Can a qubit survive being buried for 250 years?

In honor of the 250th anniversary of America’s founders signing the Declaration of Independence, a capsule filled with letters and memorabilia will be buried at Philadelphia’s Independence Hall on July 4. It is slated to be unearthed and opened again in 2276.

The time capsule is one of many initiatives of the America250 organization, a nonpartisan commission established by Congress in 2016 to plan and organize the US’s semiquincentennial.

“America’s Time Capsule is the result of an extraordinary national effort and a remarkable feat of engineering,” Rosie Rios, chair of America250, writes in an emailed statement to C&EN. “Every state and territory, all three branches of government, students, partners and institutions contributed to this national record.”

All told, the time capsule holds over 200 items. Some are mundane (a glass Coke bottle, numerous handwritten notes) while others are cutting edge (a quantum computer chip, a DNA digital archive). Together, the artifacts will remain buried for two and a half centuries. Meticulously considering the capsule’s materials—inside and out—is key to ensuring the safety of its contents.

A capsule made to withstand centuries

“Water is the number one killer of time capsules,” says Jay Nanninga, a mechanical engineer at the National Institute of Standards and Technology, Gaithersburg (NIST). So when it was decided that a time capsule would be part of the America250 celebration, Nanninga’s team of engineers was tasked with coming up with a design that would withstand 250 years of moisture.

The final cylindrical design has two components. The internal piece is the actual time capsule, where all the artifacts are held. The outer shell is a 415 kg bell jar that creates a barrier of air around the capsule. “If the water level rises around that area, the bell jar will passively keep the water from reaching the time capsule,” Nanninga explains.

Of course, water can also destroy metal. To avoid this as much as possible, Ninninga chose 316-grade stainless steel pipe to make the capsule and bell jar. This steel is particularly resistant to corrosion thanks to its molybdenum content. The pipe is frequently used by the gas industry which made it easy for Nanninga to source; all his team had to do was seal the ends.

Engineers hot-welded most the seams of the capsule and bell jar but needed to take a different approach for the lid. After all, “we didn’t want to burn up everything we just put in the time capsule when we welded the lid on,” Ninninga says. So engineers used an indium wire to cold-weld the lid into place. Indium readily bonds to itself and, under a modest amount of pressure will create an air- and watertight seal between two pieces of metal.

As to how the lid comes off , there was some concern that the people of the future may not have hex wrenches, but Ninninga couldn’t find one made of 316-grade steel. Still, he’s “confident that 250 years in the future, they’ll probably have a better way of taking it apart than I do today,” he says. “That’s my hope.”

Minimizing harmful trapped volatiles

With the water risk mitigated, the designers faced a new challenge: off-gassing from the capsule’s contents.

“This is a unique containment, and it’s not like we can open the door every now and again and let the [volatiles] disappear,” says Fenella France, chief of the preservation research and testing division at the Library of Congress.

Each item had to be vetted to ensure that it wouldn’t significantly degrade itself or its neighbors. Scientists at the library were tasked with advising on what could go into the capsule and evaluating every item sent for inclusion.

“We generally did discourage submissions of plastics,” says Andrew Davis, a chemist in France’s department. “We just can’t have the confidence that they’re going to do well in a sealed environment.” After all, plastics are notorious for releasing reactive compounds.

Instead, participants were encouraged to send items that had a history of lasting—most obviously metals and high-quality paper. “We know that cotton and rag paper from 500 years ago is still around,” France says.

The scientists advised participants to print documents and letters on archival-quality paper, which is acid-free, chemically buffered, and 100% cotton; the deterioration products of paper made from tree pulp, on the other hand, include various acids and aldehydes, which autocatalyze further decay.

In January, the Library received hundreds of time capsule submissions to evaluate. Not everything was suitable for inclusion. For example, something that was inert itself—a commemorative coin, for example—might be “put it in this beautiful varnished wooden box and glued it to a backing that was also heavily dyed,” France says.

“You could smell it,” Davis adds. “I’m like, I know there are VOCs [volatile organic compounds] coming off of that.”

The scientists strongly advised against putting such items in the capsule, but the ultimate decision was made by the America250 commission. The diverse memorabilia include an iPhone, an Olympic gold medal, pieces of Indigenous artwork, and more. Each piece was carefully packaged in acid-free-paper boxes that had been tested and chosen by the library scientists for being nonreactive.

