6 research outputs found
Dynamic Nuclear Polarization with Single Electron Spins
We polarize nuclear spins in a GaAs double quantum dot by controlling
two-electron spin states near the anti-crossing of the singlet (S) and m_S=+1
triplet (T+) using pulsed gates. An initialized S state is cyclically brought
into resonance with the T+ state, where hyperfine fields drive rapid rotations
between S and T+, 'flipping' an electron spin and 'flopping' a nuclear spin.
The resulting Overhauser field approaches 80 mT, in agreement with a simple
rate-equation model. A self-limiting pulse sequence is developed that allows
the steady-state nuclear polarization to be set using a gate voltage.Comment: related papers available at http://marcuslab.harvard.ed
Dynamic nuclear polarisation in biased quantum wires with spin-orbit interaction
We propose a new method for dynamic nuclear polarisation in a quasi
one-dimensional quantum wire utilising the spin-orbit interaction, the
hyperfine interaction, and a finite source-drain potential difference. In
contrast with current methods, our scheme does not rely on external magnetic or
optical sources which makes independent control of closely placed devices much
more feasible. Using this method, a significant polarisation of a few per cent
is possible in currently available InAs wires which may be detected by
conductance measurements. This may prove useful for nuclear-magnetic-resonance
studies in nanoscale systems as well as in spin-based devices where external
magnetic and optical sources will not be suitable.Comment: 6 pages, published versio
Triplet-Singlet Spin Relaxation via Nuclei in a Double Quantum Dot
The spin of a confined electron, when oriented originally in some direction,
will lose memory of that orientation after some time. Physical mechanisms
leading to this relaxation of spin memory typically involve either coupling of
the electron spin to its orbital motion or to nuclear spins. Relaxation of
confined electron spin has been previously measured only for Zeeman or exchange
split spin states, where spin-orbit effects dominate relaxation, while spin
flips due to nuclei have been observed in optical spectroscopy studies. Using
an isolated GaAs double quantum dot defined by electrostatic gates and direct
time domain measurements, we investigate in detail spin relaxation for
arbitrary splitting of spin states. Results demonstrate that electron spin
flips are dominated by nuclear interactions and are slowed by several orders of
magnitude when a magnetic field of a few millitesla is applied. These results
have significant implications for spin-based information processing
Recipes for spin-based quantum computing
Technological growth in the electronics industry has historically been
measured by the number of transistors that can be crammed onto a single
microchip. Unfortunately, all good things must come to an end; spectacular
growth in the number of transistors on a chip requires spectacular reduction of
the transistor size. For electrons in semiconductors, the laws of quantum
mechanics take over at the nanometre scale, and the conventional wisdom for
progress (transistor cramming) must be abandoned. This realization has
stimulated extensive research on ways to exploit the spin (in addition to the
orbital) degree of freedom of the electron, giving birth to the field of
spintronics. Perhaps the most ambitious goal of spintronics is to realize
complete control over the quantum mechanical nature of the relevant spins. This
prospect has motivated a race to design and build a spintronic device capable
of complete control over its quantum mechanical state, and ultimately,
performing computations: a quantum computer.
In this tutorial we summarize past and very recent developments which point
the way to spin-based quantum computing in the solid-state. After introducing a
set of basic requirements for any quantum computer proposal, we offer a brief
summary of some of the many theoretical proposals for solid-state quantum
computers. We then focus on the Loss-DiVincenzo proposal for quantum computing
with the spins of electrons confined to quantum dots. There are many obstacles
to building such a quantum device. We address these, and survey recent
theoretical, and then experimental progress in the field. To conclude the
tutorial, we list some as-yet unrealized experiments, which would be crucial
for the development of a quantum-dot quantum computer.Comment: 45 pages, 12 figures (low-res in preprint, high-res in journal)
tutorial review for Nanotechnology; v2: references added and updated, final
version to appear in journa