Quantum Optics Seminar by Richard Warburton – Niels Bohr Institute - University of Copenhagen

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Quantum Optics Seminar by Richard Warburton

A single electron or hole trapped inside a self-assembled quantum dot has a spin and can be used as a spin qubit. The single spin benefits from the exciton: the spin can be initialized, manipulated and subsequently read-out optically. The complex physics lies in the dephasing processes of the exciton and spin. In the presence of noisy nuclei, the electron spin qubit dephases rather rapidly via the hyperfine interaction of the electron spin with the 100,000 nuclear spins, a central spin problem. The contact part of the hyperfine interaction is strongly suppressed for a hole spin but the dipole-dipole interaction remains.  

Presented here are the results of experiments facilitated by the detection of resonance fluorescence from single spins in single quantum dots in very high quality material at low temperature [1]. The experiments reveal the sources of noise influencing both spin and exciton qubits and demonstrate the ability to invert the nuclear spins with frequency-swept nuclear magnetic resonance pulses following optical polarization. Specifically, the main results are: (i) determination of charge noise and spin noise spectra, exploiting a spectroscopic signature to distinguish charge noise from spin noise [1]; (ii) demonstration that nuclear spin noise (and not charge noise) is the main contributor to exciton dephasing at modest optical couplings; (iii) active reduction of charge noise with a classical feedback circuit; (iv) a coherent hole spin as revealed by 50 neV wide dips in a coherent population trapping experiment; (v) hole spin dephasing via charge noise; (vi) estimations of the anisotropy in the hole spin hyperfine interaction; and (vii) determination of the key parameters of the nuclear spins from frequency-swept nuclear magnetic resonance.
Prospects for engineering spin coherence further and for enhancing the spin-photon interaction with a micro-cavity will be discussed. 

[1] A. V. Kuhlmann et al., Nature Physics 9, 570 (2013)