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

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 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 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 1 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.

Results – Part 3 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 (in measuring QPC): Thermally activated tunneling Electrical noise

Results – Part 5 Vary read-out threshold to maximize visibility= 65% 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).