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Uncertainty in Quantum Measurements Reduced

Photonics.com
Mar 2012
ATLANTA, March 5, 2012 — In the weird realm of quantum physics, it is not possible to accurately specify both the momentum of an object and its exact position. For decades scientists have been able to cheat this limitation through a process called “squeezing.” Now that process has been refined.

Georgia Institute of Technology physicists have successfully “squeezed” a property called nematic tensor, which is used to describe the rubidium atoms of Bose-Einstein condensates — a unique form of matter in which all atoms have the same quantum state. Squeezing is a process that has the effect of changing how uncertainty is shown graphically.


Professor Michael Chapman with graduate students Christopher Hamley and Corey Gerving in an optical laboratory at Georgia Tech. The researchers are exploring squeezed states using atoms of Bose-Einstein condensates. (Images: Gary Meek)

“What is new about our work is that we have probably achieved the highest level of atom squeezing reported so far, and the more squeezing you get, the better,” said Michael Chapman, a physics professor at Georgia Tech. “We are also squeezing something other than what people have squeezed before.”

For the past 15 years, scientists have squeezed the spin states of atoms that have just two relevant quantum states — known as spin-half systems. The spin states of individual atoms within this collection can be added together to get a collective angular momentum that describes the entire system of atoms.


Michael Chapman with optical equipment in his laboratory.

In the Bose-Einstein condensate atoms being studied in Chapman’s group, however, the atoms have three quantum states, and their collective spin totals zero — not helpful for describing systems. To overcome this, the group learned to squeeze a more complex measure that described their systems of spin 1 atoms: nematic tensor, also known as quadrupole.

Nematicity is a measure of alignment that is important for describing liquid crystals, some high-temperature superconductors and exotic magnetic materials.

“We don’t have a spin vector pointing in a particular direction, but there is still some residual information in where this collection of atoms is pointing,” Chapman said. “That next higher-order description is the quadrupole, or nematic tensor. Squeezing this actually works quite well, and we get a large degree of improvement, so we think it is relatively promising.”


An atom-trapping apparatus, magnetic coils and some of the optical elements used to create the squeezed states in the Chapman’s lab.

Experimentally, squeezing is achieved by entangling some of the atoms, which takes away their independence. Chapman’s group accomplished this by colliding atoms in their ensemble of some 40,000 rubidium atoms. After they collide with one another, one atom becomes connected to the other, forming an entanglement, which then creates squeezing, he explained.

Reducing uncertainty in atom measurement could have implications for precise magnetic measurements. Chapman said they will need to determine experimentally whether the technique can improve the measurement of magnetic field, which could have important applications. The property could also prove useful for quantum information system applications, which could store information in the spin of atoms and their nematic tensor.

There are a lot of things you can do with quantum entanglement, and improving the accuracy of measurements is one of them,” Chapman said. “We still have to obey Heisenberg’s Uncertainty Principle, but we do have the ability to manipulate it.”

The research appeared online in Nature Physics.

For more information, visit: www.gatech.edu  


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