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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. - PowerPoint PPT Presentation
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Single-shot read-out of an Single-shot read-out of an individual electron spin in a individual electron spin in a
quantum dotquantum 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
Idea behind the paper Individual spins are carriers of
quantum information Read-out of a single spin state with
optical techniques is already possible
The paper presents a method ofelectrical read-out of a singlespin
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 GaAs/AlGaAs
heterostructure 2DEG below the
surface Dilution refrigera-
tor (T ≈ 300 mK) Magnetic field (B =
10 T)
Experimental setup Topgates deplete
the 2DEG Plungergate to
control energy-levels in the QD
Current through QPC can be measured
Experimental setup 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.
Results – Part 1holdwait tt
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 2
Probability of relaxation to spin-up-state increases with
Probable reasons of spin-relaxation at high magnetic fields: Dominated by spin-orbit interaction Smaller contributions from hyperfine
interactions with the surrounding nuclear spins.
holdwait tt
Results – Part 3
1
wait
T
t
Ce
Fit curve to:
Top: Spin-down probability for different magnetic fields.Bottom: Need to introduce constant term into fitting function.
Results – Part 4 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 (inmeasuring QPC): Thermally activated
tunneling Electrical noise
Results – Part 5 Vary read-out threshold to
maximize visibility= 65% 1
07.028.0
Measured:Visibility is maximized at the red line.
Summary and Conclusion 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 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).