Coworkers, advisors, and collaborators

Olivier Pfister

Former Advisor, University of Virginia Physics


Cassandra Fraser

Former Advisor, University of Virginia Chemistry


Mikhail Lukin

Advisor, Harvard Physics


Nathalie de Leon

Professor at Princeton University


Kristiaan De Greve

Postdoctoral Researcher at Harvard University


Alp Spiahigil

Postdoctoral Scholar at Caltech


Marko Lončar

Collaborator, Harvard University School of Engineering and Applied Sciences


Hongkun Park

Collaborator, Harvard University Department of Physics and Chemistry


Element Six

Industry Partner


My research is performed in the lab of Professor Mikhail Lukin in the Harvard University Department of Physics. In the Lukin group, I work with many excellent graduate students and postdoctoral researchers, some of whom are highlighted here.

This research is interdisciplinary, drawing on techniques in quantum optics, nanophotonics, and materials physics. We therefore have many collaborators in and beyond Harvard, including the research group of Professor Hongkun Park in the Chemistry Department and Professor Marko Lončar in the School of Engineering and Applied Science.

This research has also attracted interest from industry, resulting in a productive collaboration with British diamond research firm Element Six, who provides us with our diamond substrates and works with us on diamond materials engineering.

Research Projects

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    Nanophotonics with color centers in diamond

    We use techniques in quantum optics and nanophotonics to increase the interaction strength between photons and color centers in diamond. Specifically, we create silicon-vacancy centers in diamond nanophotonic devices and then use techniques in quantum optics to engineer and demonstrate strong light-matter interactions. This work will hopefully lead to high-speed, deterministic atom-photon interactions with applications in quantum information and nonlinear optics.

    Over the past hundred years, information technology has revolutionized science, industry, and society. The burgeoning field of quantum information technology promises another set of revolutions for the twenty-first century: guaranteed-secure communications can protect us from data theft and cyber-espionage, efficient simulation of physical and chemical systems would transform development of novel drugs and next-generation materials, and implementation of digital quantum algorithms could provide exponential speed-ups for some important general-purpose computational tasks.

    All of these goals require quantum networks in which individual quantum bits communicate. My research is designed to address this challenge by using single particles of light (photons) to link qubits by engineering strong interactions between photons and color centers in diamond. These color centers are atom-scale defects that have spatially localized transitions between discrete electronic levels, producing absorption and therefore color in an otherwise transparent crystal.

    My research with Professor Mikhail Lukin in the Harvard University Department of Physics focuses on the Silicon-Vacancy (SiV) color center in diamond, a promising optically-addressible solid-state qubit. We use nanophotonics to increase the interaction between photons and the SiV. We first fabricate nanoscale optical waveguides out of the diamond crystal, effectively focusing light very tightly around our color centers, which increases the coupling strength to the focused photons. Finally, by adding nanoscale mirrors to both ends of the waveguide, we can create a nanophotonic crystal cavity. In such a cavity, the photons are effectively trapped between the high-reflectivity nanoscale mirrors, giving rise to many roundtrips inside the cavity before the photon escapes. Just as you can see many copies of your own reflection when you stand between two mirrors, the incoming photon can “see” many copies of the color center, giving it an increased interaction probability and hence stronger coupling. These techniques increase the interaction between photons and color centers in diamond by orders of magnitude.

    We have now reached the regime where a single SiV in a diamond nanophotonic crystal cavity can produce almost complete switching of an input field depending on the internal spin degree of freedom of the SiV. Moreover, by positioning multiple SiV centers in a single device, we can create entanglement or cavity-mediated interactions between two SiV centers. These results highlight the promise of this system as a scalable, solid state platform for optical quantum information science.

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    Spectroscopy and fundamental physics of color centers in diamond

    The silicon-vacancy (SiV) center in diamond is the key component of our solid-state quantum optics platform. However, the fundamental properties of the SiV center were not well-understood until recently. A key component of our work has been to use spectroscopic techniques in atomic and solid-state physics to understand the electronic properties of the silicon-vacancy center -- as well as the related germanium-vacancy center -- in diamond, especially inside practical devices.

