1,394 research outputs found
Geometric Phases for Mixed States during Cyclic Evolutions
The geometric phases of cyclic evolutions for mixed states are discussed in
the framework of unitary evolution. A canonical one-form is defined whose line
integral gives the geometric phase which is gauge invariant. It reduces to the
Aharonov and Anandan phase in the pure state case. Our definition is consistent
with the phase shift in the proposed experiment [Phys. Rev. Lett. \textbf{85},
2845 (2000)] for a cyclic evolution if the unitary transformation satisfies the
parallel transport condition. A comprehensive geometric interpretation is also
given. It shows that the geometric phases for mixed states share the same
geometric sense with the pure states.Comment: 9 pages, 1 figur
Unification of SU(2)xU(1) Using a Generalized Covariant Derivative and U(3)
A generalization of the Yang-Mills covariant derivative, that uses both
vector and scalar fields and transforms as a 4-vector contracted with Dirac
matrices, is used to simplify and unify the Glashow-Weinberg-Salam model. Since
SU(3) assigns the wrong hypercharge to the Higgs boson, it is necessary to use
a special representation of U(3) to obtain all the correct quantum numbers. A
surplus gauge scalar boson emerges in the process, but it uncouples from all
other particles.Comment: 12 pages, no figures. To be published in Int. J. Mod. Phys.
Neutrinoless double beta decay in SO(10) inspired seesaw models
By requiring the lower limit for the lightest right-handed neutrino mass,
obtained in the baryogenesis from leptogenesis scenario, and a Dirac neutrino
mass matrix similar to the up-quark mass matrix we predict small values for the
mass and for the matrix element responsible of the
neutrinoless double beta decay, around eV and
smaller than eV, respectively. The allowed range for the
mass of the heaviest right-handed neutrino is centered around the value of the
scale of B - L breaking in the SO(10) gauge theory with Pati-Salam intermediate
symmetry.Comment: 9 pages, RevTex4. Revised, title change
Dark matter from SU(4) model
The left-right symmetric Pati-Salam model of the unification of quarks and
leptons is based on SU(4) and SU(2)xSU(2) groups. These groups are naturally
extended to include the classification of families of quarks and leptons. We
assume that the family group (the group which unites the families) is also the
SU(4) group. The properties of the 4-th generation of fermions are the same as
that of the ordinary-matter fermions in first three generations except for the
family charge of the SU(4)_F group: F=(1/3,1/3,1/3,-1), where F=1/3 for
fermions of ordinary matter and F=-1 for the 4-th generation. The difference in
F does not allow the mixing between ordinary and fourth-generation fermions.
Because of the conservation of the F charge, the creation of baryons and
leptons in the process of electroweak baryogenesis must be accompanied by the
creation of fermions of the 4-th generation. As a result the excess n_B of
baryons over antibaryons leads to the excess n_{\nu 4}=N-\bar N=n_B of
neutrinos over antineutrinos in the 4-th generation. This massive
fourth-generation neutrino may form the non-baryonic dark matter. In principle
their mass density n_{\nu 4}m_N in the Universe can give the main contribution
to the dark matter, since the lower bound on neutrino mass m_N from the data on
decay of the Z-bosons is m_N > m_Z/2. The straightforward prediction of this
model leads to the amount of cold dark matter relative to baryons, which is an
order of magnitude bigger than allowed by observations. This inconsistency may
be avoided by non-conservation of the F-charge.Comment: 9 pages, 2 figures, version accepted in JETP Letters, corrected after
referee reports, references are adde
Alternating Spin and Orbital Dimerization in Strong-coupling Two-band Models
We study a one-dimensional Hamiltonian consisting of coupled SU(2) spin and
orbital degrees of freedom. Using the density matrix renormalization group, we
calculate the phase-diagram and the ground state correlation functions for this
model. We find that, in addition to the ferromagnetic and power-law
antiferromagnetic phases for spin and orbital degrees of freedom, this model
has a gapless line extending from the ferromagnetic phase to the Bethe ansatz
solvable SU(4) critical point, and a gapped phase with doubly degenerate ground
states which form alternating spin and orbital singlets. The spin-gap and the
order parameters are evaluated and the relevance to several recently discovered
