Quantum Leap: Fluorescent Proteins Emerge as Next-Gen Qubits

For decades, researchers have utilized the green fluorescent protein (GFP) from crystal jellyfish, and similar molecules, to visualize cellular processes in biology. These widely used tools are now being developed for quantum applications.

Researchers are harnessing the quantum properties of these fluorescent proteins to function as qubits, the fundamental units of quantum computing. Peter Maurer, a quantum engineer at the University of Chicago, highlighted the potential to transform these fluorescent labels into qubits. This concept, while seemingly futuristic, is based on established physics and has been demonstrated in principle.

Peter Maurer, a quantum engineer at the University of Chicago, highlighted "the potential to transform these fluorescent labels into qubits."

Advancements in Biological Sensing

Fluorescent protein labels are essential in biological laboratories globally, used for monitoring protein location and activity, assessing cellular conditions, and evaluating drug candidates. The integration of quantum properties offers new possibilities.

Quantum sensors are known for their exquisite sensitivity to magnetic fields. Protein-based quantum sensors may enable the detection of minute signals from firing neurons, ion flows, or tiny quantities of free radicals, which can indicate cellular stress or early cancer signs. Additionally, these protein quantum sensors can be remotely activated and deactivated, making them valuable for novel imaging technologies and therapies.

Jin Zhang, a biosensor developer at the University of California, San Diego (UCSD), expressed excitement about "the potential for enhanced sensitivity," noting that fluorescent labels often present sensitivity challenges.

Ania Jayich, a physicist at the University of California, Santa Barbara, affirmed that such developments are now considered achievable.

The Broader Quantum Sensing Landscape

The development of protein quantum sensors is part of a rapidly advancing field of quantum sensing for biological applications. Researchers suggest minimal obstacles to development, as some usable proteins are readily available, and the necessary equipment is standard.

Contemporary quantum physics focuses on manipulating individual quantum properties for high-precision applications in computing, communications, and sensing. While quantum computing requires qubits isolated from external disturbances, quantum sensing relies on qubits that are influenced by external factors, allowing for measurement. Examples include magnetic resonance imaging (MRI) and superconducting quantum interference devices (SQUIDs).

Comparison with NV Diamond Centers

One widely used quantum sensor is the 'NV diamond center,' a defect in diamond crystals, known for its high sensitivity, versatility, and stability at room temperature. These sensors find applications primarily in physical sciences, such as mapping semiconductor performance.

However, applying NV diamonds in living biological systems ("warm and messy") has proven challenging. Despite this, the field of biomedical quantum sensing is gaining momentum, supported by institutions like the Chicago Quantum Institute and funding from the US National Science Foundation. Research includes using NV diamonds for nanoscale MRI, tracking magnetic tracers during surgery, and developing highly sensitive HIV tests.

NV diamond sensors have limitations in biological contexts, being comparatively large (about ten times bigger than a protein) and difficult to precisely position. Fluorescent proteins, conversely, are small and can be precisely generated within cells using genetic engineering, allowing them to be placed adjacent to the target of investigation.

The Quantum Leap for Fluorescent Proteins

David Awschalom, director of the Chicago Quantum Institute, and Peter Maurer explored molecules that could function as qubits, aiming for those compatible with cellular environments. They focused on 'enhanced yellow fluorescent protein' (EYFP), a commercially available product, due to its electron energy structure resembling existing qubits.

Traditionally, fluorescent proteins glow when excited electrons return to a relaxed energy state. Biologists link genetic instructions for these labels to proteins of interest. While variants exist for different colors and sensitivities to factors like pH or calcium ions, they do not detect magnetic fields.

Crucially, a small fraction of the time, excited electrons in these proteins enter a metastable, non-fluorescent 'triplet state,' which is now recognized as an advantage for creating a coherent superposition of spins. This state, previously seen as a drawback (causing dimming), enables a mechanism also used by NV diamond quantum sensors.

Awschalom's team successfully achieved the desired quantum superposition state with EYFP using laser light and microwaves, demonstrating a protein-based quantum sensor that functioned in living bacterial cells at room temperature. The fluoresced light intensity was affected by magnetic fields, showing approximately a 30% variation.

Future Challenges and Prospects

Ongoing challenges include the inherent fragility of fluorescent proteins, which degrade under illumination, prompting research into mitigation and sensitivity enhancement. Maurer's team is investigating ways to mitigate this and enhance sensitivity. They plan to develop variants that spend more time in the triplet state and explore the proteins' ability to reliably detect changes in pH and temperature.

Nathan Shaner, a biological engineer at UCSD, highlighted "the significant potential for directly detecting electromagnetic fields," citing the difficulty in creating robust indicators for neuronal action potentials as an example.

This advancement promises new capabilities in understanding complex biological processes.