SCIENCE sciencemag.org 25 SEPTEMBER 2020 • VOL 369 ISSUE 6511 1557
Still, in 2019 Wilson and colleagues dem-
onstrated that they could generate entangled
microwaves and use them to detect an object
within the same cryostat, as they reported in
March 2019 in Applied Physics Letters. Fink;
Shabir Barzanjeh, a physicist now at the Uni-
versity of Calgary; and colleagues performed
a similar experiment, but amplified the signal
pulse and ferried it out of the cryostat to de-
tect a room temperature object, as they re-
ported on 8 May in Science Advances.
But to really make the scheme work,
physicists must also preserve the retained
microwave pulse until the reflected pulse
(or the background replacing it) returns.
Then, both pulses can be measured together
in a way that enables the quantum waves to
interfere. So far, however, nobody has done
that. Instead, they’ve measured the retained
pulse immediately and the returning pulse
later, which in the experiments wipes out
any gain from the quantum correlations.
Even if experimenters can overcome the
technical hurdles, quantum radar would still
suffer from a fatal weakness, researchers say.
The entangled pulses of microwaves pro-
vide an advantage only when the broadcast
pulses are extremely faint. The extra quan-
tum correlations fade from prominence if
pulses contain significantly more than one
photon—which is overwhelmingly the case
in real radar. “If you crank up the power, you
won’t see any difference between the quan-
tum and the classical,” Barzanjeh says. And
cranking up the power is a much easier way
to improve the sensitivity.
Such considerations suggest quantum
radar will never be deployed for long-range
uses such as tracking airplanes, says Fabrice
Boust, a physicist at France’s aerospace
agency, ONERA, who specializes in radar.
And whatever system China may have de-
veloped, it almost certainly isn’t a quantum
radar as commonly conceived, he says. “I
am convinced that when they announced
their quantum radar it was not working,”
Boust says. “But they knew they would get
a reaction.”
Fink says his personal goal remains sci-
entific: demonstrating in the laboratory
the true advantage—however small it may
be—of entanglement for detecting objects
hidden by glare. But the dream of fielding
a quantum radar to detect stealth aircraft
will likely fade away, says Giacomo Sorelli, a
theorist at the Sorbonne University. “Taking
out the long-range application of the tech-
nology will surely take out a lot of the inter-
est of funding agencies,” he says.
Shapiro is less sure. This week, he notes,
researchers again discussed quantum radar
in a special session of the online Radar Con-
ference of the Institute of Electrical and Elec-
tronics Engineers. j
Sizing up a green carbon sink
Studies zero in on forests’ potential to fight warming
CLIMATE SCIENCEF
orests are having their moment. Be-
cause trees can vacuum carbon from
the atmosphere and lock it away in
wood, governments and businesses
are embracing efforts to fight climate
change by reforesting cleared areas and
planting trees on a massive scale. But scien-
tists have warned that the enthusiasm and
money flowing to forest-based climate solu-
tions threaten to outpace the science.
Two papers published this week seek to
put such efforts on a firmer footing. One
study quantifies how much carbon might be
absorbed globally by allowing forests cleared
for farming or other purposes to regrow. The
other calculates how much carbon could be
sequestered by forests in the United States if
they were fully “stocked” with newly planted
trees. Each strategy has prom-
ise, the studies suggest, but also
faces perils.
To get a worldwide perspec-
tive on the potential of second-
growth forests, an international
team led by ecologist Susan
Cook-Patton of the Nature Con-
servancy (TNC) assembled data
from more than 13,000 previ-
ously deforested sites where
researchers had measured re-
growth rates of young trees.
The team then trained a machine-learning
algorithm on those data and dozens of vari-
ables, such as climate and soil type, to predict
and map how fast trees could grow on other
cleared sites where it didn’t have data.
A TNC-led team had previously calcu-
lated that some 678 million hectares, an area
nearly the size of Australia, could support
second-growth forests. (The total doesn’t
include land where trees might not be desir-
able, such as farmland and ecologically valu-
able grasslands.) If trees were allowed to take
over that entire area, new forests could soak
up one-quarter of the world’s fossil fuel emis-
sions over the next 30 years, Cook-Patton and
colleagues report in Nature. That absorption
rate is 32% higher than a previous estimate,
based on coarser data, produced by the Inter-
governmental Panel on Climate Change. But
the total carbon drawdown is 11% lower than
a TNC-led team estimated in 2017.
The study highlights “what nature can
do all on its own,” Cook-Patton says. Andit represents “a lightning step forward” in
precision compared with earlier studies,
says geographer Matthew Fagan of the Uni-
versity of Maryland, Baltimore County, who
was not involved in the work.
But, Fagan adds, “Natural regrowth is not
going to save the planet.” One problem: There
is often little economic incentive for private
landowners to allow forests to bounce back.
Under current policies and market pricing,
“nobody will abandon cattle ranching or
agriculture for growing carbon,” says Pedro
Brancalion, a forest expert at the University
of São Paulo in Piracicaba, Brazil. And even
when forests get a second life, they often
don’t last long enough to store much carbon
before being cleared again. Fagan notes that
even in Costa Rica, renowned as a reforesta-
tion champion for doubling its forest cover
in recent decades, studies have found that
half of second-growth forests
fall within 20 years.
Given such realities, some ad-
vocates are pushing to expand
tree planting in existing forests.
To boost that concept, a team
of researchers at the U.S. For-
est Service (USFS) quantified
how many additional trees U.S.
forests could hold. Drawing on
a federal inventory, they found
that more than 16% of forests
in the continental United States
are “understocked”—holding fewer than 35%
of the trees they could support. Fully stock-
ing these 33 million hectares of forest would
ultimately enable U.S. forests to sequester
about 18% of national carbon emissions each
year, up from 15% today, the team reports in
the Proceedings of the National Academy of
Sciences. But for that to happen, the United
States would have to “massively” expand
its annual tree-planting efforts, from about
1 billion to 16 billion trees, says lead author
Grant Domke, a USFS research forester in St.
Paul, Minnesota.
Cook-Patton says planting trees might
make sense in some places, but natural re-
generation, where possible, provides more
bang for the buck. “For any given site,” she
says, “we should always ask ourselves first:
‘Can the forest regenerate naturally, or can
we do something to help?’” jGabriel Popkin is a journalist in Mount Rainier,
Maryland.By Gabriel Popkin“Can the forest
regenerate
naturally, or can
we do something
to help?”
Susan Cook-Patton,
the Nature Conservancy