Biomedical Engineering Reference
In-Depth Information
with the environment. It is still an open question to what extent macroscopic
systems can be in superposition states and, if not, where and why the
transition to the macroscopic world occurs. Currently, experiments are
being developed to study these phenomena. Examples of such experiments
are the observation of interference of large molecules with themselves,
coupling a small well-defined quantum system with a macroscopic system
like a tiny mirror, and the preparation of large numbers of photons in a
superposition state. In parallel to addressing these fundamental issues, the
question is also raised as to whether this high level of control over small
quantum objects can form a useful basis for technological applications. It is
too early to say whether applications will fully exploit the fundamental
resources available in nature, but major conceptual breakthroughs have
already been achieved. For example, entangled states have proven a
valuable resource for novel quantum communication protocols, such as
quantum cryptography, which is fundamentally unbreakable.
2.6.1 Molecular magnets as qubits: spin tunneling
A molecular magnet is a molecule that is typically ferromagnetic or
antiferromagnetic with an isolated spin center. Physical candidates for an
ideal molecular magnet are evaluated based on three criteria:
.
the total spin of the isolated system is high
.
they are typically large molecules with little intermolecular interaction,
utilizing the 1/r
3
dipole-dipole interaction
.
they exhibit high magnetic anisotropy.
By far the most studied molecules have been Mn
12
and Fe
8
, each being
ferromagnetic with high magnetic anisotropy and total spin S=10. The
resulting spin states of both Mn
12
and Fe
8
can be modeled as a double-well
potential, which becomes the basis for further analysis of molecular
magnets.
The most promising use of molecular magnets for both data storage and
computation is through the exploitation of the quantum phenomenon of
spin tunneling. The concept of the intrinsic angular momentum of an
electron, later referred to as its spin, was introduced by the Dutch physicists
George Uhlenbeck and Samuel Goudsmit in 1925. In retrospect, the
electron's spin was first observed in 1922 by the Stern-Gerlach experiment.
Stern and Gerlach found that a bundle of neutral silver particles is split into
two beams due to the interaction with an inhomogeneous magnetic field,
and explained this by two quantized states of the angular momentum.
A better theoretical explanation of the Stern-Gerlach experiment was
given in 1927 by Wolfgang Pauli, by taking into account the intrinsic
angular momentum of electrons, their spin. The spin of an electron is a pure
