Biography

I am an experimental physicist doing postdoctoral research in the ultracold atoms group in the physics department at the University of Toronto. I study equilibrium phases and non-equilibrium dynamics of many-body interacting spin systems. These systems are realized with clouds of potassium atoms laser-cooled to temperatures just above absolute zero.
I was previously a postdoc in the Hulet group at Rice University, using laser-cooled lithium atoms to study spin-imbalanced superfluid phases. As a Ph.D. student in the Princeton University Physics Department, I studied spin transport in optically pumped thermal vapors of cesium atoms.

A portrait of Ben Olsen

Teaching


My work teaching high-school students physics through Project-Based Learning with Tapia Camps was covered in the September, 2017 issue of National Science Teachers Association Reports.

Students at Tapia camp I teach physics courses where learning closely resembles scientific research. My goals are to equip students with a robust problem-solving toolbox including independent creative thinking, and to guide them towards finnding important, interesting problems to tackle with those tools. My students cultivate skills that they can use no matter where their talents take them: they practice defining problems, utilizing their prior knowledge, overcoming mistakes to arrive at solutions, and critically evaluating those solutions. I help students refine these skills through practical assessments and connections to modern research.

My teaching methods are informed by research on best practices. Some resources I've found helpful include:

Teaching Experience

  • Instructor Phy 326: Advanced Physics Laboratory, University of Toronto, Fall 2017
    3rd, 4th year physics majors
    Experimental lab course including NMR, magnetometry, and optical tweezers
  • Lead physics instructor & curriculum development Tapia STEM Camps, Richard Tapia Center for Excellence & Equity, Rice University, Summer 2017
    8th-12th grade
    Project-Based Learning combining physics and design for STEM graphic and oral presentations
  • Guest lecturer Physics 202: Modern Physics, Rice University, Spring 2015
    2nd year STEM majors
    (Professor: Dr. Randall G. Hulet) Lecture introducing concepts of general relativity
  • Guest lecturer Physics 311/312: Introduction To Quantum Physics I/II, Rice University, 2013-2015
    3rd year physics majors
    (Professor: Dr. Randall G. Hulet) Seven lessons blending lecture, small-group discussion, and problem solving
  • Assistant for instruction ISC 231: An Integrated, Quantitative Introduction to the Natural Sciences, Laboratory Section, Princeton University, 2010-2011
    1st year STEM majors
    (Instructor: Dr. Eva-Maria Schoetz) Biophysics lab experiments and data analysis. I designed and led a workshop on MATLAB for data analysis and simulation.
  • Instructor Physics & Science Reasoning, Princeton University Preparatory Program, 2009-2010
    12th grade
    (Director: Dr. Jason Klugman) Ten-week summer course on critical thinking and problem-solving skills in the context of physics
  • Teaching assistant Ph 6: Physics Laboratory, California Institute of Technology, 2005
    2nd year physics majors
    (Instructor: Dr. Frank Rice) Experimental lab and problem sessions on topics including mass spectrometry, Michelson interferometers, and electron diffraction
  • Teaching assistant Ph 5: Analog Electronics for Physicists, California Institute of Technology, 2005
    2nd year physics majors
    (Instructor: Dr. Virginio Sannibale) Experimental lab and problem sessions on topics including passive circuit elements, op-amps, and phase-locked loops
  • Freelance individual tutor, 2008-2014
    Undergraduate & high school
    Tutored students one-on-one in high-school physics (both calculus- and algebra-based) and mathematics (single- and multivariable calculus), ACT, SAT, and SAT Physics test preparation, undergraduate mechanics and electromagnetism.
  • Problem-solving toolbox

    Students should be equipped to encounter unfamiliar situations and apply their knowledge in new contexts.

  • Interactive learning

    Students should be actively engaged by brainstorming, discussing, drawing, and challenging their models of phenomena.

  • Career skills

    Physics class should resemble scientific research or other STEM work to prepare students for a variety of careers.

