Scientists are using lasers to cool atoms to extremely low temperatures. But what does it mean to "cool" atoms? Atoms are constantly in motion, so cooling atoms to low temperatures essentially amounts to reducing their motion or speed.
The basic principle behind laser cooling is that light, in addition to carrying energy E, also carries momentum p, with (c is the speed of light). This momentum can be transferred to an object to generate a force. For atoms, it is useful to consider the light from the laser as consisting of photons. If an atom moving in some direction absorbs a photon from a laser beam propagating in the opposite direction, the atom's velocity will decrease.
The atom subsequently emits the photon by spontaneous emission; yet the average velocity change due to this process is zero, since the photon is emitted in a random direction. By repeating this process of absorption followed by spontaneous emission many times, it is possible to significantly lower the velocity and hence kinetic energy of an atom, thereby "cooling" it down.
A major requirement of this laser cooling scheme is that photon absorption takes place preferentially, when the atoms are moving against the flow of photons from a laser beam. To ensure this happens, scientists use the Doppler shift associated with the motion of the atom. Similar to the change in pitch of a train whistle as the train approaches and passes by an observer, an atom will experience a shift in the apparent frequency of light due to the relative motion of the atom and the source of light. An atom moving towards a laser observes that the laser light is at a slightly higher frequency, due to the Doppler shift. Likewise, an atom moving away from a laser observes that the laser light is at a slightly lower frequency.
Hence, by tuning the laser to emit light at a frequency slightly lower than the frequency required to be absorbed by the atom at rest, it is more likely that the light will be absorbed if the atom is moving towards the laser than if the atom is moving away from the laser. The atom moving towards the laser will "see" a frequency that is closer to the ideal frequency for absorption, while an atom moving away from the laser will experience a frequency further away from the ideal frequency for absorption.
Optical molasses for sodium atoms is seen as a bright area at the intersection of three pairs of centimeter diameter, counter-propagating laser beams. The additional beam above the molasses decelerates a sodium atomic beam so that the atoms can be captured by the optical molasses.
Credit: Dr. Kristian Helmerson, National Institutes of Standards and Technology (NIST).
This type of laser cooling is known as, "Doppler cooling" and was first demonstrated 1985, by Steven Chu and colleagues, then at Bell Labs. Their experiment consisted of six laser beams arranged in such a manner that they cooled sodium atoms in a vacuum. The atoms in this laser beam configuration experience a viscous force, which tends to dampen their motion, prompting Chu and colleagues to refer to it as, "optical molasses". Doppler cooling typically results in temperatures for the atoms in the milliKelvin range.
In 1988, however, William D. Phillips and colleagues at National Institute of Standards and Technology (NIST) observed temperatures of atoms in optical molasses in the tens of microKelvin range. Subsequent studies and a theoretical framework developed by Claude Cohen-Tannoudji of the Ecole Normale Superierre showed that the sub-Doppler cooling temperatures were due to laser cooling mechanisms assisted by another type of atom-light interaction called stimulated emission.
Today, laser cooling provides a starting point for a number of experiments such as Bose-Einstein condensation, a form of matter where all the atoms are in the lowest possible energy state. Atomic clocks based on laser cooled atoms are the best in the world, achieving accuracy better than 1 part in 1015. For their development of methods to cool atoms to temperatures barely above absolute zero, Chu, Cohen-Tannoudji and Phillips were awarded the 1997 Nobel Prize in Physics.
The National Ignition Facility and Inertial Fusion Energy
Current nuclear power plants, which use fission, or the splitting of atoms to produce energy, have been pumping out electric power for more than 50 years. But these types of plants generate enormous amounts of radioactive waste, of which the safe storage and disposal remains a significant challenge. The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory was designed to produce nuclear fusion, a process that occurs naturally at extreme temperatures and pressures in the cores of stars and planets.
In a star, strong gravitational pressure sustains the fusion of hydrogen atoms. The light and warmth that we enjoy from the sun, a star 93 million miles away, are reminders of how well the fusion process works and the immense energy it creates. Nuclear fusion burn and gain has not yet been demonstrated to be viable for electricity production. For fusion burn and gain to occur, a special fuel consisting of the hydrogen isotopes deuterium and tritium must first "ignite".
Igniting or fusing the atomic nuclei of deuterium and tritium results in a release of energy. These isotopes are derived from water and lithium, and they are plentiful--one in every 6,500 atoms on Earth is a deuterium atom. Successful fusion ignition would generate more energy from the reaction than went into creating it. In addition, a fusion power plant would produce no climate-changing gases, as well as considerably lower amounts and less environmentally harmful radioactive waste than current nuclear power plants.
The NIF is designed to produce fusion burn and energy gain using a technique known as inertial confinement fusion. Its 192 intense laser beams, focused into a tiny gold cylinder called a hohlraum, will generate a "bath" of soft X-rays that will compress a tiny hollow shell filled with deuterium and tritium to 100 times the density of lead. In the resulting conditions (a temperature of more than 100 million degrees Celsius and pressures 100 billion times the Earth's atmosphere) the fuel core will ignite and thermonuclear burn will quickly spread through the compressed fuel, releasing ten to 100 times more energy than the amount deposited by the laser beams.
If successful, NIF will be the first facility to replicate in the laboratory the extreme conditions needed to achieve not only fusion ignition and burn, but also energy gain. As the supply of oil and natural gas steadily decline, experiments at the NIF will hopefully bring us closer to a viable source of nearly limitless, clean energy.
The interior of the NIF target chamber. The service module carrying technicians can be seen on the left. The target positioner, which holds the target, is on the right.
Credit is given to Lawrence Livermore National Security, LLC, Lawrence Livermore National Laboratory, and the Department of Energy under whose auspices this work was performed.
Special thanks to Dr. Kristian Helmerson.