INSIGHTS | PERSPECTIVES
1074 6 MARCH 2015 • VOL 347 ISSUE 6226 sciencemag.org SCIENCE
ergy levels depend on the external magnetic
field (known as the Zeeman effect), this technique provides a precise magnetic sensor
with little more experimental setup than an
optical microscope and a piece of diamond
(see the figure, panel A) In (1), single NV
centers were implanted in diamond with an
ion beam with a well-defined energy. Careful
control of the distance to the metal layer is a
key point in the experiment. Kolkowitz et al.
controlled distance by introducing a wedged
quartz spacer between the diamond and
metal, and boosted magnetic sensitivity by
using a decoherence microscopy technique
(6). Instead of measuring the energy of the
spin levels directly, they monitored the relaxation rate between levels.
This approach takes advantage of the
exquisite sensitivity of quantum systems to
their environment. Quantum decoherence,
resulting from interaction with the external
environment, is the bane of many physi-
cists’ existence, and is generally avoided at
all costs. Indeed, this pernicious interaction
is one reason quantum computers are so
difficult to build, and is also why quantum
effects are not seen in everyday life. How-
ever, Kolkowitz et al. turn the paradigm
on its head to use quantum decoherence
for their benefit: Because decoherence
is caused by the environment, it can be
used to provide information about the lo-
cal surroundings (see the figure, panel B).
Careful monitoring of the relaxation rates
unraveled details of electron transport in
a nearby metal layer. Clear differences for
the cases of diffusive motion of electrons in
polycrystalline silver (ohmic behavior) and
ballistic motion of electrons in single-crystal films were demonstrated.
So how little “touch”’ is imparted onto the
investigated metal by the team’s noncontact
method? Electrical isolation is ensured by
the quartz layer separating the silver from
the sensor, whereas physical contact can
be avoided completely by fixing the diamond to a scanning tip (7). More concerning might be the unavoidable “back-action”
that measurements produce in quantum
physics. Kolkowitz et. al. address this issue
by measuring what is an essentially classical system, where the quantum back-action
is negligible. The field produced by their
single-spin sensor is completely swamped
by the many thousands of electrons in the
metal. However, at low temperatures, where
quantum effects become relevant, quantum
interaction between NV ensembles and superconducting resonators has indeed been
demonstrated (8, 9).
The readout laser is perhaps the most
invasive part of the measurement protocol.
Illumination of metals with light can pro-
duce substantial changes in conductivity
and can even lead to ejection of electrons
(the photoelectric effect). However, all of
the information was gathered in the dark,
when the laser was turned off and stored
in the NV state. A short laser pulse was ap-
plied only at the end of the measurement to
read out the sensor. Laser illumination can
actually be used as a resource, in concert
with diamond-based readout, to manipu-
late magnetic properties at the nanoscale,
as shown recently (10).
The work of Kolkowitz et al. provides
researchers with another tool for probing
the importance of dimensionality, geometry, and topology on conductivity at the
nanoscale. It is an important complement
to well-established electrical readout techniques (11) and joins a growing repertoire
of diamond sensors being applied to a diverse range of materials. As far as metals
and conductivity are concerned, diamonds
are nearly untouchable. ■
1. S. Kolko witz et al ., Science347, 1129 (2015).
2. A. Gruber et al ., Science 276, 2012 (1997).
3. B. M. Chernobrod, G. P. Berman, J. Appl. Phys. 97, 014903
4. G. Balasubramanian et al ., Nature 455, 648 (2008).
5. J. R. Maze et al., Nature 455, 644 (2008).
6. J. H. Cole, L. C. L. Hollenberg, Nanotechnology 20, 495401
7. P. Maletinsky et al., Nat. Nanotechnol. 7, 320 (2012).
8. Y. Kubo et al., Phys. Rev. Lett. 105, 140502 (2010).
9. X. Zhu et al ., Nature 478, 221 (2011).
10. J.-P. Tetienne et al ., Science 344, 1366 (2014).
11. A. Brenneis et al ., Nat. Nanotechnol. 10, 135 (2015).
Energy | ;
Decoherence NV NV
Energy | ;
A light touch. (A) Single defects in diamond known as NV centers are placed at the surface and separated from the metal layer with a gradient quartz spacer, tens of nanometers
in thickness. The spin energy levels of the NV center can be monitored optically, and their response to magnetic fields (which can increase the energy gap) used as a sensitive
magnetometer. (B) Rather than measuring the energy levels directly, the relaxation rate between sublevels provides information about the external environment. Well-isolated
quantum systems have low relaxation rates because the environment does not provide enough energy to flip the NV spin. Bringing a noisy environment (such as moving electrons)
near the NV center induces quantum decoherence and can be used to monitor dynamics in order to differentiate diffusive electron motion from ballistic motion.
1Institute of Quantum Optics, Ulm University, Ulm, D-89069
Germany. 2School of Physics, University of Melbourne,
Melboune, 3010 Australia. 3Center for Integrated Quantum
Science and Technology (IQST), Ulm University, Ulm, D-89069
Germany. E-mail: email@example.com 10.1126/science.aaa6908 ILLU