spin-gap materials is discussed.Comment: 4 pages REVTEX and 4 Postscript figure
Baryon and Lepton Number Violation with Scalar Bilinears
We consider all possible scalar bilinears, which couple to two fermions of
the standard model. The various baryon and lepton number violating couplings
allowed by these exotic scalars are studied. We then discuss which ones are
constrained by limits on proton decay (to a lepton and a meson as well as to
three leptons), neutron-antineutron oscillations, and neutrinoless double beta
decay.Comment: 11 pages latex fil
Universal quantum Controlled-NOT gate
An investigation of an optimal universal unitary Controlled-NOT gate that
performs a specific operation on two unknown states of qubits taken from a
great circle of the Bloch sphere is presented. The deep analogy between the
optimal universal C-NOT gate and the `equatorial' quantum cloning machine (QCM)
is shown. In addition, possible applications of the universal C-NOT gate are
briefly discussed.Comment: 18 reference
Signatures of Nucleon Disappearance in Large Underground Detectors
For neutrons bound inside nuclei, baryon instability can manifest itself as a
decay into undetectable particles (e.g., ), i.e.,
as a disappearance of a neutron from its nuclear state. If electric charge is
conserved, a similar disappearance is impossible for a proton. The existing
experimental lifetime limit for neutron disappearance is 4-7 orders of
magnitude lower than the lifetime limits with detectable nucleon decay products
in the final state [PDG2000]. In this paper we calculated the spectrum of
nuclear de-excitations that would result from the disappearance of a neutron or
two neutrons from C. We found that some de-excitation modes have
signatures that are advantageous for detection in the modern high-mass,
low-background, and low-threshold underground detectors, where neutron
disappearance would result in a characteristic sequence of time- and
space-correlated events. Thus, in the KamLAND detector [Kamland], a
time-correlated triple coincidence of a prompt signal, a captured neutron, and
a decay of the residual nucleus, all originating from the same
point in the detector, will be a unique signal of neutron disappearance
allowing searches for baryon instability with sensitivity 3-4 orders of
magnitude beyond the present experimental limits.Comment: 13 pages including 6 figures, revised version, to be published in
Phys.Rev.
"Assisted cloning'' and "orthogonal-complementing" of an unknown state
We propose a protocol where one can exploit dual quantum and classical
channels to achieve perfect ``cloning'' and ``orthogonal-complementing'' of an
unknown state with a minimal assistance from a state preparer (without
revealing what the input state is). The first stage of the protocol requires
usual teleportation and in the second stage, the preparer disentangles the
left-over entangled states by a single particle measurement process and
communicates a number of classical bits (1-cbit per copy) to different parties
so that perfect copies and complement copies are produced. We discuss our
protocol for producing two copies and three copies (and complement copies)
using two and four particle entangled state and suggest how to generalise this
for N copies and complement copies using multiparticle entangled state.Comment: 7 pages, Latex, no figures, submitted to Phys. Rev. A. 1999(to be
accepted
Quantum information cannot be completely hidden in correlations: implications for the black-hole information paradox
The black-hole information paradox has fueled a fascinating effort to
reconcile the predictions of general relativity and those of quantum mechanics.
Gravitational considerations teach us that black holes must trap everything
that falls into them. Quantum mechanically the mass of a black hole leaks away
as featureless (Hawking) radiation, but if the black hole vanishes, where is
the information about the matter that made it? We treat the states of the
in-fallen matter quantum mechanically and show that the black-hole information
paradox becomes more severe. Our formulation of the paradox rules out one of
the most conservative resolutions: that the state of the in-falling matter
might be hidden in correlations between semi-classical Hawking radiation and
the internal states of the black hole. As a consequence, either unitarity or
Hawking's semi-classical predictions must break down. Any resolution of the
black-hole information crisis must elucidate one of these possibilities.Comment: We first obtained this result two years ag
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