    There are many candidates for optically-addressible qubits in the solid state, including self-assembled quantum dots, rare earth ions, and color centers in diamond and related materials. In diamond, attention has previously focused on the nitrogen-vacancy (NV) color center, primarily due to its spin degree of freedom that is optically addressible and long-lived even at room temperature. However, the optical properties of the NV are extremely unfavorable for engineering strong coupling between the NV and light. Hence, recent interest in our lab has turned towards exploring alternative color centers for solid-state quantum optics.

    Recently, the silicon-vacancy (SiV) center has attracted interest for its strong optical transition and narrow optical linewidth. However, fairly litte was known about the SiV center other than its basic electronic structure. Work in our lab focuses on determining the important properties of the SiV center for applications to quantum optics. For example, we demonstrated that the SiV can have extremely narrow optical transitions with a narrow inhomogeneous distribution even inside practical nanophotonic devices. We also explored the spin degree of freedom of the SiV and showed that under appropriate conditions, the SiV can be used as a long-lived qubit.

    Along the way, we discoveredd that the key properties of the SiV arose from its inversion symmetry. Based on this intuition, we explored a related color center in diamond, the germanium-vacancy (GeV) center which has the same symmetry. In some ways, the optical properties of the GeV are even more favorable for applications than those of the SiV.

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    Diamond surface treatments for color center stability and control

    Individual color centers in diamond can be extremely sensitive to their local electromagnetic environment. This sensitivity can be an asset when the color centers are used for sensing and metrology. However, using these color centers for quantum information or nonlinear optics usually requires exceptional pure environments. Our fabrication of nanostructures introduces additional damage and also brings the color centers close to the diamond surface, making this environmental purity even harder to achieve. To mitigate these problems, we have developed techniques to control the surface and bulk environment around the color centers, leading to superior spin and optical properties.

    Color centers in diamond are small, usually several-atom defects that have a reasonably strong optical transition, causing normally transparent diamond to absorb light and become colored. These color centers can be thought of as individual molecules "trapped" inside the diamond lattice, which (because diamond has a large bandgap) acts almost like a vacuum and often has minimal effects on many of the properties of the color center.

    However, the diamond lattice is not perfect, and color centers are often necessarily close to the diamond surface, either for near-surface sensing or in nanostructures. As a result, these color centers in diamond can interact with nearby defects or surfaces, leading to time-varying shifts in the energy of the color centers' electronic transitions; the Nitrogen-Vacancy center is especially susceptible to this problem, because it has a large ground-state dipole moment. Defects with nonzero spin can also couple to any available spin degrees of freedom, which can cause decoherence and limit the usefulness of color centers as sensors and quantum memories.

    To solve these problems, we have developed a complex process involving high temperature vacuum annealing and oxygen surface termination. The resulting surfaces are characterized with electron microscopy and various x-ray spectroscopies. This treatment results in charge-stable color centers with low spectral diffusion, even inside nanostructures.

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    Optical implementations of high-spin Bell inequalities

    Bell inequalities provide an experimental method to verify some of the counterintuitive, apparently metaphysical properties of quantum mechanics. Bell inequalities also provide a convenient practical test for the robustness or security of some quantum information protocols. As originally constructed, these inequalities apply to spin-1/2 objects, which are in many senses the simplest systems in quantum mechanics. Although these inequalities have been tested conclusively for many years, new inequalities have been developed that apply to systems of arbitrary dimension and allow us to probe the "quantumness" of a system as it becomes larger and larger. However, these more complex inequalities have never been tested. I proposed a concrete realization of a specific Bell-type inequality for high-dimensional systems using entangled photons and photon-number-resolving detectors.

    Since Bell’s original paper in 1964, a wide variety of experimental tests have overwhelmingly supported the completeness of quantum mechanics over local hidden-variable theories. However, relatively little effort has focused on systems of spins larger than 1/2; generalizing Bell’s result to higher dimensions is difficult, and the experiments needed to test these high-spin Bell inequalities are exacting. New advances in high effciency photon-number-resolving detectors suggest that experimental tests of these inequalities should be possible in the Schwinger representation, using the continuous-variable entangled (two-mode squeezed) fields produced by an optical parametric oscillator below threshold.

