Next Generation Quantum Sensors
Engineering optically addressable molecular qubit sensors
Diamond based qubits have been the main driving force for nanoscale quantum sensing, with applications ranging from condensed matter physics to geology to developmental biology. However, diamond-based technology comes with limitations. For example, diamond nanoparticles are relatively large, usually of 30-100nm size, and have complex surfaces that lead to noise and qubit dephasing. In this thrust, my lab strives for a new generation of water soluble ‘molecular’ qubit sensors that are on the order of one to two nanometer (i.e., fifty-times smaller than diamond nanocrystals) and have coherence times of 10us. These qubit sensors will be readily functionalizable and can be conjugated to a large class of target molecules.
In collaboration with the Esser-Kahn and Gagliardi group, we have been investigating biocompatible molecular qubit systems, with a large optical cross-section and optically detectable magnetic resonance (ODMR). In preliminary experiments, our team has successfully identified the ODMR signature associated with molecular spin qubits. We are currently exploring the spin physics and optical properties of these molecular systems. In parallel, our lab started a collaboration with the Awschalom lab where we investigate protein based qubits as novel field sensors. If successful these molecular qubits have the potential to replace diamond based quantum sensors on a wide range of applications.
Single-molecule platform for diamond sensing
Diamond nanocrystals can host optically addressable qubits that are small enough to be uptaken by living cells, and, to some extent, can target specific subcellular locations. In principle, this would make diamond nanocrystals an ideal system for quantum enabled bio-sensing. However, diamond nanocrystals are rarely used for sensing due to their low sensitivity, which is limited by a relatively short qubit coherence (note, nanoscale thermometry is an exception). Although these limitations in NV coherence have recently been linked to surface noise, we still do not have a good nanoscopic understanding of its origin, let alone the strategies to mitigate this noise.
In collaboration with the Esser-Kahn lab, our group has been investigating approaches to extend the NV coherence time by engineering the surface noise properties of diamond nanocrystals. In our approach, we encapsulated diamond nanocrystals with a silica shell that saturates dangling bonds and charge traps. Our results indicate that when compared to uncoated diamond nanocrystals our core-shell particles possess a significant increase in spin coherence. Using dynamical decoupling techniques, we further investigated the spectral properties of the noise for uncoated nanocrystals and their core-shell counterparts. We found that their respective noise spectra are distinctly different, with that of uncoated nanocrystals resembling a white noise spectrum while core-shell structures indicating an 1/f-noise. The observed results suggest fundamental changes in the spin dynamics of these engineered particles. In a follow up experiment, we will investigate the microscopic cause of this drastic increase in spin coherence for core-shell particles. The gained insights will also have direct applications in nanoscale temperature sensing.
Collaborators: Aaron Esser-Kahn (UChicago), Giulia Galli (UChicago) Michael Flatté (Univ. of Iowa), and Joe Heremans (ANL)
Publications:
Preparation of metrological states in dipolar-interacting spin systems
Sensitivity is one of the main factors limiting today’s application of nanoscale quantum sensing. We explore the creation of quantum entangled states that enable a significant sensitivity increase. From classical physics, one would expect that a measurement of N identical sensor qubits results in a sensitivity increase of a factor N^1/2. However, it turns out when using quantum entanglement, one can do considerably better and obtain a sensitivity that is proportional to N, which is known as the Heisenberg limit. While this sensitivity increase has far reaching applications in quantum metrology, nobody currently knows of a robust approach to create such entangled states in solid-state spins, the system of choice in nanoscale sensing.
Our lab recently developed a variational algorithm that drives dipolar interacting spin systems into highly entangled states that lead to a significant metrological gain. The algorithm can be performed on a small dipolar interacting spin ensemble without ever knowing the exact spatial distribution of the spins. Furthermore, our approach solely relies on uniform single-qubit rotations and free evolution under dipolar interaction, which are experimentally readily implementable. Depending on the circuit depth and the level of readout noise, the resulting states resemble either a Greenberger-Horne-Zeilinger state or a Spin Squeezed State. Importantly, this approach is relatively robust to experimental imperfections such as initialization and readout error and dephasing. In solid state spin systems, the creation of such states is of particular interest, since the non-Markovian aspect of the noise results in a metrological gain even in the presence of dephasing. These results are written up in a manuscript that is currently under review with npj – Quantum Information (see ref.-[1]). As a next step, we are planning to actively explore these experimental approaches to demonstrate and benchmark the proposed protocols.
