ADVANCES
18 Scientific American, October 2019
ANDRIUS PAŠUKONISStanford UniversityIllustration by Thomas FuchsANIMAL BEHAVIOR
Going
the Distance
Frog fathers ferry tadpoles
past nearby ponds to faraway
pools of water
After poison frog tadpoles hatch from
their eggs in the leaf litter, they wriggle
onto the backs of their patiently waiting
fathers, who piggyback them to water.
Scientists studying the candy-colored
amphibians, sometimes called poison dart
frogs, in the Amazon rain forest recently
discovered that frog dads often skip close-
by ponds in favor of something more dis-
tant—a move that expends precious ener-
gy. Sometimes they traveled as far as 400
meters, scientists reported in July in Evolu-
tionary Ecology. “It’s actually quite the jour-
ney,” says study author and biologist
Andrius Pašukonis of Stanford University.
Pašukonis and his colleagues affixed tiny,
diaperlike radio transmitters to the bottoms
of seven three-striped poison frogs in Peru
and 11 dyeing poison frogs in French Guiana.
The researchers used radio signals to chart
the frogs’ paths on 23 separate journeys, not-
ing each time tadpole-toting fathers passed
by water or deposited their young.
Three-striped poison frogs traveled far-
thest, traversing an average distance of
roughly 215 meters—when the nearest
available pool was on average only 52
meters away from their home territory.Dyeing dart frogs traveled approximately
39 meters on average, hopping past ponds
an average distance of 19 meters away.
Two frogs even left the forest’s shelter to
deposit their tadpoles in flooded pastures.
Despite the energy cost and higher risk
of meeting predators, dropping young
tadpoles in faraway pools may offer evolu-
tionary benefits such as decreased risk
of inbreeding and less competition for
resources, Pašukonis says. But it is difficult
to say what exactly motivates the frogs
themselves to go farther, notes neurobiol-
ogist Sabrina Burmeister of the University
of North Carolina at Chapel Hill, who stud-
ies poison frog cognition but was not in -
volved in the new research.
The findings could help protect amphib-
ians threatened by habitat loss. “Knowing
their ranges, and the types of habitats they
utilize and why, would be very important
for any type of conservation effort,” Bur-
meister says. — Jennifer LemanPHYSICS
Tiniest Sound
Isolating the phonon could boost
quantum computing
Researchers have gained control of the
elusive “particle” of sound, the phonon.
Although phonons—the smallest units of
the vibrational energy that makes up sound
waves—are not matter, they can be consid-
ered particles the way photons are particles
of light. Photons commonly store informa-
tion in prototype quantum computers,
which aim to harness quantum effects to
achieve unprecedented processing power.
Using sound instead may have advantages,
although it would require manipulating pho-
nons on very fine scales.
Until recently, scientists lacked this abili-
ty; just detecting an individual phonon
destroyed it. Early methods involved con-
verting phonons to electricity in quantum
circuits called superconducting qubits.
These circuits accept energy in specific
amounts; if a phonon’s energy matches, the
circuit can absorb it—destroying the phonon
but giving an energy reading of its presence.
In a new study, scientists at JILA (a col-
laboration between the National Institute of
Standards and Technology and the Universi-
ty of Colorado Boulder) tuned the energy
units of their superconducting qubit so pho-
nons would not be destroyed. Instead the
phonons sped up the current in the circuit,
thanks to a special material that created an
electric field in response to vibrations. Ex
per i ment ers could then detect how much
change in current each phonon caused.
“There’s been a lot of recent and impres-
sive successes using superconducting qubits
to control the quantum states of light. And
we were curious—what can you do with
sound that you can’t with light?” says Lucas
Sletten of U.C. Boulder, lead author of the
study published in June in Physical Review X.
One difference is speed: sound travels much
slower than light. Sletten and his colleagues
took advantage of this to coordinate circuit-
phonon interactions that sped up the cur-
rent. They trapped phonons of particular
wavelengths (called modes) between two
acoustic “mirrors,” which reflect sound, and
the relatively long time sound takes to make
a round trip allowed the precise coordina-
tion. The mirrors were a hair’s width apart—
similar control of light would require mirrors
separated by about 12 meters.
Sound’s “slowness” also let the experi-
menters identify phonons of more than onemode. Typically, Sletten says, quantum com-
puters increase their capacity through addi-
tional superconducting qubits. But having
just one qubit process information with mul-
tiple modes could achieve the same result.
“This is definitely a milestone,” says
Yi wen Chu, a physicist at ETH Zurich, who
was not involved in the study. Analogous
experiments with light were a first step
toward much of today’s work on quantum
computers, she notes.
Similar applications for sound are far off,
however: among other things, scientists
must find a way to keep phonons alive much
longer than they currently can—about 600
nanoseconds. Eventually, though, the
research could open new paths forward in
quantum computing. — Leila SlomanFrog father wearing a radio tracker
carries tadpoles.