The quantum world is weird. At the level of matter we operate on, an object is either here or there—you put your drink on a table, and, unless you move it, that’s where it stays. The chance of finding it in your bedroom at the same time is zero. 

But when the object is an electron, that assumption goes out the window. Quantum mechanics says an electron occupies a range of possible locations until it is measured (think Schrödinger’s cat). The electron is not simply a particle, but also a wave existing as a superposition of possible states. 

Concepts like this are abstract, a little trippy and quintessentially quantum. They require one to set aside their intuition and accept that, at the scale of atoms and subatomic particles, strange things happen.

When zooming out to the scale of biological entities and systems, however, this quantum strangeness tends to get drowned out. Biological environments are notoriously warm, wet and noisy, the opposite of what is usually required for true quantum effects to exist. Molecules are quantum mechanical in isolation, but the more complex a system gets, the more obscured those effects become. To put it plainly: scientists generally don’t need quantum mechanics to predict and understand how life works. 

Or do they? Are there biological processes whose behavior we cannot fully explain without quantum effects? Research in bacteria and beyond suggests the answer might be yes.

The ASM Mechanism Discovery Unit explores emerging scientific frontiers to deepen our understanding of how microbes function. Quantum microbiology is a compelling example of questions that arise when microbiology intersects with physics, engineering and other disciplines. By bringing together diverse scientific perspectives, the unit seeks to spark new ideas, reveal unexplored opportunities and help shape future directions in microbial science. 

What is Quantum Microbiology?  

At the most fundamental level, one could argue that everything is quantum. But there’s a difference between having quantum-ness and harnessing it to do something. Quantum biology seeks to identify and exploit quantum phenomena in biological systems, particularly as they relate to function; quantum microbiology specifically examines and controls microbial life. 

Our understanding of the world is largely based on classical physics, which allows us to solve problems like how flagella propel bacteria or the thermodynamics of chemical reactions. Most of the time, these principles work just fine for deciphering biological mechanisms. Quantum (micro)biology is founded on the supposition that, in some cases, another layer of nuance may be required. 

“We don't actually have a theory of chemical bonding without quantum mechanics. So, of course it’s crucial,” said Graham Fleming, Ph.D., a professor of chemistry at UC Berkeley. “But I think what’s meant by quantum biology is a bit more subtle, [and] that’s exploiting some of the weirdness of quantum mechanics to produce a function that you couldn't [otherwise] get.”

Parsing out this possibility is difficult. Tightly controlled conditions (e.g., extreme cold, lasers and high vacuums) are required to study quantum phenomena in simple systems. Developing probes that detect easily obscured effects—and their potential functional roles—in messy, complicated biological systems can be harder. 

Moreover, quantum mechanics may be used to describe a behavior, but whether quantum phenomena are definitively part of the behavior is tricky to establish. “There's a depressing tendency of people to lump everything they don't understand in the quantum biology camp, which I think is not helpful in any way,” Fleming noted. 

A process or behavior may exhibit “quantum-like” properties without requiring bona fide quantum effects, or quantum and classical effects may work together, requiring a hybrid framework. 

“Ultimately, this is what we want to know: Does it do something?” added Gregory Scholes, Ph.D., William S. Tod Professor of Chemistry at Princeton University. “There're phenomena which take a lot of work to explain, and [for which] we need quantum mechanics … But then, regardless, [we] say ‘Does this do nothing for function? Or is it profound for function?’”

The Quantum Dynamics of Photosynthesis

Take photosynthesis as an example. In photosynthetic bacteria and plants alike, the process relies on the transfer of light energy from complexes of pigment molecules to a reaction center, where life-supporting photochemical reactions take place. 

According to the classic model of photosynthetic light-harvesting, when a pigment molecule absorbs light, the excitation energy hops from 1 molecule to another toward the reaction center. Scholes likened the process to putting 2 tuning forks together: a molecule enters an excited state, which excites another nearby molecule, and the excitation propagates through the pigment network.

However, studies in photosynthetic systems, including green sulfur bacteria, pointed to another model based on quantum mechanics. Rather than being localized on a single molecule, excitation energy is delocalized—or “smeared out,” as Fleming put it—over multiple pigments, like a wave. Different parts of the wave stay synchronized with each other for a short period of time. This relationship, known as coherence, allows the energy to “sample” multiple paths throughout the pigment network before localizing and moving toward the reaction center. 

These discoveries were important for establishing the quantum biology field. Yet, recent research suggests the role of quantum coherence in the extreme efficiency of photosynthetic energy transfer may be more subtle than previously thought. The likely model involves an interplay between quantum electronic states and molecular vibrations in surrounding pigments and proteins. It’s not necessarily a hop versus a wave, but a mix depending on the timescale, the state of the system and the environment. 

In other words, photosynthesis is not a “smoking gun” for a uniquely quantum mechanical biological function. For that, “we would need a function that's completely impossible to explain by a classical mechanism,” Scholes said. “Maybe it'll be in bacteria.”

Understanding Electron Transfer in Bacterial Nanowires

Enter: Nikhil Malvankar, Ph.D. an associate professor, and Sibel Ebru Yalcin, Ph.D., a research scientist, of molecular biophysics and biochemistry at Yale University’s Microbial Sciences Institute. “We got interested in the bacteria because they were transferring electrons at rates [that] we just couldn't explain with classical physics, and all our quantum knowledge was telling us, ‘Oh, this is just impossible. It just can't happen,’” Malvankar said. 

