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Science

Chip-Scale Magnetic Measurements

Published Jan 13, 2005• 0 citations
Publisher

Abstract

The ability to measure tiny magnetic fields with good sensitivity can be found in many applications, from biological imaging to prospecting for buried treasure. However, the most sensitive magnetometers that operate in ambient conditions tend to be power-hungry, bulky, and heavy. Shrinking the size to just several millimeters and the power consumption to hundreds of milliwatts, Schwindt et al. have fabricated a sensitive magnetometer using microelectromechanical technology. A cloud of rubidium atoms trapped in a micromachined vapor cell is used to sense the magnetic field. The magnetic field splits the energy levels of rubidium atoms, and the extent of the splitting depends on the strength of the magnetic field. Changes in the magnetic field are then detected and tracked optically by the relative absorption changes of a laser light tuned to the split energy levels. It could be that in the not-too-distant future we could be using handheld battery-operated magnetometers. — ISO

DOI
10.1126/science.307.5707.182a

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The ability to measure tiny magnetic fields with good sensitivity can be found in many applications, from biological imaging to prospecting for buried treasure. However, the most sensitive magnetometers that operate in ambient conditions tend to be power-hungry, bulky, and heavy. Shrinking the size to just several millimeters and the power consumption to hundreds of milliwatts, Schwindt et al. have fabricated a sensitive magnetometer using microelectromechanical technology. A cloud of rubidium atoms trapped in a micromachined vapor cell is used to sense the magnetic field. The magnetic field splits the energy levels of rubidium atoms, and the extent of the splitting depends on the strength of the magnetic field. Changes in the magnetic field are then detected and tracked optically by the relative absorption changes of a laser light tuned to the split energy levels. It could be that in the not-too-distant future we could be using handheld battery-operated magnetometers. — ISO
Using the techniques of microelectromechanical systems, we have constructed a small low-power magnetic sensor based on alkali atoms. We use a coherent population trapping resonance to probe the interaction of the atoms’ magnetic moment with a magnetic field, and we detect changes in the magnetic flux density with a sensitivity of 50pTHz−1∕2 at 10Hz. The magnetic sensor has a size of 12mm3 and dissipates 195mW of power. Further improvements in size, power dissipation, and magnetic field sensitivity are immediately foreseeable, and such a device could provide a hand-held battery-operated magnetometer with an atom shot-noise limited sensitivity of 0.05pTHz−1∕2.
In cold-atom chips, atoms are guided above tracks of wires that supply the magnetic field to keep them aloft. In applications ranging from quantum information processing to metrology, any deviation in the magnetic field from point to point over the chip could influence the delicate state of the atoms. Terraciano et al. introduce a technique that takes a snapshot image of the magnetic field landscape. Using a cloud of cold rubidium atoms, whose energy levels are sensitive to magnetic field, they let the cloud fall toward the chip and probe the atoms' state with a laser beam tuned to one of the magnetic transitions. The ability to take a two-dimensional snapshot image of magnetic field variations of 30 mG/cm above the atom chip over 5 mm with 250-μm resolution should prove useful in calibrating these chips for their envisioned applications. — ISO
Using the techniques of microelectromechanical systems, we are developing chip-scale atomic sensors based on laser excitation of alkali atoms. Recently, we demonstrated a magnetometer physics package based on coherent population trapping that had a sensitivity of 50 pT/Hzfrac12at 10 Hz, had a volume of 12 mm3, and used 195 mW of power as presented by Schwindt et al. (2004). To improve the sensitivity and reduce the power consumption of the magnetometer, we are evaluating other methods of interrogating the atoms for use in microfabricated devices. One of these methods uses frequency modulated nonlinear magneto-optical rotation (FM NMOR). We demonstrate that an FM NMOR magnetometer can be made to self-oscillate, offering simple construction and low power consumption
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We demonstrate an optical magnetometer based on a microfabricated 87Rb vapor cell in a micromachined silicon sensor head. The alkali atom density in the vapor cell is increased by heating the cell with light brought to the sensor through an optical fiber, and absorbed by colored filters attached to the cell windows. A second fiber-optically coupled beam optically pumps and interrogates the atoms. The magnetometer operates on 140 mW of heating power and achieves a sensitivity below 20 fT/√Hz throughout most of the frequency band from 15 Hz to 100 Hz. Such a sensor can measure magnetic fields from the human heart and brain.
We introduce a new ultra-sensitive adiabatic magnetometer that has a broad bandwidth and can operate in the presence of magnetic fields and gradients. It follows conceptually typical implementations of atomic magnetometers based on alkali-metal vapor cells and lasers for optical pumping and optical Faraday effect detection, while its unique feature is a measurement of an oscillating magnetic field along the probe beam direction at frequencies lower than the resonant frequency, proportional to a static magnetic field along the pump beam direction. The bandwidth of the adiabatic magnetometer scales as the strength of the field along the pump beam. From our theoretical studies it is expected that the adiabatic magnetometer can reach 1 fT sensitivity with a bandwidth of 10 kHz, which any type of atomic magnetometers cannot achieve. Among anticipated various applications of this adiabatic magnetometer are biomagnetic sensing, nuclear magnetic resonance detection, and alkali-metal density measurements. We experimentally conducted alkali-metal density measurements, as an example of applications.