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).