4,015 research outputs found
Probing strongly interacting atomic gases with energetic atoms
We investigate properties of an energetic atom propagating through strongly
interacting atomic gases. The operator product expansion is used to
systematically compute a quasiparticle energy and its scattering rate both in a
spin-1/2 Fermi gas and in a spinless Bose gas. Reasonable agreement with recent
quantum Monte Carlo simulations even at a relatively small momentum k/kF>1.5
indicates that our large-momentum expansions are valid in a wide range of
momentum. We also study a differential scattering rate when a probe atom is
shot into atomic gases. Because the number density and current density of the
target atomic gas contribute to the forward scattering only, its contact
density (measure of short-range pair correlation) gives the leading
contribution to the backward scattering. Therefore, such an experiment can be
used to measure the contact density and thus provides a new local probe of
strongly interacting atomic gases.Comment: 35 pages, 11 figures; (v4) published with the new titl
Universal four-component Fermi gas in one dimension
A four-component Fermi gas in one dimension with a short-range four-body
interaction is shown to exhibit a one-dimensional analog of the BCS-BEC
crossover. Its low-energy physics is governed by a Tomonaga-Luttinger liquid
with three spin gaps. The spin gaps are exponentially small in the weak
coupling (BCS) limit where they arise from the charge-density-wave instability,
and become large in the strong coupling (BEC) limit because of the formation of
tightly-bound tetramers. We investigate the ground-state energy, the sound
velocity, and the gap spectrum in the BCS-BEC crossover and discuss exact
relationships valid in our system. We also show that a one-dimensional analog
of the Efimov effect occurs for five bosons while it is absent for fermions.
Our work opens up a very rich new field of universal few-body and many-body
physics in one dimension.Comment: 9 pages, 3 figures; (v2) Efimov effect for 5 bosons in 1D is
discussed; (v3) expanded versio
Megabits secure key rate quantum key distribution
Quantum cryptography (QC) can provide unconditional secure communication
between two authorized parties based on the basic principles of quantum
mechanics. However, imperfect practical conditions limit its transmission
distance and communication speed. Here we implemented the differential phase
shift (DPS) quantum key distribution (QKD) with up-conversion assisted hybrid
photon detector (HPD) and achieved 1.3 M bits per second secure key rate over a
10-km fiber, which is tolerant against the photon number splitting (PNS)
attack, general collective attacks on individual photons, and any other known
sequential unambiguous state discrimination (USD) attacks.Comment: 14 pages, 4 figure
Computing with cells: membrane systems - some complexity issues.
Membrane computing is a branch of natural computing which abstracts computing models from the structure and the functioning of the living cell. The main ingredients of membrane systems, called P systems, are (i) the membrane structure, which consists of a hierarchical arrangements of membranes which delimit compartments where (ii) multisets of symbols, called objects, evolve according to (iii) sets of rules which are localised and associated with compartments. By using the rules in a nondeterministic/deterministic maximally parallel manner, transitions between the system configurations can be obtained. A sequence of transitions is a computation of how the system is evolving. Various ways of controlling the transfer of objects from one membrane to another and applying the rules, as well as possibilities to dissolve, divide or create membranes have been studied. Membrane systems have a great potential for implementing massively concurrent systems in an efficient way that would allow us to solve currently intractable problems once future biotechnology gives way to a practical bio-realization. In this paper we survey some interesting and fundamental complexity issues such as universality vs. nonuniversality, determinism vs. nondeterminism, membrane and alphabet size hierarchies, characterizations of context-sensitive languages and other language classes and various notions of parallelism
Josephson Effect between Condensates with Different Internal Structures
A general formula for Josephson current in a wide class of hybrid junctions
between different internal structures is derived on the basis of the Andreev
picture. The formula extends existing formulae and also enables us to analyze
novel B-phase/A-phase/B-phase (BAB) junctions in superfluid helium three
systems, which are accessible to experiments. It is predicted that BAB
junctions will exhibit two types of current-phase relations associated with
different internal symmetries. A ``pseudo-magnetic interface effect'' inherent
in the system is also revealed.Comment: 4 pages, 2 figure
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