2019-07-01_Discover

JULY/AUGUST 2019 JULY/AUGUST 2019 .. DISCOVER DISCOVER 77 77

HOW TO GET THERE

Let Lasers Be

Your Guide

To sneak up on absolute zero,

scientists have used vacuums

and lasers in elaborate

experiments to cool atoms of

a gas. A vacuum can cool a

gas without condensing it into

a liquid or solid — as would

normally happen — but its

atoms still move. That’s where

lasers come in.

When an atom absorbs a

light particle, or photon, from a

laser, it emits another photon.

When physicists tune the

lasers in just the right way, an

atom traveling in one direction

absorbs one photon and then

emits another in a different

direction and at a higher

energy. The atom will then

slow down, photon by photon.

By catching an atom in the

crosshairs of multiple lasers,

researchers can decrease

its momentum from every

direction. This technique, first

used in the 1970s, is called

laser cooling.

But there’s a way to go

even lower. A technique called

evaporative cooling siphons off

a gas’ highest-energy atoms —

like soup cooling by releasing

heat as steam. By combining

lasers and evaporative cooling

in new ways, scientists have

chilled gases to about 50

trillionths above 0 kelvin. It’s

not zero, but it’s close.

RACE TO THE

BOTTOM

1926

Chemists first

describe a method,

called adiabatic

demagnetization,

that uses magnetic

fields to cool

materials below

1 kelvin. In 1933,

scientists employ

it to chill a salt to

0.25 kelvin. That’s

low, but not as low

as laser cooling

can go.

1978

First demonstration

of laser cooling

takes materials

to 40 kelvins;

10 years later,

physicists use laser

cooling to achieve

43 millionths of

a kelvin.

1997

Three physicists

share the Nobel

Prize for inventing

laser cooling.

2015

Stanford University

researchers chill

a gas made of

rubidium — a soft

metal used to

make solar cells

— to 50 trillionths

of a degree above

absolute zero,

setting a new

record.

2017

Physicists at the

National Institute

of Standards and

Technology in

Boulder, Colorado,

chill an aluminum

membrane to

0.00036 kelvin,

lower than theory

predicted possible

for the material.

The experiment

suggests a way

to see quantum

effects, like a single

object coexisting in

two places at once.

(D) By catching an atom in

the crosshairs of six lasers,

physicists can slow the atom

in any direction.

(C) It emits

another photon,

but in a different

direction and at

a higher energy.

The atom slows,

losing energy

photon by

emitted photon.

(B) The atom

absorbs a

photon from

the laser.

(A) A laser

beam is

a stream

of photons,

aimed at

an atom.

Laser cooling an atom

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V
ER

In order to chill

an atom, a

series of lasers

intersect to slow

its momentum.

Scientists

employ

elaborate

laser setups

like this

to study

superchilled

atoms. The

frigid matter

gives insights

into quantum

behaviors.

Laser beam

Atom