Physicists in Germany are the first to use "spin-squeezed" atoms to boost the precision of an interferometer based on interacting atoms. Their work involves entangling several hundred atoms in a way that reduces the noise in a measurement of their spin along a certain direction. If the technique can be scaled up to work with millions of atoms, it could help to boost the precision of atomic clocks.

When measuring the internal spin of an atom, the noise in the measurement obeys Heisenberg’s uncertainty principle of quantum mechanics. In other words, the noise in, say, the y component of the spin (Jy) multiplied by the noise in the z component (Jz) must always be greater than a fundamental value. If the atoms do not interact with each other, however, the noise is the same in both directions and increases as the square root of the number of atoms. This is called the "classical limit" because it resembles the noise seen in non-quantum or classical systems.

It is possible, however to, "spin-squeeze" the atoms to reduce the noise in one direction (say Jy) at the expense of boosting the noise in the other direction (Jz). This squeezing can be useful if the atoms are used to measure a particular physical quantity – such as a magnetic field – that interacts with the squeezed component of the collective atomic spin. Squeezed states of photons are already being used to boost the performance of optical interferometers.

Bose-Einstein condensate

Christian Gross and colleagues at the University of Heidelberg began their experiments with an ensemble of several hundred rubidium atoms trapped in a 1D optical lattice. The atoms are chilled to a few tens of nanoKelvin to form a Bose-Einstein condensate (BEC), in which all the atoms are in the same quantum state – and interactions between atoms become important.

The BEC is then subjected to a carefully chosen magnetic field that tunes the interaction between the atoms via a “Feshbach resonance”. This interaction causes the spins of the atoms to become correlated – a phenomenon called entanglement.

The entangled atoms are in a superposition of internal atomic states that can be used in a Ramsey inferometer, which measures the interference between two different quantum states of a system. The Heidelberg researchers then applied a phase shift to the entangled atoms, which is analogous to the application of a magnetic field.

When the two states are recombined, the resulting interference pattern differs from that created in the absence of a magnetic field. This is measured as an imbalance in the number of atoms with spins pointing up and down. Because the states are squeezed, the noise is –8.2 dB below that in the same measurement made without spin squeezing.

More precise atomic clocks

Spin-squeezed states could in future be used to boost the precision of atomic clocks. However, Gross told that the measurements were made using about 170 entangled atoms – which is much less than the millions of atoms used in an atomic clock. Another problem is that the quantum states used in the Heidelberg experiment are very sensitive to magnetic fields, which is a big drawback for atomic clocks.

The work is described online at Nature doi:10.1038/nature08919, where an independent group of physicists at the Ludwig-Maximilians-Universitaät in Germany and the Laboratoire Kastler Brossel in France report the creation of a spin-squeezed BEC condensate in a device fabricated on a silicon chip (Nature doi:10.1038/nature08988). Although Max Riedel and colleagues did not do interferometry measurements, they found that the spin noise was reduced by –3.7 dB and suggest that such chips could find use in atomic clocks.