Scientists made a biological quantum bit out of a fluorescent protein

Quantum technologies might seem incompatible with life. The quantum bits, or qubits, that make them up commonly require ultracold temperatures, and rely on hard, orderly materials like diamond or silicon, that are foreign to the squishy, wet world of biology. But a new, biological qubit is a native of that messy realm.

Made out of a fluorescent protein, the qubit is just 3 nanometers in diameter, scientists report August 20 in Nature. By hitting the protein with laser light, tweaking it with microwaves and observing its fluorescence, the researchers unleashed its quantum nature.

If the research pans out, such biological qubits could be used as sensors for making precisely targeted, delicate quantum measurements of conditions within cells, such as magnetic fields and temperature. Such capabilities could enable new types of medical imaging, for example.

Qubits are similar to the standard bits used in computing, in that they have two possible values when measured, like the 0s and 1s of traditional computers. But qubits can also exist as both 0 and 1 at the same time, in what’s known as a quantum superposition, giving them different capabilities than standard bits.

Rather than shoehorning traditional qubits into biological systems, the researchers hit on an idea: “Maybe you should turn the problem inside out,” says physicist David Awschalom of the University of Chicago. He and colleagues repurposed tools of biology to form “a quantum bit that would be very happily ensconced in another biological entity.”

The qubit is made of a bit of the protein called the fluorophore — the part that fluoresces when hit with light. That fluorophore has a spin, a quantum property that causes it to act like a magnet that can be pointing up or down, or in a quantum superposition of the two.

The researchers demonstrated that they could manipulate that spin. They produced a quantum effect called Rabi oscillations, in which the system cycles between the two spin states when hit with electromagnetic radiation, in a way that is a hallmark of a qubit.  

Using genetic engineering, the researchers produced the protein in cells in the lab, demonstrating Rabi oscillations in human cells at a temperature of 175 kelvin (– 98.15 degrees Celsius), and in Escherichia coli bacteria at room temperature.

Many types of qubits won’t function at such high temperatures. But the protein’s structure is beneficial: “Fluorescent proteins in general have the advantage that the fluorophore … where the qubit is encoded, is in this protective shell,” says study coauthor Peter Maurer, a biophysicist at the University of Chicago. That shell is key to the qubit’s warm-weather tolerance, protecting it from outside disturbances.

“It’s a fancy demonstration with a lot of promise,” says biophysicist Romana Schirhagl of University Medical Center Groningen in the Netherlands, who was not involved in the research. But, she says, its potential has yet to be proven outside of highly controlled labs. “There’s a lot of work still to be done for it to be actually useful.” Schirhagl worries the protein qubits might not be bright enough to stand out in a messy biological sample or might become dimmer with repeat laser blasts.

The new work is part of a recent flurry of effort on quantum sensing using fluorescent proteins. For decades, these proteins have been key tools in biology labs, where their glow can help visualize other proteins of interest, locating objects such as cancer cells.

Now, researchers hope to add quantum sensing to that trusty technique. “It’s really allowing us to take a similar philosophy to what’s been done with fluorescent proteins in general and use it in new ways,” says physicist Harrison Steel of the University of Oxford. Steel and colleagues reported a related quantum technique based on a different fluorescent protein in a paper published in 2024 at bioRxiv.org, which is currently undergoing peer review.

Steel and colleagues’ work harnesses a quantum phenomenon called a radical pair mechanism. It’s similar to an effect that’s thought to allow some birds to sense Earth’s magnetic field, helping them migrate long distances without a compass.

Quantum effects have traditionally been considered too delicate for the wilds of biology. But now, there’s new hope of breathing some life into quantum physics.