Moved by Light

Welcome to Quantumville. Population: uncertain. Walk down Main Street, lined with blurry cars simultaneously moving and remaining still. See the house with the curtains drawn? The television in the living room is both on and off at the same time. In this neighborhood, everyday objects do seemingly contradictory things.

You won’t drive through this far-fetched town anytime soon, but it’s not as far off the map as it used to be. In laboratories across the world, bits of metal and glass are being groomed to behave in ways that defy common sense. Objects big enough to be seen and touched — some weighing kilograms — are beginning to rebel against the physical laws that govern daily experience.

At the forefront of this effort is a growing discipline called optomechanics. Its practitioners use beams of light to do something utterly unfeasible a decade ago: make large objects colder than they would be in the void of outer space. Only at these temperatures do objects reach energies low enough to enter the realm of quantum mechanics and start behaving like subatomic particles.

“Our guiding principle is to see quantum effects in a macroscopic object,” says physicist Ray Simmonds of the National Institute of Standards and Technology in Boulder, Colo.

A number of optomechanics teams have sprung up in recent years, each cooling its own favorite bit of fairly ordinary stuff. Simmonds works with an aluminum drum (unveiled in the March 10 Nature). In Switzerland, scientists chill silica doughnuts. At Yale University, saillike membranes are the vogue.

“We’re putting the mechanics back in quantum mechanics,” says Yale physicist Jack Harris.

It’s mainly a race of tortoises creeping steadily closer to absolute zero, the coldest of the cold. But recently an interloper hare took a shortcut to the lead. And the stakes are high: The winners will test whether quantum mechanics holds at ever-larger scales and may go on to build a new generation of mechanical devices useful in quantum computing.

Cooling touch

Spend an afternoon watching sunbathers burn at the beach, and the idea of using light to refrigerate may seem counterintuitive. But light particles have a hidden cooling ability that comes from the tiny nudge they impart when bouncing off an object. This force, too weak for a beachgoer to feel, is so feeble that sunlight reflecting off a square-meter mirror delivers a pressure less than a thousandth of the weight of a small paper clip.

“It’s an incredibly tiny effect,” says physicist Steve Girvin, also of Yale.

In the 1970s scientists figured out how to use this “radiation pressure” to cool individual atoms by damping their vibrations with lasers. Now a slew of new devices leverage the punch of light and other forms of electromagnetic energy to cool objects made of trillions of atoms or more. This scaled-up cooling doesn’t suppress the vibrations of individual atoms. Instead, it quiets the inherent wobbling of an entire object, like a foot pressed to a flopping diving board.

Putting light’s cooling power to work starts with a laser beam bouncing between two mirrors. The distance between the mirrors in this “optical cavity” determines the frequency of light that will resonate — just as the length of a guitar string determines its pitch. Keep the mirrors still and properly tuned light will bounce back and forth, as constant as a metronome.

But allow one of these mirrors to wobble, and a more intricate and subtle interplay emerges. A laser beam tuned below the resonance frequency of the cavity will push against the swaying mirror and snatch away energy. By stealing vibrational energy from the mirror, the bouncing light gets a boost up to the optical cavity’s stable frequency. Robbed of energy, the mirror’s swaying weakens, and it cools.

By measuring the light leaking out of this type of system, two groups of physicists showed in 2006 that they could cool mirrors to 10 kelvins (10 degrees Celsius above absolute zero). A third used a similar technique to cool a glass doughnut to 11 kelvins, colder than the object would be if it were wobbling on the dark side of the moon.

“This demonstration that you could use laser radiation to cool a mechanical object, this started the race,” says Tobias Kippenberg, leader of the doughnut team at the Swiss Federal Institute of Technology in Lausanne. “Every year we improve our cooling by a factor of 10.”

As papers flowed in and objects neared the bottom of the thermometer, researchers competed to suck out every last drop of energy. The goal: to reach the ground state, where an object no longer possesses any packets, or quanta, of vibrational energy. In this state, motion almost completely stops and the quantum regime begins to become a reality.

But getting those last few quanta out would be a challenging task; even the mirrors at 10 kelvins still contained tens of thousands to hundreds of thousands of quanta.

