OUR REPORT “Single-proton spin detection
by diamond magnetometry” (1) presents
measurements on near-surface nitrogen-vacancy centers in diamond. In these
measurements, we observed signals that
showed all the characteristics expected
from single-proton nuclei. This interpretation was based on the three criteria of the
Zeeman effect, quantum-coherent coupling, and (in one occasion) on a before/
after-type control experiment.
We have discovered a potentially
serious issue with the main conclusion
in the paper, namely the “detection of
a single-proton spin.” Specifically, we
have recognized that resident carbon-13
nuclei within the diamond can mimic
single-proton behavior, challenging our
interpretation. Carbon-13 can produce
quantum-coherent signals at the proton
nuclear magnetic resonance frequency,
and the scaling of frequency with magnetic field is indistinguishable from that
of single protons within the measurement
error. This behavior is due to an unrecognized effect that occurs with the dynamical
decoupling sequence used for signal detection. We provide a detailed description of
this behavior in a separate article (2).
Our Science Express Report claims
single-proton spin detection in three
instances. We find that two of these
instances are ambiguous and can be
explained by either single proton or single
carbon-13, whereas the third instance
can only be explained by single proton.
Because this is only a single data point,
we are not confident that it provides
sufficient basis to support our claim of
“single-proton spin detection.” We there-
fore retract the Report.
M. Loretz,1 T. Rosskopf,1 J. M. Boss,1 S.
Pezzagna,2 J. Meijer,2 C. L. Degen1*
1Department of Physics, ETH Zurich, Otto Stern
Weg 1, 8093 Zurich, Switzerland. 2Institute
for Experimental Physics II, Department of
Nuclear Solid State Physics, Universität Leipzig,
Linnéstrasse 5, D-04103 Leipzig, Germany.
*Corresponding author. E-mail: firstname.lastname@example.org
1. M. Loretzetal .,Science 10.1126/science.1259464 (2014);
published online 16 October 2014.
2. M. Loretz et al ., “Spurious harmonic response of multipulse
quantum sensing sequences,” arXiv:1412.5768 (2014).
Protecting coffee from
SUSTAINABLE AGRICULTURAL intensification is one approach to meet food security
and biodiversity conservation goals simultaneously (1, 2). The yield increases required
to achieve these goals can be facilitated by
using improved crops (3, 4). Pest-resistant
or high-yielding crops may deliver higher
returns than unimproved crops and thus
alleviate pressures on remaining natural
habitats. However, because higher yields
increase household income, such improved
crops may also incentivize farmers to
expand cropland, which negatively affects
biodiversity and ecosystem services. Arabica
coffee, which is one of the most valuable
agricultural commodities in the world,
exemplifies this problem.
Wild Coffea arabica is a shrub native
to the understory of the moist evergreen
Afromontane forest of Ethiopia and is the
ancestor of all commercial Arabica cultivars
worldwide. In its region of origin, arabica
coffee was originally harvested from wild
populations, but over time, shade trees and
coffee shrubs have been increasingly managed by farmers to increase coffee yields (5).
Increased coffee yields improve local livelihood, but unfortunately the intensification
of coffee agriculture is also degrading forest
and causing severe biodiversity losses (6, 7).
Even more important, excessive forest management and the use of locally improved
arabica coffee cultivars are threatening the
mating system and the genetic resources of
wild Coffea arabica. Past and ongoing conversion of natural moist evergreen forest to
heavily managed forest has already resulted
in decreased pollen dispersal and increased
self-pollination in wild arabica stands (8),
and the original coffee gene pool already
shows signs of admixture with cultivars
(9). To date, three forests with wild Coffea
arabica populations have been designated
as UNESCO Biosphere Reserves in Ethiopia
and a few others are proposed as reserves,
but there are currently no guarantees that
the genetic integrity of any of these populations will be maintained.
If the worldwide coffee industry plans to
use the genetic diversity of the Ethiopian
wild coffee to adapt arabica coffee to climate
change and emerging pests and diseases
(10, 11), more conservation efforts in the
Ethiopian coffee forests are urgently needed.
The ongoing conversion of the last remaining wild coffee populations to managed
agroforests must be halted to conserve wild
coffee and its pollinators, and the use of
improved cultivars in the immediate vicinity
of these populations must be discouraged to
avoid introgression of cultivar genes into the
wild arabica gene pool.
Raf Aerts,1 Gezahegn Berecha,1,2
1Plant Conservation and Population Biology,
University of Leuven, Kasteelpark Arenberg
31-2435, BE-3001 Leuven, Belgium. 2Department of
Horticulture and Plant Science, Jimma University,
1. B. Phalan et al. , Science 333, 1289 (2011).
2. T. Garnett et al., Science 341, 33 (2013).
3. M. Tester, P. Langridge, Science 327, 818 (2010).
4. E. C. Brummer et al., Front. Ecol. Environ.9, 561 (2011).
5. R.Aerts et al., For. Ecol. Manag. 261,1034(2011).
6. K. Hundera et al., Biol. Conserv. 159, 285 (2013).
7. K. Hundera et al., Environ. Manag.51, 801 (2013).
8. G. Berecha et al. , Biol. J. Linn. Soc. 112, 76 (2014).
9. R. Aerts etal. , Evol.Appl. 6, 243 (2013).
10. M. Gross, Curr. Biol. 24, R503 (2014).
11. D. Zamir, Science 345, 1124 (2014).
Take the risk
I AGREE WITH M. McNutt that young
scientists should be evaluated on the
basis of their willingness to take risks
Edited by Jennifer Sills