## Research

My research is in experimental quantum science. I am excited about developing new strategies to control quantum systems in the lab with ever-increasing scale and fidelity in order to use them as tools for science and technology. Programmable quantum systems are unique vehicles to learn about the physics of complex many-body systems and at the same time explore how these complexities can power fundamentally new technologies such as quantum computation, quantum metrology, and quantum networking. The modern era of quantum science is particularly exciting with the rapidly developing capabilities to construct large quantum systems that are firmly beyond the regime of classical simulatability and for which entanglement and non-classical phenomena play a central role.

### Towards hardware-efficient error correction with transmon-based qubits

Standard approaches for quantum error correction pose daunting challenges for building large-scale quantum computers due to large error-correction overheads. Erasure qubits offer a compelling strategy to reduce this overhead by converting qubit errors into detectable erasures, which are flagged in real-time and can be more effectively corrected than other errors. Our team at the AWS Center for Quantum Computing demonstrated that a "dual-rail qubit" encoded in the symmetric/antisymmetric subspace of two transmons can convert T1 decay into erasure errors while also suppressing residual dephasing, leading to long, millisecond-scale coherence within the logical subspace. This strategy may enable more robust and lower-overhead error-corrected processors.

A. Kubica, A. Haim, et. al. Phys. Rev. X 13, 041022 (2023).

H. Levine, A. Haim, et. al. Phys. Rev. X 14, 011051 (2024).

### Neutral atom quantum processor with dynamically reconfigurable atoms

The neutral atom platform for quantum processing offers unique features, from scalable trapping of identical, high-coherence neutral-atom qubits to capabilities for parallel, high-fidelity gate operations. Our team at Harvard recently showed a new way to combine these features: by coherently moving atoms during a quantum circuit, we reconfigured the connectivity of the array to effectively enable arbitrary long-range gates. This coherent transport can be implemented in parallel on many atoms at once, which combines beautifully with the parallel implementation of two-qubit gates by illuminating the whole atom array with a Rydberg excitation laser: transport, entangle, transport, entangle, ... These features enable highly efficient implementation of many classes of quantum circuits, including those which are central building blocks for quantum error correction.

The two-qubit gate fidelities with neutral atoms are also steadily improving over time. Our team introduced a new gate protocol in 2019 (the "Pichler gate") that could be implemented by globally driving both atoms in the pair, enabling convenient parallel implementation across large systems. Our first gate fidelities were 97.4(3)%, and recent advances from the Harvard team including optimized variants of this gate and higher power lasers have now reached 99.5% fidelity.

D. Bluvstein, H. Levine, et. al. Nature 604, 451 (2022)

S. Evered, D. Bluvstein, et. al. Nature 622, 268 (2023)

H. Levine, A. Keesling, et. al. PRL 123, 170503 (2019)

H. Levine, A. Keesling, et. al. PRL 121, 123603 (2019)

Many-body evolution on a ruby lattice [Semeghini, et. al. Science (2021)]

Quench dynamics on a Rydberg atom array

[Bluvstein, et. al. Science (2021)]

### Programmable quantum simulation with atom arrays

Programmable atom arrays are a flexible platform for investigating many-body physics in a wide variety of regimes. Strong atomic interactions are switched on by coherent coupling to Rydberg states, introducing a rich spin-Hamiltonian whose behavior depends strongly on the geometry of the interacting atoms. By arranging atoms in different structures and programming the Hamiltonian parameters by controlling the laser excitation field, we have studied:

Many-body ground-states, including topological spin-liquid states, Schrödinger cat states, and ordered phases on many lattice geometries.

Non-equilibrium dynamics, including the surprising phenomena of many-body scarring which first emerged in an experiment where we quenched the Hamiltonian on a 1D atom array and observed persistent oscillations rather than the expected thermalization.

Universal dynamics of quantum phase transitions which manifest with experimental signatures according to the Quantum Kibble-Zurek mechanism.

Mapping between the Rydberg Hamiltonian and the cost function for a combinatorial optimization problem, the Maximum Independent Set. Finding the many-body ground state is equivalent to solving this computational problem, and an active area of research is to compare the quantum vs. classical hardness of different problem instances to identify regimes of quantum speedup.

G. Semeghini, et. al. Science 374, 1242 (2021)

S. Ebadi, et. al. Nature 595, 227 (2021)

A. Omran, et. al. Science 365, 570 (2019)

H. Bernien, et. al. Nature 551, 579 (2017)

D. Bluvstein, et. al. Science 371, 1355 (2021)

A. Keesling, et. al. Nature 568, 207 (2019)

S. Ebadi, et. al. Science 376, 1209 (2022)

Level structure for analyzing stimulated Raman transitions

### Fundamentals of atom-light interactions: new strategies for controlling atoms

Atom-light interactions are a central part of the AMO toolbox. Understanding these interactions from many perspectives can offer useful intuition that motivates new experimental methods. As one example, we considered the physics of stimulated Raman transitions which are used to drive transitions between atomic qubit states. While Raman transitions are often understood as two-photon processes which can be computed based on the frequency spectrum of the drive laser, we developed an alternate semi-classical interpretation based on the notion that an off-resonant laser induces a so-called "vector light shift" that acts as a fictitious magnetic field. In this framework, atomic spin transitions are driven by modulating the vector light shift through laser intensity modulation, without any need to consider the detailed frequency spectrum.

With this intuition, the task of building a suitable laser system for driving Raman transitions is to induce gigahertz-scale amplitude modulation on a laser. We developed a new method for such high-frequency amplitude modulation by first applying phase modulation with an electro-optic modulator and then using a highly dispersive optical element (volumetric chirped Bragg grating) to convert the phase modulation into amplitude modulation.

H. Levine, et. al. PRA 105, 032618 (2022)