Scientists
Store and Retrieve Data Inside an Atom

In a paper entitled:
“Solid-state quantum memory using the 31P nuclear spin,”
published in the October 23 issue of the journal Nature,
the team described an experiment in which exceptionally pure and
isotopically controlled crystals of silicon were precisely doped
with phosphorus atoms. Quantum information was processed in
phosphorus electrons, transferred to phosphorus nuclei, then
subsequently transferred back to the electrons. This is the first
demonstration that a single atomic nucleus can serve as quantum
computational memory.

John Morton of Oxford
University was the lead author. Co-authoring the paper from
Berkeley Lab were Thomas Schenkel, Eugene Haller and Joel Ager.
Other co-authors were Richard Brown, Brendon Lovett and Arzhang
Ardavan of Oxford University, and Alexei Tyryshkin, Shyam Shankar
and Stephen Lyon, of Princeton, University.

The immediate lure of quantum
computing is blinding speed: a quantum computer would be able to
perform certain mathematical tasks, such as factoring, many
billions of times faster than the most powerful supercomputers of
today. Beyond that, quantum computing should make it possible to
engage calculations that cannot be considered with current
“classical” computing technology. The secret behind
quantum computing is the weird, counterintuitive but demonstrably
real properties of quantum mechanics.

In classical computing,
information is processed and stored based on the charge of an
electron, and represented in a binary digit or “bit.”
Each bit carries a value of 0 (no charge) or 1 (charge). Quantum
computing utilizes an intrinsic quantum property called “spin,”
in which certain particles can act as if they were tiny bar
magnets. Spin is assigned a directional state of either “up”
or “down,” which can be used to encode data in 0s and
1s. However, unlike charge in classical computing, which is
either present or not, spin can be up, down or both, thanks to a
quantum effect called “superposition.”

Superpositioning exponentially
expands the storage capabilities of a quantum data bit or
“qubit.” Whereas a byte of classical data, made up of
three bits, can represent only one of the eight possible
combinations of 0s and 1s, a quantum equivalent (sometimes called
a qubyte) can represent all eight combinations at once.
Furthermore, thanks to another quantum property called
“entanglement,” operations on all eight combinations
can be performed simultaneously.

Of the many challenges facing
quantum computing, one of the biggest has been finding a way to
preserve the integrity of data while it is stored. Although the
spin of electrons has proven well-suited for data processing, it
is too fragile to be used as memory – the data quickly
becomes corrupted by the influence of other electrons. To
overcome this obstacle, the co-authors of this experiment turned
to the more protected environs of the atomic nucleus.

“In this exciting
collaboration with colleagues from Oxford and Princeton, we have
reported on a very important demonstration of coherent
information transfer between the electron spin (processing qubit)
and the nuclear spin (memory qubit) of phosphorus atoms in
isotopically enriched silicon crystals,” said co-author
Schenkel, a physicist in Berkeley Lab’s Accelerator and
Fusion Research Division, who has been a leader in the use of ion
beams for the development of quantum computer test structures.

“The electron spin
information was faithfully stored in the nuclear spin for nearly
two seconds (thousands of times longer than ever reported for
similar studies), then transferred back to the electron spin with
about 90-percent fidelity,” Schenkel said.

In this study, the co-authors
created a superposition state in electron spin and transferred it
to nuclear spin using a combination of microwave and
radio-frequency pulses, which they applied to phosphorus-31. This
stable isotope of phosphorus is the ideal electron donor for
silicon-28, the stable isotope of silicon that is the basis for
today’s computer technology. Said lead author Morton in a
statement, “The electron acts as a middle-man between the
nucleus and the outside world. 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.”

Crucial to the success of this
study were the exceptionally pure silicon-28 crystals created by
co-authors Haller and Ager. Haller is a world authority on
crystal growth and purification and is credited with launching
the modern era of isotopically enriched semiconductor research.
Ager designed and built a one-of-its-kind reactor for creating
isotopically enriched and chemically pure silicon, featuring a
high conversion efficiency.

Said Haller, “Crystals of
natural silicon contain 4.7-percent of the isotope silicon-29, in
addition to silicon-28 and silicon-30. For this study we needed
silicon crystals that were not only chemically pure, but
isotopically pure as well because silicon-29 has a nuclear spin
that would interfere with the readout of the electron and nuclear
spins of the phosphorus.”

Since the silicon crystals to
be doped would consist of billions of atoms, creating
isotopically pure crystals of silicon-28 was a painstaking
process. Once these exceptionally pure crystals were created,
they then had to be doped with phosphorus-31 in specific areas of
the crystal and to just the right amount – an undertaking
that Ager compared to adding one extra person to Earth’s
population at one particular address.

Now that it has been
demonstrated that electron spin data can be stored and retrieved
via nuclear spin, future steps will require improving spin
control and readout mechanisms. Also, while the quantum memory
time observed in this study is exceptionally long by previous
standards, it should still be possible to significantly extend
this time.

“The good news is that
there are no know physical limits that would prevent quantum
memory time in nuclear spin from being longer,” said Ager.
“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 Berkeley Lab portion of
this research was supported in part by the U.S. Department of
Energy’s Office of Science, through the Materials Sciences
and Engineering Division of its Basic Energy Sciences programs,
and in part
by the National Security Agency.

Berkeley Lab is a U.S.
Department of Energy national laboratory located in Berkeley,
California. It conducts unclassified scientific research and is
managed by the University of California.