Flip Channels Research Articles

Quantum capacity of Pauli channels with memory

The amount of coherent quantum information that can be reliably transmitted down the memory Pauli channels with Markovian correlated noise is investigated. Two methods for evaluating the quantum capacity of the memory Pauli channels are proposed to try to trace the memory effect on the transmissions of quantum information. We show that the evaluation of quantum capacity can be reduced to the calculation of the initial memory state of each successive transmission. Furthermore, we derive quantum capacities of the memory phase flip channel, bit flip channel and bit-phase flip channel. Also, a lower bound of the quantum capacity of the memory depolarizing channel is obtained. An increase of the degree of memory of the channels has a positive effect on the increase of their quantum capacities.

Magnetic correlations in the spin iceHo2−xYxTi2O7as revealed by neutron polarization analysis

We present single-crystal neutron-diffuse-scattering measurements with polarization analysis on the spin ice Ho2-xYxTi2O7 (x = 0, 0.3, and 1). At 2 K, the spin flip scattering patterns are typical of a nearest-neighbor spin ice and the effects of Y doping are seen in the width of the pinch points. At low-T, the spin flip patterns are characteristic of a dipolar spin ice and are unaffected by Y doping. In the non-spin flip channel, we observe the signature of strong antiferromagnetic correlations at the same T as the dipolar spin ice correlations appear in the spin flip channel. These experimental observations have been well reproduced by our Monte Carlo simulations.

The temperature evolution of the magnetic correlations in pure and diluted spin ice Ho2−xYxTi2O7

Diffuse polarized neutron scattering studies have been carried out on single crystals of pyrochlore spin ice Ho2−xYxTi2O7 (x=0, 0.3, and 1) to investigate the effects of doping and anisotropy on spin correlations in the system. The crystals were aligned with the (1−10) orientation coincident with the direction of neutron polarization. For all the samples studied the spin flip (SF) diffuse scattering (i.e. the in-plane component) reveals that the spin correlations can be described using a nearest-neighbour spin ice model (NNSM) at higher temperatures (T=3.6K) and a dipolar spin ice model (DSM) as the temperature is reduced (T=30mK). In the non-spin flip (NSF) channel (i.e. the out-of-plane component), the signature of strong antiferromagnetic correlations is observed for all the samples at the same temperature as the dipolar spin ice behaviour appears in the SF channel. Our studies show that the non-magnetic dopant Y does not significantly alter SF or NSF scattering for the spin ice state, even when Y doping is as high as 50%. In this paper, we focus on the experimental results of the highly doped spin ice HoYTi2O7 and compare our results with pure spin ice Ho2Ti2O7. The crossover from a dipolar to a nearest-neighbour spin ice behaviour and the doping insensitivity in spin ices are briefly discussed.

Flip and Flop: A Molecular Determinant for AMPA Receptor Channel Opening

Alternative splicing in the extracellular ligand binding domain of the AMPA receptors generates two variants, i.e., flip and flop. The flop variant of the GluR2 AMPA receptor is known to desensitize faster than the flip counterpart, whereas the GluR1 flip and flop variants exhibit the same rate of desensitization. However, whether the alternative splicing affects the channel opening kinetic properties of these receptors is unknown. Using a laser-pulse photolysis technique, we have characterized the channel opening kinetic mechanism for the flip and flop channels of GluR1 and GluR2, respectively. We find that the flop variant of GluR2 opens the channel, following the binding of glutamate, with the same rate as the flip channel, but closes its channel more rapidly. The difference in the kinetic properties between the two receptor isoforms can be described by a model we proposed previously in which the channel closing rate, a measure of the stability of the open channel state, controls an apparent tendency of the channel to desensitize, most likely, through the closed channel state. Specifically, the flop sequence of GluR2 promotes the channel to close more rapidly and consequently to desensitize with a faster rate than the flip sequence. For GluR1, the alternative splicing does not seem to affect the channel opening kinetics, since the flip and flop variants of GluR1 have the same channel opening rate, and the same channel closing rate. As expected and indeed observed, the flop variant desensitizes with the same rate as the flip variant does. On the basis of these results, we hypothesize that the flip/flop sequence cassette of AMPA receptors, in a sequence-dependent manner, regulates the rate of the channel closing process, in the microsecond time domain, through which it further regulates the channel desensitization in the millisecond time region.

