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  • Atom Optics Comes of Age

Photonics.com
Oct 2012
GREENBELT, Md., Oct. 19, 2012 — NASA is funding atom interferometry, a variation on the 200-year-old optical technique, in the belief that the emerging, highly precise measurement technology will allow the detection of ripples in space-time and other events.

A team at NASA's Goddard Space Flight Center and at California-based Stanford University and AOSense Inc. recently won funding under the NASA Innovative Advanced Concepts (NIAC) program to advance atom optics, which some believe to be a technological panacea for everything from measuring gravitational waves to steering submarines and airplanes.

"I've been following this technology for a decade," said Bernie Seery, a Goddard executive who was instrumental in establishing Goddard's strategic partnership with Stanford and AOSense two years ago. "The technology has come of age, and I'm delighted NASA has chosen this effort for a NIAC award. With this funding and other support, we can move ahead more quickly now."

He said the US military has invested heavily in the technology to dramatically improve navigation. "It opens up a wealth of possibilities."

So far, the researchers have concentrated on using seed funding from Goddard and NASA to develop advance sensors that could find theoretically predicted, but never directly detected, gravitional waves.

Einstein predicted gravity waves in his general theory of relativity, but to date, these ripples in the fabric of space-time have never been observed.
Einstein predicted gravity waves in his general theory of relativity, but to date, these ripples in the fabric of space-time have never been observed. Now, a scientific research technique called atom interferometry is trying to rewrite the canon. In conjunction with researchers at Stanford University, scientists at NASA Goddard are developing a system to measure the faint gravitational vibrations generated by movement of massive objects in the universe. The scientific payoff could be important, helping to better clarify key issues in our understanding of cosmology. But application payoff could be substantial, too, with the potential to develop profound advances in fields such as geolocation and timekeeping. In this video, we examine how the system would work and the scientific underpinnings of the research effort. Courtesy of NASA/Goddard Space Flight Center.

Predicted by Albert Einstein's general theory of relativity, gravitational waves occur when massive celestial objects move and disrupt the fabric of space-time around them. By the time these waves reach Earth, they are so weak that the planet expands and contracts less than an atom in response. This makes their detection with ground-based equipment more challenging because environmental noise, as with ocean tides and earthquakes, can easily swamp their faint murmurings.

To date, no instrument or observatory, including the ground-based Laser Interferometer Gravitational-Wave Observatory in Livingston, La., has ever directly detected them.

The discovery of gravitational waves would revolutionize astrophysics, giving scientists a new tool for studying everything from black holes to the early universe, they say, and they believe atom interferometry holds the key.

Atom interferometry obtains its highly accurate measurements just as optical interferometry does, by comparing light that has been split in two by a beamsplitter. One beam reflects off a mirror that is fixed in place and hits a camera or detector. The other shines through something scientists want to measure, then reflects off a second mirror, back through the beamsplitter and onto a camera or detector. The resulting interference pattern from the two beams is then inspected to obtain highly precise measurements.

Atom interferometry involves quantum mechanics. Just as light acts as both waves and photon particles, atoms can be coaxed into acting like waves if cooled to near absolute zero. Once the atom's velocity is slowed to nearly zero by firing a laser at it, another series of laser pulses aimed at the cooled atoms puts them into a superposition of states. In this state, the atoms have different momenta, permitting them to separate spatially and be manipulated to fly along different trajectories. Eventually, they cross paths and recombine at the detector — just as in a conventional interferometer.

"Atoms have a way of being in two places at once, making it analogous to light interferometry," said Mark Kasevich, a Stanford professor and team member credited with pushing the frontiers of atom optics.

The power of atom interferometry is its precision: If the path an atom takes varies by even a picometer, an atom interferometer would be able to detect the difference.

Goddard physicist Babak Saif, along with researchers from Stanford University and AOSense Inc., has received NASA funding to advance a potentially revolutionary technology — atom optics — to detect theoretically predicted gravitational waves.
Goddard physicist Babak Saif, along with researchers from Stanford University and AOSense Inc., has received NASA funding to advance a potentially revolutionary technology — atom optics — to detect theoretically predicted gravitational waves. Courtesy of NASA/Pat Izzo.

Given its atomic-level precision, "gravitational-wave detection is arguably the most compelling scientific application for this technology in space," said physicist Babak Saif, who is leading the effort at Goddard.

So far, the team has designed a powerful, narrowband fiber optic laser system that it plans to test at one of the world's largest atom interferometers — a 33-ft drop tower in the basement of a Stanford physics lab. Close scientifically to what the team would need to detect theoretical gravitational waves, the technology would be used as the foundation for any atom-based instrument created to fly in space, Saif said.

During the test, the team will insert a cloud of neutral rubidium atoms inside the tower. As gravity asserts a pull on the cloud and the atoms begin to fall, the new laser system will be used to fire pulses of light to cool them. Once in the wavelike state, the atoms will encounter another round of laser pulses that allow them to separate spatially. Their trajectories then can be manipulated so that their paths cross at the detector, creating the interference pattern.

The team also is fine-tuning a gravitational-wave mission concept it has formulated. Similar to the Laser Interferometer Space Antenna (LISA), the concept calls for three identically equipped spacecraft placed in a triangle-shaped configuration. Unlike LISA, however, the spacecraft would come equipped with atom interferometers and would orbit much closer to one another — between 500 and 5000 km apart, compared with LISA's 5 million km separation. Should a gravitational wave roll past, the interferometers would be able to sense the minuscule movement.

"I believe this technology will eventually work in space," Kasevich said. "But it presents a really complicated systems challenge that goes beyond our expertise. We really want to fly in space, but how do you fit this technology onto a satellite? Having something work in space is different than the measurements we take on Earth."

For more information, visit: www.nasa.gov 


GLOSSARY
atom optics
The area of optics in which the wave nature of a particle is exploited to carry out very accurate interferometry and other optical techniques with atoms instead of photons. In this process, particles are cooled via laser cooling to a temperature at which the energy is reduced, such that the particle reaches the size of its de Broglie wavelength.  At this point one can then observe the wave nature of the particle by observing its fringes, and thus carry out photon-based experiments and...
beamsplitter
An optical device for dividing a beam into two or more separate beams. A simple beamsplitter may be a very thin sheet of glass inserted in the beam at an angle to divert a portion of the beam in a different direction. A more sophisticated type consists of two right-angle prisms cemented together at their hypotenuse faces. The cemented face of one prism is coated, before cementing, with a metallic or dielectric layer having the desired reflecting properties, both in the percentage of reflection...
camera
A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector. The camera generally contains a lens of variable aperture and a shutter of variable speed to precisely control the exposure. In an electronic imaging system, the camera does not use chemical means to store the image, but takes advantage of the sensitivity of various detectors to different bands of the electromagnetic spectrum. These sensors are transducers...
detector
1. A device designed to convert the energy of incident radiation into another form for the determination of the presence of the radiation. The device may function by electrical, photographic or visual means. 2. A device that provides an electric output that is a useful measure of the radiation that is incident on the device.
interferometer
An instrument that employs the interference of lightwaves to measure the accuracy of optical surfaces; it can measure a length in terms of the length of a wave of light by using interference phenomena based on the wave characteristics of light. Interferometers are used extensively for testing optical elements during manufacture. Typical designs include the Michelson, Twyman-Green and Fizeau interferometers. The basic interferometer components are a light source, a beamsplitter, a reference...
laser interferometer
An interferometer that uses a laser as its light source. The purely monochromatic nature of the laser results in improved efficiency and overall performance.
quantum mechanics
The science of all complex elements of atomic and molecular spectra, and the interaction of radiation and matter.
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