Thermometry and Refrigeration in a Two-Component Mott Insulator of Ultracold Atoms

Interesting spin Hamiltonians can be realized with ultracold atoms in a two-component Mott insulator (2CMI). It was recently demonstrated that the application of a magnetic field gradient to the 2CMI enables new techniques of thermometry and adiabatic cooling. Here we present a theoretical description which provides quantitative analysis of these two new techniques. We show that adiabatic reduction of the field gradient is capable of cooling below the Curie or N\'eel temperature of certain spin ordered phases.Comment: 5 pages, 5 figures (v4): Added journal referenc

Review and reinterpretation of Rio Grande silvery minnow reproductive ecology using egg biology, life history, hydrology, and geomorphology information

To inform management actions to recover the endangered Rio Grande silvery minnow (Hybognathus amarus, RGSM), we (1) calculated the terminal settling velocities of newly expelled and water-hardened RGSM eggs for the observed range of suspended sediment concentrations and water temperatures in the Rio Grande, New Mexico, USA, and (2) reviewed RGSM reproductive ecology in the context of egg biology, the species’ life history, and the historic and contemporary hydrology and geomorphology of the Rio Grande. Results show that in a naturally functioning riverine environment, the location and timing of spawning, the ontogenic stage of egg development, and habitat-specific differences in sediment and temperature that influence egg-settling rates interact to (1) prevent egg suffocation, (2) promote egg entrainment in clear, warm, productive floodplain habitats, and (3) limit downstream population displacement. Our research suggests that the RGSM is primarily a demersal, floodplain spawning species that evolved eggs that are secondarily buoyant in high-sediment environments rather than a main channel, pelagic broadcast-spawning species with an evolved long-distance, downstream drift phase, as previously reported. The current high magnitude of egg drift is hypothesized to be an artefact of contemporary river management and channelization, leading to reduced lateral connectivity, floodplain abandonment, and habitat degradation. Conservation actions implemented to restore historic channel form and reconnect low-velocity backwater and floodplain habitats are recommended. In the absence of a documented upstream migration of adult fish, removal of barriers to a presumed upstream movement is unlikely to provide immediate benefits to RGSM

Thermometry and cooling of ultracold atoms in an optical lattice

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 127-132).Ultracold atoms of 7Rb were prepared in a mixture of two hyperfine states, F 1, mF = -1 > and 2, -2 >. This two-component system was then studied in the presence of a magnetic field gradient and an optical lattice. The presence of a magnetic field gradient separated the atoms into regions of opposite spin, with a boundary region of mixed spin in the center. In the presence of an optical lattice, the width of this region was found to be proportional to the system's temperature and inversely proportional to the strength of the magnetic field. This allowed the measurement of the size of the boundary region to act as a thermometer for the system, representing the first demonstration of spin gradient thermometry. This thermometer represents the first practical method for thermometry in the Mott insulator, and has features of high dynamic range and tunable sensitivity. Given sufficient optical resolution and control over the magnetic field gradient, the lower limit of this thermometer is set by quantum magnetic ordering effects. The dynamic response of this system to changes in magnetic field gradient was studied, both in the weak and strong lattice regimes. The result of these studies was the development of spin gradient demagnetization cooling. By performing an adiabatic drop in gradient strength while still in the superfluid, significant cooling of the entire system was observed. When the same process was performed in the Mott insulator, the spin temperature was cooled dramatically, while remaining out of equilibrium with the remaining degrees of freedom of the system. By reversing the gradient direction, inverted spin populations with negative temperatures have been produced. Spin gradient demagnetization has produced the closest approach to absolute zero yet recorded: 300 pK for the equilibrated system, and spin temperatures of 75 pK as well as -75 pK. The ability to achieve these temperatures puts studies of quantum magnetism in optical lattices within reach.by Patrick Medley.Ph.D

Spin gradient thermometry for ultracold atoms in optical lattices

We demonstrate spin gradient thermometry, a new general method of measuring the temperature of ultracold atoms in optical lattices. We realize a mixture of spins separated by a magnetic field gradient. Measurement of the width of the transition layer between the two spin domains serves as a new method of thermometry which is observed to work over a broad range of lattice depths and temperatures, including in the Mott insulator regime. We demonstrate the thermometry in a system of ultracold rubidium atoms, and suggest that interesting spin physics can be realized in this system. The lowest measured temperature is 1 nK, indicating that the system has reached the quantum regime, where insulating shells are separated by superfluid layers.Comment: 5 pages, 3 figures, minor edits for clarit

