Entangled Images Point to Better Quantum Data, Optical Measurements

Alexander E. Braun, Senior Editor -- Semiconductor International, 6/16/2008 8:25:00 AM

Researchers at the Joint Quantum Institute (JQI) of the National Institute of Standards and Technology (NIST, Gaithersburg, Md.) and the University of Maryland (College Park) used a convenient and flexible method for creating twin light beams to produce “quantum images,” which are pairs of information-rich visual patterns whose features are “entangled” or inextricably linked by the laws of quantum physics.

Besides promising better detection of faint objects and improved amplification and positioning of light beams, the technique for producing quantum images — unprecedented in its simplicity, versatility and efficiency — might be useful for storing data patterns in quantum computers and transmitting highly secure encrypted information. The research team, led by JQI’s Paul Lett, described its results in a recent edition of Science Express.

“Images have always been a preferred method of communication because they carry so much information in their details,” said Vincent Boyer, lead author of the 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.”

1. Montage of quantum images. Two laser beams coming from the bright glare (top right) transmit images of a cat-like face at two slightly different frequencies (orange and purple colors). The twisted lines indicate that the seemingly random fluctuations that occur in any part of the orange image are interconnected or 'entangled' with the purple image’s corresponding parts. (Source: Vincent Boyer, JQI)

Conventional cameras only record the color and intensity of a lightwave striking a surface. A hologram additionally records the lightwave’s “phase” — the precise locations of its crests and valleys. However, much more takes place in a lightwave. Even the most stable laser beam randomly brightens and dims over time because, as quantum mechanics has shown, light has inherent “uncertainties” or “noise” in its features, manifested as moment-to-moment fluctuations in its properties. Controlling these fluctuations improves detection of faint objects, produces better amplified images, and allows laser beams to be more accurately positioned.

Quantum mechanics provides subtle ways of reducing light’s noise to values lower than physicists ever imagined. It cannot be completely eliminated, but it can be rearranged to improve images’ desired features. “Squeezing,” a quantum-mechanical technique, allows noise to be reduced in one property — such as intensity — at the expense of increasing it in a complementary property, such as phase. This also opens new applications for images, such as transferring large amounts of encrypted data and performing parallel processing of information for quantum computers.

Strikingly, the quantum images produced are born in pairs. Transmitted by two light beams originating from the same point, the 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, and it simultaneously exhibits very similar random fluctuations, even if the two are far apart. They are “entangled” — their properties are linked in such a way that they exist as a unit rather than individually. Moreover, they are squeezed: Matching up both quantum images and subtracting their fluctuations, their noise is lower — and the information content potentially higher — than with any two classical images.

2. A laser beam ('probe') passes through a mask that imprints a visual pattern. Along with a second beam ('pump'), it enters a cell containing a gas or rubidium atoms. Beam and gas interactions produce an amplified version of the image as well as a second version, rotated 180°. The bottom panel shows, left to right, an incoming probe beam imprinted with the letters 'N' and 'T,' an outgoing probe beam with an amplified image, and a reversed version of the letters. The middle image is entangled with the rightmost, and their changes over time are highly related. (Source: Vincent Boyer, JQI)

To create quantum images, the researchers use a method known as “four-wave mixing,” a technique in which incoming lightwaves enter a gas and interact to produce outgoing lightwaves. 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 gas atoms interact with the light, absorbing energy and reemitting an amplified version of the original image. Additionally, 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, rotated 180° with respect to the pump beam and in a slightly different color.

One breakthrough is that each image is made of up to 100 distinct regions, akin to a digital image’s pixels, each with its own independent optical and noise properties. A pixel on one image forms a partnership with its counterpart on the other.

If one observed two unrelated pixels — for example, one in the top row of the first image and a pixel in the top row of the second — they appear to behave randomly. But, being entangled, their random fluctuations over time are eerily similar — it becomes possible to predict many of the properties in the second pixel just by looking at the first.

Previous efforts at making quantum images have been limited to building them with “photon counting” — collecting one photon at a time over a long period or having specialized “images,” such as something that could only be constructed from a dot and ring. In contrast, the new method produces an entire image at one time and can make a wide variety of images in any shape. Moreover, those earlier efforts were difficult to implement — some setups requiring light to bounce back and forth between tightly controlled, precisely spaced mirrors. By contrast, the four-wave mixing approach requires easy-to-prepare laser beams and a small cell of rubidium vapor.

The researchers now plan to produce quantum images with slowed-down light; such images could be used in information storage and processing and communications applications.