More precision from less predictability: A new quantum trade-off

May 29, 2013

An important advance in the quantitative understanding and experimental verification of complementarity; arguably the most important foundational principle of quantum mechanics.

Researchers at Griffith University's Centre for have demonstrated that, contrary to what the Heisenberg uncertainty relation may suggest, particle properties such as position and momentum can be measured simultaneously with high precision.

But it comes at a cost.

The findings have been published in of Universal Complementarity Relations in the prestigious journal Physical Review Letters.

Co-author Dr Michael Hall said the work represents an important advance in the quantitative understanding and of complementarity; arguably the most important foundational principle of quantum mechanics.

" is often thought to imply that you can estimate precisely how fast an electron is moving, or exactly where it is, but not both at the same time," Dr Hall said.

"The argument is that properties such as speed and position require physically incompatible or 'complementary' devices for their and therefore, any device used to make a simultaneous measurement will give inherently imprecise estimates," he said.

"This argument was challenged by Einstein in 1935, who gave an example where the position and speed could be measured accurately at the same time, by exploiting with a second particle."

Professor Geoff Pryde, co-author and leader of the experimental team, said it is important to note that this is not in direct conflict with the well-known Heisenberg uncertainty relation, which requires only that the position and speed cannot both be predicted accurately beforehand, but it does leave open the important question of whether any quantum restrictions apply to simultaneous measurements.

"We have verified experimentally that Einstein was correct by using polarisation properties of photons rather than position and speed," Professor Pryde said.

"But we have also shown that a high degree of joint precision does not come for free; it is possible only if the measurement outcomes are sufficiently unpredictable, as quantified by a new generalisation of the Heisenberg uncertainty relation.

"As the uncertainty principle underlies many aspects of quantum information technology, ranging from entanglement verification to random number generation to the security of quantum cryptography, our work could have implications in all these areas."

Explore further: Mapping the optimal route between two quantum states

More information: Paper: prl.aps.org/abstract/PRL/v110/i22/e220402

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EyeNStein
1 / 5 (4) May 29, 2013
if measurement outcomes are sufficiently unpredictable

Does this mean that if an electron is fired from a precise electron gun assembly at a precise time: then its finally measured speed and position, though initially predictable, is ultimately more uncertain?
This is highly counter intuitive even for quantum behaviours.
vacuum-mechanics
1.8 / 5 (5) May 29, 2013
Professor Geoff Pryde, co-author and leader of the experimental team, said it is important to note that this is not in direct conflict with the well-known Heisenberg uncertainty relation, which requires only that the position and speed cannot both be predicted accurately beforehand, but it does leave open the important question of whether any quantum restrictions apply to simultaneous measurements….

Actually, what which seems to be the problem with 'Heisenberg uncertainty relation' is because we do not understand its physical working mechanism; understanding the mechanism (as below) then there are no more problems…
http://www.vacuum...19〈=en
antialias_physorg
3.7 / 5 (3) May 29, 2013
Does this mean that if an electron is fired from a precise electron gun assembly at a precise time: then its finally measured speed and position, though initially predictable, is ultimately more uncertain?

It means that it's initially UNpredictable but some time after firing precisely measurable.
This is a very good indication that the Uncertainty principle is an expression of information you can gain from an experiment.

Information is the degree of correlation between a priori encoding and a posteriori measurement... so if there is a limit on the correlation - the uncertainty - then a better a posteriori measurement means you have to get it at the cost of lesser exactness in a priori encoding (read: you measure speed and momentum more exactly but at the cost of having the measurement distributed over a wider range)

Seems like we just can't cheat information theory (which is sorta cool because its a purely mathematical construct).
MRBlizzard
1 / 5 (1) May 30, 2013
I have thought of Information Theory as associated with the Grand Canonical Ensemble in Thermodynamics. Could some thermodynamic principles be brought to bear on this QM situation/experiment?
Jitterbewegung
1 / 5 (2) May 30, 2013
I can't see how there could be a problem measuring a photon's position and speed at the same time. If you measure photon's position it still travels at the speed of light doesn't it?