Research

Developing microscopic control of complex quantum systems and leveraging this control to realize entanglement-based measuring devices is a central theme of modern quantum science. Progress here is expected to revolutionize computing, allow novel and secure multiparty communication protocols, and usher vast improvements in the performance of sensors. Surprisingly, the stabilization of such quantum systems can rise out of their own complexity, originating from exotic non-equilibrium dynamics or intrinsic disorder in the system. Moreover, on the experimental front, recent advances enable one to directly build up such complexity via the control of individual quantum systems, such as single photons, isolated atoms and solid-state spins.

Our research program focuses on the latter. More broadly, our interests fall within the growing landscape associated with the creation and control of optically active spin defects in solid-state materials. This landscape ranges from the manipulation of naturally occurring defects in diamond and silicon carbide, to the discovery of single-photon sources in van der Waals heterostructures, to the search for spin-qubits in rare-earth-doped oxides. At first glance, each of these platforms appears extremely different from one another. However, they share a number of key experimental advantages: they are mechanically stable and robust to a wide range of physical conditions, can be generated in a scalable and precise way, and exhibit a mechanism for optical pumping.

To this end, our group will employ Nitrogen-Vacancy (NV) and Silicon-Vacancy (SiV) centers in diamond, as well as novel spin defects in hexagonal boron nitride (hBN), for three overarching directions: (1) quantum-enhanced sensing, (2) simulation of non-equilibrium quantum dynamics, and (3) quantum information and computation.

Quantum-enhanced sensing

Recently quantum sensors based on solid-state spin defects have emerged as localized probes for a myriad of signals, including magnetic and electric fields, strain and temperature. Compared to conventional methods, such platform offers nanoscale spatial resolution and the ability to operate under a wide variety of external conditions — e.g. ranging from cryogenic (∼ 10 mK) to high temperatures (∼ 600 K), from ambient to megabar pressures and in the case of biological samples, directly in-vivo. Our group is interested in employing solid-state quantum sensors as an unique and versatile tool to explore novel phenomena in physical, chemical and biological systems.

Quantum sensing at extreme pressures. (A) Schematic of a diamond anvil cell (DAC) integrated with NV centers for quantum sensing at high pressures. (B) The DAC chamber filled with a sample of interest and a ruby (pressure calibrant). A ∼50-nm layer of NV centers is embedded into the diamond anvil directly below the sample chamber as our sensors. (C) A representative optically detected magnetic resonance (ODMR) spectrum from an NV center ensemble under an applied magnetic field. (D) A “pink” diamond anvil containing a dense ensemble of NV centers. (E)

Related works
1. Optically enhanced electric field sensing, Phys. Rev. Applied 16, 024024 (2021)
2. Imaging stress and magnetism at high pressures, Science 336, 6471 (2019)
3. Imaging local charge environment in diamond, PRL 121, 246402 (2018)

Non-equilibrium quantum dynamics

While quantum sensing employs the coupling between a defect and its external environment, quantum simulation utilizes the interplay between defects themselves. In a sample with extremely high spin defect concentration, the average spin-spin spacing can be as small as a few nanometers, leading to strong dipolar interactions. This provides a natural playground to experimentally investigate quantum many-body dynamics in regimes that are especially difficult to study via analytical calculations or numerical simulations.

Nanoscale spin diffusion in a long-range interacting quantum system. (A) Schematic depicting the emergence of hydrodynamics in a strongly interacting dipolar spin ensemble.
Optical pumping (green arrow) of the NV center (red) enables it to behave as a spin sink for nearby dense electronic spin defects (blue), resulting in the preparation of a local, inhomogenous spin-polarization profile. Quantum dynamics under dipolar interactions then lead to the spreading of this profile as a function of time. (B) The measured survival probability S_p(t) as a function of time. After an initial transient, S_p(t) approaches a robust power-law decay ~t^{-3/2}, inevitably demonstrating the late-time diffusive nature of the many-body system. (C) A robust non-Gaussian polarization profile emerges from the semi-classical model for all experimentally accessible time-scales, indicating non-conventional diffusion arising from the presence of strong disorders (both positional and on-site) in the system.

Related works
1. Emergent hydrodynamics in a strongly interacting dipolar spin ensemble. Nautre 597, 45-50 (2021).
2. Probing many-body noise in a strongly interacting two-dimensional dipolar spin system. arXiv: 2103.12742 (2021).

Quantum information and computation

Thanks to their atomic-like properties: long-coherence quantum states and well-defined optical transitions, spin defects in diamond (e.g. NV and SiV centers) have recently emerged as one of the most promising physical systems for realization of quantum information processing. Using a combination of optical and microwave fields, we have successfully demonstrated the entanglement between single NV electronic spin and nearby nuclear spins (e.g. carbon-13 and nitrogen-14).

Realization of noise-resilient universal geometric quantum gates. (A) Illustration of a NV center in diamond with a proximal C13 nuclear spin as our two-qubit system. (B) The level structure of the electron and the nuclear spins for the geometric CNOT gate and the microwave and radio-frequency coupling configuration. (C) The density operator of a maximally entangled two-qubit state reconstructed through quantum state tomography after the geometric controlled-NOT gate is applied.

Related works
1. Storage quantum entanglement in decoherence-free subspace, PRB 96, 134314 (2017)
2. Quantum teleportation from photons to macroscopic vibrational modes in diamond, Nature Communicaition 7, 11736 (2016)
3. Realization of universal geometric quantum gates, Nature 514, 72 (2014)