A fluorescent-protein spin qubit (2411.16835v2)

Abstract: Optically-addressable spin qubits form the foundation of a new generation of emerging nanoscale sensors. The engineering of these sensors has mainly focused on solid-state systems such as the nitrogen-vacancy (NV) center in diamond. However, NVs are restricted in their ability to interface with biomolecules due to their bulky diamond host. Meanwhile, fluorescent proteins have become the gold standard in bioimaging, as they are genetically encodable and easily integrated with biomolecules. While fluorescent proteins have been suggested to possess a metastable triplet state, they have not been investigated as qubit sensors. Here, we realize an optically-addressable spin qubit in the Enhanced Yellow Fluorescent Protein (EYFP) enabled by a novel spin-readout technique. A near-infrared laser pulse allows for triggered readout of the triplet state with up to 44% spin contrast. Using coherent microwave control of the EYFP spin at liquid-nitrogen temperatures, we measure a spin-lattice relaxation time of $(141 \pm 5)\, \mathrm{\mu s}$, a $(16 \pm 2)\, \mathrm{\mu s}$ coherence time under Carr-Purcell-Meiboom-Gill (CPMG) decoupling, and a predicted oscillating (AC) magnetic field sensitivity with an upper bound of $183 \, \mathrm{fT}\, \mathrm{mol}^{1/2}\, \mathrm{Hz}^{-1/2}$. We express the qubit in mammalian cells, maintaining contrast and coherent control despite the complex intracellular environment. Finally, we demonstrate optically-detected magnetic resonance at room temperature in aqueous solution with contrast up to 3%, and measure a static (DC) field sensitivity with an upper bound of $93 \, \mathrm{pT}\, \mathrm{mol}^{1/2}\, \mathrm{Hz}^{-1/2}$. Our results establish fluorescent proteins as a powerful new qubit sensor platform and pave the way for applications in the life sciences that are out of reach for solid-state technologies.

• The paper presents an optically-addressable EYFP spin qubit achieving up to 44% spin contrast, with measured coherence times of 16 ±2 µs.
• It utilizes a near-infrared laser pulse for efficient spin-readout of the metastable triplet state at both low temperatures and in biological environments.
• The research paves the way for bio-compatible quantum sensors by leveraging genetic encodability and molecular self-assembly for advanced qubit design.

Advances in Molecular Quantum Sensing: A Fluorescent-Protein Spin Qubit

The paper presents a pivotal development in the field of molecular quantum sensing, specifically through the demonstration of an optically-addressable spin qubit utilizing the enhanced yellow fluorescent protein (EYFP). This work leverages the potential of fluorescent proteins, widely used in bioimaging, to function as qubit sensors, thereby addressing the limitations posed by solid-state systems like nitrogen-vacancy centers in diamond when interfaced with biological entities.

Summary of Experimental Findings

The researchers achieved optical addressability of the EYFP's metastable triplet state, utilizing a spin-readout technique that incorporates a near-infrared laser pulse. This method permits efficient triggered readout with a significant spin contrast of up to 44%. The investigation at liquid-nitrogen temperatures yielded a spin-lattice relaxation time of $141 \pm 5 \, \mu s$ and a coherence time of $16 \pm 2 \, \mu s$ through Carr-Purcell-Meiboom-Gill (CPMG) decoupling. Notably, the predicted sensitivity to oscillating (AC) magnetic fields reaches an upper bound of 183 femtotesla per sqrt(Hz).

The authors extended this research to test EYFP's performance in the mitochondrial environment of mammalian cells, demonstrating its robustness despite the complexities of intracellular conditions. Furthermore, optically-detected magnetic resonance (ODMR) at room temperature in aqueous solutions was shown with contrast up to 3% and a static (DC) field sensitivity limit of 93 picotesla per sqrt(Hz).

Implications and Future Directions

This research indicates that fluorescent proteins can serve as a versatile platform for quantum sensing applications previously unattainable with solid-state technology. The EYFP qubit is poised to significantly impact the life sciences, offering utility in applications ranging from fundamental biological research to advanced medical diagnostics.

The findings outlined emphasize the potential of fluorescent proteins to function as molecule-scale qubit sensors, facilitating ultra-sensitive measurements in biological systems. The engineering of complex molecular systems, as evidenced in this paper, opens avenues for the synthesis of improved quantum sensors through molecular self-assembly and directed evolution, given the customizable nature of fluorescent proteins. These developments could potentially culminate in the enhancement of spin coherence through host-matrix control or optimization via directed evolution.

Conclusion

The paper underscores the efficacy of EYFP as a qubit sensor, highlighting its superior compatibility with biological systems and its promising applicability compared to traditional NV centers in diamonds. The genetic encodability of fluorescent proteins presents a substantial advantage, and the integration of existing fluorescent protein libraries can enable a seamless transition to novel quantum sensing applications.

The comprehensive approach demonstrated within this research not only provides a foundational methodology for the advancement of fluorescent-protein-based spin qubits but also suggests a roadmap for future research incorporating computational, biophysical, and quantum engineering techniques. This multifaceted strategy holds promise for transformative advancements at the intersection of quantum information sciences and bioengineering.