# Abraham Asfaw

## Department of Electrical Engineering Princeton University B205 Engineering Quadrangle Princeton, NJ 08544

Update: I am presently in the job market.

Selam! I am a Ph.D. candidate at Princeton University in the Department of Electrical Engineering. My research interests are primarily in quantum computation and condensed matter physics. For graduate work, I have focused on electron spin resonance experiments on donor spins in silicon and electrons floating on superfluid helium as a member of Lyon lab.

Beyond research, I have worked on projects involving cryptography and reversible computing. More recently, I have been interested in blockchains and cryptocurrency markets.

Formerly, I was a student in the Department of Electrical and Computer Engineering at Manhattan College. Before that, I spent 6 years at Saint Joseph School in Addis Ababa, Ethiopia.

Outside academics, I enjoy swimming, DJing at Dbar, and writing code in Python.

# Journal Publications

For an up-to-date list of publications, please visit my Google Scholar page.
• A. T. Asfaw, E. I. Kleinbaum, T. M. Hazard, A. Gyenis, A. A. Houck and S. A. Lyon, SKIFFS: Superconducting Kinetic Inductance Field-Frequency Sensors for Sensitive Magnetometry in Moderate Background Magnetic Fields [arXiv]

We describe sensitive magnetometry using lumped-element resonators fabricated from a superconducting thin film of NbTiN. Taking advantage of the large kinetic inductance of the superconductor, we demonstrate a continuous resonance frequency shift of 27 MHz for a change in magnetic field of $$1.8~\mu$$T within a perpendicular background field of 60 mT. By using phase-sensitive readout of microwaves transmitted through the sensors, we measure phase shifts in real time with a sensitivity of 1 degree/nT. We present measurements of the noise spectral density of the sensors, and find their field sensitivity is at least within one to two orders of magnitude of superconducting quantum interference devices operating with zero background field. Our superconducting kinetic inductance field-frequency sensors enable real-time magnetometry in the presence of moderate perpendicular background fields up to at least 0.2 T. Applications for our sensors include the stabilization of magnetic fields in long coherence electron spin resonance measurements and quantum computation.

• A. T. Asfaw, E. I. Kleinbaum and S. A. Lyon, Transport Measurements of Surface Electrons in 200 nm Deep Helium-Filled Microchannels Above Amorphous Metallic Electrodes [arXiv]

We report transport measurements of electrons on helium in a microchannel device where the channels are 200 nm deep and $$3~\mu$$m wide. The channels are fabricated above amorphous metallic Ta$$_{40}$$W$$_{40}$$Si$$_{20}$$, which has surface roughness below 1 nm and minimal variations in work function across the surface due to the absence of polycrystalline grains. We are able to set the electron density in the channels using a ground plane. We estimate a mobility of 300 cm$$^2$$/V$$\cdot$$s and electron densities as high as 2.56$$\times10^{9}\text{ cm}^{-2}$$. We demonstrate control of the transport using a barrier which enables pinchoff at a central microchannel connecting two reservoirs. The conductance through the central microchannel is measured to be 10 nS for an electron density of 1.58$$\times10^{9}\text{ cm}^{-2}$$. Our work extends transport measurements of surface electrons to thin helium films in microchannel devices above metallic substrates.

• T. M. Hazard, A. Gyenis, A. Di Paolo, A. T. Asfaw, S. A. Lyon, A. Blais and A. A. Houck, Nanowire Superinductance Fluxonium Qubit [arXiv]

Disordered superconducting materials provide a new capability to implement novel circuit designs due to their high kinetic inductance. Here, we realize a fluxonium qubit in which a long NbTiN nanowire shunts a single Josephson junction. We explain the measured fluxonium energy spectrum with a nonperturbative theory accounting for the multimode structure of the device in a large frequency range. Making use of multiphoton Raman spectroscopy, we address forbidden fluxonium transitions and observe multilevel Autler-Townes splitting. Finally, we measure lifetimes of several excited states ranging from $$T_1=620$$ ns to $$T_1=20~\mu$$s, by applying consecutive $$\pi$$-pulses between multiple fluxonium levels. Our measurements demonstrate that NbTiN is a suitable material for novel superconducting qubit designs.