Mutual Information of Pauli Channels with Correlated Noise

A general formula for the mutual information of the Pauli channels with memory modelled by correlated noise is derived. It is shown that the mutual information depends on the channel shrinking factor, the input state parameter and the channel memory coefficient. The analyses based on the general formula reveal that the entanglement is always a useful resource to enhance the mutual information of some Pauli channels, such as the bit flip channel and the bit-phase flip channel. Our analyses also show that the entanglement is not advantageous to the reliable transmission of classical information for some Pauli channels at any time, such as the phase flip channel and the phase damping channel.

Two mechanisms of action of the adamantane derivative IEM‐1460 at human AMPA‐type glutamate receptors

1. Antagonizing glutamatergic neurotransmission by blockade of AMPA-type glutamate receptors (GluR) is a promising pharmacological strategy for neuroprotection in neurodegenerative diseases and acute treatment of stroke. 2. We investigated the interaction of the adamantane derivative IEM-1460 with human wild-type and mutant AMPA-type GluR channels. Different recombinant homooligomeric human AMPA-type GluR channels and a rat nondesensitizing mutant GluR (GluR2 L504Y) channel were expressed in HEK293 cells and investigated using the patch-clamp technique in combination with ultrafast agonist application. 3. When IEM-1460 was coapplied with glutamate, an open channel block mechanism was observed at slow desensitizing GluR2 flip (>/=0.1 mM IEM-1460) and nondesensitizing GluR2 L504Y channels (>/=1 microM IEM-1460). 4. A competitive block of AMPA-type channels was observed with IC(50) values for the dose block curves of 0.1 mM IEM-1460 at human unmutated and 10 microM IEM-1460 at mutant GluR channels. 5. Nondesensitizing GluR2 L504Y channels were used to further characterize the block mechanism. After equilibration with the agonist, a current decay upon coapplication of glutamate and IEM-1460 was observed. The recovery from block was independent of the glutamate and IEM-1460 concentration. The extent of current inhibition as well as the time constant of current decay upon addition of the blocker to the test solution were dependent on agonist concentration; this strongly points to an additional competitive-like block mechanism of IEM-1460 at human AMPA-type GluR channels. 6. The data were interpreted in the frame of a molecular scheme with two binding sites of IEM-1460 at the receptor, one at the unliganded resting and the other at the fully liganded open state of the channels.

Desensitization and resensitization are independently regulated in human recombinant GluR subunit coassemblies

AMPA-type glutamate receptor (GluR) channels are the most abundant excitatory transmitter receptors of the central nervous system. Four subunits with different posttranscriptional modifications and flip/flop splice variants are known. In vivo they occur as tetrameric heteromeric receptors. In the present study we analyzed the time course of desensitization (tau(D)) and resensitization (tau(rec)) kinetics of different homomeric (coassembly of splice or editing variants of one subunit) and heteromeric (coassembly of different subunits) GluR channels. We found that tau(D) had intermediate values depending on the amount of cDNA of the respective subunit at all heteromeric and homomeric GluR channels tested. The same holds true for tau(rec) except GluR2 flip channels were coexpressed with GluR1 channels. In this case, tau(rec) had values close to that of fast resensitizing GluR2 flip channels, even in the case of an abundance of GluR1 cDNA.

Measurement and simulation of polarized neutron reflectivity and off-specular scattering from evolving magnetic domain structure in Co/Cu multilayers

We report on the measurement and simulation of the field-dependent polarized neutron reflectivity (specular) and scattering (off-specular) for an antiferromagnetically coupled Co/Cu multilayer (ML) with number of bilayers N = 40 . The first-order Bragg peak is visible in the non-spin flip (NSF) channels representing the bilayer periodicity. We observe off-specular intensity around the AF Bragg peak at the half-order position in both SF and NSF channels. These Bragg sheets are most intense at lower fields and gradually disappearing at higher fields as the system attains saturation. We attribute the Bragg sheets to vertically correlated AF domains along the ML stack which get bigger in size with field strength. Those interpretations are confirmed by simulation of the data within the distorted wave born approximation (DWBA).

Channel-opening kinetics of GluR2Q(flip) AMPA receptor: a laser-pulse photolysis study.

