As an undergraduate at Oxford University in the mid-1970s, K. Birgitta Whaley struggled to choose between chemistry and physics. Now, as a professor at the University of California, Berkeley, and director of its Quantum Information and Computation Center, she doesn’t have to: Her research interests span all realms quantum, including both chemistry and physics, as well as computer science and her newest pursuit, quantum biology, where physics meets the life sciences.
Whaley turned her attention to biology in 2007 after experimentalists demonstrated that green sulphur bacteria can synthesize sugar from light by biologically controlling quantum mechanical effects at temperatures up to 80 degrees Fahrenheit. As a theorist, Whaley is interested in learning how these living organisms can process quantum information so efficiently, because she is seeking clues on how to design a robust quantum computer. But unlike the green bacteria, which can process quantum information at room temperature in nature, our best quantum computer prototypes are limited to controlling quantum effects in the laboratory at temperatures verging on absolute zero.
Moving beyond simple bacteria, birds are now thought to map their travels using quantum mechanics, and that may have applications to quantum science.
Humans have not yet figured out how it is that nature can stabilize a coherent electronic quantum state in such complex systems for such long periods of time.
Biology emerges from chemistry, which in turn emerges from how atoms and molecules interact in the microscopic realms ruled by quantum probabilities. The basic tool of quantum mechanics is the wave equation published in 1926 by Erwin Schrödinger, which is used to list all the properties of a particular quantum object or system, such as the entire range of nonidentical spatial positions that a single electron can simultaneously occupy. This counter-intuitive, yet well-proven, capacity for an atomic particle or biological molecule to concurrently inhabit multiple places, times or energy states is called a superposition.
Another important concept in quantum biology is entanglement. Saying that two or more atomic particles are entangled means that information can be transferred between them instantaneously, no matter how far apart they are, even light-years. (But to understand the transferred information, an observer would also need to receive some decoding instructions that could only be transmitted at or below the speed of light).
And then there is entropy: the tendency of isolated systems to approach stasis (a state of heat death or maximum disorder). In his 1944 book, “What is Life?,” Schrödinger focused on how organisms, such as fruit flies, employ quantum mechanical effects to combat entropy by producing order from disorder.
Think of order as consisting of how units of information or quantities of energy are arranged inside a closed system: As the energy in a system dissipates, information is lost to the system as disorder sets in. But the ability of a closed system to increase its information or energy content by accessing its environment is tantamount to a restoration of order. Schrödinger called the process of reordering the energy in a system “negative entropy.” He wrote that the struggle of life “consists in continually sucking orderliness from the environment.”
Learning how to control superpositions and entanglements without losing information to the environment is a sine qua non for building a viable quantum processor that can run calculations using arrays of atoms and molecules as transistors. Whaley has high hopes that continuing discoveries in the burgeoning field of quantum biology will result in a breakthrough design for novel quantum devices.
In March, Whaley explicated the basics of quantum control of biological systems to a gathering of high school teachers at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara. More recently, she sat for a two-hour interview with Quanta Magazine. This is a condensed and edited version of that conversation.
QUANTA MAGAZINE: Is quantum biology a new new thing or an old new thing?
K. BIRGITTA WHALEY: Schrödinger’s bio-physics book “What is Life?” appeared years before the discovery of the atomic structure of DNA. In it, he made the case that quantum physics governs the evolution of “gene molecules” containing the “code” for life. And he proposed that because living systems are subject to entropy and decay, they must continually pull energy from their quantum environments or die.
To buttress his arguments, Schrödinger made extensive use of experimental research conducted by Max Delbrück during the 1930s. Delbrück was a physicist-turned-biologist who recognized that the chemical stability of animate material is determined by the fact that organic molecules must jump over energy barriers for the reactions of life to occur. The height of these energy barriers is determined by quantum interactions between the electrons, atoms and molecules composing the life form.
How did this play out in the lab?
Delbrück bombarded clumps of fruit fly chromosomes with X-rays to induce and study rates of genetic mutation, but his probes did not allow exploration of the atomic scale quantum dynamics in real time. The advent of lasers in the 1960s made that possible. Now, we measure crisscrossing pulses of laser light with spectrometers to follow the molecular dynamics of biological objects in real time, as measured in quadrillionths of a second.
By probing the chemistry of plants with lasers, we can observe the interplay between the quantum components in living organisms and their local surroundings, the environmental “bath.” But an “open” quantum system and its bath in a living organism are not really separate; they continuously influence each other by trading quantities of energy and information back and forth.
It turns out that organic systems with tailor-made molecules are highly tunable. The trick is to not lose the input data.
What attracted you to quantum biology?
I got hooked six years ago after the spectacular experiment by Graham Fleming showed the existence of quantum coherence during photosynthesis in super-chilled green sulphur bacteria. Subsequent experiments have tracked quantum interactions at ambient temperatures.
What is quantum coherence?
Coherence is the concerted dynamics of quantum states, either with themselves at different times and places or with other states. The opposite of coherence is decoherence: When isolated quantum systems open up and interact energetically with their atomic environments, they rapidly decohere: They lose their quantum mechanical concerted nature — their coherence — and start to behave classically, macroscopically. Decoherence is the main obstacle to building a quantum computer.
Why?
Machine or plant, it is hard to keep a closed quantum system isolated from its bath — or so we thought until the experimenters began catching real-time coherence events in photosynthesis. They saw coherent superpositions of electronic excitations in the bacteria.
