Quantum Optics Seminar by A. Mark Fox

Picosecond control of excitons and spins in InGaAs quantum dots

Recent progress on the control of excitons and spins in self-assembled InGaAs quantum dots by picosecond laser pulses will be reviewed. I will begin by describing photocurrent techniques to read out the state of the system following the optical excitation. Coupling of excitons to phonons enables pumping via sidebands, and at positive detuning the two-level system can be inverted even though the pumping mechanism is incoherent [1,2]. This inversion occurs by rapid thermalization of the optically dressed states, and is relatively insensitive to small fluctuations in the excitation pulse area or frequency. For negative detunings, ultrafast de-excitation has been demonstrated, opening possibilities for ultrafast switching of the exciton state [3].

Photocurrent techniques also permit the initialization of single holes spins by ionization of excitons after circularly-polarized excitation. By using sequences of time-delayed pulses, it has been possible to demonstrate picosecond preparation, full control, and read-out of a single hole spin [4]. Very high fidelity initialization has been demonstrated in a dot with small fine-structure splitting (FSS) in a regime where the hole spin lifetime exceeds the T₂^* dephasing time [5].

Resonance fluorescence techniques provide an alternative method for reading out the state of the system at the end of the pulse, with the advantage of avoiding dephasing by tunnelling. The suppression of the laser scatter is more challenging with pulsed excitation, but by using a nano-cavity to enhance the signal, usable signal to noise ratios can be achieved. In this way, it has been possible to develop a double-p-pulse technique to measure the radiative lifetime of a nano-cavity with a very large Purcell factor, and to implement an ultrafast on-chip, on-demand single-photon source [6]. 

[1]   J. H. Quilter, A. J. Brash, F. Liu, M. Glässl, A. M. Barth, V. M. Axt, A. J. Ramsay, M. S. Skolnick and A. M. Fox, in Phys. Rev. Lett. 114, 137401 (2015).

[2]   A. J. Brash, L. M. P. P. Martins, A. M. Barth, F. Liu, J. H. Quilter, M. Glässl, V. M. Axt, A. J. Ramsay, M. S. Skolnick and A. M. Fox, J. Opt. Soc. Am. B, 33, C115 (2016).

[3]   F. Liu, L. M. P. Martins, A. J. Brash, A. M. Barth, J. H. Quilter, V. M. Axt, M. S. Skolnick and A. M. Fox, Phys. Rev. B 93, 161407 (R) (2016).

[4]   T. M. Godden, J. H. Quilter, A. J. Ramsay, Y. Wu, P. Brereton, S. J. Boyle, I. J. Luxmoore, J. Puebla-Nunez, A. M. Fox and M. S. Skolnick, Phys. Rev. Lett. 108, 017402 (2012).

[5]   A.J. Brash, L. M. P. P. Martins, F. Liu, J. H. Quilter, A. J. Ramsay, M. S. Skolnick and A. M. Fox, Phys. Rev. B 92, 121301 (R) (2015).

[6]   F. Liu, A. J. Brash, J. O’Hara, L. M. P. P. Martins, C. L. Phillips, R. J. Coles, B. Royall, C. Bentham, N. Prtljaga, I. E. Itskevich, L. R. Wilson, E. Clarke, M. S. Skolnick, and A. M. Fox, arXiv:1706.0442