A Picture Is Worth 4.6 Terabits

A serendipitous discovery

Around 2000, Clark and Solomon, along with a new colleague, Eric Rosenthal, began working as independent consultants to NASA and the U.S. Department of Defense. Rosenthal had been a vice president of research and development at Walt Disney Imagineering and general manager of audiovisual systems engineering at ABC television prior to joining forces with Clark and Solomon.

While working on one DARPA-funded project, Solomon stumbled upon a page in a century-old optics textbook that caught his eye. It described a method developed by noted physicist Gabriel Lippmann for producing color photographs. Instead of using film or dyes, Lippmann created photos by using a glass plate coated with a specially formulated silver halide emulsion.

When exposed to a bright, sunlit scene, the full spectrum of light reflected off a mercury-based mirror coating on the back of the glass. It created standing waves inside the emulsion layer of the colors detected. The silver grains in the brightest parts of the standing wave became oxidized, as if remembering the precise colors they saw. (It was in stark contrast to traditional color photographs and television, which store only red, green, and blue parts of the spectrum.) Then, chemical processing turned the oxidized silver halide grains black, leaving the light waves imprinted in the medium in a way that is nearly impossible to tamper with. Lippmann received the 1908 Nobel Prize in Physics for his work.

Lippmann’s photography technique did not garner commercial success, because there was no practical way to duplicate the images or print them. And at the time, the emulsions needed the light to be extremely bright to be properly imprinted in the medium.

Nevertheless, Solomon was impressed with the durability of the resulting image. He explained the process to his colleagues, who recognized the possibility of using the technique to store information for archival purposes. Johnson saw Lippmann’s old photographs at the Museum for Photography, in Lausanne, Switzerland, where he noticed that the colors appeared clear and intense despite being more than a century old.

The silver halide method stuck with Solomon, and in 2013 he and Johnson returned to Lippmann’s emulsion photography technique.

“We got to talking about how we could take all this information we knew about color and use it for something,” Johnson says.

Data in space and on land

While Rosenthal was visiting the International Space Station headquarters in Montgomery, Ala., in 2013, a top scientist said, “‘The data stored on the station gets erased every 24 hours by cosmic rays,’” Rosenthal recalls. “‘And we have to keep rewriting the data over and over and over again.’” Cosmic rays and solar flares can damage electronic components, causing errors or outright erasures on hard disks and other traditional data storage systems.

Rosenthal, Johnson, and Solomon knew that properly processed silver halide photographs would be immune to such hazards, including electromagnetic pulses from nuclear explosions. The team examined Lippmann’s photographic emulsion anew.

Solomon’s son, Brian Solomon, a professional photographer and a specialist in making photographic emulsions, also was concerned about the durability of conventional dye-based color photographs, which tend to start fading after a few decades.

The team came up with an intriguing idea: Given how durable Lippmann’s photographs appeared to be, what if they could use a similar technique—not for making analog images but for storing digital data? Thus began their newest engineering endeavor: changing how archival data—data that doesn’t need to be overwritten but simply preserved and read occasionally—is stored.

Conventionally stored data sometimes is protected by making multiple copies or continuously rewriting it, Johnson says. The techniques require energy, though, and can be labor-intensive.

The amount of data that needs to be stored on land is also growing by leaps and bounds. The market for data centers and other artificial intelligence infrastructure is growing at an annual rate of 44 percent, according to Data Bridge Market Research. Commonly used hard drives and solid-state drives consume some power, even when they are not in use. The drives’ standby power consumption varies between 0.05 and 2.5 watts per drive. And data centers contain an enormous number of drives requiring tremendous amounts of electricity to keep running.

Johnson estimates that about 25 percent of the data held in today’s data centers is archival in nature, meaning it will not need to be overwritten.

The ‘write once, read forever’ technology

The technology Johnson, Solomon, and their collaborators have developed promises to overcome the energy requirements and vulnerabilities of traditional data storage for archival applications.

The design builds off of Lippmann’s idea. Instead of taking an analog photograph, the team divided the medium into pixels. With the help of emulsion specialist Yves Gentet, they worked to improve Lippmann’s emulsion chemistry, making it more sensitive and capable of storing multiple wavelengths at each pixel location. The final emulsion is a combination of silver halide and extremely hardened gelatin. Their technique now can store up to four distinct narrow-band, superimposed colors in each pixel.

The standing wave storage technique can store up to four colors out of a possible 32 at each pixel location. This adds up to an astounding storage capacity of 4.6 terabits (or roughly 300 movies) in the area of a single photograph. Wave Domain

“The textbooks say that’s impossible,” Solomon says, “but we did it, so the textbooks are wrong.”

For each pixel, they can choose four colors out of a possible 32 to store.

That amounts to more than 40,000 possibilities. Thus, the technique can store more than 40,000 bits (although the format need not be binary) in each 10-square-micrometer pixel, or 4.6 terabits in a 10.16 centimeter by 12.7 cm modified Lippmann plate. That’s more than 300 movies’ worth of data stored in a single picture.

To write on the SWS medium, the plate—coated with a thin layer of the specially formulated emulsion—is exposed to light from an array of powerful color LEDs.

That way, the entire plate is written simultaneously, greatly reducing the writing time per pixel.

The plate then gets developed through a chemical process that blackens the exposed silver grains, memorizing the waves of color it was exposed to.

Finally, a small charged-couplet-device camera array, like those used in cellphones, reads out the information. The readout occurs for the entire plate at once, so the readout rate, like the writing rate, is fast.

“The data that we read is coming off the plate at such a high bandwidth,” Solomon says. “There is no computer on the planet that can absorb it without some buffering.”

The entire memory cell is a sandwich of the LED array, the photosensitive plate, and the CCD. All the elements use off-the-shelf parts.

“We took a long time to figure out how to make this in a very inexpensive, reproducible, quick way,” Johnson says. “The idea is to use readily available parts.” The entire storage medium, along with its read/write infrastructure, is relatively inexpensive and portable.

To test the durability of their storage method, the team sent their collaborators at NASA some 150 samples of their SWS devices to be hung by astronauts outside the International Space Station for nine months in 2019. They then tested the integrity of the stored data after the SWS plates were returned from space, compared with another 150 plates stored in Rosenthal’s lab on the ground.

“There was absolutely zero degradation from nine months of exposure to cosmic rays,” Solomon says. Meanwhile, the plates on Rosenthal’s desk were crawling with bacteria, while the ISS plates were sterile. Silver is a known bactericide, though, so the colors were immune, Solomon says.

Their most recent patent, granted earlier this year, describes a method of storing data that requires no power to maintain when not actively reading or writing data. Team members say the technique is incorruptible: It is immune to moisture, solar flares, cosmic rays, and other kinds of radiation. So, they argue, it can be used both in space and on land as a durable, low-cost archival data solution.

Passing on the torch

The new invention has massive potential applications. In addition to data centers and space applications, Johnson says, scientific enterprises such as the Rubin Observatory being built in Chile, will produce massive amounts of archival data that could benefit from SWS technology.

“It’s all reference data, and it’s an extraordinary amount of data that’s being generated every week that needs to be kept forever,” Johnson says.

Johnson says, however, that he and his team will not be the ones to bring the technology to market: “I’m 94 years old, and my two partners are in their 70s and 80s. We’re not about to start a company.”

He is ready to pass on the torch. The team is seeking a new chief executive to head up Wave Domain, which they hope will continue the development of SWS and bring it to mass adoption.

Johnson says he has learned that people rarely know which new technologies will eventually have the most impact. Perhaps, though few people are aware of it now, storing big data using old photographic technology will become an unexpected success.