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WHAT'S HOT IN... PHYSICS, Jan./Feb. 2008
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Spin Controlled in Semiconductor Quantum
Dots
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by Simon Mitton
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Physics Top Ten
Papers
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Rank
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Papers
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Cites Jul-Aug
07
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Rank
May-Jun 07
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1
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K.S. Novoselov, et al., "Two-dimensional
gas of massless Dirac fermions in graphene,"
Nature, 438(7065): 197-200, 10 November 2005. [U.
Manchester, U.K.; Inst. Microelect. Tech., Chernogolovka,
Russia; Radboud U., Nijmegen, Netherlands] *982BV
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50
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1
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2
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Y.-B. Zhang, et al., "Experimental
observation of the quantum
Hall
effect and Berry’s phase in
graphene," Nature, 438(7065): 201-4,
10 November 2005. [Columbia U., New York, NY] *982BV
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48
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2
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3
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P. Astier, et al., "The Supernova Legacy
Survey: measurement of OM , O^ and
w from the first year data set,"
Astron. & Astrophys., 447(1): 31-48, February
2006. [18 institutions worldwide] *007GS
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33
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3
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4
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M.F. Skrutskie, et al., "The Two Micron
All Sky Survey (2MASS)," Astronom. J.,
131(2): 1163-83, February 2006. [11 U.S. institutions]
*010RX
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28
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4
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5
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W. Ma, et al., "Thermally stable,
efficient polymer solar cells with nanoscale control of the
interpenetrating network morphology," Adv.
Funct. Materials, 15(10): 1617-22, October 2005. [U.
Calif., Santa Barbara] *976VL
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27
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6
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6
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J.R. Petta, et al., "Coherent manipulation
of coupled electron spins in semiconductor
quantum
dots," Science,
309(5744): 2180-4, 30 September 2005. [Harvard U.,
Cambridge, MA; Weizmann Inst., Rehovot, Israel; U.
Calif., Santa Barbara] *970NX
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23
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10
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7
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G. Li, et al., "High-efficiency solution
processable polymer photovoltaic cells by self-organization
of polymer blends," Nature Materials,
4(11): 864-8, November 2005. [U. Calif., Los Angeles; Natl.
Renewable Energy Lab., Golden, CO] *979GS
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23
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†
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8
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D.J. Eisenstein, et al., "Detection of the
baryon acoustic peak in the large-scale correlation
function of SDSS luminous red galaxies,"
Astrophys. J., 633(2): 560-74, 10 November 2005.
[29 institutions worldwide] *983NK
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22
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5
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9
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M. Sirianni, et al., "The photometric
performance and calibration of the Hubble Space
Telescope Advanced Camera for Surveys,"
Pub. Astro. Soc. Pacific, 117(836): 1049-1112,
October 2005. [7 U.S. and European institutions] *968QC
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20
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†
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10
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S. Cole, et al., The 2dF Galaxy Redshift
Survey: power-spectrum analysis of the final data set and
cosmological implications, Mon. Not. Roy.
Astron. Soc., 362(2): 505-34, 11 September 2005. [18
institutions worldwide] *959HX
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19
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†
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SOURCE: Thomson Scientific's
Hot Papers
Database. Read the
Legend.
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Quantum dots are solid-state structures that are
capable of confining a very small number of electrons. They have long
been thought of as artificial atoms because of their discrete atom-like
states. An important driver for research on the quantum control of
solid-state systems is the application of quantum coherence as a
platform for information processing.
In a classical computer, information is stored in bits, whereas in quantum
computation two-state systems known as qubits (quantum
bits) are used. In principle, quantum mechanisms can be used to
perform operations much faster than a classical computer because
superposition of quantum states within the qubit allows an infinite number
of states.
Research on the properties of quantum dots as candidates for solid-state
qubits continues at a frantic pace. Physicists have shown that spin in
quantum dots is a promising candidate for holding quantum information.
That’s because a single electron spin in a quantum dot can have a
relaxation time of tens of milliseconds.
Such relative stability combined with ultrafast optical manipulation should
permit representation and control of quantum information in a solid-state
system. The leap from theory to application is, however, still enormous.
Physicists will need to learn far more about how to control spin systems in
the solid-state in order to fabricate scalable devices.
Several groups have managed to make the smallest possible dot, containing
one electron. This might be expected to have simple properties, like the
electron in the Bohr model of hydrogen. In the latter case the single spin
of the electron couples to the spin of one proton. That’s not the
case with a quantum dot hosted by the semiconductor GaAs because the
electron is subject to about 106 spins carried by Ga and As
nuclei in the crystal lattice, which wrecks the quantum coherence of the
electronic spin.
Hot Paper #6, from Charlie Marcus’ experimental condensed-matter lab
at Harvard University, deftly solves this problem by using a double quantum
dot, which can be imagined as an artificial H2 molecule. Double
dots have been around for years, but Marcus’s group is the first to
make a device in which the electron spins are precisely controlled by a
gate voltage and a varying external magnetic field.
To fabricate the new structures, the Harvard team used molecular beam
epitaxy. Their double-well device has six electrodes biased with negative
voltages. Varying the input potentials controls the number of electrons in
each well. The low-energy electronic states of the two-electron system are
tunable in a variety of ways. The upshot is that there are two states that
offer the chance of making a coded qubit in a solid-state device.
The construction of a scalable quantum processor requires coherent control
of quantum coherence in a large-scale system. The device described in #6
features robust control technology with local electrical signals.
In #6 Principal Investigator Marcus and his colleagues demonstrate coherent
control of the spin states in a double quantum dot, and this allows state
preparation, coherent manipulation, and projective readout. The electrical
control of spins in semiconductors is the breakthrough message here, and it
may pave the way to quantum computation becoming reality.
For ScienceWatch.com, team member Mikhail Lukin explains, "It was
an important advance because for the first time we demonstrated control of
quantum spins in a quantum dot. That enabled us to investigate coherence
properties."
Lead author Jason Petta, who is now at Princeton University, adds: "This
paper has had a large impact, since it is the first demonstration of
quantum control of coupled electron spins in a semiconducting device.
Electron spin qubits in GaAs were proposed in 1998 by Loss and DiVincenzo.
The prospects for making this proposal a reality seemed remote in 1998
since there was a large gap between the proposal and the experimental state
of the art.
"For this reason, electron spin qubits were viewed by some as the dark
horse of quantum computing. Our worked showed that electron spins can be
prepared in a well-defined quantum state, coherently manipulated, and
measured, using all-electrical methods."
Dr. Simon Mitton is a Fellow of St. Edmund’s College,
Cambridge, U.K.
What's Hot In... : What's Hot in Physics Menu : Spin Controlled in Semiconductor Quantum Dots - Physics - Jan/Feb 2008
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