272 research outputs found
Flatness-based control of a single qubit gate
This work considers the open-loop control problem of steering a two level
quantum system from an initial to a final condition. The model of this system
evolves on the state space SU(2), having two inputs that correspond to the
complex amplitude of a resonant laser field. A symmetry preserving flat output
is constructed using a fully geometric construction and quaternion
computations. Simulation results of this flatness-based open-loop control are
provided.Comment: Submitted to IEEE AC. Simulation code available at
http://cas.ensmp.fr/~rouchon/publications/PR2007/CodeMatlabScilabQubit.zi
Life after charge noise: recent results with transmon qubits
We review the main theoretical and experimental results for the transmon, a
superconducting charge qubit derived from the Cooper pair box. The increased
ratio of the Josephson to charging energy results in an exponential suppression
of the transmon's sensitivity to 1/f charge noise. This has been observed
experimentally and yields homogeneous broadening, negligible pure dephasing,
and long coherence times of up to 3 microseconds. Anharmonicity of the energy
spectrum is required for qubit operation, and has been proven to be sufficient
in transmon devices. Transmons have been implemented in a wide array of
experiments, demonstrating consistent and reproducible results in very good
agreement with theory.Comment: 6 pages, 4 figures. Review article, accepted for publication in
Quantum Inf. Pro
Exponentially Localized Magnetic Fields for Single-Spin Quantum Logic Gates
An infinite array of parallel current-carrying wires is known, from the field
of neutral particle optics, to produce an exponentially localized magnetic
field when the current direction is antiparallel in adjacent wires. We show
that a finite array of several tens of superconducting Nb nanowires can produce
a peak magnetic field of 10mT that decays by a factor of 10^4 over a length
scale of 500nm. Such an array is readily manufacturable with current
technology, and is compatible with both semiconductor and superconducting
quantum computer architectures. A series of such arrays can be used to
individually address single single-spin or flux qubits spaced as little as
100nm apart, and can lead to quantum logic gate times of 5ns.Comment: 5 pages, incl. 4 figure
Internal Consistency of Fault-Tolerant Quantum Error Correction in Light of Rigorous Derivations of the Quantum Markovian Limit
We critically examine the internal consistency of a set of minimal
assumptions entering the theory of fault-tolerant quantum error correction for
Markovian noise. These assumptions are: fast gates, a constant supply of fresh
and cold ancillas, and a Markovian bath. We point out that these assumptions
may not be mutually consistent in light of rigorous formulations of the
Markovian approximation. Namely, Markovian dynamics requires either the
singular coupling limit (high temperature), or the weak coupling limit (weak
system-bath interaction). The former is incompatible with the assumption of a
constant and fresh supply of cold ancillas, while the latter is inconsistent
with fast gates. We discuss ways to resolve these inconsistencies. As part of
our discussion we derive, in the weak coupling limit, a new master equation for
a system subject to periodic driving.Comment: 19 pages. v2: Significantly expanded version. New title. Includes a
debate section in response to comments on the previous version, many of which
appeared here http://dabacon.org/pontiff/?p=959 and here
http://dabacon.org/pontiff/?p=1028. Contains a new derivation of the
Markovian master equation with periodic drivin
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