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Topic Name: A new approach to quantum information system
Category: Optoelectronics
Research persons: Mikhail D. Lukin,Professor of physics in Harvard's Faculty of Arts and Sciences.
Location: 17 Oxford Street,Cambridge, MA 02138, United States
Details
Surmounting several distinct hurdles to quantum computing, physicists at
Harvard University have found that individual carbon-13 atoms in a diamond
lattice can be manipulated with extraordinary precision to create stable quantum
mechanical memory and a small quantum processor, also known as a quantum
register, operating at room temperature. The finding brings the futuristic
technology of quantum information systems into the realm of solid-state
materials under ordinary conditions.The results, described this week in Science, could revolutionize scientists'
approach to quantum computing, which is built on the profound eccentricity of
quantum mechanics and could someday far outperform conventional supercomputers
in solving certain problems.
"These experiments lay the groundwork for development of a new approach
to quantum information systems," says Mikhail D. Lukin, professor of
physics in Harvard's Faculty of Arts and Sciences.
Earlier advances in quantum computing have occurred inside high vacuums
cooled to fractions of a degree above absolute zero. Individual quantum bits, or
qubits - the building blocks of a quantum computer, encoding information much as
a conventional computer bit stores information as zeroes and ones - are
extremely fragile. Usually they decay very rapidly, losing quantum information
within a tiny fraction of a second unless the qubit is suspended in high vacuum
under these specialized, extreme conditions. This short "coherence
time" has been a major impediment to advances in quantum computing.
Quantum mechanics dictates that coherence is destroyed - and quantum
information lost - through contact with virtually anything, which is why
previous attempts at quantum computing have occurred under such extreme
circumstances. This need for absolute isolation has vexed scientists for more
than a decade, not only because it is difficult to achieve experimentally - not
to mention in a practical computer - but also because it has complicated the
ability to manipulate a quantum computer's input or read its output.
The new advance makes use of spinning properties of atomic nuclei,
fundamental building blocks of matter with sub-nanometer dimensions, to encode
quantum bits. Acting as tiny magnets, such nuclear spins are well known for
their exceptional stability. But in practice the very weak interactions of
nuclear spins with their surroundings - the very reason for their near-perfect
isolation - means that it is essentially impossible to address and manipulate
individual nuclei, and harder still to control interactions between them. For
instance, many billions of nuclei are required in conventional MRI machines,
which work by detecting signals from spinning nuclei.
"The problem is, what makes single nuclear spin so stable - its weak
interaction with its surroundings - also prevents us from directly manipulating
it," Lukin says. "How do you control something that can't interact
with anything?"
You do it gingerly and indirectly, the Harvard physicists report in Science.
They found that nuclear spins associated with single atoms of carbon-13 - which
make up some 1.1 percent of natural diamond - can be manipulated via a nearby
single electron whose own spin can be controlled with optical and microwave
radiation. The excitation of an electron by focusing laser light on a nitrogen
vacancy center, a stable defect in a diamond lattice where nitrogen replaces an
atom of carbon and develops an electronic spin in its ground state, causes the
single electron's spin to act as a very sensitive magnetic probe with
extraordinary spatial resolution.
Using the nitrogen center as an intermediary, a single carbon-13 atom's
nuclear spin is cooled to near absolute zero, creating in the process a single,
isolated quantum bit with a coherence time that approaches seconds. The
controlled interaction between the electron and nuclear spins allows the latter
to be used as very robust quantum memory.
The Harvard physicists also observed and manipulated coupling between
individual nuclear spins, thus demonstrating a way to increase the number of
qubits working in the quantum register. Because the electron spin and nuclear
spin are controlled independently, the experiments lay the groundwork for
development of larger, scalable systems in which such quantum registers are
connected via optical photons.
"Beyond specific applications in quantum information science," the
authors write, "our measurements show that the electron spin can be used as
a sensitive local magnetic probe that allows for a remarkable degree of control
over individual nuclear spins."
Research Persons:
Mikhail D. Lukin,
Professor of physics in Harvard's Faculty of Arts and Sciences.
Lyman 231
17 Oxford Street
Cambridge, MA 02138
((617) 495-2862 , lukin@physics.harvard.edu
Other Researchers Involved:
Lukin's co-authors on the Science paper are
M.V. Gurudev Dutt, Lilian I.
Childress, Liang Jiang, Emre Togan, Jeronimo Maze, and Alexander S. Zibrov, all
at Harvard; Fedor Jelezko at Universität Stuttgart; and Phillip R. Hemmer of
Texas A&M University.
Funded:
Their work was supported by the National Science
Foundation, the Army Research Office's Multi University Research
Initiative, and
the Packard and Hertz
Foundations. Work at Stuttgart was additionally supported
by the Deutsche Forschungsgemeinschaft and
the European Commission.
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