A common theme of my past research experience in physics is the use of experimental atomic physics techniques to perform precise measurements for a a wide variety of applications, especially answering fundamental physics questions.

Quantum sensing for neutrino and dark matter detection

As a staff scientist in the Quantum Technology Center (QTC) at the University of Maryland, I have continued to develop quantum sensors for fundamental physics, focusing on using nitrogen-vacancy (NV) centers in diamond construct a directional neutrino and dark matter detector. Weakly Interacting Massive Particles (WIMPs) are one of the leading candidates for dark matter, as their existence is also predicted by many supersymmetric theories. Current WIMP experiments typically use a large mass of several tons of material (such a liquid noble gas or semiconductors) to look for WIMP interactions. Decades of conventional searches have not found any WIMPs, and current methods will soon run into the so-called “solar neutrino fog”, where constant backgrounds from solar neutrinos passing through the Earth will overwhelm a dark matter signal. The most viable method to overcome this barrier is to add the ability to detect the direction of incoming particles, as the solar neutrino and dark matter winds flowing through the Earth are predicted to have different directions. Recent advances in quantum sensing with NVs in diamond make them attractive candidates for directional detection.

A full-scale detector would consist of a ~m³ volume of mm-scale diamond segments. When a 1-100 GeV WIMP or solar neutrino interacts with the detector, it will kick a carbon nucleus in the diamond that will lead to secondary recoils, leaving a ~10-100 nm frozen damage track in the crystal lattice structure of the diamond whose orientation is strongly correlated with the initial direction of the particle. Detection then proceeds in three stages:

• Stage 1: conventional semiconductor-based techniques (such as charge, phonon, or photon collection) are used to register the event in the diamond and identify which segment contains the diamond track.
• Stage 2: The identified segment is removed and scanned with state-of-the-art NV quantum strain spectroscopy techniques to locate the track to ~1 μm precision.
• Stage 3: nanoscale imaging techniques (such as NV superresolution microscopy or high-resolution X-ray diffraction microscopy) are used to scan the micron-scale voxel and characterize the length, shape, and orientation of the track. This information can be correlated with the timestamp from stage 1 to identify whether the signal came from a WIMP or neutrino. Stages 2 and 3 must be completed within less than a week of imaging time in order to keep up with the anticipated event rate.

At the UMD QTC, we are developing techniques for each detection stages above, with a particular focus on stages 2 and 3. In the past, the required strain measurement precision has been demonstrated using NV quantum strain spectroscopy and scanning X-ray diffraction microscopy (performed at Argonne National Labs). Our current focus is to demonstrate that these techniques to observe artificial damage tracks in diamond made using precision ion implantation (in collaboration with Sandia National Lab), improve the scanning speed for stage 2 by developing 3D widefield quantum diamond microscopy techniques, and develop NV superresolution techniques for full 3D strain sensing. In the next few years, we aim to build a ~100-1000 mg detector which will integrate these techniques in order to demonstrate the viability of this technique and readiness to scale to larger mass detectors.

Past Research

Measurement of the Electron Electric Dipole Moment in ThO (2015-23)

This was the focus of my PhD dissertation research at Harvard University, which can be downloaded here: Progress towards an improved measurement of the electric dipole moment of the electron.

Despite the immense experimental and theoretical success of the Standard Model, several crucial questions remain. One of the most puzzling is explaining the current abundance of matter over antimatter, the so-called baryon asymmetry problem. According to the Sakharov conditions, baryogenesis requires a sufficient amount of charge-parity (CP) violation, which we have only found in limited amounts in nature. A non-zero electric dipole moment (EDM) would violate CP, and thus constitute a powerful probe for new physics.

The ACME EDM (Advanced Cold Molecule Electron EDM) experiment utilizes the state-of-the-art AMO methods to perform a precise measurement of the electron EDM. The apparatus consists of a cryogenic buffer gas cooled beam (CBGB) of thorium monoxide (ThO). Various techniques are used to transfer as many molecules as possible into the EDM-sensitive -electronic state, where we perform a spin precession measurement. The molecules are prepared into a superposition of states, which precesses as it goes through applied electric and magnetic fields in the interaction region. The strong internal electric field inside ThO amplifies the effective field experience by the electron, boosting the sensitivity of our measurement by many orders of magnitude. The phase of the precessed state is then measured, from which we can deduce the contribution from the electron EDM.

In 2014, the ACME I experiment improved the previous upper limit on the eEDM by an order of magnitude. I joined the experiment in 2015. In 2018, the ACME II experiment successfully improved upon this limit by another order of magnitude: . I made important contributions towards this result, specifically in the areas of data acquisition, analysis, and experimental control. Since then, the ACME III campaign has begun, aiming to further improve upon this result. Various upgrades to the experiment have been researched and demonstrated. I was the lead author on an improved measurement of the EDM-sensitive H-state radiative lifetime, which found its value to be 4.2 (5) ms, several times longer than the precession time used in ACME II. This opened the way to increase by several times, significantly improving the sensitivity. I also designed improved collection optics for the experiment, upgraded the data acquisition and experimental control systems, and designed magnetic field coils for the new experiment apparatus, in addition to contributing various other development efforts. Overall, the ACME III experiment is projected to improve upon ACME II sensitivity by at least an order of magnitude () while also reducing known systematic errors by a commensurate amount. More information can be found at www.electronedm.info.

Search for Long Range Spin-Spin Interactions (2012-15)

This was my research focus during my undergraduate degree at Amherst College in the lab of Professor Larry Hunter. Details on this research can be found in my senior bachelor’s thesis, In Search of New Geometries for Probing Spin-Spin Interactions.

The great success of the Standard Model was partially due to the discoveries of
the gauge bosons (W, Z, and gluons) which mediate the weak and strong interactions. Naturally, searches for physics beyond the Standard Model (BSM) have included extensive experimental and theoretical investigations into the existence of additional gauge bosons – both the spin-1 (vector boson) variety as well as their more exotic cousins (scalar and pseudoscalar bosons). The existence of some of these new particles would have potential to explain dark matter or the strong CP problem.

In the Hunter Lab, we use a cesium-mercury co-magnetometer in a Bell-Bloom configuration to look for evidence of certain interactions mediated by BSM vector bosons, specifically long-range spin-spin interactions (LRSSIs). The apparatus consists of an Hg vapor cell sandwiched between two Cs vapor cells immersed in a magnetic field. The atoms in the cells are pumped to a coherent spin state by optically pumping them with circularly polarized modulated light. Afterwards, the pump laser is turned off and a weaker laser tuned several GHz off resonance is used to probe the frequency of the spin precession via detection of Faraday rotation. The apparatus is mounted on a rotating table to allow for different orientations of the spin-polarized atoms. If LRSSIs existed, then the spin-polarized electrons inside the Earth would perturb the spins in the lab apparatus.

This technique of using polarized geoelectrons to look for LRSSIs was pioneered by Larry Hunter in 2013. This technique also allowed experimental bounds to be set for the first time on a variety of velocity-dependent LRSSIs. I spent time during my sophomore and junior years at Amherst assisting Prof. Hunter with these calculations.

During my senior year at Amherst, I did work on testing upgrades on the apparatus to reduce systematics from AC Stark shifts. These upgrades are expected to increase sensitivity to LRSSIs by an order of magnitude. For more information, contact Larry Hunter at Amherst College.