    In this paper, we explore the realistic experimental implementation of this proposal to violate Mermin’s high-spin inequalities. We demonstrate that violation for spin values greater than 1 should be attainable under a range of feasible experimental conditions that include finite squeezing and nonideal detection effciency.

    This work was performed in my third and fourth year as an undergraduate at the University of Virginia with Professor Olivier Pfister.

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    Synthesis and characterization of multi-luminescent boron biomaterials

    Fluorescence microscopy is one of the most powerful and ubiquitous tools to characterize biological and chemical systems. Developing new fluorescent dyes that are bright, multifunctional, biocompatible, and photostable can therefore have applications in a wide range of systems. In the Fraser lab at UVa, I synthesized and characterized a family of fluorescent dyes based on small molecules surrounded by polymer nanoparticles. By tuning the structure of these dyes, I was able to tune the color and properties of their fluorescence, allowing small variations on a single basic design to be used as a simple fluorescent dye, a self-calibrating oxygen probe, and a highly precise sensor for aqueous magnesium.

    Previous work has shown that difluoroboron dibenzoylmethane-polylactide (BF2dbmPLA) shows intense phosphorescence and a rare oxygen-sensitive room-temperature phosphorescence. Although this material has many good qualities as a potential imaging agent for a variety of purposes, improvements can still be made to increase the dye’s effectiveness and range of applicability. Fortunately, the modular nature of the diketone synthesis permits facile variation of the substituents on the diketone core.

    Building on our previous understanding of this system, I synthesized new BF2dbm-based dyes with a variety of substituents ranging from heavy atoms (which increase phosphorescence intensity through spin-orbit coupling) to large aromatic spacers (which increase donor-acceptor character) and beyond. These effects were characterized with a variety of optical spectroscopies including time- and frequency-resolved absorption, fluorescence, and phosphorescence, as well as typical chemical characterization techniques like UV/VIS and NMR.

    This research was performed in my first through third years as an undergraduate with Professor Cassandra Fraser in the University of Virginia Department of Chemistry.

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    Computational modeling of small fluorescent molecules

    Although fluorescent dyes are ubiquitous in many applications (see above), designing a dye for a particular application is a challenging task with imprecise, intuitive methods. As a result, determining the ideal chemical structure for a desired set of properties usually requires several rounds of synthesizing and characterizing a range of molecules before an acceptable dye is found. Even with experience, this process can take years.

    Fortunately, computational chemistry techniques are now sufficiently powerful and accessible to allow for many of the key properties of these molecules to be calculated. In the Fraser lab at UVa, I applied these techniques to our fluorescent dyes, complementing our existing synthetic chemical approach and accelerating dye development.

    Even small changes in the structure of a molecule can lead to drastically different optical and electronic properties. Unfortunately, exactly solving the Schrödinger Equation for even simple systems—much less molecules containing hundreds of electrons—is impossible. In the past, chemists and physicists usually had to resort to direct synthesis of a compound of interest to gain information about its properties.

    In the last few decades, however, computational techniques to gain accurate approximate models of molecular electronic energies have become widespread. Using techniques from this field of computational chemistry, I led a research effort to understand our small fluorescent molecules from a fundamental physical viewpoint and even predict the properties of new systems before synthesis.

    For example, some of our dyes show intense solvatochromism while others emit at more or less the same wavelength regardless of solvent. This reason for this was not well understood. However, early studies I’ve performed indicate that the parent dye, dibenzoylmethane (with only a phenolic aromatic group) shows very little donor acceptor character; the electron density distribution appears similar in the HOMO and the LUMO. However, as the aromatic ring size increases, the HOMO becomes centered more and more on the aromatic substituent while the LUMO remains centered on the diketone core.

    Although quantitatively predicting emission wavelengths in complicated polymeric systems is difficult, these and other model studies helped to point us in the right direction guided us in developing new analogues with predictable properties.

    This research was performed in my second and third years as an undergraduate with Professor Cassandra Fraser in the University of Virginia Department of Chemistry, with advice from Professor Carl Trindle.