Pedagogical Training

Students Mentored

  • 2017- - Wilson Wu (Undergraduate student, University of Toronto)
    Physics Mentorship Program, University of Toronto
  • 2016-2017 - Edward Bredin (Undergraduate student, University of Toronto)
    Physics Mentorship Program, University of Toronto
  • 2013 - Matthias Schmitt (Graduate student, Stuttgart University)
    Summer Research, Rice University
  • 2012 - Chris Akers (Undergraduate student, Texas A&M University)
    Summer Research, Rice University

Research

Spin dynamics in interacting Fermi gases

Transverse spin diffusivity as a function of interaction strength in a 2D Fermi gasSeveral emerging technologies including spintronics and quantum computing, as well as more fundamental studies of many-body systems, are governed by the motion of the constituent spins. The interactions between pairs of spins modify the evolution of the global magnetization of a large sample. Different parameters can lead to spin transport that is ballistic or diffusive. In a Fermi gas with strong interactions, we measured diffusive spin transport, and found that it obeys a conjectured quantum bound for both 3D and 2D systems [A1].
We found the timescale for demagnetization, measured using a Ramsey interferometer sequence, was consistent with the timescale for growth of the contact, measured with rf spectroscopy. The contact probes the contribution to the total energy of the system due to interactions.

[A1] Physical Review Letters 118, 130405 (2017) (pdf)


Strongly-interacting spin-polarized superfluids

Quantum gases of fermions underlie many natural phenomena and technologies, including neutron stars and superconductors. In most of these systems, the fermions interact based on their spin. Particles with opposite spins will form pairs: based on the interaction strength, either loosely-bound Cooper pairs or tightly-bound molecules. In clouds with equal populations of the two spin states, the entire cloud then forms a superfluid.

Superfluid critical polarization as a function of interaction strength In a 3D gas, the balanced superfluid resides at the core of the cloud, shrinking in size as the imbalance between spin populations increases, until it disappears altogether. This critical imbalance for suppression depends on the interaction strength between the two species. We measured this dependence using a cloud of Lithium-6 atoms cooled to ~100 nK, where we tuned the interactions using a magnetic Feshbach resonance [B1].

In a 1D gas, a polarized phase is always found in the center of the cloud. This polarized phase is predicted to be an exotic type of magnetically ordered superfluid known as FFLO. FFLO forms more readily in lower dimensions, but the signal from a single 1D Fermi gas is very weak. Experimentally, we create an array of roughly 100 x 100 such gases arranged in a square array using an optical lattice. We also induce a weak coupling between the 1D gases with the goal of phase-locking their magnetic ordering. As this coupling is varied, the character of the ensemble varies between 1D-like and 3D-like [B2]. In this 1D-3D crossover region, the FFLO ordering in neighboring tubes may lock phase, enabling their direct detection.

[B1] Physical Review A 92, 063616 (2015) (pdf)
[B2] Physical Review Letters 117, 235301 (2016) (pdf)


Scientific apparatus design and optimization

Many experiments in modern atomic physics are performed using table-top apparatus involving lasers, vacuum chambers, electromagnets, and other electronic devices, all computer controlled with microsecond (and sometimes nanosecond) scale timing. Advances in device technology and laser cooling and trapping techniques can increase the long-term stability of such experiments while reducing their complexity and cost.
One common source of drift in frequency-locked lasers is their sensitivity to fluctuations in temperature. Throughout the day, even in lab environments, the ambient temperature can change enough to destabilize common laser locks. We developed a scheme using commercial, off-the-shelf components that can be used to lock the frequency of a laser near a potassium resonance, but with no first-order sensitivity to temperature [C1].


Enhancement of Cs NMR from optical pumping

Another technique I am exploring is machine-learning optimization of experimental sequences; in a complicated apparatus, the optimum parameters for laser cooling can be challenging and time-consuming to find manually. However, using modern machine learning including genetic algorithms, such optimum conditions can be found automatically in a short time.

[C1] Review of Scientific Instruments 82, 033114 (2011) (pdf)


Spin dynamics in thermal gases

Clouds and beams of atoms with Maxwell-Boltzmann velocity distributions (or thermal gases) are used in a variety of sensors, including atomic clocks, magnetometers, and gyroscopes. Many of these devices enclose vapors of alkali atoms in glass cells to prevent them from oxidizing, and polarize the atoms' spins using optical pumping. The spins, however, are scrambled after collisions with the glass cell walls, reducing the overall spin polarization of the gas, and the resulting signal.

Nuclear and electron spin gradients in a vapor cell We measured the nuclear and electronic spin polarization in a vapor cell filled with optically pumped cesium atoms as a function of distance to a glass cell wall, and found that the length scale for depolarization of the two types of spins differs. In these cells, the nuclear and electronic spin gradients lead to a flux of spin to the glass walls [D1].


