Development of a TG-FTIR system for investigations with condensable and corrosive gases

A thermogravimetric analyzer and a Fourier-transform infrared (FTIR) spectrometer were combined and redesigned for investigations with corrosive and condensable reactive gases. The standard gas inlet and outlet of the thermogravimetric analyzer were changed in order to heat the gas tubes, which are lead through the flanges, and avoid condensation in these parts of the system. Furthermore, all tubes upstream and downstream of the thermogravimetric analyzer were trace heated up to 180°C. The gas measuring cell of the FTIR spectrometer was designed such that an optimum compromise between the small flow rates through the thermogravimetric analyzer and a short residence time of the gases in the gas measuring cell could be achieved. The gas supply allows the dosage of different gas compositions containing nitrogen, oxygen, water, NH3, and NO2, for example. The system was validated by analyzing the composition of a diesel particulate matter (PM) sample with a temperature-programmed desorption followed by oxidation (TPD/O) experiment, which showed good agreement with the established analysis methods. The reactivity of the PM sample was investigated by temperature-programmed oxidation (TPO) experiments with different reactive gas mixtures of oxygen, water, and NO2 in nitrogen. By adding NO2, the soot oxidation started at lower temperatures and the addition of water lead to a shift of the maxima of the carbon oxidation rates to lower temperatures. The ratio of formed CO2 and CO was shifted to higher values by the addition of NO2 and water whereby the influence of water was much more pronounce

Tuning the dipolar interaction in quantum gases

We have studied the tunability of the interaction between permanent dipoles in Bose-Einstein condensates. Based on time-dependent control of the anisotropy of the dipolar interaction, we show that even the very weak magnetic dipole coupling in alkali gases can be used to excite collective modes. Furthermore, we discuss how the effective dipolar coupling in a Bose-Einstein condensate can be tuned from positive to negative values and even switched off completely by fast rotation of the orientation of the dipoles.Comment: 4 pages, 3 figures. Submitted to PRL. (v3: Figure 3 replaced

Controlling a mesoscopic spin environment by quantum bit manipulation

We present a unified description of cooling and manipulation of a mesoscopic bath of nuclear spins via coupling to a single quantum system of electronic spin (quantum bit). We show that a bath cooled by the quantum bit rapidly saturates. Although the resulting saturated states of the spin bath (``dark states'') generally have low degrees of polarization and purity, their symmetry properties make them a valuable resource for the coherent manipulation of quantum bits. Specifically, we demonstrate that the dark states of nuclear ensembles can be used to coherently control the system-bath interaction and to provide a robust, long-lived quantum memory for qubit states.Comment: 4 pages, 3 figure

Solid-state circuit for spin entanglement generation and purification

We show how realistic charge manipulation and measurement techniques, combined with the exchange interaction, allow for the robust generation and purification of four-particle spin entangled states in electrically controlled semiconductor quantum dots. The generated states are immunized to the dominant sources of noise via a dynamical decoherence-free subspace; all additional errors are corrected by a purification protocol. This approach may find application in quantum computation, communication, and metrology.Comment: 5 pages, 2 figures; corrected minor typo

Universality of Uhrig dynamical decoupling for suppressing qubit pure dephasing and relaxation

The optimal $N$-pulse dynamical decoupling discovered by Uhrig for a spin-boson mmodel [Phys. Rev. Lett, {\bf 98}, 100504 (2007)] is proved to be universal in suppressing to $O(T^{N+1})$ the pure dephasing or the longitudinal relaxation of a qubit (or spin-1/2) coupled to a generic bath in a short-time evolution of duration $T$. It is also found that for the purpose of suppressing the longitudinal relaxation, an ideal Uhrig $\pi$-pulse sequence can be generalized to a sequence consisting of the ideal one superimposed with finite-duration pulses satisfying certain symmetry requirements.Comment: 4 pages, 1 figure

