Correcting for Quantum Collapse

Source:  Summaries of recent literature by Science editors.

RESEARCH HIGHLIGHTS 8 MARCH 2013 VOLUME 339, ISSUE 6124

PHYSICS

Ian S. Osborne

The predicament faced by Schrödinger's cat is, perhaps, a broadly known if not wholly understood example of quantum mechanics that illustrates well the weirdness of the quantum world and the kinds of barriers faced by those now trying to control and manipulate it. The rules of quantum mechanics dictate that the mere measurement of a quantum system, describable mathematically in terms of a wavefunction, results in the irreversible collapse of that wavefunction and forces a definitive answer—the cat being either dead or alive. In quantum computing, wavefunction collapse presents a real issue because errors induced by unavoidable interactions with its environment look very much like measurements and can lead to a breakdown of the very quantum state you are trying to do a computation with. Methods have been introduced to correct those errors, which are then fed back into the system to keep the quantum state functional. Schindler et al. show that such error correction strategies can be used to undo a quantum measurement. Using a system of cold atoms and a series of laser pulses, they distributed some of the knowledge they had of the quantum state of the atom of interest across the whole system. In doing so, they have shown that the state of a particular atom can be measured, but that it can be returned to the same superposition state (being both alive and dead) that it was in before the measurement. Such manipulations should lead to Schrödinger's cats with more than nine lives and, perhaps, simpler architectures for quantum processors.

Phys. Rev. Lett. 110, 070403 (2013).
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