Enhancement of Cs NMR from optical pumping If the walls of the cell are coated with a thin layer of alkali salt (such as CsH or CsCl), the spin flux from the vapor can induce a spin polarization in the nuclei of the salt, which can be detected using NMR [D2]. Our measurements showed that the nuclear spin current in the vapor was primarily responsible for this spin transfer. In the salts, spin polarization is much longer-lived, and could potentially have applications in remote sensing or medical diagnostic imaging.

[D1] Physical Review A 84, 063410 (2011) (pdf)
[D2] Physical Review A 83, 063410 (2011) (pdf)


Publications

  1. Observation of Quantum-Limited Spin Transport in Strongly Interacting Two-Dimensional Fermi Gases
    C. Luciuk, S. Smale, F. Boettcher, H. Sharum, B. A. Olsen, S. Trotzky, T. Enss, and J. H. Thywissen
    Physical Review Letters 118, 130405 (2017) doi:10.1103/PhysRevLett.118.130405 (pdf)
  2. 1D to 3D Crossover of a Spin-Imbalanced Fermi Gas
    M. C. Revelle, J. A. Fry, B. A. Olsen, and R. G. Hulet
    Physical Review Letters 117, 235301 (2016) doi:10.1103/PhysRevLett.117.235301 (pdf)
  3. Phase diagram of a strongly interacting spin-imbalanced Fermi gas
    B. A. Olsen, M. C. Revelle, J. A. Fry, D. E. Sheehy, and R. G. Hulet
    Physical Review A 92, 063616 (2015) doi:10.1103/PhysRevA.92.063616 (pdf)
  4. Spin-velocity correlations of optically pumped atoms
    R. Marsland III, B. H. McGuyer, B. A. Olsen, and W. Happer
    Physical Review A 86, 023404 (2012) doi:10.1103/PhysRevA.86.023404 (pdf)
  5. Cusp kernels for velocity-changing collisions
    B. H. McGuyer, R. Marsland III, B. A. Olsen, and W. Happer
    Physical Review Letters 108, 183202 (2012) doi:10.1103/PhysRevLett.108.183202 (pdf)
  6. Optical pumping and spectroscopy of Cs vapor at high magnetic field
    B. A. Olsen, B. Patton, Y.-Y. Jau, and W. Happer
    Physical Review A 84, 063410 (2011) doi:10.1103/PhysRevA.84.063410 (pdf)
  7. Transfer of spin angular momentum from Cs vapor to nearby Cs salts through laser-induced spin currents
    K. Ishikawa, B. Patton, B. A. Olsen, Y.-Y. Jau, and W. Happer
    Physical Review A 83, 063410 (2011) doi:10.1103/PhysRevA.83.063410 (pdf)
  8. Temperature-insensitive laser frequency locking near absorption lines
    N. Kostinski, B. A. Olsen, R. Marsland III, B. H. McGuyer, and W. Happer
    Review of Scientific Instruments 82, 033114 (2011) doi:10.1063/1.3574221 (pdf)

Outreach

Science Rendezvous

Ben giving a lab tourAs part of a Canada-wide festival celebrating STEM, the Physics Department at the University of Toronto hosted an open house in Mclennan Physical Laboratory. In addition to assisting visitors with interactive demos, I led a tour of the Ultracold Atoms Group lab facilities. Over 50 guests ranging in age from about 8 to about 80 got to see how we use lasers and magnets to create some of the coldest matter in the known universe, and how we use that matter to understand the basics of superconductivity and magnetism.

Let's Talk Science Challenge

Ben judging a design challenge At an all-day STEM event for students in 6th-8th grades, teams of 4 compete in a STEM trivia contest and a hands-on design challenge. Students also learn about science at several interactive demonstration booths. I supervised two teams and helped judge the design challenge.

Quantum Mechanics for 8th Graders

In the 8th grade science classes of Susan Kalmbach and Bill Merritt at John Witherspoon Middle School in Princeton, NJ, I gave an interactive lesson with demonstrations on waves, sound, light, spectroscopy and the development of quantum theory.

North Jersey Regional Science Fair

I served as a judge for physics projects from students in 8th-12th grades, hosted at Rutgers University in New Brunswick, NJ