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Quantum Enhanced Single-Molecule Biosensing: Harnessing Entangled Photons with Whispering Gallery Mode Resonators


Prof Frank Vollmer, Dr Charles Downing, University of Exeter

External collaborator: Prof Antonio Vidiella-Barranco, University of Campinas, Brazil

In this pioneering project, you'll harness the power of quantum optics to enhance the precision of biosensing. Prof Vollmer’s group specialises in utilising whispering gallery mode (WGM) microsphere sensors which are micron-scale interferometers (Figure a, b.) for the ultra-sensitive detection of single molecules, spanning diverse applications in biology and biochemistry from tracking neurotransmitter release to monitoring single enzyme activity [1, 2]. This project seeks to fuse these cutting-edge sensors with quantum optical sensing techniques, enabling measurements with precision beyond classical limits and pushing the boundaries in optical biosensing. The challenge of single molecule detection using quantum optics remains open, promising unparalleled insights into molecular interactions and processes with unprecedented accuracy.

With our developed source of entangled photon pairs (Figure c.), originating from a nonlinear optical crystal PPKTP, you'll explore a well-established quantum sensing paradigm that leverages photon pairs for enhanced phase measurements within a Mach-Zehnder interferometer setup. Your task will involve optimising the photon pair source to generate narrow-spectrum photon pairs suitable for seamless integration with WGM microspheres via tapered optical fibres. Through quantum sensing experiments involving WGM microsphere sensors, including the exploration of a combined WGM resonator - Mach-Zehnder interferometer setup, you'll not only enhance sensing capabilities but also unveil other quantum optical phenomena associated with entangled photons in optical resonators, such as quantum interference (HOM interference). This primary aspect of the project offers abundant experimental avenues, from constructing experimental setups using free space and fibre optics to programming control systems and analysing data, all while employing single-photon detectors for highly sensitive light detection in controlled environments.

There is also an opportunity to work on the theoretical part of this project by doing quantum optics theory and simulations. We have initial results showing that a WGM resonator in the Mach-Zehnder interferometer with photon pairs at the input shows a dramatically different transmission spectrum compared with input classical light (Figure d.). This is the signal we propose to use for enhanced WGM sensing. There is a lot more to explore in the theory of photon pairs coupled to WGM resonators: you could work on developing the full model of the experiment in the Python package qutip, investigate other states of light such as squeezed states (used at the LIGO gravitational wave observatory), and make theory predictions to test directly in the experiment. The theory part of the project is supported by Dr Charles Downing [3] and a collaboration with the group of Prof Antonio Vidiella-Barranco [4] at the University of Campinas, Brazil. These new collaborations can be strengthened by additional EPSRC funding.


Please contact Prof Frank Vollmer for further information:


Please apply by 06 May 2024


  1. Yu, D. et al. Whispering-gallery-mode sensors for biological and physical sensing. Nature Reviews Methods Primers 1, 83 (2021).

  2. Xavier, J., Yu, D., Jones, C., Zossimova, E. & Vollmer, F. Quantum nanophotonic and nanoplasmonic sensing: towards quantum optical bioscience laboratories on chip.  10, 1387-1435 (2021).

  3. Downing, C. A. & Vidiella-Barranco, A. Parametrically driving a quantum oscillator into exceptionality. Scientific Reports 13, 11004 (2023).

  4. Sousa, E. H. S., Vidiella-Barranco, A. & Roversi, J. A. Generation and transfer of entangled states between two connected microtoroidal cavities: Analysis of different types of coupling. Optik 271, 170016 (2022).


Figure a,b: Whispering-Gallery Mode (WGM) microsphere interferometer c: source of entangled photon pairs d: WGM interferometer with photon pairs at the input

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