Physicists have demonstrated a way to get around Heisenberg's Uncertainty Principle, one of the central discoveries of 20th century physics. To achieve this they entangled two small – but nevertheless macroscopic – vibrating drums, a remarkable advance in itself.
The uncertainty principle states that it is impossible to know the position and momentum of an object perfectly at the same time. The imprecision in the measurement of each, multiplied together, must be larger than half the Planck constant. Measuring one will always create a disturbance that produces uncertainty about the other by a process known as quantum backaction. While this makes for plenty of jokes about car journeys or billiard balls, on a human scale this effect is insignificant. On the other hand, when dealing with objects of very small mass, particularly subatomic particles, it places major restrictions on our ability to know the world.
However, Dr Mika Sillanpää of Aalto University, Finland has demonstrated the principle can be avoided when two objects are linked to each other by the phenomenon known as quantum entanglement. Sillanpää and colleagues created tiny aluminum drumheads and used microwaves to make them vibrate out of phase billions of times a second.
Although not classically connected, the drumheads are entangled so that changes to one affect the other. "One of the drums responds to all the forces of the other drum in the opposing way, kind of with a negative mass", Sillanpää said in a statement.
Entangling these two is a significant achievement in itself. Indeed, the same edition of Science that carries Sillanpää's paper has a report from a team led by Dr Shlomi Kotler of the University of Colorado that achieved something very similar.
Both teams used drumheads that are small – 10 µm, or a fifth of the width of a human hair, but still large enough to see without a microscope. Quantum entanglement has been demonstrated for decades with objects the size of a few atoms or smaller, but has proven difficult to scale up. Claims of quantum entanglement at a macroscopic scale have been made before, but these have relied on inferences that have left room for doubt. Kotler in particular has been able to measure the entanglement more directly, creating greater confidence in the results.
More significantly, however, Sillanpää also demonstrated the absence of quantum backaction in his system. Measuring the position of one drumhead did not destroy our knowledge of the momentum of the system as a whole.
The idea that entangling objects in this way could provide the sort of certainty Heisenberg considered impossible isn't new but demonstrating it in practice required removing any disturbances that could disrupt the entanglement. Since heat is a major potential disruptor, the experiment was done at a hundredth of a degree in Kelvin (-273º C), well below ambient temperatures even in a Helsinki winter.
In addition to confirming our understanding of quantum mechanics the experiment could have practical uses. Precise measurements of entangled vibrations could be used to increase the sensitivity of gravitational wave detectors, for example, the paper points out. Meanwhile, Kotler's version is thought to have potential for quantum computing.