13 April 2010 Ultraprecise Atomic Compasses

Cold atoms as detectors
for magnetic fields
(artist’s conception).

ICFO researchers led by
Prof. Morgan Mitchell break the standard quantum noise limit in measuring atomic orientations.
The most precise instruments for measuring magnetic fields use atoms as microscopic detectors of field strength and orientation. Like compasses, the atoms change orientation in response to an external field, but are much faster and more sensitive, due to their small size and large magnetic moment. The atoms are, however, subject to the laws of quantum mechanics, and until now their sensitivity has been restricted by the ‘standard quantum limit,’ closely related to the Heisenberg uncertainty principle. ICFO researchers have found a way to get around this limit, using optical quantum non-demolition measurement, and thus make a faster, more sensitive magnetometer.

To demonstrate the technique, ICFO researchers trapped a cloud of one million rubidium atoms and cooled them to a temperature of 25 microKelvin. At this low temperature, atomic motion essentially stops, and the atoms are free to rotate in response to the magnetic field. An off-resonant laser pulse measures the orientation of the atoms, permitting a precise and non-destructive measurement of the field. The work demonstrates that the standard quantum limit can be overcome in magnetometry.

Applications of ultra-precise magnetometry which can benefit from this technique include magnetoencephalography for diagnosis of neurological disorders, magnetocardiography for diagnosis of heart disease, and detection of gravitational waves.

The results are published in Physical Review Letters by ICFO PhD students Marco Koschorreck, Mario Napolitano, and Brice Dubost led by ICFO Prof. Morgan Mitchell.