[Paper Review] A fluorescent-protein spin qubit

Joseph Feder, Benjamin S. Soloway+13 more|arXiv (Cornell University)|Nov 25, 2024

Electron Spin Resonance Studies4 citations

TL;DR

This paper demonstrates the first optically-addressable spin qubit in a fluorescent protein—Enhanced Yellow Fluorescent Protein (EYFP)—using a near-infrared laser pulse for spin-state readout with up to 44% contrast. It achieves coherent microwave control at liquid-nitrogen temperatures, with a spin-lattice relaxation time of 141 ± 5 μs and a coherence time of 16 ± 2 μs under CPMG decoupling, enabling magnetic field sensing with sensitivities down to 183 fT·mol⁻¹/²·Hz⁻¹/² in cells and 93 pT·mol⁻¹/²·Hz⁻¹/² at room temperature.

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 $(141 \pm 5)\, \mathrm{μs}$, a $(16 \pm 2)\, \mathrm{μs}$ 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.

Motivation & Objective

• To establish fluorescent proteins as viable platforms for optically addressable spin qubits in biological environments.
• To overcome the limitations of solid-state spin qubits like NV centers in diamond, which are incompatible with biomolecular integration due to their bulky hosts.
• To enable coherent control and high-contrast spin readout in EYFP under biologically relevant conditions, including in mammalian cells.
• To demonstrate room-temperature optically detected magnetic resonance in aqueous solution with measurable sensitivity.
• To develop a novel spin-readout technique using near-infrared laser pulses to selectively probe the triplet state of EYFP.

Proposed method

• Utilized a near-infrared laser pulse to trigger and read out the triplet spin state of EYFP, achieving up to 44% spin contrast.
• Employed coherent microwave control at 77 K to manipulate the spin qubit, enabling Rabi oscillations and spin-echo measurements.
• Applied Carr-Purcell-Meiboom-Gill (CPMG) sequence to extend spin coherence time to 16 ± 2 μs.
• Measured spin-lattice relaxation time (T₁) as 141 ± 5 μs using microwave pulses and fluorescence detection.
• Quantified magnetic field sensitivity using the spin qubit’s response to AC and DC magnetic fields, with upper bounds of 183 fT·mol⁻¹/²·Hz⁻¹/² and 93 pT·mol⁻¹/²·Hz⁻¹/², respectively.
• Expressed the EYFP qubit in mammalian cells and confirmed retained spin contrast and coherent control in complex intracellular environments.

Experimental results

Research questions

• RQ1 Can a fluorescent protein host a stable, optically addressable spin qubit suitable for biological applications?
• RQ2 What is the coherence time and relaxation behavior of the EYFP triplet state under microwave control at cryogenic temperatures?
• RQ3 Can high-contrast spin readout be achieved in EYFP using a near-infrared laser pulse?
• RQ4 To what extent can the EYFP spin qubit maintain functionality in living mammalian cells?
• RQ5 What is the magnetic field sensitivity of the EYFP qubit at room temperature in aqueous solution?

Key findings

• The EYFP spin qubit achieved a spin contrast of up to 44% using a near-infrared laser pulse for triggered readout.
• The spin-lattice relaxation time (T₁) was measured at 141 ± 5 μs at 77 K, indicating long-lived spin states.
• Under CPMG decoupling, the spin coherence time (T₂) reached 16 ± 2 μs, demonstrating effective dephasing suppression.
• The qubit exhibited an upper bound magnetic field sensitivity of 183 fT·mol⁻¹/²·Hz⁻¹/² for AC fields at 77 K.
• At room temperature in aqueous solution, the qubit achieved 3% spin contrast and a DC field sensitivity upper bound of 93 pT·mol⁻¹/²·Hz⁻¹/².
• The EYFP qubit maintained coherent control and measurable contrast when expressed in mammalian cells, confirming functionality in complex biological environments.