Quantum Entangled Images

Paul Lett [photo courtesy: Joint Quantum Institute]

Conventional photographic films or digital camera sensors only record the color and intensity of a light wave striking their surfaces. A hologram additionally records a light wave’s “phase”—the precise locations of the crests and valleys in the wave. However, much more happens in a light wave. Even the most stable laser beam brightens and dims randomly over time because, as quantum mechanics has shown, light has inherent “uncertainties” in its features, manifested as moment-to-moment fluctuations in its properties. Controlling these fluctuations—which represent a sort of “noise”—can improve detection of faint objects, produce better amplified images, and allow workers to more accurately position laser beams.

Quantum mechanics has revealed light’s unavoidable noise, but it also provides subtle ways of reducing it to values lower than physicists once imagined possible. Researchers can’t completely eliminate the noise, but they can rearrange it to improve desired features in images. A quantum-mechanical technique called “squeezing” (read our past postings on squeezed light) lets physicists reduce noise in one property—such as intensity—at the expense of increasing the noise in a complementary property, such as phase. Modern physics not only enables useful noise reduction, but also opens new applications for images—such as transferring heaps of encrypted data protected by the laws of quantum mechanics and performing parallel processing of information for quantum computers.

In a recent communication published in Science Express, a team of researchers led by Paul Lett of the Joint Quantum Institute (JQI) of the Commerce Department’s National Institute of Standards and Technology (NIST) and the University of Maryland reported a convenient and flexible method for creating twin light beams to produce “quantum images,” pairs of information-rich visual patterns whose features are “entangled,” or inextricably linked by the laws of quantum physics. In addition to promising better detection of faint objects and improved amplification and positioning of light beams, the researchers’ technique for producing quantum images—unprecedented in its simplicity, versatility, and efficiency—may someday be useful for storing patterns of data in quantum computers and transmitting large amounts of highly secure encrypted information.

“Images have always been a preferred method of communication because they carry so much information in their details,” says Vincent Boyer, lead author of the new paper. “Up to now, however, cameras and other optical detectors have largely ignored a lot of useful information in images. By taking advantage of the quantum-mechanical aspects of images, we can improve applications ranging from taking pictures of hard-to-see objects to storing data in futuristic quantum computers.”

Perhaps most strikingly, the quantum images produced by these researchers are born in pairs. Transmitted by two light beams originating from the same point, the two images are like twins separated at birth. Look at one quantum image, and it displays random and unpredictable changes over time. Look at the other image, and it exhibits very similar random fluctuations at the same time, even if the two images are far apart and unable to transmit information to one another. They are “entangled”—their properties are linked in such a way that they exist as a unit rather than individually. Together, they are squeezed: Matching up both quantum images and subtracting their fluctuations, their noise is lower—and their information content potentially higher—than it is from any two classical images.

A laser beam (marked as “probe”) first passes through a mask that imprints a visual pattern. Along with a second laser beam (marked “pump”), it enters a cell containing a gas of rubidium atoms. Interactions between the rubidium gas and the beams produce an amplified version of the imprinted image as well as a second version of the image, rotated 180 degrees around the pump. The bottom panel shows, from left to right, an incoming probe beam imprinted with the letters “N” and “T,” an outgoing probe beam with an amplified image, and an upside-down version of the letters. The middle image is “entangled” with the rightmost image; the images’ changes over time are highly related to one another [Credit: Vincent Boyer et al., JQI]

To create quantum images, the researchers use a simple yet powerful method known as “four-wave mixing,” a technique in which incoming light waves enter a gas and interact to produce outgoing light waves. In the setup, a faint “probe” beam passes through a stencil-like “mask” with a visual pattern. Imprinted with an image, the probe beam joins an intense “pump” beam inside a cell of rubidium gas. The atoms of the gas interact with the light, absorbing energy and re-emitting an amplified version of the original image. In addition, a complementary second image is created by the light emitted by the atoms. To satisfy nature’s requirement for the set of outgoing light beams to have the same energy and momentum as the set of incoming light beams, the second image comes out as an inverted, upside-down copy of the first image, rotated by 180 degrees with respect to the pump beam and at a slightly different color.

In this photo montage of actual quantum images, two laser beams coming from the bright glare in the distance transmit images of a cat-like face at two slightly different frequencies (represented by the orange and the purple colors). The twisted lines indicate that the seemingly random changes or fluctuations that occur over time in any part of the orange image are strongly interconnected or “entangled” with the fluctuations of the corresponding part in the purple image. Though false color has been added to the cats’ faces, they are otherwise actual images obtained in the experiment. [Credit: Vincent Boyer/JQI]





Reference
"Entangled Images from Four-Wave Mixing" by V. Boyer, A. Marino, R. Pooser, and P. Lett,
Science Express, 12 June 2008, Abstract Link.

[We thank Media Relations, National Institute of Standards and Technology (NIST) for materials used in this posting. -- 2Physics.com]