Quantum information technologies require reliable sources of correlated/entangled
photons. To realize and use these technologies in real life rather than just in
laboratory, high efficiency and stable sources of entangled photons are needed.
This work considers the design of various semiconductor heterostructures, based on
Al$_x$Ga$_{1-x}$As/GaAs, that rely on intersubband transitions in the conduction or
in the valence band, to deliver the second order nonlinear process, known as
spontaneous parametric down-conversion (SPDC). The second-order SPDC can produce
Bell state entangled photons.
Second harmonic generation (SHG) is a well known and broadly discussed process,
and using the fact that SHG is the reverse process of SPDC, initial studies of SHG
are used to support the validity and accuracy of the methodology employed for
nonlinear susceptibility calculations.
In designing the heterostructures, genetic optimization is used to reduce the
computational cost in finding the best structure. The heterostructures designed by
considering the intersubband transitions in the conduction band constitute good
sources of spectrally entangled photons. The efficiency of the process is estimated
and the Schmidt number calculation shows that the structure can produce
twin photons with a reasonably good degree of entanglement.
Alternatively, using intersubband transitions in the valence band can deliver the
polarization entangled photons, which cannot be achieved with conduction intersubband
transitions. The genetic optimization is again used to design the best structures for
this purpose, and the efficiency of the process is also calculated.
We then extend our work to multiparticle entangled states, also known as Greenberger-Horne-Zeilinger (GHZ) states,
by considering the third order nonlinear SPDC (TOSPDC) process in designing the heterostructures
for this purpose. This designed structure can be a good candidate as a direct TOSPDC
source, since the second order nonlinearity is here suppressed by considering symmetric
structures only. The efficiency of the process is also calculated and discussed in the
thesis.
