33 research outputs found
Analysis of the SN1987A two-stage explosion hypothesis with account for the MSW neutrino flavour conversion
Detection of 5 events by the Liquid Scintillation Detector (LSD) on February,
23, 1987 was interpreted in the literature as the detection of neutrinos from
the first stage of the two-stage supernova collapse. We pose rigid constraints
on the properties of the first stage of the collapse, taking into account
neutrino flavour conversion due to the MSW-effect and general properties of
supernova neutrino emission. The constraints depend on the unknown neutrino
mass hierarchy and mixing angle \theta_{13}.Comment: presented at "Rencontres de Moriond EW 2007", 10-17 March 200
Perpetual motion of a mobile impurity in a one-dimensional quantum gas
Consider an impurity particle injected in a degenerate one-dimensional gas of
noninteracting fermions (or, equivalently, Tonks-Girardeau bosons) with some
initial momentum . We examine the infinite-time value of the momentum of
the impurity, , as a function of . A lower bound on
is derived under fairly general conditions. The derivation,
based on the existence of the lower edge of the spectrum of the host gas, does
not resort to any approximations. The existence of such bound implies the
perpetual motion of an impurity in a one-dimensional gas of noninteracting
fermions or Tonks-Girardeau bosons at zero temperature. The bound has an
especially simple and useful form when the interaction between the impurity and
host particles is everywhere repulsive
A necessary condition for quantum adiabaticity applied to the adiabatic Grover search
Numerous sufficient conditions for adiabaticity of the evolution of a driven
quantum system have been known for quite a long time. In contrast, necessary
adiabatic conditions are scarce. A practicable necessary condition well-suited
for many-body systems has been proven recently in [Phys. Rev. Lett. 119, 200401
(2017)]. Here we tailor this condition for estimating run times of quantum
adiabatic algorithms. As an illustration, the condition is applied to the
adiabatic algorithm for searching in an unstructured database (adiabatic Grover
search algorithm). We find that thus obtained lower bound on the run time of
this algorithm reproduces scaling ( being the number of database
entries) of the explicitly known optimal run time. This observation highlights
the merits of the new adiabatic condition and its potential relevance to
adiabatic quantum computing
Constraints on neutrino mixing angle theta_13 and Supernova neutrino fluxes from the LSD neutrino signal from SN1987A
Detection of 5 events by the Liquid Scintillation Detector (LSD) on February,
23, 1987 was recently interpreted as a detection of the electron neutrino flux
from the first stage of the two-stage Supernova collapse. We show that, if
neutrino mass hierarchy is normal, such interpretation excludes values of
neutrino mixing angle \theta_{13} larger than 3\cdot 10^{-2}, independently of
the particular Supernova collapse model. Also constraints on the original
fluxes of neutrinos and antineutrinos of different flavours are obtained.Comment: 6 pages, no figure
A remark on the notion of independence of quantum integrals of motion in the thermodynamic limit
Studies of integrable quantum many-body systems have a long history with an
impressive record of success. However, surprisingly enough, an unambiguous
definition of quantum integrability remains a matter of an ongoing debate. We
contribute to this debate by dwelling upon an important aspect of quantum
integrability -- the notion of independence of quantum integrals of motion
(QIMs). We point out that a widely accepted definition of functional
independence of QIMs is flawed, and suggest a new definition. Our study is
motivated by the PXP model -- a model of spins possessing an
extensive number of binary QIMs. The number of QIMs which are independent
according to the common definition turns out to be equal to the number of
spins, . A common wisdom would then suggest that the system is completely
integrable, which is not the case. We discuss the origin of this conundrum and
demonstrate how it is resolved when a new definition of independence of QIMs is
employed
Neutrino magnetic moment signatures in the supernova neutrino signal
It is known that if neutrino is a Dirac fermion with magnetic moment, then
\nu_L -> \nu_R -> \nu_L transition of supernova neutrinos may occur. The first
stage of such transition is due to the neutrino spin flip inside the hot dense
supernova core, while the second one - due to the neutrino spin precession in
the galactic magnetic field on the way from the supernova to terrestrial
detectors. This can result in the detection of 60-200 MeV neutrinos
simultaneously with the "normal" supernova neutrino signal, which would be a
smoking gun for the Dirac neutrino magnetic moment, \mu. We argue that in case
of a nearby supernova explosion (~10 kpc away from the Earth) one may observe
such high-energy events in Super-Kamiokande if \mu \gtrsim 10^{-13} \mu_B, and
in a Mt-scale detector if \mu \gtrsim 0.5*10^{-13} \mu_B. Such an observation
by itself, however, may be not sufficient to determine the value of the
magnetic moment, because of the ignorance of the interstellar magnetic field.
