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Creation of spin-entanglement in semiconductor nanostructures
Quantum correlations and entanglement are a fundamental resource for quantum computing and
quantum communication. Against our most profound intuition, these phenomena allow distant
partners to share some instantaneous information –although only of probabilistic nature [1].
The experimental demonstration of entanglement that is useful for semiconductor qubits is
presently one of the big challenges in Physics, and has motivated a number of theoretical
proposals for the creation of entangled spin qubits.
The difficult task is not so much to find quantum correlations - which often occur in the most
stable configuration of quantum systems (such as e.g. the binding state in a diatomic
molecule) - but rather to design a way to separate them while maintaining their entanglement.
For this purpose, we have proposed to use semiconductor quantum dots - tiny regions of around
200nm where the electrostatic potential can allow exactly one electron to pass. By tuning the
energy levels of the dots, one can transform them into energy filters, which select only
electrons with specific energies. This trick is then used to split the correlated pair into
two distant partners.
For instance, one can use the singlet state (a pair of electrons with zero total spin) that
exists in the ground state of a quantum dot as the source of the quantum correlation. Using
two secondary dots as energy filters, one can ensure that the pair will split and leave as
two electrons, each one in a separate drain lead, carrying with them their entanglement [2].
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Figure 1: Left: Creation of spin-entanglement with three coupled
quantum dots. Because of energy conservation, the correlated pair present
in the central dot will split coherently and travel via the secondary dots
to the drain leads. Right: A superconductor provides pairs of electrons in
the spin-singlet state, while the Coulomb repulsion in the carbon nanotubes
separates them. |
Another type of proposals uses s-wave superconductors as the source of correlation [3,4]. The
goal is to extract the Cooper pairs (electrons with opposite momentum in a singlet state)
responsible for the superconducting properties. One can use quantum dots to act as energy
filters or, alternatively, carbon nanotubes. Indeed, electrons confined in such quasi-one
dimensional channels repel each other strongly because of their electric (Coulomb)
interaction. Hence, electrons will dominantly escape from the superconductor into two
different nanotubes.
A third proposal [5] makes use of a very general quantum interference phenomenon: because of
quantum indistinguishability, only electrons with the right orbital symmetry (in the
spin-singlet state) can emerge from a pair collision at right angle. One can use quantum
point contacts to create beams of electrons in semiconductors, as well as to select only
right angle scattering events. Advanced experimental techniques for the imaging of electron
flow on a two-dimensional semiconductor could also be used to probe such collisions.
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Figure 2: Collisions between electrons in a two-dimensional electron
gas (2DEG). The electrons are injected from two reservoirs with the help
of quantum point contacts (QPC), which create a narrow channel supporting
only one transverse mode. The electrons detected at a scattering angle of
Θ=π/2 are entangled. |
Contact:
Daniel Saraga |
Daniel Loss |
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Institute of Physics
University of Basel
Switzerland
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