Defense Intelligence Reference Document Quantum Computing And Utilizing Organic Molecules In Automation Technology

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Two special schemes that operate differently than digital analogues of gate-based technology
are adiabatic and cluster-state quantum computation. In an adiabatic design, the answer is
the ground state of a complex network of interactions and the interactions are slowly turned
on to evolve qubits from the initial state to the final ground state. In the cluster-state
scheme, the system is placed in a particular state through use of a small set of control gates
and the output is repeatedly measured using arbitrary basis (the fault-tolerance mechanism).
In the adiabatic case, the computation is 'programmed' in the setup of interactions. In the
cluster-state case, the calculation produces a superposition of states that need several
measurements to ensure correct interpretation. Both schemes have been shown to be
equivalent to gate-based circuit technologies. (15-17)
INITIALIZATION AND MEASUREMENT
Implied in the discussions above is the ability to set the quantum computer into a known
initial state, measure various states during computation if needed, and output the final state.
These processes can be tricky while maintaining isolation and low entropy. The initialization
and measurement techniques are discussed with each technology.
The Quantum Dot Approach
A major obstacle in the quest to design and construct a radically new kind of inorganic
quantum computer has been finding a way to manipulate the single electrons that are likely
to constitute the new machines' qubits. The ability to manipulate and alter a single electron
without disturbing the trillions of electrons in the immediate surroundings has become a
research focus for many studies. (18) (19)(20) A candidate is to utilize properties of the
intrinsic spin of the electron. In 1925, Austrian physicist Wolfgang Pauli proposed that an
electron in a quantum state can assume only one of two states-"spin-up" or "spin down."
(21) One approach to manipulate spin state and electrical charge independently for use in
quantum computing has arisen in the quantum dot.
Quantum dots (QDs) are tiny islands within a solid state lattice where electrons experience
charging effects as well as quantum confinement, like an electron in an energy level around a
nucleus. (22) Besides fundamental insights into matter, these artificial atoms can also work
as building blocks for the control of electronics at the single electron level. The so called
single-electron transistors are able to switch on and off electron transport through a dot by
means of electrical gates using the effect of Coulomb blockade.
Conversely, one can use spin rather than charge to control electrical conduction in
mesoscopic-scale electronics; such "spin-controlled electronic devices," and their
development and study, are termed spintronics. (23) Exploiting the spin degree of freedom,
a quantum dot can act as a spin filter (24)(25)(26) or as a spin-blockade device. (27)
Quantum dots have been proposed as host for an electron-spin qubit. Arrays of such
quantum dots with tunable tunnel-couplings between them would work as a universal
quantum computer. (28) Recent progress in this field using GaAs-based two-dimensional
electron gases is impressive (29), though decoherence can arise from spin-orbit and
hyperfine interaction with the nuclear-spin(s) of the host material (30) (31) (32). These
decoherence effects can be reduced by using a lattice structure with low magnetic moment.
Carbon has near zero magnetic moment due to the six each paired protons and neutrons in
carbon isotope 12C; a small net magnetic moment comes from the natural 1% contamination
of 13C.
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