News in Science

Uncertainty principle certainly big enough to see

Certainly uncertain Physicists have performed a rare practical demonstration that shows a principle of quantum physics at work on an object large enough to be seen by the naked eye.

The landmark study shows the so-called Heisenberg uncertainty principle at work in an experiment involving laser light bouncing between a pair of mirrors.

The Heisenberg principle is an overarching law of quantum mechanics which sets a limit to the accuracy with which certain pairs of a particle's properties, such as its position and momentum, can be measured. The more accurately you measure one of those properties, the less precisely the other can be known.

Although the principle is often thought of in conceptual terms, the latest study, reported in today's issue of Science, shows how it hindered measurement in a practical way.

The experiment by researchers from the University of Colorado is one of the few approaches that demonstrate the uncertainty principle in action, writes Gerard Milburn from the University of Queensland (UQ) in an accompanying editorial in Science.

The researchers set up an experimental system with two small mirrors facing each other in a chamber. The mirrors were carefully positioned so that a beam of laser light scientists shone into the chamber bounced between the mirrors at a resonant frequency.

Between the mirrors, the scientists positioned a thin refractive membrane attached to the apparatus in a way that allowed it to move back and forth. They then cooled the set-up to far below 0°C, to minimise the amount of natural quantum vibration within the membrane.

As they sent the laser bouncing between the mirrors, some photons were reflected by the membrane, while others passed through it. Each time the membrane reflected a photon, the membrane vibrated, effectively changing the resonant frequency of the light bouncing between the mirrors.

"When you play a guitar, you can put a little piece of steel on the strings, and moving the steel up and down changes the length of the string, and that changes the frequency.That's what this moving membrane is doing," explains Milburn, from UQ's Centre for Engineered Quantum Systems.

Whether individual photons passed through the membrane without disturbing it, or "bounced" off its surface and made it move was a random occurrence, creating an effect known as radiation pressure shot noise.

Measuring uncertainty

The next element in the experimental set up was a detector placed outside the pair of mirrors, which could detect photons that passed through the mirrors. In theory, scientists could measure the movement of the membrane by detecting changes in the small amount of light hitting the sensors.

But when the level of light in the chamber was low, the number of photons that passed through the mirrors to the detectors was also random.

"If the motion of the membrane is pretty small, those ordinary fluctuations in intensity just coming from the granular nature of light will mask the movement of the mirror," explains Milburn.

"The way to fix that is to turn the intensity of the laser up higher and higher, so the photon flux is enormous and the tiny changes in the membrane position can be seen above the random shot noise on the detector."

But turning up the intensity of the laser created another limit. The 'kicks' that photons were giving to the membrane inside became too large, again losing accuracy in the measurement.

"Balancing those two demands is precisely what leads to the Heisenberg uncertainty principle in this experiment," says Milburn.

"In trying to monitor the position of this mirror as carefully as they possibly can, they have to keep cranking up the power so they get really good signal to noise ratio. But then, as the mirror vibrates, the intensity changes by a lot, and the net effect is that this random force gets bigger."

In other words, trying to measure one property of the photons more precisely limited the scientists' ability to measure the other property.

"This new optomechanical technology allows them to get right down to the quantum ground state of system where they are at the Heisenberg limit at the very beginning, so they can see this trade-off happening right away," says Milburn.

"We often get told that quantum mechanics applies to atoms and doesn't apply to everything else," he notes. "The nice thing here is it's just a vibrating membrane changing the apparent length of a cavity and it's big you know, it's not an atom."