25 research outputs found
Magnetic frustration and fractionalization in oligo(indenoindenes)
Poly(indenoindenes) are {\pi}-conjugated ladder carbon polymers with
alternating hexagons and pentagons hosting one unpaired electron for each
five-membered ring in the open-shell limit. Here we study the main magnetic
interactions that are present in finite oligo(indenoindenes) (OInIn),
classifying the six possible isomers in two different classes of three isomers
each. One class can be rationalized by frustrated S = 1/2 Heisenberg chains,
with ferromagnetic interactions between neighbour sites and antiferromagnetic
interactions between the next neighbours. The other class is characterized by
the more trivial antiferromagnetic order. Employing several levels of theory we
further show that the ground state of one of the isomers is a valence-bond
solid (VBS) of ferromagnetic dimers (S = 1). This is topologically similar to
that of the Affleck-Kennedy-Lieb-Tasaki (AKLT) model, which is known to show
fractional S = 1/2 states at the edges
Interfacing nuclear spins in quantum dots to cavity or traveling-wave fields
We show how to realize a quantum interface between optical fields and the
polarized nuclear spins in a singly charged quantum dot, which is strongly
coupled to a high-finesse optical cavity. An effective direct coupling between
cavity and nuclear spins is obtained by adiabatically eliminating the (far
detuned) excitonic and electronic states. The requirements needed to map qubit
and continuous variable states of cavity or traveling-wave fields to the
collective nuclear spin are investigated: For cavity fields, we consider
adiabatic passage processes to transfer the states. It is seen that a
significant improvement in cavity lifetimes beyond present-day technology would
be required for a quantum interface. We then turn to a scheme which couples the
nuclei to the output field of the cavity and can tolerate significantly shorter
cavity lifetimes. We show that the lifetimes reported in the literature and the
recently achieved nuclear polarization of ~90% allow both high-fidelity
read-out and write-in of quantum information between the nuclear spins and the
output field. We discuss the performance of the scheme and provide a convenient
description of the dipolar dynamics of the nuclei for highly polarized spins,
demonstrating that this process does not affect the performance of our
protocol.Comment: 37 pages, 14 figure
Superradiance-like Electron Transport through a Quantum Dot
We theoretically show that intriguing features of coherent many-body physics
can be observed in electron transport through a quantum dot (QD). We first
derive a master equation based framework for electron transport in the
Coulomb-blockade regime which includes 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. We find that, for
an initially polarized nuclear system, the proposed setup leads to a strong
current peak, in close analogy with superradiant emission of photons from
atomic ensembles. This effect could be observed with realistic experimental
parameters and would provide clear evidence of coherent HF dynamics of nuclear
spin ensembles in QDs.Comment: 21 pages, 10 figure
Multistability and spin diffusion enhanced lifetimes in dynamic nuclear polarization in a double quantum dot
The control of nuclear spins in quantum dots is essential to explore their
many-body dynamics and exploit their prospects for quantum information
processing. We present a unique combination of dynamic nuclear spin
polarization and electric-dipole-induced spin resonance in an electrostatically
defined double quantum dot (DQD) exposed to the strongly inhomogeneous field of
two on-chip nanomagnets. Our experiments provide direct and unrivaled access to
the nuclear spin polarization distribution and allow us to establish and
characterize multiple fixed points. Further, we demonstrate polarization of the
DQD environment by nuclear spin diffusion which significantly stabilizes the
nuclear spins inside the DQD
Large nuclear spin polarization in gate-defined quantum dots using a single-domain nanomagnet
The electron-nuclei (hyperfine) interaction is central to spin qubits in
solid state systems. It can be a severe decoherence source but also allows
dynamic access to the nuclear spin states. We study a double quantum dot
exposed to an on-chip single-domain nanomagnet and show that its inhomogeneous
magnetic field crucially modifies the complex nuclear spin dynamics such that
the Overhauser field tends to compensate external magnetic fields. This turns
out to be beneficial for polarizing the nuclear spin ensemble. We reach a
nuclear spin polarization of ~50%, unrivaled in lateral dots, and explain our
manipulation technique using a comprehensive rate equation model
Nuclear spin cooling using Overhauser field selective coherent population trapping
Hyperfine interactions with a nuclear spin environment fundamentally limit
the coherence properties of confined electron spins in the solid-state. Here,
we show that a quantum interference effect in optical absorption from two
electronic spin states of a solid-state emitter can be used to prepare the
surrounding environment of nuclear spins in well-defined states, thereby
suppressing electronic spin dephasing. The evolution of the coupled
electron-nuclei system into a coherent population trapping state by optical
excitation induced nuclear spin diffusion can be described in terms of Levy
flights, in close analogy with sub-recoil laser cooling of atoms. The large
difference in electronic and nuclear time scales simultaneously allow for a
measurement of the magnetic field produced by nuclear spins, making it possible
to turn the lasers that cause the anomalous spin diffusion process off when the
strength of the resonance fluorescence reveals that the nuclear spins are in
the desired narrow state.Comment: 11 pages, 3 figure