Of the over 200 items packed into the capsule, a few have particularly strong connection to chemistry.

California showcases science and technology

Many of California’s additions to the time capsule celebrate the state’s leadership in innovation and research. Its contributions include a NASA satellite photo of California, a $1 coin featuring Steve Jobs, an artificial intelligence prediction about what the state will be like in 250 years, and part of a fusion superconductor made by General Atomics. There is also a qubit (quantum bit) chip from the University of California, Berkeley (UCB).

When the state started discussing ideas for the time capsule last year, the government relations team at UCB suggested they include a qubit, the basic building block of a quantum computing device.

The university has been actively promoting the importance of quantum research to state lawmakers, says Claire Cramer, the executive director of the Berkeley Quantum hub. She says that when she got an email asking if the center could provide a qubit for the time capsule, “we said ‘OK, of course we can,’ because what else would you say?”

Qubit chips can take weeks to fabricate and are usually made in large batches, but UCB physicist Irfan Siddiqi fortunately happened to have one he was willing to part with.

The superconducting chip, fabricated in Siddiqi’s lab in collaboration with Kan-Heng Lee of the Lawrence Berkeley National Laboratory, is made of aluminum and niobium on silicon. Cramer says the chip is functional in principle but has to be cooled to near absolute zero and connected to the right control hardware to work.

“It’s like if you took your laptop and you yank the motherboard out and said, ‘Here I have a computer,’ ” she says. “It might be really interesting in 250 years if they take it and try to make it work again. I’m sure the device will work. The question is how far will we have progressed?”

Should future researchers want to use the qubit in a quantum computer, the chip’s circuit design is clearly visible under a microscope. “You probably could reverse engineer it,” even if all the information about how it works were lost, Cramer says.

America’s past and present, written in DNA

The Library of Congress contributed visitor notecards and a small vial—about as tallas the diameter of a US quarter coin—filled with synthetic DNA. Rather than a genetic blueprint, however, the DNA encodes digital copies of selected items from the library’s collections.

Over the past couple of years, the library has been exploring novel solutions for storing the 1.25 billion (and counting) items in its digital holdings, says project lead Vincent Coltellino. The semiquincentennial time capsule provided a perfect platform to showcase this work.

The library team assembled 1 gigabyte (GB) of data spanning multiple media types, including a draft of the Declaration of Independence, Pierre L’Enfant’s 1791 city plan for Washington DC, an audio recording of “The Star-Spangled Banner” by John Philip Sousa’s band, a pre-1600 Indigenous American codex, and a 3D rendering of Abraham Lincoln’s right hand. Once the library had chosen its contributions to the capsule, the Molecular Information Systems Lab at University of Washington (UW) encoded, synthesized, and validated the DNA.

Coltellino says his team tried to pick collection items that were a good representation of the library and the celebration, as well as “diverse enough that we could get a good feeling of what is in the realm of possible with the actual processes of DNA data storage.”

There are several reasons DNA would be useful for archival work. The first is longevity. “Nature stores data in DNA for millions of years,” says Robertas Skliaustas of the DNA Data Storage Alliance, which develops standards and best practices for DNA data storage. And there are no electricity or special storage conditions required once the strands have been encoded. DNA is also incredibly information-dense: 1 g can store over 200 petabytes (200 million GB) of data.

The decoding instructions to turn the DNA sequence back into digital files are in an online database as well, on a microchip packaged alongside the DNA vial, and inscribed on the metal container. Coltellino says the library and UW teams thought a lot about how people 250 years from now would be able to retrieve the information. They had to assume that English would still be a valid language and that whoever unearthed the capsule would have access to DNA sequencing technology. Otherwise “there’s three or four layers of redundancy” to ensure that the data is recoverable if, for instance, the online database were to cease to exist, Coltellino says.

Opening the time capsule

In 2276, America’s 500th birthday, if the time capsule buried in 2026 hasn’t been lost or destroyed, it will be dug up. Someone will crack the indium seal and reveal the messages within.

From the capsule’s contents, “you get a slice of life,” says Davis at the Library of Congress.

“That creativity, that imagination, that’s what I hope they get out of it,” Davis says of the future openers of the time capsule. “But first and foremost, I hope there’s something to open.”