A modified wire clamp system for thirty-liter Niskin bottles

A modified clamping system for 30-liter Niskin bottles, consisting of a wire stop, a socket block, and a toggle clamp, has been designed and has been tested at sea. The modified system makes deployment and recovery of the Niskin bottles considerably easier than it is with the standard clamps .Funding was provided by the National Science Foundation under grant Number OCE 84-17910, and by the United States Department of Energy under contract Number DE-AC02-76EV03566

A large-volume, deep-sea submersible pumping system

Eight self-contained, in-situ pumps have been used effectively and routinely by our group for the past six years to collect both particulate and dissolved phases from large volumes of sea water. Multiple pumps are rapidly and easily deployed on the same wire, to any ocean depth, in almost any weather. Each is capable of drawing up to 200 liters per hour through four large Nuclepore™ filters, then through three cartridge filters. Pumping is controlled by a Sharp™ pocket computer suitably interfaced with the pump motor and flow meter. Endurance is about 15 hours. Total flow and flow rate are recorded, respectively, by a mechanical flow meter and the computer.Funding was provided by the National Science Foundation under Grant Number NSF OCE-8800620 and the Department of Energy under Grant DE-FG02-88ER60681

Mapping the inner regions of the polar disk galaxy NGC4650A with MUSE

[abridged] The polar disk galaxy NGC4650A was observed during the commissioning of the MUSE at the ESO VLT to obtain the first 2D map of the velocity and velocity dispersion for both stars and gas. The new MUSE data allow the analysis of the structure and kinematics towards the central regions of NGC4650A, where the two components co-exist. These regions were unexplored by the previous long-slit literature data available for this galaxy. The extended view of NGC~4650A given by the MUSE data is a galaxy made of two perpendicular disks that remain distinct and drive the kinematics right into the very centre of this object. In order to match this observed structure for NGC4650A, we constructed a multicomponent mass model made by the combined projection of two disks. By comparing the observations with the 2D kinematics derived from the model, we found that the modelled mass distribution in these two disks can, on average, account for the complex kinematics revealed by the MUSE data, also in the central regions of the galaxy where the two components coexist. This result is a strong constraint on the dynamics and formation history of this galaxy; it further supports the idea that polar disk galaxies like NGC~4650A were formed through the accretion of material that has different angular momentum.Comment: 14 pages, 10 figures; accepted for publication in Astronomy & Astrophysic

The Planetary Nebulae Population in the Nuclear Regions of M31: the SAURON view

Following a first study of the central regions of M32 that illustrated the power of integral-field spectroscopy (IFS) in detecting and measuring the [O III]{\lambda}5007 emission of PNe against a strong stellar background, we turn to the very nuclear PN population of M31, within 80 pc of its centre. We show that PNe can also be found in the presence of emission from diffuse gas and further illustrate the excellent sensitivity of IFS in detecting extragalactic PNe through a comparison with narrowband images obtained with the Hubble Space Telescope. We find the nuclear PNe population of M31 is only marginally consistent with the generally adopted form of the PNe luminosity function (PNLF). In particular, this is due to a lack of PNe with absolute magnitude M5007 brighter than -3, which would only result from a rather unfortunate draw from such a model PNLF. We suggest that the observed lack of bright PNe in the nuclear regions of M31 is due to a horizontal-branch population that is more tilted toward less massive and hotter He-burning stars, so that its progeny consists mostly of UV-bright stars that fail to climb back up the asymptotic giant branch (AGB) and only of few, if any, bright PNe powered by central post-AGB stars. These results are also consistent with recent reports on a dearth of bright post-AGB stars towards the nucleus of M31, and lend further support to the idea that the metallicity of a stellar population has an impact on the way the horizontal branch is populated and to the loose anticorrelation between the strength of the UV-upturn and the specific number of PNe that is observed in early-type galaxies. Finally, our investigation also serves to stress the importance of considering the same spatial scales when comparing the PNe population of galaxies with the properties of their stellar populations.Comment: 11 pages, 10 figures, accepted for publication on Monthly Notices of the Royal Astronomical Societ

