Popping a quantum balloon

When a tiny, quantum-scale, hypothetical balloon is popped in a vacuum, what happens to the particles inside?

This is a deceptively complicated question, the subject of debate among theoretical physicists studying nature at the subatomic level. Predicting if chaos or regularity prevails there is also important to scientists who are trying to harness quantum's bizarre wave and particle like behavior to advance nanotechnology.

Dr. Maxim Olchanyi, associate professor at the University of Massachusetts Boston, Department of Physics, thinks the answer is chaos … sort of. Writing in the April 17, 2008 edition of Nature, he reported that when an observer attempts to measure the energies of particles coming out of a quantum balloon, the interference caused by the attempt throws the system into a final, “relaxed” state analogous to the chaotic scattering of air molecules. The result is the same for any starting arrangement of particles since the act of measuring wipes out the differences between varying initial states.

“It’s enough to know the properties of a single stationary state of definite energy of the system to predict the properties of the thermal equilibrium (the end state),” Olchanyi said in a press release issued by the University of Southern California, where he began his research.

The measurement – which must involve interaction between observer and observed, such as light traveling between the two – disrupts the “coherent” state of the system, Olchanyi said. In mathematical terms, the resulting interference reveals the final state, which had been hidden in the equations describing the initial state of the system.

“The thermal equilibrium is already encoded in an initial state,” Olchanyi said. “You can see some signatures for the future equilibrium. They were already there but more masked by quantum coherences.”

Olchanyi’s finding level extends into the world of applications, where scientists require reliable predictions in order to develop quantum-scale semiconductors. Quantum computing is gaining attention as manufacturers rapidly reach the limit on how much smaller chips can be. Because quantum particles can exist in multiple states at the same time, they could be used to carry out many calculations at once, factoring hugh numbers in just seconds. But to exploit this power, researchers must prevent coherent systems from falling into the chaos of thermal equilibrium.

Paolo Zanardi, an associate professor of physics studying quantum information at USC College, said in the USC interview: “Finding such ‘unthermalizable’ states of matter and manipulating them is exactly one of those things that quantum information/computation folks like me would love to do. Such states would be immune from ‘decoherence’ (loss of quantum coherence induced by the coupling with environment) that’s still the most serious, both conceptually and practically, obstacle between us and viable quantum information processing.”

Modern technology already operates at a scale where quantum effects are significant. Examples include the laser, the transistor, the electron microscope, and magnetic resonance imaging. But further exploitation at the nanoscale is only in its infancy.

The National Science Foundation and the Office of Naval Resarch funded the research of Olchanyi and his co-authors, postdoctoral researchers Marcos Rigol and Vanja Dunjko.