Collaborators: Liang Jiang (UChicago), Aashish Clerk (UChicago), and Fred Chong (UChicago)
Publications:
Applications to Biological Targets
Interfacing intact biomolecules with coherent quantum sensors
NV centers in diamond promise sensitivity needed to detect the nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) signature of small ensembles and even individual biomolecules. Such spectroscopic measurements could provide structural and dynamical information that are not available from fluorescent measurements. Although diamond-based quantum sensing has an unmatched sensitivity, spectroscopy on intact biological molecules has remained elusive, because of challenges in interfacing biological and quantum systems. One of the main difficulties has been: how to immobilize intact biological molecules within the 10nm sensing range of a diamond qubit sensor while maintaining the sensors’ exceptional coherence properties.
Our work addresses this challenge through a unique combination of techniques borrowed from three distinct fields: quantum engineering, single-molecule biophysics and material processing. Using single-molecule and diamond processing tools, we showed that our diamond surface modification technique allows us to precisely control the density of immobilized proteins and DNA molecules on a diamond sensor. Importantly, our surface modification process maintained the exceptional coherence properties of near surface (depth: 3–7nm) NV centers with T2-times approaching 100us. Furthermore, the developed functionalization architecture remains chemically stable under physiological conditions for over five days, making our technique compatible with most biophysical and biomedical applications. The results of this work have been published in PNAS [1]. We are currently working with SomaLogic, a Colorado based biotech startup, to extend our immobilization technique to an aptamer based binding assay that will eventually allow us to perform molecular pull-down of up to 7,000 different proteins on our quantum sensor array.
Collaborators: Nathalie de Leon (Princeton Univ.), Niels Quack (Univ. of Sydney), and Jason Cleveland (CTO – SomaLogic)
Publications:
Nanoscale NMR spectroscopy
Spectroscopic techniques, most notably nuclear magnetic resonance (NMR), provide detailed structural and dynamical information at an atomic scale. However, these techniques do not yet possess the sensitivity and spectral resolution to probe small volumes and concentrations at physiological relevant levels. In this thrust, my lab develops a diamond based nanoscale NMR spectroscopy platform that allows us to probe the spectroscopic signature of molecular structures and dynamical processes of small ensembles of biomolecules in their physiological environment. Our approach relies on diamond based nanoscale magnetic field sensing to probe nuclear spin resonances of molecules within a few nanometers from our quantum sensor.
Existing diamond sensing techniques have enabled NMR spectroscopy with chemical resolution and femtomol sensitivity. However, these proof-of-principle experiments have only been performed on simple test systems such as xylene in DMSO and with spectral resolutions on the order of a few parts per million. For meaningful experiments in the life sciences, such as the detection of the metabolomic profile of a single cell, requires a further increase in sensitivity and spectral resolution. As part of a recently funded NSF-QLCI, my lab, in collaboration with Joe Sachleben (UChicago – NMR facilities) and Alex High, has started to address these challenges by developing a NV NMR system that operates at a magnetic field of 14T. When compared to existing NV experiments, this will allow us to increase the sensitivity and spectral resolution by a factor of 150x. Operating at those fields comes with several substantial engineering challenges such as coherent microwave manipulation at frequencies of (2)392GHz. This high-field diamond NMR system will be capable of performing ‘high-resolution’ NMR spectroscopy of pL sample volumes, the size of a single cell. The resulting technology will allow us to map the metabolomic profile of living individual cells with concentrations in the range of milli- to micromolar. Profiling metabolic activities at a single cell level will directly benefit biological research ranging from understanding cancer progression, aging, and the action mechanism of pharmaceuticals. In parallel, we will also push capabilities to interface our nanoscale NMR sensors with biological immobilization assays (see Thrust-2.1) that will allow us to perform spectroscopy on intact biomolecules at sub-picomolar concentrations. The capability of performing spectroscopy in this range will enable us to study protein modifications, such as individual phosphorylation events, and conformational changes of a target protein within the environment of a cell.
Collaborators: Alex High (UChicago) and Joe Sachleben (UChicago – NMR facilities)