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Single spin detection
Maksym Sladkov
Top master nanoscience symposium
June 23, 2005
Outline
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Introduction
Magnetic Resonance Force Microscopy
Quantum Dot detection of single spin
Other detection techniques
Summary
What is spin?
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Fundamental property
of elementary particles
“Intrinsic angular
moment”
Purely QM nature
Only either “up” or
“down” orientation can
be measured
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Precess around
direction of static
magnetic field
Leads to the Zeeman
splitting
Has tendency to
resonance in presence
of periodic MF
Obey Pauli exclusion
principle
Application: MRI & MD visualization
MRI imaging of human brain
Dynamics of small paramagnetic
particles in the cell
Require large number of spins. 105!
Applications: Quantum computing
1.Initial state:
  a y yes  an no
2.Perform a well defined sequence of quantum
operations (Quantum gates):
U (t )   e 
iHt
3. Read final state: Single Spin Detection!
Magnetic Resonance Force
Microscopy
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Mass-loaded high
sensitive cantilever
(10-15 N)
Changing in resonant
frequency of cantilever
is the detectedion
parameter
Presence of resonance
slice
Spin adiabatic motion
Rugar D. et al. (2004) Nature
Spin-tip interaction
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Spin-tip interaction
changing the resonant
frequency of cantilever
Sign of the frequency
shift depends on the
relative orientation of
spin with respect to
magnetic field
Adiabatic reversal of the spin and
cantilever frequency shift detection
Related movie at:
http://www.almaden.ibm.com/st/nano
scale_science/asms/mrfm/
Single spin observation
Signal/noise ~ 0.06
 <[Signal][Noise]> = 0
 Time averaging of
energy allows extract
signal from noise:
<(x+dx)2>=<x2>+<dx2>
 Averaging time 13 h per
point
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The future of MRFM
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Not only electron spin, but any magnetic moment
can be detected
Sensitivity (for nuclei spin detection) and read-out
time can be improved by increasing of field gradient
and by lowering the temperature
Read-out of single-spin quantun state would be
possible
MRFM also holds the potential to map 3-D images
of molecules (e.g. proteins) in situ with high
resolution and atomic scale characterization of
nanodevices.
Quantum Dot detection of single spin
Elzerman JM et al. (2004) Nature
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QD is created in 2DEG by
applying negative voltage to
T,M & R electrodes.
VP changes relative position
of dot energy levels with
respect to EF
Depending on the spin
orientation and potential on
the dot electron can tunnel
off or on the dot
IQPC senses the charge on
QD
Two-pulse technique
1.
2.
3.
Empty the dot
Injection & waiting –
twait
Read-out – tread
IQPC=f(VP+charge on the
dot)
Spin-down – detected as
charachteristic step
durring the tread.
T1 – measurements
Fraction of spin-down
traces vs. waitng time.
625 traces for each from
15 different waiting times
Future of QD single spin detection
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Principal electrical detection
Can be used in quantum computing
Shows possibility of studying spin relaxation
processess on the single-spin scale
Other detection techniques
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FET Detection of single spin: Xiao M et al.
(2004) Nature.
STM: Durkan C. (2004) Contemp. Phys.
Optical quantum dot detection, nanoSQUID…etc.
Summary
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Detection of single electron spin is possible
It opens a possibility of creation of spin-based
quantum computer
Can lead to the prominent new 3-D imaging
techniques
It allows to get deeper understanding of spinrelaxation processes in solids