25,039 research outputs found
Population inversion of driven two-level systems in a structureless bath
We derive a master equation for a driven double-dot damped by an unstructured
phonon bath, and calculate the spectral density. We find that bath mediated
photon absorption is important at relatively strong driving, and may even
dominate the dynamics, inducing population inversion of the double dot system.
This phenomenon is consistent with recent experimental observations.Comment: 4 Pages, Added Reference [30] to Dykman, 1979, available at
http://www.pa.msu.edu/people/dykman/pub/Sov.J.LowTemp.Phys_5.pd
Universality of Decay out of Superdeformed Bands in the 190 Mass Region
Superdeformed nuclei in the 190 mass region exhibit a striking universality
in their decay-out profiles. We show that this universality can be explained in
the two-level model of superdeformed decay as related to a strong separation of
energy scales: a higher scale related to the nuclear interactions, and a lower
scale caused by electromagnetic decay. Furthermore, we present the results of
the two-level model for all decays for which sufficient data are known,
including statistical extraction of the matrix element for tunneling through
the potential barrier.Comment: 7 pages, 3 figures. v2: some minor clarifications, minor correction
to Fig.
Preparing multi-partite entanglement of photons and matter qubits
We show how to make event-ready multi-partite entanglement between qubits
which may be encoded on photons or matter systems. Entangled states of matter
systems, which can also act as single photon sources, can be generated using
the entangling operation presented in quant-ph/0408040. We show how to entangle
such sources with photon qubits, which may be encoded in the dual rail,
polarization or time-bin degrees of freedom. We subsequently demonstrate how
projective measurements of the matter qubits can be used to create entangled
states of the photons alone. The state of the matter qubits is inherited by the
generated photons. Since the entangling operation can be used to generate
cluster states of matter qubits for quantum computing, our procedure enables us
to create any (entangled) photonic quantum state that can be written as the
outcome of a quantum computer.Comment: 10 pages, 4 figures; to appear in Journal of Optics
Manifestation of three-body forces in f7/2-shell nuclei
The traditional nuclear shell model approach is extended to include many-body
forces. The empirical Hamiltonian with a three-body force is constructed for
the identical nucleons on the 0f7/2 shell. Manifestations of the three-body
force in spectra, binding energies, seniority mixing, particle-hole symmetry,
electromagnetic and particle transition rates are investigated. It is shown
that in addition to the usual expansion of the valence space within the
tranditional two-body shell model, the three-body component in the Hamiltonian
can be an important part improving the quality of the theoretical approach.Comment: 5 pages, 1 figur
Coupling of spacetime atoms and spin foam renormalisation from group field theory
We study the issue of coupling among 4-simplices in the context of spin foam
models obtained from a group field theory formalism. We construct a
generalisation of the Barrett-Crane model in which an additional coupling
between the normals to tetrahedra, as defined in different 4-simplices that
share them, is present. This is realised through an extension of the usual
field over the group manifold to a five argument one. We define a specific
model in which this coupling is parametrised by an additional real parameter
that allows to tune the degree of locality of the resulting model,
interpolating between the usual Barrett-Crane model and a flat BF-type one.
Moreover, we define a further extension of the group field theory formalism in
which the coupling parameter enters as a new variable of the field, and the
action presents derivative terms that lead to modified classical equations of
motion. Finally, we discuss the issue of renormalisation of spin foam models,
and how the new coupled model can be of help regarding this.Comment: RevTeX, 18 pages, no figure
Selective spin coupling through a single exciton
We present a novel scheme for performing a conditional phase gate between two
spin qubits in adjacent semiconductor quantum dots through delocalized single
exciton states, formed through the inter-dot Foerster interaction. We consider
two resonant quantum dots, each containing a single excess conduction band
electron whose spin embodies the qubit. We demonstrate that both the two-qubit
gate, and arbitrary single-qubit rotations, may be realized to a high fidelity
with current semiconductor and laser technology.Comment: 5 pages, 3 figures; published version, equation formatting improved,
references adde
Quantum computation via measurements on the low-temperature state of a many-body system
We consider measurement-based quantum computation using the state of a
spin-lattice system in equilibrium with a thermal bath and free to evolve under
its own Hamiltonian. Any single qubit measurements disturb the system from
equilibrium and, with adaptive measurements performed at a finite rate, the
resulting dynamics reduces the fidelity of the computation. We show that it is
possible to describe the loss in fidelity by a single quantum operation on the
encoded quantum state that is independent of the measurement history. To
achieve this simple description, we choose a particular form of spin-boson
coupling to describe the interaction with the environment, and perform
measurements periodically at a natural rate determined by the energy gap of the
system. We found that an optimal cooling exists, which is a trade-off between
keeping the system cool enough that the resource state remains close to the
ground state, but also isolated enough that the cooling does not strongly
interfere with the dynamics of the computation. For a sufficiently low
temperature we obtain a fault-tolerant threshold for the couplings to the
environment.Comment: 9 pages, 3 figures; v2 published versio
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