Better lasers and equipment refinements allowed three groups, publishing in Nature Physics in 2009, to reach 63, 37 and 30 quanta. Keith Schwab of Caltech bombarded a wobbling object with microwaves that drained away all but about four quanta. He and his colleagues reported in Nature in 2010 that they had put their object into its ground state 21 percent of the time — tantalizingly close to the consistency needed to test for quantum effects.

Then in April 2010, a shot rang out. An object had been spotted entering its ground state over and over again — by an outsider who wasn’t even using light.

“I wanted to get to the ground state in the quickest and most efficient way possible and have there be no question that I was there,” says Andrew Cleland, a physicist at the University of California, Santa Barbara, who reported his team’s achievement in Nature (SN: 4/10/10, p. 10).

Cleland’s secret: While other scientists built stuff that shook thousands or millions of times a second, he created a ceramic wafer 30 micro-meters long that expanded and contracted 6 billion times per second. The faster an object’s natural quiver, the easier it is to remove energy, meaning less cooling needed to reach the ground state. Using a state-of-the-art liquid-helium refrigerator capable of achieving millikelvin temperatures, Cleland’s team put the wafer in its ground state 93 percent of the time.

By measuring the electric fields produced by this object, Cleland and his colleagues showed that they could nudge the wafer into a state of superposition — both moving and still at the same time.

“There can be no doubt that we achieved superposition,” Cleland says. This first demonstration of quantum effects in a fairly ordinary object was named the 2010 Breakthrough of the Year by Science.

But Cleland’s sprint to the front of the pack has some long-term disadvantages. His technique is blind to the actual position of a fluctuating object, for one thing, and thus he can’t spot one of the consequences of quantum mechanics: zero-point energy, which gives an object residual motion even in its ground state. Experimentalists using optomechanics hope to detect this motion and verify that it is proportional to how fast an object normally wobbles.

Back in front

Girding themselves for the long haul, optomechanics teams have now begun to catch up to Cleland’s hare strategy. On March 21 in Dallas at the American Physical Society meeting, members of the NIST team presented data showing that their drumlike membrane had reached the ground state about 60 percent of the time.

The aluminum skin of this drum — in technical terms, a resonator — moves up and down much more slowly than Cleland’s object, vibrating less than 11 million times per second. Reaching the ground state at this slower wobble couldn’t be done with Cleland’s refrigerator; it required the cooling nudge of microwaves.

The payoff for going the extra mile: time. The slower an object wobbles, the longer it tends to stay in its ground state. For Cleland, the ground state lifetime was about 6 nano-seconds. “The difference with our system, our resonator, is that it has a very long lifetime, about 100 microseconds,” says Simmonds. “That’s the key element that sets it apart.”

With the results unpublished, the team won’t say whether any quantum effects have been seen. But the stability could give the researchers an advantage for using optomechanical devices to store and relay information.

A “killer app,” some say, would be playing interpreter between different wavelengths of light or other electro-magnetic energy. A resonator in its ground state could theoretically be designed to absorb photons of just about any kind of light, stored as packets of vibrational energy.

Cool the resonator back to its ground state, and it could release this energy as light of a different wavelength. So gigahertz microwave energy that sets a stick to wobbling could be reemitted at optical frequencies hundreds of thousands of times higher, for instance. Such devices could bridge quantum computing systems that use different frequencies of light to transmit bits of information.

At Caltech, applied physicist Oskar Painter is taking steps toward realizing this light-to-light conversion at higher temperatures. He designs nanometer-scale optomechanical crystals that convert higher-frequency light to lower-frequency vibrations. A zipperlike object described in 2009 in Nature, for instance, could one day be useful for converting optical light into microwaves.

Optomechanical techniques, such as those used by Painter, could also shave the sensitivities of force detectors. At Yale, engineer Hong Tang develops sensors out of light-cooled resonators that promise unprecedentedly low levels of background noise.

“We want to make better accelerometers and better inertia sensors,” Tang says. These devices, similar to those that sense the motion of a Wii controller, could measure tiny changes in movement and direction.

Like many other optomechanics researchers, Painter and Tang receive funding from the Defense Advanced Research Projects Agency. DARPA hopes to use laser-cooled sensors to improve the ability of vehicles to navigate underwater, says DARPA program manager Jamil Abo-Shaeer. “We want to push these things to the limits of quantum mechanics, the ultimate limit,” he says.

