A team of Australian engineers at the University of New South Wales (UNSW)
has demonstrated a quantum bit based on the nucleus of a single atom in
silicon, promising dramatic improvements for data processing in ultra-powerful
quantum computers of the future. Quantum bits, or qubits, are the building
blocks of quantum computers, which will offer enormous advantages for searching
expansive databases, cracking modern encryption, and modelling atomic-scale
systems such as biological molecules and drugs.
The world-first result, to be published in
Nature on April 18,
brings these machines one-step closer, describing how information was stored
and retrieved using the magnetic spin of a nucleus.
"We have adapted magnetic resonance technology, commonly known for its
application in chemical analysis and MRI scans, to control and read-out the
nuclear spin of a single atom in real time," says Associate Professor
Andrea Morello from the School of Electrical Engineering and Telecommunications
at UNSW.
The nucleus of a phosphorus atom is an extremely weak magnet, which can
point along two natural directions, either "up" or "down."
In the strange quantum world, the magnet can exist in both states
simultaneously -- a feature known as quantum superposition.
The natural positions are equivalent to the "zero" and
"one" of a binary code, as used in existing classical computers. In
this experiment, the researchers controlled the direction of the nucleus, in
effect "writing" a value onto its spin, and then "reading"
the value out -- turning the nucleus into a functioning qubit.
"We achieved a read-out fidelity of 99.8 per cent, which sets a new
benchmark for qubit accuracy in solid-state devices," says UNSW Scientia
Professor Andrew Dzurak, who is also Director of the Australian National
Fabrication Facility at UNSW, where the devices were made.
The accuracy of the UNSW team's nuclear spin qubit rivals what many consider
to be today's best quantum bit -- a single atom in an electromagnetic trap inside
a vacuum chamber. The development of this "Ion Trap" technology was
awarded the 2012 Nobel Prize in physics.
"Our nuclear spin qubit operates at a similar level of accuracy, but
it's not in a vacuum chamber -- it's in a silicon chip that can be wired up and
operated electrically like normal integrated circuits," says Morello.
"Silicon is the dominant material in the microelectronics industry, which
means our qubit is more compatible with existing industry technology and is
more easily scaleable."
Morello's PhD student Jarryd Pla is the lead experimental author of the
work, which was conducted in collaboration with the groups led by Dzurak and
Professor David Jamieson at the University of Melbourne. Morello, Dzurak and
Jamieson are all Program Managers in the ARC Centre of Excellence for Quantum
Computation and Communication Technology.
In September 2012, the same UNSW team reported in
Nature the first
functional quantum bit based on an electron bound to a phosphorus atom embedded
in silicon, "writing" information onto its spin and then
"reading" the spin state back out.
With their latest result, the team has dug even deeper into the atomic
structure to manipulate and measure the spin of its nucleus. This is the core
of an atom, containing most of its mass, but its diameter is only about
one-millionth that of the atom's diameter.
"This means it's more challenging to measure, but it's almost
completely immune to disturbances from the outside world, which makes it an
exceptional quantum bit," says UNSW engineering PhD student Jarryd Pla.
"Our nuclear spin qubit can store information for longer times and with
greater accuracy. This will greatly enhance our ability to carry out complex
quantum calculations once we put many of these qubits together."
Electron spin qubits will likely act as the main "processor" bits
for quantum computers of the future, coupled with other electrons to perform
calculations. But nuclear spin qubits could also be integrated and could
provide a useful memory function or help implement two-bit logic gates between
the electronic qubits, the researchers say.
Demonstrating quantum memories and two-qubit
logic gates is the main focus of the UNSW team for the near future. They are
also exploring ways of improving the accuracy of their nuclear and electron
spin qubits even further, by moving to a purer form of silicon.