Quantum Biology: Part 2 – Photosynthesis

by Amara Graps

Photosynthesis – The Use Case that Keeps on Giving

The most famous ‘Use Case’ for Quantum Biology is the process of photosynthesis, in which the field seeks to answer the question: “Why is photosynthesis so efficient?” This question refers to the fact that a particle of light or photon is absorbed and converted to energy with near 100% efficiency. 

Seth Lloyd set out to disprove the Engel et al., 2007 work: Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems and ended up proving it instead, in his exquisite paper: Lloyd, 2011 in Quantum coherence in biological systems. See also the excellent video explanation on the Up and Atom channel at YouTube. The research focus was on the ‘exciton transport process’. Quantum mechanical? Or not? 

To explain: The photosynthesis process begins with the absorption of the photon by a photocomplex, a set of molecular structures embedded in the membrane of a plant or bacterial cell. Part of the energy of the photon is converted into heat in the form of molecular vibrations, but most of it is captured as a so-called Frenkel exciton, a bound electron-hole pair that resides in a chromophore, a ‘light-carrying’ molecule such as a plant or bacterial chlorophyll. Energy transfer occurs via the dipolar interaction between chromophores. The exciton must propagate through hundreds or thousands of chromophores to reach the reaction center, a special molecular complex, in which charge separation takes place, and the energy is dumped. 

Engel et al., 2007 reasoned that interference between the various paths, or so-called ‘quantum beats’, should be visible if there was indeed coherence among excitons. Without wasting energy on random hopping, such coherence would enable the excitation to determine the best path to the reaction center.

The exciton transport process was confirmed by Lloyd to be a quantum mechanical process. His work showed that the efficiency of excitonic transport had a remarkable 77K dependence on temperature. Moreover, he showed that a ‘quantum walk’, where the walker takes a quantum superposition of paths through the network, best describes the exciton transport process.

However, the exciton transport process as quantum mechanical was not confirmed universally in the quantum biology field, and the debates have heated up in the last decade. See Philip Ball’s 2018 award winning Physics World article Is photosynthesis quantum-ish? In the year after Lloyd’s work,  David Jonas and colleagues contended that the “beats” were actually due to the process of Raman scattering, which involves molecular vibrations, rather than quantum coherence among excited electronic states. 

If there is a consistent feature of the photosynthesis research over the years, it is that a group will publish new research, in a year or two, placing their marker on the years-long photosynthesis research journey labeling such work as ‘Revisited’ or ‘Updated’.  See the mega-2020 study by Cao et al. Quantum biology revisited  (useful summary: here)  which showed that inter-exciton coherences are too transitory for photosynthetic energy transfer. And the section three of Kim et al., 2021,  Quantum Biology: An Update and Perspective, which concluded (using 100 references) that coherence is present over short durations, but that quantum coherence has yet to be proven. Instead, future instrumental advancements are needed to show quantum mechanics’ importance in photosynthesis. 

Subsequent research includes Runeson et al., 2022 Explaining the Efficiency, which highlights conditions where quantum mechanics can enter, and Schouten et al., 2023  Exciton-Condensate-Like Amplification of Energy Transport in Light Harvesting, which highlights the possibility of an exciton-condensate-like amplification, for an efficient energy exciton transfer. 

It’s fair to say that this one question is a large driver of Quantum Biology at a number of research institutions and has tremendous commercial applications if it can be harnessed.

Technologically-Made Quantum Sensors Again

The main tools for probing nature-made, quantum sensors are technologically-made, quantum sensors. 

For the photosynthesis Use Case, as Duan explained, the time scale for the transfer of one absorption site to another is ~100 femtoseconds (10^-13 seconds) with overall energy transfer time to the reaction center taking several picoseconds (10^-12 seconds). With femtosecond 2D photon echo methods or 2D spectroscopy, one can directly observe the electronic couplings between chlorophylls or exciton basis. Such analytic tools are available today. 

GQI’s Quantum Sensing Stack

To help the wider ecosystem track and foresee the commercial potential of quantum sensors, the strategies that will be necessary to bring them to market, and the funding milestones that will have to be passed, GQI has defined the quantum sensing stack.  

Physics Package

1. System – the physics platform upon which the sensor is based
   (e.g., superconducting circuits, neutral atoms, trapped ions, spins, photonics)
2. Probe component – the cell or other system component where sensing takes place
   (e.g., SQUID, NV Tip, Vapor Cell, MOT, RF Trap, SPAD etc.)
3. Probe state – they physical state or mechanism used for sensing 

Control Package

1. Control & Readout – the key hardware components used for control & readout
2. Environment – the engineering severity level (ESL) at which system operates
3. Package – the size weight and power (SWaP) of the system
4. Space qualified – a key hurdle for satellite and other space applications.

Control Logic 

1. Physical Protocol – the physical measurement protocol implementation, including techniques used for error suppression. (e.g., dynamic decoupling, optimal control etc.)
2. Logical Protocol – the logical measurement protocol, designed to return the desired measurement result (e.g., initialize, transform, evolve, transform, project, repeat).

Framework

1. Overall Calibration – the management of the sensor infrastructure, including periodic external or self-calibration of the sensor.
2. Signal Processing – the immediate classical post-processing of the raw sensor readings to form a signal value, including techniques used for error mitigation (e.g., averaging, integration)

Modality (Basic Application)

1. Time – clocks and frequency standards
2. Magnetic Field – magnetometers; inc. sensing of DC and RF magnetic fields
3. Electric Field – electrometers (e.g., voltmeters), inc, sensing of DC and RF electric fields
4. Gravity/Acceleration – gravimeters & gravity gradiometers, accelerometers
5. Rotation – gyrometers (gyros)
6. Other – inc. temperature, pressure, strain, and others

Advanced Applications 

1. Sensor Fusion – intelligently combining classical, quantum and hybrid sensor inputs
2. Computational Imaging – advanced signal processing 
3. Decision Support – diagnostics and decision support systems (e.g., INS)

The magnetic field sensor to measure the magnetic field would be in the column with the red rectangle. 

(*) GQI’s  Quantum Sensing Outlook Report (*), is a 114-page report on Sensors, including five pages about the quantum sensors to measure magnetic fields, and nine pages on quantum sensors for timing (clocks). GQI also has a 48-Slide “State of Play” for Quantum Sensing State of Play. It is one of six presentation slide-decks for customers which describe the State of Play in various sectors: Quantum Technology Introduction, Quantum Hardware, Quantum Safe, Quantum Sensing, Imaging, and Times, Quantum Software, and Quantum Landscape. If you are interested to learn more, please don’t hesitate to contact [email protected]. 

October 30, 2024