385,878 research outputs found
Valence holes as Luttinger spinor based qubits in quantum dots
We present a theory of valence holes as Luttinger spinor based qubits in
p-doped self-assembled quantum dots within the 4-band formalism. The
two qubit levels are identified with the two chiralities of the doubly
degenerate ground state. We show that single qubit operations can be
implemented with static magnetic field applied along the and
directions, acting analogously to the and
operators in the qubit subspace respectively. The coupling of two dots and
hence the double qubit operations are shown to be sensitive to the orientation
of the two quantum dots. For vertical qubit arrays, there exists an optimal
qubit separation suitable for the voltage control of qubit-qubit interactions
Study of multi black hole and ring singularity apparent horizons
We study critical black hole separations for the formation of a common
apparent horizon in systems of - black holes in a time symmetric
configuration. We study in detail the aligned equal mass cases for ,
and relate them to the unequal mass binary black hole case. We then study the
apparent horizon of the time symmetric initial geometry of a ring singularity
of different radii. The apparent horizon is used as indicative of the location
of the event horizon in an effort to predict a critical ring radius that would
generate an event horizon of toroidal topology. We found that a good estimate
for this ring critical radius is . We briefly discuss the
connection of this two cases through a discrete black hole 'necklace'
configuration.Comment: 31 pages, 21 figure
Better Complexity Bounds for Cost Register Automata
Cost register automata (CRAs) are one-way finite automata whose transitions have the side effect that a register is set to the result of applying a state-dependent semiring operation to a pair of registers. Here it is shown that CRAs over the tropical semiring (N U {infinity},min,+) can simulate polynomial time computation, proving along the way that a naturally defined width-k circuit value problem over the tropical semiring is P-complete.
Then the copyless variant of the CRA, requiring that semiring operations be applied to distinct registers, is shown no more powerful than NC^1 when the semiring is (Z,+,x) or (Gamma^*,max,concat). This relates questions left open in recent work on the complexity of CRA-computable functions to long-standing class separation conjectures in complexity theory, such as NC versus P and NC^1 versus GapNC^1
A precise CNOT gate in the presence of large fabrication induced variations of the exchange interaction strength
We demonstrate how using two-qubit composite rotations a high fidelity
controlled-NOT (CNOT) gate can be constructed, even when the strength of the
interaction between qubits is not accurately known. We focus on the exchange
interaction oscillation in silicon based solid-state architectures with a
Heisenberg Hamiltonian. This method easily applies to a general two-qubit
Hamiltonian. We show how the robust CNOT gate can achieve a very high fidelity
when a single application of the composite rotations is combined with a modest
level of Hamiltonian characterisation. Operating the robust CNOT gate in a
suitably characterised system means concatenation of the composite pulse is
unnecessary, hence reducing operation time, and ensuring the gate operates
below the threshold required for fault-tolerant quantum computation.Comment: 9 pages, 8 figure
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