By Patricia Daukantas
Can you do quantum mechanics with everyday-sized objects? And could such macroscopic quantum objects help detect the most elusive predictions of Einstein’s theory of general relativity?
Yes and yes, according to Nergis Mavalvala, a physics professor at the Massachusetts Institute of Technology (U.S.A.). She and her colleagues are using lasers for cooling and trapping gram-sized and even kilogram-sized interferometer mirrors--just like optical traps for cooling atoms. In a few years, instruments operating at the standard quantum limit will start hunting for gravitational waves.
Mavalvala, one of two OSA members who won a John D. and Catherine T. MacArthur Foundation “genius grant” in 2010, recently gave a public lecture about her work on the Laser Interferometer Gravitational-Wave Observatory (LIGO) project at the Arlington, Va., headquarters of the U.S. National Science Foundation.
“If you could do astrophysics with a gravitational wave, it’s like turning on a new sense,” Mavalvala said. “You’ve had eyes all along, and suddenly you have ears and you turn on hearing. It’s bound to provide some very different information.”
Albert Einstein’s theory of general relativity predicted the existence of gravity waves traveling at the speed of light. Such waves, if they exist, would stretch and squeeze spacetime transverse to the direction of propagation. The amplitude of a gravitational wave, also known as strain or h, is a dimensionless quantity defined as the change in length per length, similar to tidal forces. In other words, gravitational stretch and squeeze spacetime by fractional amounts proportional to the distance between two objects.
Gravitational waves, if they exist, would have frequencies of 10 kHz or less and would interact only weakly with matter. Einstein was morose over his own calculations when he realized the difficulty of detecting such waves. Two decades after his death, however, radio astronomers provided the first clue to their existence.
In 1974, University of Massachusetts (U.S.A.) researchers Russell Hulse and Joseph Taylor Jr. found a binary pulsar consisting of two neutron stars orbiting each other at 0.15 percent of light speed. Since the pulsars contain more than the mass of the sun packed within a 10-km radius, they have extremely strong gravitational fields.
Taylor and Hulse showed that their orbits are shrinking at exactly the rate that Einstein’s theory would predict for the emission of gravitational waves from the system. Further observations up to the present day continue to confirm Einstein’s theory. The binary pulsar’s energy loss is widely accepted as evidence for gravitational waves, and led to the 1993 Nobel Prize in Physics for Hulse and Taylor, Mavalvala said.
Direct detection of gravitational waves on Earth, however, is incredibly difficult because the strain on an interferometer would be on the order of 10–21. So the two interferometers of the LIGO project, located in Hanford, Wash., and Livingston, La. (U.S.A.), have 4-km-long arms.
Scientists could measure gravitational waves by measuring the phase shifts of light in an interferometer. However, external forces also push the mirrors around much more than a gravitational wave can push them around, and laser light has fluctuations in phase and amplitude, so both sources of noise must be reduced. The LIGO team designed their interferometers with optical cavities in each arm to increase the instruments’ sensitivity to mirror displacement and thus to gravitational waves.
Advanced LIGO: More sensitivity, more issues
The first-generation detectors known as Initial LIGO, which are now being removed, led to much interesting astrophysical research, but no positive detections of gravitational waves (yet). Construction of the next-generation Advanced LIGO detectors, designed to be 10 times more sensitive than their predecessors, began in October.
But with Advanced LIGO, there’s a catch: The detectors will bump up against the standard quantum limit. According to Mavalvala, the team has to get around the quantum limitations by injecting squeezed states of light, with more precise measurements of phase at the expense of knowledge about the amplitude of the light, in order to reduce the quantum shot noise limit.
To deal with the other type of quantum noise--radiation pressure--the Advanced LIGO scientists are using optomechanical coupling to trap and cool macroscopic mirrors down to very low quantum states--the way lasers trap and cool atoms.
Initially Mavalvala and colleagues tested a 1-gram mirror suspended from a pair of specially made glass fibers (designed with fewer impurities and flaws than commercial optical fibers). Starting at room temperature, they shifted the mirror’s oscillator frequency from 10 Hz in the mechanical regime to 500 Hz, and cooled it to just under 1 mK. In other words, the cooled mirror has 35,000 quanta, instead of 109 quanta in its normal state.
“We are not yet in the quantum regime, a factor of 5 or 35,000 quanta is not yet quantum, but it’s getting close,” Mavalvala said.
In the most recent experiment, the team took one of the 2.7-kg mirrors from the Initial LIGO experiment, with a resonant frequency of just under 1 Hz at room temperature (and containing about 1,026 atoms), and shifted its resonance out to about 150 Hz and cooled it town to 1.4 μK--corresponding to only 200 quanta. (When that number equals 1, the kg-scale object will have reached its quantum ground state.)
Advanced LIGO, slated to begin around 2014, will operate at the standard quantum limit. Scientists expect that the project will detect signals that could be gravitational waves several times per year, Mavalvala said.