Light SourceMetal Sheet ScreenScience
A Quantum
Leap in the
Classical World
In 1801, physicist Thomas Young conducted the
first double-slit experiment, shooting a beam of
light toward a barrier with two slits in it. Instead
of forming two lines on a screen behind the bar-
rier—in the same way that particles might—the beam formed a pattern of interference
as if a wave had been pushed through the two slits. In 1908, Geoffrey Ingram Taylor
repeated the experiment using a single photon. Even though the photon was a single
particle, the wave interference pattern still appeared. That was strange enough, but
then it got really weird: When scientists tracked the individual particles as they move
through the slits, the monitored particles abandoned their wave-like state and showed
up as two separate lines on the screen. It’s as if they knew they were being watched.THE DOUBLE SLIT
EXPERIMENTP
HYSICISTS HAVE LONG
struggled with a per-
plexing conundrum:
Why do tiny particles
such as atoms, photons,
and electrons behave
in ways that bacteria,
bees, and bowling balls do not?
In a phenomenon called quan-
tum superposition, for example,
individual units (say, of light)
exist in two states at once. They
are both waves and particles, only
settling on one or the other if you
specifically test for it.
This is not something that
will happen to an object like
your desk. It won’t turn solid
when you set your coffee cup on
it, or liquid if you try to drink
it. Superposition has only been
observed in the smallest units
of matter, which made physicist
Markus Arndt of the University
of Vienna curious about where
the line is. Does quantum weird-
ness stop at some particular
size? If so, which?
To find out, Arndt and his
team created a souped-up versionof the famed double-slit experi-
ment (see below), which can show
whether individual particles are
also behaving like waves. Then
they worked their way toward
increasingly massive objects. The
synthetic molecules Arndt’s col-
leagues at the University of Basel
in Switzerland developed for the
study are the largest particles
ever tested in such an experi-
ment. Each contained as many as
2,000 atoms, according to research
published in the journal Nature
Physics. The molecules, which
have a mouthful of a chemical
formula (C 707 H 260 F 908 N 16 S 53 Zn 4 ),
“had to be massive, stable, and yet
volatile enough to f ly in a directed
beam,” Arndt says.
Next, the scientists built a
special instrument, a macromol-
ecule interferometer called the
Long-Baseline Universal Matter-
Wave Interferometer, or LUMI.
With a baseline length of two
meters, it’s the longest macro-
molecule interferometer ever
built and is specially tuned to
compensate for a number of tech-nical challenges (for example,
gravity and the rotation of the
Earth).
Inside the interferometer, the
team used a nanosecond laser
pulse of light to propel the mol-
ecules through an ultra-high
vacuum tube, which shot them
toward a series of slotted barri-
ers to reveal patterns in a screen
behind. To Ardnt’s delight, the
mammoth molecules created
the same interference pattern
as smaller objects. Though they
were particles, they were also
acting like waves.
The push and pull between
what we know of the quantum
and classical worlds has per-
plexed physicists for nearly
a century. Concepts such as
superposition are cornerstones
of quantum physics. “And yet,
we never find ourselves in such
states that we colloquially
describe as an object being in t wo
positions at once,” Arndt says.
In the hunt for a connection
between the quantum and classi-
cal world, Arndt aims to push the
limits even further, testing larger
and more massive particles.
“Why not see how far you can
go?” physicist Herman Batelaan
of the University of Nebraska–
Lincoln, who was not involved in
the study, tells Popular Mechan-
ics. “It’s a beautiful motivation
to do this work.”// BY JENNIFER LEM A N //5
16 March/April 2020