Decoherence-Free Subspaces for Multiple-Qubit Errors: (I) Characterization

Coherence in an open quantum system is degraded through its interaction with a bath. This decoherence can be avoided by restricting the dynamics of the system to special decoherence-free subspaces. These subspaces are usually constructed under the assumption of spatially symmetric system-bath coupling. Here we show that decoherence-free subspaces may appear without spatial symmetry. Instead, we consider a model of system-bath interactions in which to first order only multiple-qubit coupling to the bath is present, with single-qubit system-bath coupling absent. We derive necessary and sufficient conditions for the appearance of decoherence-free states in this model, and give a number of examples. In a sequel paper we show how to perform universal and fault tolerant quantum computation on the decoherence-free subspaces considered in this paper.Comment: 18 pages, no figures. Major changes. Section on universal fault tolerant computation removed. This section contained a crucial error. A new paper [quant-ph/0007013] presents the correct analysi

Theory of Decoherence-Free Fault-Tolerant Universal Quantum Computation

Universal quantum computation on decoherence-free subspaces and subsystems (DFSs) is examined with particular emphasis on using only physically relevant interactions. A necessary and sufficient condition for the existence of decoherence-free (noiseless) subsystems in the Markovian regime is derived here for the first time. A stabilizer formalism for DFSs is then developed which allows for the explicit understanding of these in their dual role as quantum error correcting codes. Conditions for the existence of Hamiltonians whose induced evolution always preserves a DFS are derived within this stabilizer formalism. Two possible collective decoherence mechanisms arising from permutation symmetries of the system-bath coupling are examined within this framework. It is shown that in both cases universal quantum computation which always preserves the DFS (*natural fault-tolerant computation*) can be performed using only two-body interactions. This is in marked contrast to standard error correcting codes, where all known constructions using one or two-body interactions must leave the codespace during the on-time of the fault-tolerant gates. A further consequence of our universality construction is that a single exchange Hamiltonian can be used to perform universal quantum computation on an encoded space whose asymptotic coding efficiency is unity. The exchange Hamiltonian, which is naturally present in many quantum systems, is thus *asymptotically universal*.Comment: 40 pages (body: 30, appendices: 3, figures: 5, references: 2). Fixed problem with non-printing figures. New references added, minor typos correcte

Universal Quantum Computation with the Exchange Interaction

Experimental implementations of quantum computer architectures are now being investigated in many different physical settings. The full set of requirements that must be met to make quantum computing a reality in the laboratory [1] is daunting, involving capabilities well beyond the present state of the art. In this report we develop a significant simplification of these requirements that can be applied in many recent solid-state approaches, using quantum dots [2], and using donor-atom nuclear spins [3] or electron spins [4]. In these approaches, the basic two-qubit quantum gate is generated by a tunable Heisenberg interaction (the Hamiltonian is $H_{ij}=J(t){\vec S}_i\cdot{\vec S}_j$ between spins $i$ and $j$), while the one-qubit gates require the control of a local Zeeman field. Compared to the Heisenberg operation, the one-qubit operations are significantly slower and require substantially greater materials and device complexity, which may also contribute to increasing the decoherence rate. Here we introduce an explicit scheme in which the Heisenberg interaction alone suffices to exactly implement any quantum computer circuit, at a price of a factor of three in additional qubits and about a factor of ten in additional two-qubit operations. Even at this cost, the ability to eliminate the complexity of one-qubit operations should accelerate progress towards these solid-state implementations of quantum computation.Comment: revtex, 2 figures, this version appeared in Natur

Direct experimental evidence of free fermion antibunching

Fermion antibunching was observed on a beam of free noninteracting neutrons. A monochromatic beam of thermal neutrons was first split by a graphite single crystal, then fed to two detectors, displaying a reduced coincidence rate. The result is a fermionic complement to the Hanbury Brown and Twiss effect for photons.Comment: 4 pages, 2 figure

Exchange Interaction Between Three and Four Coupled Quantum Dots: Theory and Applications to Quantum Computing

Several prominent proposals have suggested that spins of localized electrons could serve as quantum computer qubits. The exchange interaction has been invoked as a means of implementing two qubit gates. In this paper, we analyze the strength and form of the exchange interaction under relevant conditions. We find that, when several spins are engaged in mutual interactions, the quantitative strengths or even qualitative forms of the interactions can change. It is shown that the changes can be dramatic within a Heitler-London model. Hund-Mulliken calculations are also presented, and support the qualititative conclusions from the Heitler-London model. The effects need to be considered in spin-based quantum computer designs, either as a source of gate error to be overcome or a new interaction to be exploited.Comment: 16 pages, 16 figures. v3: Added Hund-Mulliken calculations in 3-dots case. A few small corrections. This version submitted to PR