A fluorescent-protein spin qubit
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.
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Plain‑English Summary of “A fluorescent‑protein spin qubit”
What this paper is about
The paper shows that a common glowing protein used in biology, called EYFP (enhanced yellow fluorescent protein), can act like a tiny quantum sensor. In simple terms, the researchers turned a protein into a “qubit” (a controllable quantum bit) that can sense very small magnetic fields. Because EYFP can be made directly by living cells (it’s genetically encodable), this opens the door to quantum sensing inside real biological systems, not just in solid crystals like diamond.
1) Main topic or purpose
The authors prove that a fluorescent protein can work as a quantum sensor (a spin qubit) that you can control and read using light and microwaves. They also show it works in water, at room temperature, and even when the protein is made inside mammalian cells.
2) Key questions the paper asks
In everyday terms, the team set out to answer:
- Can a fluorescent protein store quantum information in a way we can control (like a tiny compass needle we can flip and read)?
- Can we read its state quickly and clearly using light, without waiting a long time?
- Is it stable and coherent enough to be useful as a sensor?
- Will it still work in realistic biological environments, like inside cells or in water at room temperature?
- How sensitive could it be to tiny magnetic fields?
3) What they did (methods explained simply)
Think of the protein as a tiny machine with “energy levels” and a built‑in, little magnetic needle called “spin.”
- Two colored laser pulses and microwaves:
- A blue‑green laser (488 nm) “starts” the protein by exciting it.
- The protein sometimes takes a side path to a “triplet” state. You can imagine this as a special parking spot where its spin (tiny magnetic needle) lives for a while.
- A microwave pulse is used to nudge or flip the spin, like tapping a compass needle so it points differently.
- A near‑infrared laser (912 nm) then “opens a fast exit” from that parking spot. This makes the protein emit a delayed flash of light that depends on the spin’s direction. Reading that flash tells you which way the spin was pointing. This fast, on‑demand readout method is called optically activated delayed fluorescence (OADF).
- Why OADF matters:
- Without OADF, you’d have to wait for the slow triplet state to decay on its own, which is too long for good sensing.
- OADF lets them trigger the readout quickly and get a strong, spin‑dependent light signal.
- Measuring and control:
- They used a custom microscope with the two lasers and a tiny microwave device.
- They performed ODMR (optically detected magnetic resonance), which is like tuning a radio: when the microwave frequency matches the spin’s “station,” the emitted light changes, revealing the spin transition.
- They did “coherent control,” meaning they could smoothly rotate and echo the spin using well‑timed microwave pulses (Hahn echo and CPMG sequences) to test how long it stays stable.
- Extra checks:
- Computer simulations (quantum chemistry) predicted the energy levels and splitting of the triplet state and agreed with what they measured.
- They tested at cold temperatures (around liquid nitrogen) for best performance, and also at room temperature, in water, and inside living cells.
Analogy:
- Blue laser: sends the protein up a level.
- Triplet state: a special, long‑lived “holding area” with a tiny magnetic needle (spin).
- Microwave: turns/rotates that needle.
- Infrared laser: opens a quick “door” that releases a telltale flash of light—your readout signal.
4) Main findings and why they matter
- Strong optical readout of spin:
- Using OADF, they got a very clear signal that depends on the spin—up to about 44% contrast at 80 K (very strong for molecules).
- Quantum control and lifetimes:
- They flipped the spin back and forth (Rabi oscillations), showing precise control.
- The spin‑lattice relaxation time (T1, how long it stays polarized) was about 141 microseconds at 80 K.
- The coherence time (T2, how long it stays in a well‑defined quantum state) was:
- About 1.5 microseconds with a simple echo (Hahn echo) at zero magnetic field.
- Improved to about 16 microseconds using a more advanced method (CPMG) with many pulses, which protects the spin from noise.
- Magnetometry (sensing magnetic fields):
- At low temperature, they estimate an upper‑bound sensitivity for alternating (AC) magnetic fields of roughly 183 femtotesla per sqrt(Hz) per molecule (very sensitive).
- At room temperature, in water, they still saw a clear ODMR signal with up to about 3% contrast.
- They demonstrated direct current (DC) magnetic field sensing at room temperature with an upper‑bound sensitivity around 93 picotesla per sqrt(Hz) per molecule.
- For a sense of scale: a single proton (the nucleus of a hydrogen atom) about 5 nanometers away makes a magnetic field around 20 nanotesla—well within reach as this platform improves.
- Works in living cells:
- They expressed EYFP inside mammalian cells and still detected the spin signals and performed coherent control. This shows the method is compatible with real biology, not just purified samples.
- Agreement with theory:
- The measured “zero‑field splitting” (which sets the spin’s natural frequencies) matched their calculations well, giving confidence they understand the physics.
Why it matters:
- This is the first time a widely used fluorescent protein has been turned into an optically addressable spin qubit and used for quantum sensing tasks.
- Because fluorescent proteins are genetically encodable, you can attach them to specific biomolecules or put them in certain parts of a cell. That makes it much easier to bring quantum sensing to biology than using bulky diamond nanoparticles.
5) What this could mean in the future
- Biology‑friendly quantum tools:
- Since EYFP and related proteins are already a standard in biology, researchers can now imagine measuring tiny magnetic signals inside cells where specific proteins live and work.
- Better sensitivity on the horizon:
- Longer coherence times: swapping nearby hydrogen for deuterium (a heavier hydrogen) can reduce magnetic noise and potentially push coherence times toward 100 microseconds.
- Brighter readout: improving how the protein is excited and combining OADF with additional “cycling” of light could greatly boost the number of detected photons, sharpening the readout.
- Optical improvements: better microscopes and collection optics could further increase signal.
- Together, these steps could improve sensitivity by hundreds of times.
- Expand to other molecules:
- Many fluorescent proteins and dyes have similar triplet states. With the same OADF trick, more “molecular qubits” might be found and optimized.
- Protein engineering and directed evolution (a lab method to “breed” better proteins) could tune these qubits for brighter signals, longer coherence, or special sensing functions.
- New science:
- Arrays of such spins could help study complex quantum effects in soft materials.
- Single‑molecule quantum sensing might be possible, enabling detection and study of individual biomolecules with quantum precision.
In short, the authors turned a famous bioimaging protein into a tiny, controllable quantum magnetometer that works in realistic biological conditions. This could bring quantum sensing directly into living systems, helping scientists study life’s processes at the nanoscale in ways that were previously very hard or impossible.
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