attain millikelvin, microkelvin, and nanokelvin temperatures, at which the de Broglie wavelength of the atoms becomes commensurable with the interatomic separation, even in dilute gases. The interaction of atoms in ultracold dilute gases becomes essentially quantum mechanical. The physics of such quantum gases is more akin to the well-developed physics of condensed matter at low temperatures. It is in essence a boundary field between atomic, laser, and condensed-matter physics. Following the first, pioneering experiments on the Bose–Einstein condensation of alkali-metal atoms, this field has continued to develop at a very fast pace, which can be explained by the vast variety of particles (atoms, isotopes, and molecules) that can be laser cooled and trapped under degeneracy conditions. This is much more than physicists had at their disposal previously (3,4He). Moreover, laser radiation and magnetic fields additionally make it possible to control not only the intrinsic quantum state of the interacting ultracold particle species, but also the interaction of the particles. All this makes the research in the field of quantum gases a great breakthrough, both in atomic–molecular physics and in many-body-interaction physics, which heretofore was the prerogative of low-temperature condensed-matter physics.
Physicists know how to produce gases at a few billionths of a degree above absolute zero. The cooling methods apply not only to atoms but also to ions and molecules. This field of research has three times been awarded the Nobel Prize. The field experienced remarkable growth when experimentalists learned how to vary at will the interactions between particles, trapping them with optical tweezers or in optical gratings with adjustable geometry. Artificial crystals made of atoms or molecules can be built to simulate the structure of matter and elucidate some of its magnetic properties, hopefully contributing to the understanding of high-temperature superconductivity. The phenomenon of quantum entanglement is the basis for new devices for the storage and transmission of quantum information. Spectacular progress is constantly being made in metrology. For example, ultra-cold atom or ion clocks measure time to better than one second over the lifetime of the Universe. New types of industrial gravimeters and gyroscopes are improving the sensitivity of seismology and navigation in space. In addition, the extreme precision of the measurements allows tests of the fundamental laws of physics, such as quantum electrodynamics, Lorentz invariance or possible variations of the fundamental constants. The field of ultra-cold particles has now reached the stage where it provides insights in the fields of condensed matter, chemistry and even cosmology.