Condensed Matter Seminar

Jason R. Petta
Princeton University

Thursday, October 18, 2007
1:00 pm in SPL 52

Controlling quantum coherence in semiconducting nanostructures

Abstract: Semiconductor quantum dots have many similarities with real atoms: they exhibit discrete energy states, follow Hund's rules, and can be coupled to create artificial molecules. Quantum dots are attractive candidates for building quantum circuits based on electron spin. Single spins can be coupled using gigahertz frequency control of voltages applied to electrostatic gates. By separating a spin singlet state on a chip, we measure an ensemble averaged spin dephasing time T2* of 10 ns, limited by the contact hyperfine interaction with the GaAs host nuclei [1]. We use electrical control of the exchange interaction to drive coherent spin rotations and to implement a “singlet-triplet spin echo” pulse sequence, which leads to a spin coherence time, T2, exceeding 1 microsecond. We show that nuclear spins can be polarized by controlling two-electron spin states near the anti-crossing of the singlet (S) and triplet (T+). An initialized S state is cyclically brought into reson ance with the T+ state, where hyperfine fields drive rapid rotations between S and T+, 'flipping' an electron spin and 'flopping' a nuclear spin [2]. The resulting Overhauser field approaches 80 mT, in agreement with a simple rate-equation model. A self-limiting pulse sequence is developed that allows the steady-state nuclear polarization to be set using a gate voltage.

[1] J. R. Petta, A. C. Johnson et al., Science 309, 2180 (2005).
[2] J. R. Petta, J. M. Taylor et al., cond-mat/0709.0920.