• A. T. Asfaw, A. J. Sigillito, A. M. Tyryshkin, T. Schenkel and S. A. Lyon, Multi-frequency spin manipulation using rapidly tunable superconducting coplanar waveguide microresonators, Appl. Phys. Lett. 111, 032601 (2017). [doi:10.1063/1.4993930] [arXiv] Selected as an APL Editor's Pick

In this work, we demonstrate the use of frequency-tunable superconducting NbTiN coplanar waveguide microresonators for multi-frequency pulsed electron spin resonance (ESR) experiments. By applying a bias current to the center pin, the resonance frequency ($$\sim$$7.6 GHz) can be continuously tuned by as much as 95 MHz in 270 ns without a change in the quality factor of 3000 at 2K. We demonstrate the ESR performance of our resonators by measuring donor spin ensembles in silicon and show that adiabatic pulses can be used to overcome magnetic field inhomogeneities and microwave power limitations due to the applied bias current. We take advantage of the rapid tunability of these resonators to manipulate both phosphorus and arsenic spins in a single pulse sequence, demonstrating pulsed double electron-electron resonance (DEER). Our NbTiN resonator design is useful for multi-frequency pulsed ESR and should also have applications in experiments where spin ensembles are used as quantum memories.

# Presentations

• A Asfaw, A.J. Sigillito, A.M. Tyryshkin, T. Schenkel and S.A. Lyon, Multi-Frequency Pulsed EPR and DEER Using Rapidly Tunable Superconducting Microresonators, July 2018, EPR Oral Session Talk, 59th Rocky Mountain Conference on Magnetic Resonance (Snowbird, Utah)
• A Asfaw and S.A. Lyon, Superconducting Kinetic Inductance Field-Frequency Sensors: High-Sensitivity Magnetic Field Sensing in Moderate Background Fields, March 2018, Contributed Talk, APS March Meeting (Los Angeles, California)
• A Asfaw and S.A. Lyon, Superconducting Kinetic Inductance Field-Frequency Sensors: High-Sensitivity Magnetometry in Moderate Background Fields, February 2018, Poster, Princeton Center for Complex Materials Annual Poster Night (Princeton, New Jersey)
• A Asfaw and S.A. Lyon, Superconducting Kinetic Inductance Devices for Electron Spin Resonance Applications, December 2017, Talk, Quantum Group Meeting (Princeton, New Jersey)
• A Asfaw, AJ Sigillito, AM Tyryshkin, and SA Lyon, Current-Tunable NbTiN Coplanar Photonic Bandgap Resonators, March 2017, Contributed Talk, APS March Meeting (New Orleans, Louisiana)
• A.T. Asfaw, A.M. Tyryshkin, and S.A. Lyon, Tracking Magnetic Field Fluctuations in Electron Spin Resonance, August 2016, Poster, Gordon Research Conference: Defects in Semiconductors, (New London, NH)
• A.T. Asfaw, A.M. Tyryshkin, and S.A. Lyon, Tracking Magnetic Field Fluctuations in Electron Spin Resonance, August 2016, Poster, Gordon Research Seminar: Defects in Semiconductors, (New London, NH)
• A.T. Asfaw, A.M. Tyryshkin, and S.A. Lyon, Tracking Field Fluctuations in Pulsed EPR, July 2016, EPR Oral Session Talk, 58th Rocky Mountain Conference on Magnetic Resonance, (Breckenridge, CO)
• A Asfaw, A Tyryshkin, and S Lyon, Dynamic field-frequency lock for tracking magnetic field fluctuations in electron spin resonance experiments, March 2016, Contributed Talk, APS March Meeting (Baltimore, Maryland)
• A. Asfaw, Introduction to Quantum Computation and Quantum Algorithms, 2-day workshop presented at 4-kilo campus, Addis Ababa University (2015). [day 1 slides] [day 2 slides]
• A Asfaw, G Wolfowicz, JJL Morton, A Tyryshkin, and S Lyon, Spin Ensembles as Sensitive Probes of Environmental Magnetic Field Noise, March 2015, Contributed Talk, APS March Meeting (San Antonio, Texas)
• A.T. Asfaw, A.M. Tyryshkin, and S.A. Lyon, Suppressing Effects of Magnetic Field Noise in Long Echo Decay Measurements, July 2014, EPR Oral Session Talk, 56th Rocky Mountain Conference on Magnetic Resonance, (Copper Mountain, CO)

• A. Asfaw and P. Boothe, Computing Backwards, 7th Annual Spuyten Duyvil Undergraduate Mathematics Conference (NSF Grant DMS-0846477) (2012)
• A. Asfaw, Computability and Turing Machines, Department of Mathematics and Computer Science, Manhattan College (2011)
• A. Asfaw, Mersenne Primes and the Global Internet Mersenne Prime Search (GIMPS), 6th Annual Spuyten Duyvil Undergraduate Mathematics Conference (NSF Grant DMS-0846477) (2011)
• A. Asfaw, Moments of Velocity in Arbitrary Dimension, 5th Annual Spuyten Duyvil Undergraduate Mathematics Conference (NSF Grant DMS-0846477) (2010)