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Sensor Peers into Electrons

Photonics.com
Oct 2008
CAMBRIDGE, Mass., Oct. 1, 2008 -- Physicists have exploited a special "flaw" in diamond crystals, manipulating them into monitoring magnetic signals from individual electrons and atomic nuclei and allowing scientists to spy on some of the universe's tiniest building blocks.

The work dramatically sharpens the basic approach used in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), which ascertain chemical structures and image inside human bodies by scanning the magnetic activity of billions of individual nuclei. The new diamond-based magnetic sensor could enable novel forms of imaging, marrying NMR's noninvasive nature with atomic-scale spatial resolution, potentially benefiting fields ranging from materials science, spintronics, and quantum information to structural biology, neuroscience, and biomedicine.
MagneticSensor.jpg
Schematic of a novel approach to nanoscale magnetic imaging. When single spin impurity in diamond nano-sized crystal is illuminated with pulses of green light and microwave radiation, it serves as a very sensitive probe of local magnetic field, produced e.g. by complex, individual molecules. To measure magnetic field, impurity's fluorescence is being monitored. (Image: Jonathan Hodges)
Among other applications, the new research could make it possible to peer inside proteins, map the structure of impossibly intricate molecules, closely observe the dynamics of microscopic biochemical processes, monitor the activity of neural circuits, or use single electrons and nuclei for storing and processing information. Some of these applications were recently described by the authors Sept. 14 online in the journal Nature Physics.

"Although some existing magnetic field sensors have higher sensitivity, they probe magnetic fields over large volumes of space," said Mikhail D. Lukin, professor of physics in Harvard University's Faculty of Arts and Sciences. "The combination of excellent sensitivity and nanoscale spatial resolution that we demonstrate is completely unique. Potentially, it may allow one to image single nuclei in individual molecules."

The collaborative research, led by Lukin and Harvard physicists Amir Yacoby and Ronald L. Walsworth, involved scientists from Harvard, the Smithsonian Institution, the Massachusetts Institute of Technology, and the University of Pittsburgh.

The work builds on a Science paper published last year by Lukin and colleagues. That paper reported that single atoms of carbon-13 -- which make up some 1.1 percent of natural diamond -- can be manipulated via a nearby single electron that can, in turn, be controlled by focusing laser light on a diamond lattice flaw where nitrogen replaces an atom of carbon. Such excitation using optical and microwave radiation causes the diamond flaw's electron spin to act as a very sensitive magnetic probe with extraordinary spatial resolution.

An electron's spin, or intrinsic angular momentum, acts like a tiny magnet, providing one of the few outwardly detectable signs of an atom's location. An atomic nucleus can also have a spin, but because a nucleus is much heavier than an electron, its magnetic field is a thousand times smaller, making it much harder to detect.
magneticmicroscope.jpg
Realization of a novel magnetic sensor at Harvard. An optical microscope is used to illuminate small diamond crystal, surrounded by magnetic coils, with green light. (Photo: Jonathan Hodges)
"Our magnetic sensor is based on a single electronic spin associated with an impurity or flaw in a small diamond crystal. We managed to turn our understanding of quantum information physics into an extraordinary measuring apparatus," said Yacoby, professor of physics at Harvard. "A nanocrystal of diamond containing this specific type of impurity could be placed on the tip of a needle as a minuscule probe of extremely weak magnetic fields, such as those generated by the spin of an electron or even an atomic nucleus."

The 2007 work effectively brought the futuristic technology of quantum information systems into the realm of solid-state materials under ordinary conditions; the current research builds on that advance to develop new nanometer-scale magnetic sensors that could have important new implications in imaging of a variety of materials, biological compounds, and tissues.

"Precision sensing of magnetic fields is at the forefront of a wide range of scientific fields -- from nanoscience to bioimaging," said Walsworth, senior lecturer on physics at Harvard and senior physicist at the Smithsonian. "Potential nanoscale applications of the diamond magnetic sensor include detection of individual electron and nuclear spins in complex biological molecules, and serving as a universal 'quantum magnetic head' for addressing and readout of quantum bits of information encoded in an electron or nuclear spin memory."

Accompanying the paper on this work that appears in the current issue of Nature is a report from scientists at the University of Stuttgart who've obtained the first scanning images using a diamond magnetic sensor.

"This is a case where the sum of two contributions is really greater than their parts," Lukin said. "Together, they really jump-start a new research field."

Joining Lukin, Yacoby, and Walsworth as co-authors on the paper are Jeronimo Maze, Sungkun Hong, Liang Jiang, Emre Togan, and Alexander Zibrov, all at Harvard; Paul Stanwix of the Smithsonian; Jonathan Hodges of Harvard and MIT; Jacob Taylor of MIT; and M.V. Gurudev Dutt of Pittsburgh. The work was supported by the National Science Foundation, DARPA, the Packard Foundation, and Harvard's Center for Nanoscale Systems.

For more information, visit: www.harvard.edu


GLOSSARY
electron
A charged elementary particle of an atom; the term is most commonly used in reference to the negatively charged particle called a negatron. Its mass at rest is me = 9.109558 x 10-31 kg, its charge is 1.6021917 x 10-19 C, and its spin quantum number is 1/2. Its positive counterpart is called a positron, and possesses the same characteristics, except for the reversal of the charge.
microscopic
Characteristic of an object so small in size or so fine in structure that it cannot be seen by the unaided eye. A microscopic object may be rendered visible when examined under a microscope.
microwave
An electromagnetic wave lying within the region of the frequency spectrum that is between about 1000 MHz (1 GHz) and 100,000 MHz (100 GHz). This is equivalent to the wavelength spectrum that is between one millimeter and one meter, and is also referred to as the infrared and short wave spectrum.
optical
Pertaining to optics and the phenomena of light.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
quantum
Smallest amount into which the energy of a wave can be divided. The quantum is proportional to the frequency of the wave. See photon.
radiation
The emission and/or propagation of energy through space or through a medium in the form of either waves or corpuscular emission.
sensor
1. A generic term for detector. 2. A complete optical/mechanical/electronic system that contains some form of radiation detector.
spatial resolution
In a vision system, the linear dimensions (X and Y) of the field of view, as measured in the image plane, divided by the number of pixels in the X and Y dimensions of the system's imaging array or image digitizer, expressed in mils or inches per pixel.
spin
Acronym for self-aligned polysilicon interconnect N-channel. A metal-gate process that uses aluminum for the metal-oxide semiconductor (MOS) gate electrode as well as for signal and power supply connectors.
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