Critical fluctuations and pseudogap observed in the microwave conductivity of Bi2Sr2CaCu2O8+δ, Bi2Sr2Ca2Cu3O10+δ, and YBa2Cu3O7-δ thin films

Critical fluctuations have been studied in the microwave conductivity of Bi2Sr2CaCu2O8+δ, Bi2Sr2Ca2Cu3O10+δ, and YBa2Cu3O7-δ thin films above T-c. It is found that a consistent analysis of the real and imaginary parts of the fluctuation conductivity can be achieved only if an appropriate wave vector or energy cutoff in the fluctuation spectrum is taken into account. In all of the three underdoped superconducting films one observes strong fluctuations extending far above T-c. The coherence length inferred from the imaginary part of the conductivity exhibits the static critical exponent ν=1 very close to T-c, and a crossover to the region with ν=2/3 at higher temperatures. In parallel, our analysis reveals the absence of the normal conductivity near T-c, i.e., fully opened pseudogap. Following the crossover to the region with ν=2/3, the normal conductivity is gradually recovered, i.e., the closing of the pseudogap is monitored

Resurrection of Schrodinger Cat

Quantum decoherence is the major obstacle in using the potential of engineered quantum dynamics to revolutionize high-precision measurements, sensitive detection, or information processing. Here we experimentally demonstrate that quantum state of a system can be recovered after the state is destroyed by uncontrollable natural decoherence. Physical system is a cluster of seven dipolar-coupled nuclear spins of single-labeled 13C-benzene in liquid crystal. 13C spin plays a role of a device for measuring protons' "cat" state, a superposition of states with six spins up (alive) and six spins down (dead). Information about the state, stored in the 13C spin, is used to bring the protons' subsystem into the alive state, while the excess entropy produced by decoherence is transferred to the "measuring device", the 13C spin.Comment: 16 pages including 5 figure

Suppression of electron spin decoherence in a quantum dot

The dominant source of decoherence for an electron spin in a quantum dot is the hyperfine interaction with the surrounding bath of nuclear spins. The decoherence process may be slowed down by subjecting the electron spin to suitable sequences of external control pulses. We investigate the performance of a variety of dynamical decoupling protocols using exact numerical simulation. Emphasis is given to realistic pulse delays and the long-time limit, beyond the domain where available analytical approaches are guaranteed to work. Our results show that both deterministic and randomized protocols are capable to significantly prolong the electron coherence time, even when using control pulse separations substantially larger than what expected from the {\em upper cutoff} frequency of the coupling spectrum between the electron and the nuclear spins. In a realistic parameter range, the {\em total width} of such a coupling spectrum appears to be the physically relevant frequency scale affecting the overall quality of the decoupling.Comment: 8 pages, 3 figures. Invited talk at the XXXVII Winter Colloquium on the Physics of Quantum Electronics, Snowbird, Jan 2007. Submitted to J. Mod. Op

What is going wrong with community engagement? How flood communities and flood authorities construct engagement and partnership working

In this paper, we discuss the need for flood risk management in England that engages stakeholders with flooding and its management processes, including knowledge gathering, planning and decision-making. By comparing and contrasting how flood communities experience ‘community engagement’ and ‘partnership working’, through the medium of an online questionnaire, with the process’s and ways of working that the Environment Agency use when ‘working with others’, we demonstrate that flood risk management is caught up in technocratic ways of working derived from long-standing historical practices of defending agricultural land from water. Despite the desire to move towards more democratised ways of working which enable an integrated approach to managing flood risk, the technocratic framing still pervades contemporary flood risk management. We establish that this can disconnect society from flooding and negatively impacts the implementation of more participatory approaches designed to engage flood communities in partnership working. Through the research in this paper it becomes clear that adopting a stepwise, one-size-fits-all approach to engagement fails to recognise that communities are heterogenous and that good engagement requires gaining an understanding of the social dimensions of a community. Successful engagement takes time, effort and the establishment of trust and utilises social learning and pooling of knowledge to create a better understanding of flooding, and that this can lead to increasing societal connectivity to flooding and its impacts