Australasian Science: Australia's authority on science since 1938

Heisenberg Principle a Little Less Uncertain

By Stephen Luntz

It may be possible to measure the location and speed of a particle more accurately than previously recognised.

It may be possible to measure the location and speed of a particle more accurately than previously recognised, according to University of Queensland physicist Dr Cyril Branciard. A more precise understanding of the potential accuracy may help to prevent eavesdropping on messages encoded using quantum cryptography.

Werner Heinsenberg’s famous uncertainty principle says that it is not possible to know the momentum and location of a particle without the uncertainty of the two, multiplied together, being larger than a defined constant. This constant is very well-known, but the minimum uncertainty occurs when many identical particles in the same state are measured, some for position and some for uncertainty.

Another version says you can’t measure the location and velocity of the same system at one time without a larger uncertainty. “Measuring one necessarily disturbs the other, all the more so as the accuracy of the measurement is increased,” says Branciard. “This uncertainty has not been well-known until now.”

While previous calculations had demonstrated that certain levels of precision were impossible when measuring a single particle or wave, Branciard claims in the Proceedings of the National Academy of Sciences to have found that some values thought to be forbidden are, in fact, allowed. “Interestingly, the disturbance may not necessarily be as important as Heisenberg – and many physicists after him – thought,” he says.

Quantum cryptography relies on transmitting information using the laws of quantum physics. Since anyone intercepting the information en route will necessarily change the information in the process of observing the transmitted particles, a knowledge of how much interference will be caused would allow the transmitter and receiver to compare the measured distortion and work out how much of the conversation the eavesdropper has had access to.