MSci Project Proposal Form 2010-2011
Project Code: EXSS-Slavcheva-1
Project title: Coherent optical control of spin dynamics in p-doped quantum dots: towards qubit
implementation
Supervisor: Dr Gaby Slavcheva
Assessor: Prof Ray Murray
Telephone: Ex. 4 7623
Telephone: Ex. 4 7578
E-mail: g.slavcheva
E-mail: r.murray
Research Group: EXSS
Research Group: EXSS
Project Summary: Among the huge variety of proposed schemes to realize quantum computing, one
of the front runners is now considered to be the charged quantum dot (QD). Owing to the threedimensional carrier localisation and the discrete-level electronic structure the electron spin localised in
a QD is far less affected by the semiconductor environment compared to the quantum wells and bulk,
resulting in relatively long spin-coherence times (the time within which the spin loses its phase
information). This ensures a long-lived quantum state required for performing a large number of spin
manipulations (quantum operations) during which coherence needs to be retained. An all-optical
scheme using ultrashort pulses for spin initialisation and readout is of particular interest since it would
allow spin manipulation within the spin decoherence time, before the system could be affected by its
environment. Such a scheme would take full advantage of the cutting edge ultrafast laser technologies and
thus enable the implementation of ultrafast schemes for quantum computation.
An optical excitation of a p-doped QD into excited states using circularly polarised light creates hot
positive trion states, consisting of two holes and an electron, with a complex discrete energy level structure.
Photoluminescence experiments on QD ensembles have revealed an increased spin polarisation contrast in
the excited state emission compared to the ground state. This implies that spin manipulation of excited
trion states may prove more efficient. Spin relaxation mechanisms that become dominant are the
anisotropic electron-hole exchange interaction and the hyperfine interaction of the electron and hole spins
with the randomly distributed lattice ions nuclear spins, causing spin precession and decoherence.
The project aims to theoretically assess the potential of using excited trion states in schemes for spin
generation and readout, using ultrashort optical pulses. We shall consider ultrashort (ps) quasi-resonant
circularly-polarised excitation into the p-shell in the presence of a resident s-shell hole, and subsequent
relaxation to the luminescing ground trion singlet state. The hot trion discrete-level structure can be mapped
onto the SU(6) group. The project will involve the following steps:
(i) developing a group theoretical model of the QD energy level structure based on SU(N) Lie algebra
(ii) Derivation of system Hamiltonian and master equation for the density matrix (pseudo-spin
equations) including the complex spin relaxation dynamics
(iii)
Coupling of pseudo-spin equations to vector Maxwell equations via medium polarisation
(iv)
Self-consistent numerical solution in 1D for a realistic structure geometry
(v)
Comparison with time-resolved photoluminescence experiments
Experimental component:
Computational component:
Theoretical component:
0%
60 %
40 %
Is the MSci Project eligible for students on the MSci
Physics with Theoretical Physics Degree: YES
Are workshop facilities required: NO
Suggested reading (at least one relevant reference):
[1] F.T. Hioe and J. H. Eberly, Phys. Rev. Lett. 47, 838 (1981)
[2] Warming T et al. Phys. Rev. B 79, 125316 (2009)
[3] Cortez S et al. Phys. Rev. Lett. 89, 207401 (2002)
[4] Slavcheva G, Phys. Rev. B 77, 115347 (2008)
Imperial College of Science, Technology, and Medicine