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The feature story of the May 14, 2007 issue of New Scientist features Assistant Professor Andrew Jordan's work on reversing quantum measurements, published with co-author Alexander Korotkov in Physical Review Letters 97, October 2006. Jordan defines experiments to physically undo a measurement of an unknown quantum state. In the case of Schrodinger's cat, this means that he has figured out how to monitor the state (dead or alive) of the classic "cat in a box," then undo any damage caused by the monitoring.

"Quantum measurement is usually taught in textbooks as an instantaneous process," Jordan explains in New Scientist. "What we've learned in the last few years is that real measurements don't work that way. In nature, all processes take a finite time."

According to textbook quantum measurements, wavefunction collapse is essentially an irreversible process; the measurement record is indelible. Contrary to this conventional wisdom, the authors show that continuous quantum measurements are written in pencil, not in pen.

The authors give explicit experimental procedures to undo quantum measurement. Step one is to fire a microwave pulse at a loop of superconducting wire known as a phase qubit. This puts the qubit into an equal superposition of both of its possible energy states, and essentially, makes the qubit "behave" like Schrodinger's cat. At this point, the scientists start to measure the state, and the superposition moves either towards "dead" or "alive." To avoid killing the cat, or in this case, the qubit, the next step is to test whether the qubit has undergone quantum tunneling. The key is to catch the qubit before it tunnels and collapses to a higher energy state. This is done by making a second continuous measurement and waiting until the combined detector output gives no information about the initial quantum state.

The information obtained from the first measurement is erased by the second, fully restoring the initial quantum state. The catch is that the undoing procedure is not always successful, so that as the strength of the first measurement grows (giving a particular answer with increasing certainty), the probability of undoing it decreases, finally reaching zero for a textbook quantum measurement. Continuous wavefunction collapse has recently been experimentally observed [Katz, et al., Science 312 in 2006, providing a promising candidate for verification of this prediction in the near future. The overall process is illustrated in a picture on the following web page: http://web1.pas.rochester.edu/~jordan/undo.html. The New Scientist article is at http://web1.pas.rochester.edu/~jordan/Quantum%20Undemolition.pdf. (lhg)