Quantum dots for quantum information processing

Electron spins confined in quantum dots (QDs) are among the leading
contenders for implementing quantum information processing. In this
Thesis we address two of the most significant technological
challenges towards developing a scalable quantum information
processor based on spins in quantum dots: (i) decoherence of the
electronic spin qubit due to the surrounding nuclear spin bath, and
(ii) long-range spin-spin coupling between remote qubits. To this
end, we develop novel strategies that turn the unavoidable coupling
to the solid-state environment (in particular, nuclear spins and
phonons) into a valuable asset rather than a liability. In the
first part of this Thesis, we investigate electron transport
through single and double QDs, with the aim of harnessing the
(dissipative) coupling to the electronic degrees of freedom for the
creation of coherence in both the transient and steady-state
behaviour of the ambient nuclear spins. First, we theoretically
show that intriguing features of coherent many-body physics can be
observed in electron transport through a single QD. To this end, we
first develop a master-equation-based formalism for electron
transport in the Coulomb-blockade regime assisted by hyperfine (HF)
interaction with the nuclear spin ensemble in the QD. This general
tool is then used to study the leakage current through a single QD
in a transport setting. When starting from an initially
uncorrelated, highly polarized state, the nuclear system
experiences a strong correlation buildup, due to the collective
nature of the coupling to the central electron spin. We demonstrate
that this results in a sudden intensity burst in the electronic
tunneling current emitted from the QD system, which exceeds the
maximal current of a corresponding classical system by several
orders of magnitude. This gives rise to the new paradigm of
electronic superradiance. Second, building upon the insight that
the nuclear spin dynamics are governed by collective interactions
giving rise to coherent effects such as superradiance, we propose a
scheme for the deterministic generation of steady-state
entanglement between the two nuclear spin ensembles in an
electrically defined double quantum dot. Because of quantum
interference in the collective coupling to the electronic degrees
of freedom, the nuclear system is actively driven into a two-mode
squeezedlike target state. The entanglement buildup is accompanied
by a self-polarization of the nuclear spins towards large
Overhauser field gradients. Moreover, the feedback between the
electronic and nuclear dynamics is shown to lead to intriguing
effects such as multistability and criticality in the steady-state
solutions. In the second part of this Thesis, our focus turns
towards the realization of long-range spin-spin coupling between
remote qubits. We propose a universal, on-chip quantum transducer
based on surface acoustic waves in piezo-active materials. Because
of the intrinsic piezoelectric (and/or magnetostrictive) properties
of the material, our approach provides a universal platform capable
of coherently linking a broad array of qubits, including quantum
dots, trapped ions, nitrogen-vacancy centers or superconducting
qubits. The quantized modes of surface acoustic waves lie in the
gigahertz range, can be strongly confined close to the surface in
phononic cavities and guided in acoustic waveguides. We show that
this type of surface acoustic excitations can be utilized
efficiently as a quantum bus, serving as an on-chip, mechanical
cavity-QED equivalent of microwave photons and enabling long-range
coupling of a wide range of qubits. In summary, this thesis
provides contributions towards developing a scalable quantum
information processor based on spins in quantum dots in two
different aspects. The first part is dedicated to a deeper
understanding of the nuclear spin dynamics in quantum dots. In the
second part we put forward a novel sound-based strategy to realize
long-range spin-spin coupling between remote qubits. This completes
a broad picture of spin-based quantum information processing which
integrates different perspectives, ranging from the single-qubit
level to a broader quantum network level.