16 research outputs found
Parametric amplification of the mechanical vibrations of a suspended nanowire by magnetic coupling to a Bose-Einstein condensate
We consider the possibility of parametric amplification of a mechanical
vibration mode of a nanowire due to its interaction with a Bose-Einstein
condensate (BEC) of ultracold atoms. The magneto-mechanical coupling is
mediated by the vibrationally modulated magnetic field around the
current-carrying nanowire, which can induce atomic transitions between
different hyperfine sublevels. We theoretically analyze the limitations arising
from the fact that the spin inverted atomic medium which feeds the mechanical
oscillation has a finite bandwidth in the range of the chemical potential of
the condensate
Magnetic noise spectrum measurement by an atom laser in gravity
Bose-Einstein condensates of ultracold atoms can be used to sense
fluctuations of the magnetic field by means of transitions into untrapped
hyperfine states. It has been shown recently that counting the outcoupled atoms
can yield the power spectrum of the magnetic noise. We calculate the spectral
resolution function which characterizes the condensate as a noise measurement
device in this scheme. We use the description of the radio-frequency
outcoupling scheme of an atom laser which takes into account the gravitational
acceleration. Employing both an intuitive and the exact three-dimensional and
fully quantum mechanical approach we derive the position-dependent spectral
resolution function for condensates of different size and shape
P\'olya number of continuous-time quantum walks
We propose a definition for the P\'olya number of continuous-time quantum
walks to characterize their recurrence properties. The definition involves a
series of measurements on the system, each carried out on a different member
from an ensemble in order to minimize the disturbance caused by it. We examine
various graphs, including the ring, the line, higher dimensional integer
lattices and a number of other graphs and calculate their P\'olya number. For
the timing of the measurements a Poisson process as well as regular timing are
discussed. We find that the speed of decay for the probability at the origin is
the key for recurrence.Comment: 8 pages, no figures. Accepted for publication in Physical Review
Quantum walks: a comprehensive review
Quantum walks, the quantum mechanical counterpart of classical random walks,
is an advanced tool for building quantum algorithms that has been recently
shown to constitute a universal model of quantum computation. Quantum walks is
now a solid field of research of quantum computation full of exciting open
problems for physicists, computer scientists, mathematicians and engineers.
In this paper we review theoretical advances on the foundations of both
discrete- and continuous-time quantum walks, together with the role that
randomness plays in quantum walks, the connections between the mathematical
models of coined discrete quantum walks and continuous quantum walks, the
quantumness of quantum walks, a summary of papers published on discrete quantum
walks and entanglement as well as a succinct review of experimental proposals
and realizations of discrete-time quantum walks. Furthermore, we have reviewed
several algorithms based on both discrete- and continuous-time quantum walks as
well as a most important result: the computational universality of both
continuous- and discrete- time quantum walks.Comment: Paper accepted for publication in Quantum Information Processing
Journa