Personal Website of Reuben Wang
R. R. W. Wang and J. L. Bohn, “Prospects for thermalization of microwave-shielded ultracold molecules”, Physical Review Research (2024)
DIGEST:
In the hopes of achieving extremely cold temperatures where quantum effects are dominant, one of the techniques utilized is referred to as evaporative cooling. This is where hotter molecules are allowed to evaporate away, leaving behind only the less energetic ones (much like how a cup of coffee cools down with its lid off). But for the overall temperature to actually decrease, the remaining molecules must come to equilibrium so that all of them know that they are collective colder. Such equilibrium is brought about by collisions that spread energy evenly throughout the gas, so it stands that collisions must be well understood to achieve the greatest reduction in temperature with the least number of molecules lost. In this work, we explore collisions between diatomic molecules in the presence of a time-varying electric field, and how such collisions affect the efficiency to which evporation cools the gas. To do so in a manner that doesn’t require enormous calculations by a computer, we utilize a machine learning tool that grants us accurate results with much less effort than usually needed.
J. L. Bohn and R. R. W. Wang, “Probability distributions of atomic scattering lengths”, submitted to Canadian Journal of Physics (2023)
DIGEST:
At the low temperatures less than a millionth of a Kelvin above absolute zero, it is so cold that all the usual dynamical stuff within an atom comes to a stand still. When this happens, atoms that bump into one another can have their collisional behavior completely described by a single parameter, the scattering length. A famous result by Gribakin and Flambaum showed that without more knowledge, one should expect the scattering length to only depend on the residual interactions of electronic charge distributions between atoms. We extend this result by also looking at ways in which colliding atoms could be jostled a little more than usual, resulting in the atoms leaving the collisions a little worse for wear. This “degraded” condition of exiting atoms amounts to an imganiary part of the scattering length, for which we derive a statistical distribution for what one might expect its value to be.
R. R. W. Wang and J. L. Bohn, “Viscous dynamics of a quenched trapped dipolar Fermi gas”, Physical Review A (2023)
DIGEST:
As more molecules are added to a finite volume, an initially gaseous sample can have its behavior transit to one that more closely resembles a fluid. Such a dense gas is said to be “hydrodynamic”, and is best described as a continuous substance rather than roaming particulates. Our work leverages this hydrodynamic description of dense gases, providing a means describe how ultracold samples of diatomic molecules behave when perturbed out of equilibrium. Specifically, we study these ultracold fluids when they are confined in parabolic (optical) containments, and establish that equilibrium is restored as a consequence of viscosity. Viscosity results from the remnant gaseous behavior within each fluid patch, manifesting as a source of damping to collective fluid motion. By employing several approximations and numerical tools, we have developed a simplified model that well describes particular aspects of the fluid motion we term as “weltering”, linekened to the irregular motion of waves on the unquiet ocean.
A. M. Polloreno, R. R. W. Wang, N. A. Tezak, “A Note on Noisy Reservoir Computation”, ArXiv (2023)
DIGEST:
Contrary to the usual notion of modern-day electronic digital computers, the term “computer” can, in general, be used as reference to any system which processes input information. One such class of these generalized computing machines is the so-called “reservoir computer”, which takes a given dynamical system (the “reservoir”) as it is (e.g. a bucket of water or an artificial neural network), and exploits its innate complexity to aid with computational tasks. In this note, we show that if a reservoir or its inputs were to be subject to a small degree of randomness, this noise can only serve to degrade the amount of useful information processed by it.
H. Li, E. Halperin, R. R. W. Wang, J. L. Bohn, “Out-of-Time-Order-Correlator for van der Waals potential”, Physical Review A (2023)
DIGEST:
The notion of chaos in physics is often characterized by largely diverging trajectories over time despite very similar starting points, the so-called “butterfly effect”. These divergent paths are well quantified in classical systems, using a metric known as the “Lyapunov exponent”. In quantum systems however, the definition of chaos is less clear and in fact an ongoing topic of discussion. In this work, we analyze a proposed metric for quantum chaos referred to as the “out-of-time-order-correlator” (OTOC), in the context of a simple molecular system. We show that despite our molecule of interest being definitively not chaotic, the OTOC still behaves with the quantum equivalent of a Lyapunov exponent, putting into question the capacity of OTOCs as a useful indicator of quantum chaos.
R. R. W. Wang and J. L. Bohn, “Anisotropic acoustics in dipolar Fermi gases”, (Editor’s Suggestion) Physical Review A (2022)
DIGEST:
Sound travels through a gas by successive compressions and rarefactions along its direction of propagation. In ultracold gases of polar molecules (i.e. molecules that have effective oppositely charged poles separated by a distance), the intermolecular interactions can result in a distortion of sound waves, dependent on the direction in which a listener hears it. This directional dependence also persists in the way sound damps out in such gases, primarily due to the effects of viscosity–a form of friction in the gas. Because our theory encapsulates all of these effects, we have proposed a means for experimentalists to measure the viscosity in these polar molecular gases via a particular type of flow motion, known as shear waves.