AMPA receptors mediate fast excitatory neurotransmission in the central nervous system. GluR2 is an AMPA receptor subunit that controls some key heteromeric AMPA receptor properties, such as calcium permeability. The kinetic properties of GluR2, relevant to the time scale of its channel opening, however, are poorly understood. Here, to measure the channel-opening kinetics, we use a laser-pulse photolysis technique, which permits glutamate to be liberated photolytically from gamma-O-(alpha-carboxy-2-nitrobenzyl)glutamate (caged glutamate) with a time constant of approximately 30 micros. We show that GluR2Q(flip), an unedited and Ca(2+) permeable isoform, is by far the fastest ligand-gated channel with the channel-opening and -closing rate constants being (8.0 +/- 0.49) x 10(4) and (2.6 +/- 0.20) x 10(3) s(-1), respectively. Therefore, the shortest rise time (20-80% of the receptor current response) or the fastest observed time by which the GluR2Q(flip) channel can open is predicted to be 17 micros. The minimal kinetic mechanism for the channel opening is further consistent with the binding of two glutamate molecules with the channel-opening probability of 0.96. These results suggest that GluR2 is a temporally, highly efficient receptor to transduce the binding of chemical signals (i.e., glutamate) into an electrical impulse.

ENTANGLEMENT-ASSISTED CLASSICAL CAPACITIES OF SOME SINGLE QUBIT QUANTUM NOISY CHANNELS

In this paper, we calculate the entanglement-assisted classical capacities of the depolarizing channel, the phase damping channel, the phase flip channel, the bit flip channel, the bit-phase flip channel, the two-Pauli channel and the amplitude channel, and discuss the analytical results obtained. The Stokes papametrization representation of a qubit and the characteristic of unitary covariance of some quantum noisy channels are used in the calculations.

Comparison of alternate solenoid and long solenoid cooling channels

Performances of three cooling channels are compared: (A) FOFO, (B) alternate solenoid channel with additional correcting coils of small radius, (C) long solenoid channel with single or double field flip. A realistic input provided by precooling part with 16GeV proton beam and carbon target is used in all the cases. After an optimization, muon/proton ratio of the cooled beam is about 0.1–0.12 being maximal in FOFO channel. The double field flip channel allows to reach minimal transverse emittance 2.1mm, single flip and (B)-channels provide 3.6–3.7mm, and FOFO—5.4mm.

Spin accumulation in the semi classical and quantum regimes

We consider spin accumulation at a ferromagnet-normal metal interface in the presence of magnetic scattering in the normal metal. In the classical regime, we discuss the inverse Drude scaling of the conductance as a function of the interface transparencies. We present a treatment based on an exact solution of the Boltzmann equation. In the quantum regime, we solve a single impurity “spin-flip Fabry Perot interferometer” for quantum coherent multiple scatterings, in which we find a resonance in the spin flip channels. This resonance appears to be the quantum analog of the semi classical inverse Drude scaling of the conductance.

Magnetic structure of Cr overlayers on Fe(100) (abstract)

Epitaxial films of Cr were grown at 200 °C on Fe(100). The magnetic order of the Cr overlayers was studied by spin polarized electron energy loss spectroscopy (SPEELS). The exchange scattering asymmetry is large (negative) for the bare Fe surface and rapidly drops with increasing Cr coverage in the submonolayer range changing sign at around one monolayer Cr. This shows directly the antiferromagnetic coupling of the first Cr layer to the Fe substrate. With further increase in the Cr thickness the exchange asymmetry oscillates with a period of about 2 layers, thus proving directly the layer-by-layer antiferromagnetic order in the Cr films. The oscillation amplitude shows variations with thickness resembling features found in the magnetic coupling in Fe/Cr/Fe sandwiches. The SPEELS spectra show a maximum in the spin flip channels around 1.9 eV. This structure broadens with increasing Cr thickness. This large spin flip energy suggests a surface enhanced exchange splitting and an enhanced Cr surface moment.

Miami–Dade's Encounter with Technology

1 V liami-Dade Junior College the master control room: electronic circuits, monitors, television tape recorders, walla. Four programs, running at once, are being piped to classrooms by closed circuit cables: The Central Nervous System, The Concept of Culture, So That Men Are Free, The Thread of Life. The programs are in living color slide-tapes, films, CBS Reports, the music of Simon and Garfunkel. This, says a technician, a problem of packaging. don't have other stations to compete with, the kid can't flip channels, but we hope to keep him from going to sleep. The talk is about efficiency, inputs, programming, dubs, systems. Miami-Dade, one of those instant colleges, has grown from zero to 25,000 students on two campuses in eight years. Super-pragmatic and super-audacious, it is running ahead of its own institutional identity crisis, testing itself not by grace or elegance, but by numbers and dropouts, by senior college admissions, and by the economic usefulness of its graduates. We're only number two a junior college, says Franklin Bouwsma, the chief guru of hardware. We have to try harder.