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
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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.
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