R. R. W. Wang and J. L. Bohn, “Thermoviscous Hydrodynamics in Non-Degenerate Dipolar Bose Gases”, Physical Review A (2022)
DIGEST:
Molecules comprised of different constituent atoms (e.g. OH instead of O2) can have a highly non-uniform charge distributions. That is to say, one end of the molecule could have a much greater charge than the other, a characteristic referred to as being polar. When 2 polar molecules collide, their resultant trajectories are now highly dependent on the relative orientation of approach, unlike the case of 2 perfectly spherical objects. On a larger scale, these peculiar collisional interactions can result in entire gases of polar molecules exhibiting novel behaviours, motivating its study. To do so, we formulate a theory of polar molecular gases that describes them as a viscous fluid, where viscosity arises directly from these polar collisions.
R. R. W. Wang and J. L. Bohn, “Thermal Conductivity of an Ultracold Paramagnetic Bose Gas”, Physical Review A (2022)
DIGEST:
Fluid mechanics studies macroscopic phenomena by means of coarse graining over all the microscopic particles which actually make up the fluid. However, certain properties of the fluid do require a deep understanding of the microscopic underpinnings from which they arise. One such property is the thermal conductivity, which measures the rate at which heat spreads throughout the fluid. In this work, we study a fluid made up of highly magnetic atoms that results in thermal conduction that varies with the direction relative to the atomic-magnetic orientation. This opens up opportunities to steer the flow of heat through these fluids, by changing the atomic-magnetic orientations with applied magnetic fields.
A. Patscheider, L. Chomaz, G. Natale, D. Petter, M. J. Mark, S. Baier, B. Yang, R. R. W. Wang, J. L. Bohn, F. Ferlaino, “Determination of the scattering length of erbium atoms”, Physical Review A (2022)
DIGEST:
At extremely cold temperatures, atoms interact with each other quantum mechanically, quantified by a property known as the scattering length (analogous to the diameter of colliding billiard balls). Magnetic fields can be applied to change the scattering length, permitting a tuning knob for varying atomic interactions. This work presents precise experimental measurements of the scattering length for Erbium atoms, over a broad range of magnetic fields. These were extracted by careful comparisons of experimental data to our theoretical model.
R. R. W. Wang and J. L. Bohn, “Anisotropic thermalization of dilute dipolar gases”, Physical Review A (2021).
DIGEST:
Just as the temperature of a room eventually evenly rises with the sudden introduction of heat, so too is the same for an isolated gas of ultracold atoms. The route to this new equilibrium temperature, is however highly dependent on the properties of the individual atoms. In this work, we derive concise formulas for how strong magnetism in atoms can affect the route to which gases reach their equilibrium temperature. These formulas will thus allow experimentalist to determine magnetic properties in atoms, simply by temperature measurements alone.
J. Li, W. G. Tobias, K. Matsuda, C. Miller, G. Valtolina, L. De Marco, R. R. W. Wang, L. Lassablière, G. Quéméner, J. L. Bohn and J. Ye, “Tuning of dipolar interactions and evaporative cooling in a three-dimensional molecular quantum gas”, Nature Physics (2021).
DIGEST:
Molecules have highly complex internal structures, making them extremely hard to control. In particular, gaseous molecules tend to chemically react as they bump against each other, disallowing the gas from cooling down through evaporation (like how a cup of coffee cools with its lid off). This work demonstrates the experimental ability to shield Potassium-Rubidium (KRb) molecules from chemically reacting, using an incredibly precise and strong electric field. We resultantly measure the characteristic heat redistribution signature of such non-chemically reactive gases, and it’s promising capacity to evaporatively cool to ultralow temperatures.
R. R. W. Wang, A. G. Sykes, and J. L. Bohn, “Linear response of a periodically driven thermal dipolar gas”, Physical Review A (2020).
DIGEST:
Cooling gases to ultra low temperatures allows in inter-atom collisions that leave the internal state of atoms unchanged. In this work, we study such a gas where the atoms are additionally highly magnetic. When periodically pumped, the gas heats up but at different rates in different directions, depending on the enforced orientation of the magnets. This work studies and characterizes the directionality of heat propagation through such magnetic atomic gases.
R. R. W. Wang, B. Xing, G. G. Carlo, and D. Poletti, “Period doubling in period-one steady states”, Physical Review E (2018).
DIGEST:
Much like ordinary crystals that have an orderly structure in space, time crystals are unique phases where matter develops order also in time. In this work, we show the emergence of time crystal signatures known as period doubling, with a quantum system of many particles known as Bosons. These Bosons are pumped with energy in cycles, while also dissipating energy to their environment so that the result is a long-lived, robust state of time crystalline order.