Candidate:
Turner
Garrow
Title:
Techniques
to
improve
quantum
dot
emission:
Resonant
two-photon
excitation
and
optical
frequency
shifting
via
electro-optic
modulation
Date:
October
12,
2021
Time:
11:00
am
Place:
MS
Teams
Supervisor(s):
Reimer,
Michael
Abstract:
The
ability
to
create
pairs
of
entangled
photons
is
a
requirement
for
many
near-future
quantum
technologies.
Despite
this,
the
current
state-of-the-art
entangled
photon
sources
are
fundamentally
limited
in
their
performance
by
their
probabilistic
nature.
Recently,
semiconductor
quantum
dots
have
gained
a
great
deal
of
interest
as
candidates
for
next-generation
entangled
photon
sources
since
quantum
dots
produce
photon
pairs
deterministically,
and
are
therefore
not
fundamentally
limited
in
their
performance. The
results
presented
here
are
from
two
separate
experiments
relating
to
the
emission
properties
of
a
quantum
dot.
The
first
is
resonant
two-photon
excitation
of
the
quantum
dot,
a
scheme
of
optically
exciting
the
dot
which
is
expected
to
outperform
all
other
optical
excitation
methods.
By
directly
exciting
charges
within
the
quantum
dot
through
two-photon
excitation,
the
charge noise
is
decreased,
which
reduces
both
re-excitation
of
the
dot
and
dephasing
over
the
lifetime
of
the
excited
state.
Quantum
state
tomography
of
the
emitted
pairs
reveals
a
peak
concurrence
of
0.87(4),
and
represents
the
first
ever
tomography
measurement
of
a
nanowire
quantum
dot
excited
with
this
excitation
scheme.
The second experiment presented here is related to an all-optical method of eliminating the quantum dot fine structure splitting. The fine structure splitting is an energy difference between the intermediate states of the optical cascade, and is an unwanted property of semiconductor quantum dots introduced in the fabrication process. The proposed method uses a pair of electro-optic modulators to shift the energy of the emitted photons to compensate for this energy difference. Here, we present results from a lithium niobate electro-optic modulator, capable of frequency conversion of circularly polarized light with an average efficiency of 82.2%. This demonstration shows that an all-optical fine structure eraser is feasible, and leaves us well-positioned for an experimental demonstration in the near future.