What is quantum mechanical about photosynthesis?
In photosynthesis, bacteria and plants convert sunlight into electrons and then into chemical energy. Here is the model: Photons are first absorbed by chlorophyll molecules embedded in protein scaffolds. These light-harvesting “antennae” transmit this photonic energy as excitations of electrons through a series of quantum mechanically linked chlorophyll molecules to a reaction site where the trapped energy catalyzes the manufacture of energy-storing sugars.
Until Fleming’s experiments, it was thought that in light harvesting, the electron excitations diffused randomly, inefficiently, through the antenna structure, losing much of the captured solar input during a wandering process of transmission.
We can now show that a single electronic excitation acting as a probability amplitude wave can simultaneously sample the various molecular paths connecting the antenna cells to the reaction center. The excitation effectively “picks” the most efficient route from leaf surface to sugar conversion site from a quantum menu of possible paths. This requires that all possible states of the traveling particle be superposed in a single, coherent quantum state for tens of femtoseconds.
We have seen this remarkable phenomenon in the green sulphur bacteria, but humans have not yet figured out how it is that nature can stabilize a coherent electronic quantum state in such complex systems for such long periods of time.
Can we use this lesson from nature to build an artificial light-harvesting machine?
Labs all over the world are working on building chemical solar cell prototypes that are modeled upon natural photosynthesis. It turns out that organic systems with tailor-made molecules are highly tunable. The trick is to not lose the input data: Each photon that is captured by the green sulphur bacteria is utilized. Imitating this biological feat could set the stage for making a robust, controllable, quantum mechanically enhanced photon harvesting device.
Those of us struggling to design scalable quantum computers are fascinated by how nature so efficiently controls energy flow — information transfer, really — through an open quantum system like the green sulphur bacteria.
The main problem with quantum information processors is that their microscopic operating system must be kept “closed” — immune to degenerative environmental influence — while they are calculating with superposed “qubits,” or atom-sized processors. So far, engineers can only dream of fashioning an open quantum system that can compute with its qubits kept in a coherent state long enough to not lose data to the bath via decoherence.
Remarkably, it seems that these photosynthesizing bacteria can actually use decoherence to speed up the transfer of electronic information by accessing vibrational energies in the protein bath surrounding the biological-quantum wire without losing the integrity of the information.
Nature does not appear to show the typical paradoxes associated with information processing in quantum physics, and that bodes well for the future of quantum computers.
Are these bacteria — proto-plants, really — quantum computers?
Plants cannot internally process information nearly as fast as we predict that a true quantum computer will be able to. But the bacteria that we have studied do transmit information with a very high rate of efficiency with quantum mechanical tricks that we cannot yet replicate in machines.
Has quantum mechanics influenced the evolution of life at the macro scale?
It is likely that plants and bacteria are subject to intense selection for highly efficient capture of the energy from light. This may explain why the photosynthetic systems we have today are typically so efficient that we can detect the quantum processes underlying this harvesting of light energy.
How do birds use quantum mechanics?
Migrating birds take advantage of the fact that the inclination of the Earth’s magnetic field changes as a function of the latitude, or how far north the bird is located. At the equator, the magnetic field is tangential to the earth. At the North Pole, it is perpendicular. As the bird flies long distances, the inclination of the magnetic field changes relative to the plane of the local surface of the Earth below.
It seems that quantum mechanical processes in the avian eye send signals to the brain that are sensitively dependent on the angle of change in magnetic field inclination, thereby allowing the bird to map routes. The hypothesis is that pairs of light-absorbing molecules in the bird retina produce quantum mechanically entangled electrons whose quantum mechanical state depends on the angular inclination of the field and which catalyze chemical reactions that send differently valued signals to the brain depending upon the degree of inclination.
How does that help birds to select the correct destination?
They seem to come genetically hard-wired with the quantum mechanical ability to compute directionality, but during their first migration, they are guided to the ancestral winter home by older, experienced birds. It’s probably similar to humans learning language.
Is a bird brain a controllable quantum system?
It would be if we knew the identity and location of both of the entangled molecules in the retina. We do know the location of the photon-trapping molecule, but we have not yet found the molecule that provides the second electron in the radical pair which initiates the mapping signal.
Why not?
It is hard to get research money to study bird brains. Plus, one has to kill them to get a clear picture of what is going on at the molecular level, and a lot of people like birds. On the other hand, cockroaches may be doing it, too.
Let’s circle back to Schrödinger’s brain. In 1953, he proposed a paradox: According to his wave equation, macroscopic objects are composed of microscopic atoms and molecules. Since the small objects can be found in wavy, oscillating, reversible or “coherent” superpositions, then why aren’t the large objects also superposed? What keeps biological objects from turning into what Schrödinger termed “quantum jellyfish”?
We avoid “jellyfishification” because large superpositions are washed out incredibly fast by decoherence. In photosynthesis, the chemical reaction quickly introduces irreversibility into the quantum process of energy transfer. There is always some thermodynamic or entropic force that is driving the overall biological dynamic. Bacteria and plants and humans do not turn into jellyfish because there is a structure, an organization in the biological dynamics.
We do not understand all the details, but in the biological domain, nature does not appear to show the typical paradoxes associated with information processing in quantum physics: And that bodes well for the future of quantum computers, provided we explore open, biological quantum systems as engineering models.