 “The bacteria” here are members of the Geobacter genus, namely Geobacter sulfurreducens, which can live in oxygen-deficient environments miles below the Earth’s surface. Other types of cells, including our own, use oxygen to accept electrons released by the electron transport chain during cellular respiration. But Geobacter doesn’t have that option. 

Instead, it evolved a mechanism to release electrons outside of the cell, where they are accepted by metallic minerals or partner bacteria. The electrons travel to their destination via long protein appendages called nanowires. The “impossible” part of this process is how fast and far those electrons travel; we’re talking 10 billion electrons per second moving over distances up to 100 times the length of the bacterial cell. These rates challenge what can be accounted for with classical frameworks. 

Quantum physicists by training, Malvankar and Yalcin found themselves revisiting their roots to investigate this phenomenon. Collaborating with Ed Egelman, Ph.D., a professor biochemistry and molecular genetics the University of Virginia School of Medicine, and Allon Hochbaum, Ph.D., an associate professor of materials science and engineering at University of California, Irvine, they discovered that Geobacter nanowires contain a core of heme molecules (like those in hemoglobin) arranged in a straight line. 

The unique architecture resembles a metal wire, which the researchers’ computations suggest allows electrons to surf like a coherent quantum wave along the wire, rather than hopping randomly from 1 site to the next. Their measurements indeed show that electrons move among hemes at an ultrafast rate, less than a trillionth of a second. 

Electron transfer through the nanowires is, thus, extremely efficient; the same way surfing is smoother and quicker than doing a bunny hop. 

“It's like water filled in a pipe,” Malvankar explained. “If you start putting water at 1 end, water from the other end of the pipe can start pouring out. We found something very similar is happening in these bacterial nanowires.” 

But is the mechanism really quantum mechanical? Electron hopping is a thermally activated process, meaning if temperature increases, the electron transfer rate in the nanowires should increase. If the electrons are moving like a wave, higher temperatures hinder this process, and electron transfer rates should decrease. Malvankar and Yalcin found the latter —higher temperatures decreased electron transfer by roughly 300 times.

The group is building on their initial measurements, using new probes that accelerate Geobacter’s growth (e.g., electric and magnetic fields, light and cooling) to better understand how electrons in the nanowires are responding as quantum versus classical particles. The results so far suggest the quantum effects are inherently linked to the function of the nanowires—additional research is needed to clarify that picture.

How Are Quantum Microbial Mechanisms Useful?

A natural question that arises from all of this is: who cares? Does it really matter if a biological process is quantum, classical or something in between? If nature has evolved in certain cases to exploit quantum mechanics, it could transform how we understand and leverage biology. 

For instance, characterizing Geobacter nanowires may allow scientists to direct bacterial growth, communication and attachment, not just through, say, tweaking nutrient availability, but by controlling electron transfer. This capacity extends beyond G. sulfurreducens, too. “It's not just 1 bacterium; many bacteria are making these nanowires [within] the microbial kingdom,” noted Yalcin, who has been developing tools to both measure and image electrons in nanowire-harboring bacteria. 

There is also the possibility of combining quantum microbiological insights with technological innovation. The emerging field of quantum computing harnesses the oddities of quantum mechanics to solve complex problems quicker and more efficiently than standard computers. To work, quantum computers need tightly controlled (and cold) environments. Scaling up quantum computing would require the ability to operate at room temperature in noisy environments. Microbes may offer clues about how to meet this need. 

Indeed, the Malvankar team postulates that systemic protein fluctuations in bacterial nanowires that interfere with quantum effects move much slower than electrons—that is, the electrons effectively “outrun” any disturbances. This could be why quantum effects are preserved in an environment where, theoretically, they shouldn’t be. Researchers could apply that understanding to synthetic systems.

Given how much is unknown, there are a lot of bridges between fundamental quantum discoveries and their applications that scientists aren’t even aware they could build. “There [could] be a completely surprising and unthought-of approach or way of exploiting quantum mechanics for function that we [haven’t] realized, and that we’d probably never guess,” Scholes highlighted. “So, it’s high risk, high gain.”

The Future of the Field: Come One, Come All

What is known for sure is that the future of quantum microbiology—and quantum biology, more generally—is not localized to 1 set of scientists. Rather, like a quantum wave, it is spread among those from diverse fields and disciplines. 

“I can have the most amazing technology, but if I don't have the understanding of biology or handling of biology, then I am nothing,” said Yalcin, who has spent years supplementing her physics background with a strong microbiological skillset. On the other hand, “biologists might feel the physics is difficult. So, I think we both need each other, to appreciate each other and to respect each other. I'm very happy that the field is moving that way.” 

Malvankar agreed. He emphasized that quantum science is “not just a monopoly of physicists,” but is relevant to living processes. One cannot answer questions about the quantum effects in bacteria if they cannot grow or manipulate the bacteria; conversely, one cannot study quantum effects in bacteria without integrating quantum physics. 

“We really need more microbiologists to recognize that the science is becoming more and more interdisciplinary,” Malvankar said. “If more microbiologists get interested in quantum physics, they’ll realize that it's not something alien; it's something we all are working with every day, and it's something they can learn.”