We point out that if in addition a deficit of the neutronization burst
neutrinos is established, it would be possible to extract the value of the
magnetic moment from observations. We also briefly discuss a possible Majorana
magnetic moment signature due to \nu_e -> \bar{\nu}_{\mu,\tau} flip inside the
supernova core
Large quantum superpositions of a nanoparticle immersed in superfluid helium
Preparing and detecting spatially extended quantum superpositions of a
massive object comprises an important fundamental test of quantum theory. These
quantum states are extremely fragile and tend to quickly decay into incoherent
mixtures due to the environmental decoherence. Experimental setups considered
up to date address this threat in a conceptually straightforward way -- by
eliminating the environment, i.e. by isolating an object in a sufficiently high
vacuum. We show that another option exists: decoherence is suppressed in the
presence of a strongly interacting environment if this environment is
superfluid. Indeed, as long as an object immersed in a pure superfluid at zero
temperature moves with a velocity below the critical one, it does not create,
absorb or scatter any excitations of the superfluid. Hence, in this idealized
situations the decoherence is absent. In reality the decoherence will be
present due to thermal excitations of the superfluid and impurities
contaminating the superfluid. We examine various decoherence channels in the
superfluid He. It is shown that the total decoherence time can be as large
as tens of seconds for a amu nanoparticle delocalized over nm in
helium at mK. Performing interference experiments in superfluid helium can
provide certain practical advantages compared to conventional schemes, e.g.
compensation of gravity by the buoyancy force and effective cooling
Perpetual motion and driven dynamics of a mobile impurity in a quantum fluid
We study the dynamics of a mobile impurity in a quantum fluid at zero
temperature. Two related settings are considered. In the first setting the
impurity is injected in the fluid with some initial velocity ,
and we are interested in its velocity at infinite time, .
We derive a rigorous upper bound on for
initial velocities smaller than the generalized critical velocity. In the limit
of vanishing impurity-fluid coupling this bound amounts to which can be regarded as a rigorous proof of the
Landau criterion of superfluidity. In the case of a finite coupling the
velocity of the impurity can drop, but not to zero; the bound quantifies the
maximal possible drop. In the second setting a small constant force is exerted
upon the impurity. We argue that two distinct dynamical regimes exist --
backscattering oscillations of the impurity velocity and saturation of the
velocity without oscillations. For fluids with (where and are the Landau critical velocity and sound velocity,
respectively) the latter regime is realized. For fluids with both regimes are possible. Which regime is realized in this case depends
on the mass of the impurity, a nonequilibrium quantum phase transition
occurring at some critical mass. Our results are equally valid in one, two and
three dimensions.Comment: v4: published versio
Decoherence at the level of eigenstates
An eigenstate decoherence hypothesis states that each individual eigenstate
of a large closed system is locally classical-like. We extend this hypothesis
to account for a typically extremely short time scale of decoherence. The
extension implies that nondiagonal matrix elements of certain operators -
quantumness witnesses - are suppressed as long as the energy difference between
corresponding eigenstates is smaller than the inverse decoherence time
Entangling problem Hamiltonian for adiabatic quantum computation
Adiabatic quantum computation starts from embedding a computational problem
into a Hamiltonian whose ground state encodes the solution to the problem. This
problem Hamiltonian, , is normally chosen to be diagonal in the
computational basis, that is a product basis for qubits. We point out that
can be chosen to be non-diagonal in the computational basis. To be
more precise, we show how to construct in such a way that all its
excited states are entangled with respect to the qubit tensor product
structure, while the ground state is still of the product form and encodes the
solution to the problem. We discuss how such entangling problem Hamiltonians
can improve the performance of the adiabatic quantum computation.Comment: v2: entanglement entropy of excited states addressed numericall