An international team of scientists—including researchers from the Lawrence Berkeley National Laboratory—has succeeded in storing quantum data in an atomic nucleus for nearly two seconds, then retrieving it for processing.
Although two seconds is a short period, it is thousands of times longer than reported in previous studies and marks a milestone in quantum computing.
Researchers have calculated that if data could be stored in a quantum system for at least one second, error correction could protect the data indefinitely.
As techniques for creating the specialized storage environment improve, retention times could be extended, according to Joel Ager of the Berkeley Lab team.
“The good news is that there are no known physical limits that would prevent quantum memory time in nuclear spin from being longer,” Ager said. “With even greater isotopic and chemical purity of our silicon crystals, we should be able to store data in the nucleus for an arbitrarily long period of time, maybe even in terms of years.”
The achievement was reported in this week’s issue of the journal Nature.
The team also included researchers from Princeton University in the United States and Oxford University in the United Kingdom. The work was funded in part by the National Security Agency, the National Science Foundation and the Energy Department, the parent agency of the Berkeley Lab.
Quantum mechanics offers the possibility of computing power and speed billions of times greater than possible with traditional computers, in which data is stored and processed in digital bits, represented by either a 1 or 0. Quantum bits, or qubits, encode the data in property of subatomic particles called spin, which can be either up or down.
What makes quantum computing potentially so powerful is that a qubit can exist in both states at once. In traditional data, a byte made up of three bits can represent only one of eight possible combinations of 1s and 0s. But a qubyte can represent all eight at once, and operations can be performed simultaneously on all eight.
The challenge in practical quantum computing is to isolate the qubit from the noisy environment and protect it so it can be measured and manipulated. The spin of electrons has proven well-suited to processing quantum data, but is too fragile for memory because data quickly becomes corrupted by other electrons. To overcome this, the researchers moved the data into the nucleus of the atom, which is quieter and more protected than the electron cloud.
The team used phosphorous atoms embedded in specially developed crystals of exceptionally pure silicon-28. This was important, because natural silicon crystals contain 4.7 percent of the isotope silicon-29, which has a nuclear spin that would interfere with the readout of data from the phosphorous, said Berkeley Lab’s Eugene Haller, an authority on crystal growth and purification. The silicon crystals were grown at the Berkeley Lab, where Ager designed and built a one-of-a-kind reactor for the process.
“With crystals painstakingly grown by the Berkeley team and very careful measurements, we were delighted to see memory times exceeding the threshold” of one second, said Steve Lyon, leader of the Princeton team.
The scientists established a state in the electron spin of the phosphorous atom and transferred it to nuclear spin using a combination of microwave and radio frequency pulses. That data was stored in the nucleus spin for 1.75 seconds and then transferred back to the electron spin with about 90 percent accuracy.
“The electron acts as a middle man between the nucleus and the outside world,” said John Morton, research fellow at Oxford’s St. John College and lead author of the article. “It gives us a way to have our cake and eat it—fast processing speeds from the electron and long memory times from the nucleus.”
Future steps in quantum computing will require improving spin control and readout mechanisms, and testing the limits of memory time for nuclear storage.