633 research outputs found
Quantum Zeno and anti-Zeno effects by indirect measurement with finite errors
We study the quantum Zeno effect and the anti-Zeno effect in the case of
`indirect' measurements, where a measuring apparatus does not act directly on
an unstable system, for a realistic model with finite errors in the
measurement. A general and simple formula for the decay rate of the unstable
system under measurement is derived. In the case of a Lorentzian form factor,
we calculate the full time evolutions of the decay rate, the response of the
measuring apparatus, and the probability of errors in the measurement. It is
shown that not only the response time but also the detection efficiency plays a
crucial role. We present the prescription for observing the quantum Zeno and
anti-Zeno effects, as well as the prescriptions for avoiding or calibrating
these effects in general experiments.Comment: 4 pages, 3 figure
Duality and integer quantum Hall effect in isotropic 3D crystals
We show here a series of energy gaps as in Hofstadter's butterfly, which have
been shown to exist by Koshino et al [Phys. Rev. Lett. 86, 1062 (2001)] for
anisotropic three-dimensional (3D) periodic systems in magnetic fields
\Vec{B}, also arise in the isotropic case unless \Vec{B} points in
high-symmetry directions. Accompanying integer quantum Hall conductivities
can, surprisingly, take values
even for a fixed direction of \Vec{B}
unlike in the anisotropic case. We can intuitively explain the high-magnetic
field spectra and the 3D QHE in terms of quantum mechanical hopping by
introducing a ``duality'', which connects the 3D system in a strong \Vec{B}
with another problem in a weak magnetic field .Comment: 7 pages, 6 figure
Disentanglement in a quantum critical environment
We study the dynamical process of disentanglement of two qubits and two
qutrits coupled to an Ising spin chain in a transverse field, which exhibits a
quantum phase transition. We use the concurrence and negativity to quantify
entanglement of two qubits and two qutrits, respectively. Explicit connections
between the concurrence (negativity) and the decoherence factors are given for
two initial states, the pure maximally entangled state and the mixed Werner
state. We find that the concurrence and negativity decay exponentially with
fourth power of time in the vicinity of critical point of the environmental
system.Comment: 8 pages, 6 figure
Magneto-optical properties of multilayer graphenes
The magneto-optical absorption properties of graphene multilayers are
theoretically studied. It is shown that the spectrum can be decomposed into
sub-components effectively identical to the monolayer or bilayer graphene,
allowing us to understand the spectrum systematically as a function of the
layer number. Odd-layered graphenes always exhibit absorption peaks which
shifts in proportion to sqrt(B), with B being the magnetic field, due to the
existence of an effective monolayer-like subband. We propose a possibility of
observing the monolayer-like spectrum even in a mixture of multilayer graphene
films with various layers numbers.Comment: 9 pages, 7 figure
Phase Diagram for the Hofstadter butterfly and integer quantum Hall effect in three dimensions
We give a perspective on the Hofstadter butterfly (fractal energy spectrum in
magnetic fields), which we have shown to arise specifically in
three-dimensional(3D) systems in our previous work. (i) We first obtain the
`phase diagram' on a parameter space of the transfer energies and the magnetic
field for the appearance of Hofstadter's butterfly spectrum in anisotropic
crystals in 3D. (ii) We show that the orientation of the external magnetic
field can be arbitrary to have the 3D butterfly. (iii) We show that the
butterfly is beyond the semiclassical description. (iv) The required magnetic
field for a representative organic metal is estimated to be modest (
T) if we adopt higher Landau levels for the butterfly. (v) We give a simpler
way of deriving the topological invariants that represent the quantum Hall
numbers (i.e., two Hall conductivity in 3D, , in
units of ).Comment: 8 pages, 8 figures, eps versions of the figures will be sent on
request to [email protected]
Full Quantum Analysis of Two-Photon Absorption Using Two-Photon Wavefunction: Comparison with One-Photon Absorption
For dissipation-free photon-photon interaction at the single photon level, we
analyze one-photon transition and two-photon transition induced by photon pairs
in three-level atoms using two-photon wavefunctions. We show that the
two-photon absorption can be substantially enhanced by adjusting the time
correlation of photon pairs. We study two typical cases: Gaussian wavefunction
and rectangular wavefunction. In the latter, we find that under special
conditions one-photon transition is completely suppressed while the high
probability of two-photon transition is maintained.Comment: 6 pages, 4 figure
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