Biomedical Engineering Reference
In-Depth Information
possible source of entangled photon pairs [
3
]. Furthermore, carrier transfer in DQD
structures can be used for initializing spin states of a dopant Mn atom localized in
one of the dots [
4
].
Pairs of vertically stacked QDs with a spatial separation down to single nanome-
ters can be obtained in a two-layer self-assembled growth process, where the strain
distribution favors the nucleation of QDs in the second layer directly on top of the
QDs formed in the first layer [
5
,
6
]. The development of manufacturing technologies
[
7
,
8
] has made it possible to achieve DQD structures made of nearly identical dots,
with the splitting of ground state transition energies down to single meV (possibly
interaction-limited) [
9
,
10
]. The state space of such a double quantum dot is
obviously richer than that of a single QD [
11
,
12
] and allows, e.g., for the formation
of entanglement between the dots. Also the recombination and relaxation processes
in DQDs show many features which cannot appear in individual QDs. The quantum
coherence of carrier states in DQDs is affected by interference and collective effects
that appear in the interaction of such systems with their radiative environment
(electromagnetic vacuum) and with the surrounding crystal lattice (phonons).
Compared to single QDs, DQDs display more complexity when the decoherence
resulting from the charge carrier-phonon coupling is studied. The interaction with
the phonon bath leads to pure dephasing in both types of systems, which has been
experimentally observed as the decay of the nonlinear optical response in a four-
wave mixing experiment with ultrashort pulses [
13
,
14
]. A characteristic feature of
the phonon-induced dephasing in QDs is that it is always only partial, i.e., after
a few picoseconds of carrier-phonon dynamics, the degree of coherence (i.e., the
amplitudes of the off-diagonal elements of the density matrix) reaches a certain
finite level, depending on the system geometry on and temperature [
15
,
16
]. In DQDs
and regular QD arrays, the degree of dephasing may be reduced by encoding the
logical qubit values into many-exciton states over a QD array [
17
]. Phonon-induced
dephasing is also detrimental to entanglement in DQDs and larger QD arrays. The
impact of the partial pure dephasing on entanglement is very strong, since it is more
prone to dephasing than local coherence. It turns out that entanglement may be
completely destroyed by the interaction with phonons even though the decoherence
is always only partial [
18
-
20
]. Moreover, because of the importance of the delicate
inter-subsystem coherences, phonon-induced entanglement decay strongly depends
on the nature of the interaction with the environment, meaning that an interaction
with separate reservoirs disentangles QD subsystems effectively than an interaction
with a common reservoir [
21
].
Optical properties of DQDs may be strongly modified due to the collective
interaction of sufficiently closely spaced QDs with the electromagnetic (EM) field.
These collective effects have been extensively studied for atomic systems [
22
]
where they manifest themselves by superradiant emission [
23
]. A signature of
superradiant behavior was also observed in ensembles of QDs [
24
]. On the other
hand, the collective interaction leads to the appearance of subradiant states which
are decoupled from the environment and, therefore, do not undergo decoherence. It
has been proposed to use these states for noiseless encoding of quantum information
[
25
,
26
].
