Transcript Principal Methods For Measuring Perfusion

Single-shot read-out of an
individual electron spin in a
quantum dot
J. M. Elzerman, R. Hanson, L. H. Willems van Beveren,
B. Witkamp, L. M. K. Vandersypen, L. P. Kouwenhoven
Delft University of Technology
Nature 430, 431 (2004)
Talk held by Kevin Inderbitzin and Lars Steffen
Outline
Idea behind the paper
 Quantum dot & spin-to-charge
conversion
 Experimental setup
 Measurements & Results
 Conclusion
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Idea behind the paper
Individual spins are carriers of
quantum information
 Read-out of a single spin state with
optical techniques is already
possible
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The paper presents a method of
electrical read-out of a single
spin
Quantum dot
confines the motion of conduction
band electrons
 has a discrete quantized energy
spectrum
 contains a small integer number of
conduction band electrons

Experimental setup
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GaAs/AlGaAs
heterostructure
2DEG below the
surface
Dilution refrigerator (T ≈ 300 mK)
Magnetic field (B =
10 T)
Experimental setup
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Topgates deplete
the 2DEG
Plungergate to
control energylevels in the QD
Current through
QPC can be
measured
Experimental setup
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Gate voltages such that the QD contains
either zero or one electron
Current through QPC is influenced by the
charge on the dot and the plungergate
voltage
Voltages on gates T, M, R and Q are
constant through the experiment
Spin-to-charge conversion
What can happen?
Expected measurements
only for spin-down
Expected measurements
The two specific possible cases again.
Top: Compare to the expected measurements in the last slide. Bottom: Different measured
„spin-down“ signals (only the read-out stage). They show the stochastic nature of the
tunneling events. Red lines = read-out threshold.
Results – Part 1
twait  thold
Results – Part 2
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Probability of relaxation to spin-upstate increases with twait  thold
Probable reasons of spin-relaxation at
high magnetic fields:
 Dominated by spin-orbit interaction
 Smaller contributions from hyperfine
interactions with the surrounding
nuclear spins.
Top: Spin-down probability for different magnetic fields.
Bottom: Need to introduce constant term into fitting function.
Results – Part 3
Fit curve to:
  Ce
 t wait
T1
Results – Part 4
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is the „dark count“ probability = prob.
that even though the electron has spin-up
the current exeeds the spin-down
threshold in the read-out stage.
Reasons (in
measuring QPC):
 Thermally activated
tunneling
 Electrical noise
Visibility is maximized at the red line.
Results – Part 5
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Measured:
  0.07
  0.28
Vary read-out threshold to
maximize visibility=1      65%
Summary and Conclusion
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Electrical single-shot spin read-out has
been demonstrated (2004).
With a measurement visibility of  65%
Very long single-spin energy relaxation
times:  0.85 ms for 8 Tesla field
Encouraging results for the use of
electron spins as qubits!
Outlook
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Necessary future steps for improving the
spin measurement visibility:
 Lower electron temperature
 Achieve faster charge measurement
More experiments necessary to confirm
theoretical predictions for the reasons of
electron spin relaxation at high magnetic
fields (mainly spin-orbit).