While DARPA funds the development of devices that can’t even be seen without a microscope, other scientists are putting optomechanics to work cooling some of the largest detectors in the world: the gravitational wave detectors of the LIGO project, built to search for gentle ripples in space-time thought to be produced by (among other cosmic events) colliding black holes.

Chasing ever greater sensitivities, these researchers use lasers to still the vibrations of their detectors’ giant mirrors — the behemoths of the optomechanical world, weighing in at more than 10 kilograms. Despite their immense size, these mirrors have now been cooled to 234 quanta, MIT quantum physicist Nergis Mavalvala and LIGO colleagues reported in 2009 in the New Journal of Physics. “Our challenges are really the same as everyone else’s, but we need to somehow cool our gram and kilogram-sized objects to nanokelvins,” says Mavalvala.

Working on another gravitational wave detector called AURIGA, researchers in Italy set the record for largest object effectively cooled via optomechanics. An aluminum bar weighing more than 1 ton reached a mere 4,000 quanta, the team reported in Physical Review Letters in 2008.

Whether such large mirrors and bars could ever demonstrate quantum effects, though, is an open question. In principle, some physicists say, quantum mechanics should hold for objects of any size. “We don’t know of any fundamental limit,” Harris says.

Practical considerations may ultimately limit the size of quantum objects, though. Any observation, be it by a pair of eyes or a stray, colliding air molecule, can destroy a quantum state. The larger an object is, the harder it is to keep isolated. But that isn’t stopping researchers with bigger objects from lining up behind Cleland and the NIST team to stretch the bounds on quantum effects.

“If we can prove that quantum mechanics holds for larger and larger objects, that would be quite spectacular,” says Dirk Bouwmeester of UC Santa Barbara. “But it would also be spectacular if we can prove that it doesn’t. New theories would be needed.”

One of the slowest tortoises in the race, Bouwmeester’s pace is deliberate. His mirrors, tens of micrometers across, vibrate a mere 10,000 or so times per second and promise an extended quantum lifetime. This durability, he says, is needed to test a controversial idea that gravity and quantum weirdness can’t coexist for long at everyday scales.

More than three-quarters of a century of research has made scientists more comfortable with quantum mechanics at small scales, but supersizing it can seem as bizarre today as it did to Erwin Schrödinger. In 1935, he poked fun at the idea in his famous thought experiment: a cat in a box that could be both alive and dead at the same time, as long as no one peeked inside the box and forced a choice, killing with curiosity.

Perhaps it is still too much to imagine Schrödinger’s cat behind the drawn curtains of Quantumville’s homes, simultaneously nibbling Purina in three different rooms at once. But as researchers continue to cool knickknack after knickknack in their optomechanical grab bag, they may catch at least a faint echo of a meow.

---

Early pushes
Interest in the pressure exerted by light goes back centuries.

From left: Ragesoss/Wikimedia Commons; Loveless/Wikimedia Commons; Haade/Wikimedia Commons

1619 Johannes Kepler suggests that the pressure of sunlight explains why comets’ tails (above) always appear to point away from the sun.

1746 Leonhard Euler shows theoretically that the motion of a longitudinal wave might produce pressure in the direction it is propagating.

1873 James Clerk Maxwell (above) uses electromagnetic theory to show that light reflecting off a surface or absorbing into it would create pressure. Bright sunlight, he calculates, would press on the Earth with a force of about 4 pounds per square mile.

1873 That same year, Sir William Crookes invents the radiometer, or light mill (above), incorrectly suggesting that the mill spins because of the pressure of light. Scientists now understand that the heat transferred by light is responsible for the mill’s spinning.

1876 Adolfo Bartoli, unaware of Maxwell’s work, infers radiation pressure’s existence from the second law of thermodynamics.

1900 Russian physicist Pyotr Lebedev announces at a meeting in Paris that he had measured the pressure of light on a solid body.

1903 Ernest Fox Nichols and Gordon Ferrie Hull measure the pressure to an accuracy within less than 1 percent, publishing the work in the Astrophysical Journal.