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
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
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
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
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.