A Light Touch That Is Really Cool


Scientists are using lasers as the ultimate refrigerators. They are cooling objects large and small to extraordinarily low temperatures. They can now make everyday objects so cold that they behave according to the rules of quantum mechanics. Until now, quantum behavior had been seen only in the micro-world of atoms and particles.
Laser cooling is enabling the world’s most accurate clocks and the most sensitive magnetic, inertial, and gravitational sensors. Such sensors would improve navigation for submariners who can’t use GPS far below the waves. Building macroscopic objects that behave quantum mechanically is also a necessary step toward quantum computing, which promises immensely greater computing power than is possible with conventional technology. A key component of quantum computing may be laser-cooled devices that convert light energy or electromagnetic energy from one frequency to another.
Lasers can burn through steel, so how can hitting something with a laser beam make it colder? The magic ingredient is human ingenuity and perseverance.
Just how cold are we talking about? Try 1/3000tho F above absolute zero, which is minus 459.67o F. Absolute zero, called 0 Kelvin, is the coldest possible temperature—the temperature at which matter has no heat energy at all. Water freezes at 273 K and boils at 373 K; we set our thermostats to about 300 K. The temperature of outer space, which is bathed in the afterglow of the Big Bang, is 2.727 K; outer space has about 100 times less heat than ice.
Every type of motion entails kinetic energy, and the random jitter and oscillation of atoms or molecules is what we call heat. To make something colder, one must slow the atoms down. As we approach absolute zero, it becomes apparent that heat energy is quantized, as are other forms of energy in the quantum world.
Surprisingly, scientists can slow atoms down and cool them to record low temperatures by exposing them to an energy source, a precisely tuned laser beam. The process starts with a mirrored box. Light waves will resonant inside the box if its size is an exact multiple of their wavelength; just as violin strings resonant at certain frequencies that depend


on the string length, which violinists change by positioning their fingers. When atoms collide with the photons in a laser beam, energy is exchanged—the atoms can either gain energy and become hotter or lose energy and become colder. The trick is to make it more likely that the atoms lose energy. Scientists shoot a laser beam into the box and ensure that its photons have slightly less than the resonant energy. Since gaining a small amount of energy allows the photons to enter the much more favorable resonant state, quantum rules tilt the balance that way. When the photons collide with jittering atoms, they are more likely to receive rather than give energy. The photons’ gain is the atoms’ loss, and the atoms get colder.
Laser cooling has enabled NIST, the U.S. National Institute of Standards and Technology, to produce the most accurate clocks ever made, good to one second in 4 billion years. Now we’ll know exactly when the government reaches its debt limit.
Scientists have taken this a step further and are cooling substantial objects, not just individual atoms. NIST has laser cooled a “drum” that is 0.000,6 inches across and beats 11 million times per second. They removed so much heat that the drum became a quantum object, vibrating with so little energy that its heat energy is quantized. Like steps on a stairway, the heat energy of quantum oscillators come in units called quanta. Heat energy can be increased or decreased only by whole quanta, just like elevation on a staircase can change only by whole steps. Also, quantum rules don’t permit an oscillator to have zero energy—there’s an irreducible “zero-point” energy that can never be extracted. NIST’s drum reaches its zero-point 60% of the time, and remains there for durations of 0.000,1 seconds. While humans can’t do much in 1/10,000th of a second, for electronics that’s a long time, long enough to open the door to quantum computing.
Laser cooling is employed at LIGO, the U.S. observatory searching for gravity waves. The 22-pound LIGO mirrors are laser cooled to 234 quanta of heat.
The Italian gravity wave detector, AURIGA, currently holds the world record for super-cooling large objects. The detector’s core, a one-ton aluminum bar, is cooled to 4000 quanta of heat.

Best Regards,

    Never too young or too old
        to wonder: Why?

Dr. Robert Piccioni
Author of "Everyone's Guide to Atoms, Einstein, and the Universe"
and "Can Life Be Merely An Accident?"