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State-of-the-art sensors such as MRI scanners and atomic clocks are capable of taking measurements with exquisite precision. MRI is used to image tissues deep within the human body and tell us whether we might be suffering from an illness, while atomic clocks are extremely precise timekeepers used for GPS, internet synchronisation and long baseline interferometry in radio astronomy. One might think that these two instruments have nothing in common, but they do: both technologies are based on the precise measurement of an atom’s spin, that is, the gyroscope-like motion of the electrons and the nucleus. In MRI, for example, the pointing angle of the spin gives information about where in the body the atom is located, while the amount of spin (the amplitude) is used to distinguish different kinds of tissue. Combining these two pieces of information, the MRI can make a 3D map of the tissues in the body.

The sensitivity of this kind of measurement was long thought to be limited by Heisenberg's uncertainty principle, which states that accurately measuring one property of an atom limits the precision with which you can measure another property. For example, if we measure an electron's position with high precision, Heisenberg’s principle limits the accuracy of the measurement of its momentum. Since most atomic instruments measure two properties (spin amplitude and angle), the principle seems to say that the readings will always contain some quantum uncertainty. This long-standing expectation has now been disproven by ICFO researchers Giorgio Colangelo, Ferran Martin Ciurana, Lorena C. Bianchet and Dr Robert J. Sewell, led by the ICREA professor at the ICFO Morgan W. Mitchell. In their article Simultaneous tracking of spin angle and amplitude beyond classical limits, published this week in Nature, they describe how a properly designed instrument can almost completely avoid quantum uncertainty.

In their study, the ICFO team cooled down a cloud of atoms to a few micro-degrees Kelvin, applied a magnetic field to produce spin motion as in MRI and illuminated the cloud with a laser to measure the orientation of the atomic spins. They observed that both the spin angle and uncertainty can be continuously monitored with a degree of sensitivity beyond the previously expected limits, although the measurements still obey Heisenberg’s principle.

The results of the study are of paramount importance since this new technique shows that it is possible to measure atomic spins even more accurately, which opens up a new path to the development of far more sensitive instruments that can detect signals such as gravitational waves and brain activity with unprecedented accuracy.