Spin gradient demagnetization cooling of ultracold atoms

A major goal of ultracold atomic physics is quantum simulation of spin Hamiltonians in optical lattices. Progress towards this goal requires the attainment of extremely low temperatures. Here we demonstrate a new cooling method which consists of applying a time-varying magnetic field gradient to a spin mixture of ultracold atoms. We have used this method to prepare isolated spin distributions at positive and negative spin temperatures of +/-50 picokelvin. The spin system can also be used to cool other degrees of freedom, and we have used this coupling to reduce the temperature of an apparently equilibrated sample of rubidium atoms in a Mott insulating state to 350 picokelvin. These are the lowest temperatures ever measured in any system.Comment: 4 pages, 4 figures; (v4) Shortened, added journal re

Phase diagram for a Bose-Einstein condensate moving in an optical lattice

The stability of superfluid currents in a system of ultracold bosons was studied using a moving optical lattice. Superfluid currents in a very weak lattice become unstable when their momentum exceeds 0.5 recoil momentum. Superfluidity vanishes already for zero momentum as the lattice deep reaches the Mott insulator(MI) phase transition. We study the phase diagram for the disappearance of superfluidity as a function of momentum and lattice depth between these two limits. Our phase boundary extrapolates to the critical lattice depth for the superfluid-to-MI transition with 2% precision. When a one-dimensional gas was loaded into a moving optical lattice a sudden broadening of the transition between stable and unstable phases was observed.Comment: 4 figure

Continuous and Pulsed Quantum Zeno Effect

Continuous and pulsed quantum Zeno effects were observed using a $^{87}$Rb Bose-Einstein condensate(BEC). Oscillations between two ground hyperfine states of a magnetically trapped condensate, externally driven at a transition rate $\omega_R$, were suppressed by destructively measuring the population in one of the states with resonant light. The suppression of the transition rate in the two level system was quantified for pulsed measurements with a time interval $\delta t$ between pulses and continuous measurements with a scattering rate $\gamma$. We observe that the continuous measurements exhibit the same suppression in the transition rate as the pulsed measurements when $\gamma\delta t=3.60(0.43)$, in agreement with the predicted value of 4. Increasing the measurement rate suppressed the transition rate down to $0.005\omega_R$.Comment: 5 pages, 4 figure

Imaging the Mott Insulator Shells using Atomic Clock Shifts

Microwave spectroscopy was used to probe the superfluid-Mott Insulator transition of a Bose-Einstein condensate in a 3D optical lattice. Using density dependent transition frequency shifts we were able to spectroscopically distinguish sites with different occupation numbers, and to directly image sites with occupation number n=1 to n=5 revealing the shell structure of the Mott Insulator phase. We use this spectroscopy to determine the onsite interaction and lifetime for individual shells

Atom trapping with a thin magnetic film

We have created a $^{87}$Rb Bose-Einstein condensate in a magnetic trapping potential produced by a hard disk platter written with a periodic pattern. Cold atoms were loaded from an optical dipole trap and then cooled to BEC on the surface with radiofrequency evaporation. Fragmentation of the atomic cloud due to imperfections in the magnetic structure was observed at distances closer than 40 $\mu$m from the surface. Attempts to use the disk as an atom mirror showed dispersive effects after reflection.Comment: 4 pages, 5 figure

Quantification and determinants of the amount of respiratory syncytial virus (RSV) shed using real time PCR data from a longitudinal household study.

Background A better understanding of respiratory syncytial virus (RSV) epidemiology requires realistic estimates of RSV shedding patterns, quantities shed, and identification of the related underlying factors. Methods RSV infection data arise from a cohort study of 47 households with 493 occupants, in coastal Kenya, during the 2009/2010 RSV season. Nasopharyngeal swabs were taken every 3 to 4 days and screened for RSV using a real time polymerase chain reaction (PCR) assay. The amount of virus shed was quantified by calculating the 'area under the curve' using the trapezoidal rule applied to rescaled PCR cycle threshold output. Multivariable linear regression was used to identify correlates of amount of virus shed. Results The median quantity of virus shed per infection episode was 29.4 (95% CI: 15.2, 54.2) log10 ribonucleic acid (RNA) copies. Young age (<1 year), presence of upper respiratory symptoms, intra-household acquisition of infection, an individual's first infection episode in the RSV season, and having a co-infection of RSV group A and B were associated with increased amount of virus shed. Conclusions The findings provide insight into which groups of individuals have higher potential for transmission, information which may be useful in designing RSV prevention strategies