561 research outputs found
One-spin quantum logic gates from exchange interactions and a global magnetic field
It has been widely assumed that one-qubit gates in spin-based quantum
computers suffer from severe technical difficulties. We show that one-qubit
gates can in fact be generated using only modest and presently feasible
technological requirements. Our solution uses only global magnetic fields and
controllable Heisenberg exchange interactions, thus circumventing the need for
single-spin addressing.Comment: 4 pages, incl. 1 figure. This significantly modified version accepted
for publication in Phys. Rev. Let
Direct Characterization of Quantum Dynamics
The characterization of quantum dynamics is a fundamental and central task in
quantum mechanics. This task is typically addressed by quantum process
tomography (QPT). Here we present an alternative "direct characterization of
quantum dynamics" (DCQD) algorithm. In contrast to all known QPT methods, this
algorithm relies on error-detection techniques and does not require any quantum
state tomography. We illustrate that, by construction, the DCQD algorithm can
be applied to the task of obtaining partial information about quantum dynamics.
Furthermore, we argue that the DCQD algorithm is experimentally implementable
in a variety of prominent quantum information processing systems, and show how
it can be realized in photonic systems with present day technology.Comment: 4 pages, 2 figures, published versio
Dressed Qubits
Inherent gate errors can arise in quantum computation when the actual system
Hamiltonian or Hilbert space deviates from the desired one. Two important
examples we address are spin-coupled quantum dots in the presence of spin-orbit
perturbations to the Heisenberg exchange interaction, and off-resonant
transitions of a qubit embedded in a multilevel Hilbert space. We propose a
``dressed qubit'' transformation for dealing with such inherent errors. Unlike
quantum error correction, the dressed qubits method does not require additional
operations or encoding redundancy, is insenstitive to error magnitude, and
imposes no new experimental constraints.Comment: Replaced with published versio
Few-body spin couplings and their implications for universal quantum computation
Electron spins in semiconductor quantum dots are promising candidates for the
experimental realization of solid-state qubits. We analyze the dynamics of a
system of three qubits arranged in a linear geometry and a system of four
qubits arranged in a square geometry. Calculations are performed for several
quantum dot confining potentials. In the three-qubit case, three-body effects
are identified that have an important quantitative influence upon quantum
computation. In the four-qubit case, the full Hamiltonian is found to include
both three-body and four-body interactions that significantly influence the
dynamics in physically relevant parameter regimes. We consider the implications
of these results for the encoded universality paradigm applied to the
four-electron qubit code; in particular, we consider what is required to
circumvent the four-body effects in an encoded system (four spins per encoded
qubit) by the appropriate tuning of experimental parameters.Comment: 1st version: 33 pages, 25 figures. Described at APS March Meeting in
2004 (P36.010) and 2005 (B17.00009). Most figures made uglier here to reduce
file size. 2nd version: 19 pages, 9 figures. Much mathematical detail chopped
away after hearing from journal referee; a few typos correcte
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