Dr Krishna Coimbatore Balram

PhD.(Stanford), MSc(Stanford)

  • BS8 1UB

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Personal profile

Research interests

My primary research interest is the development of novel nanofabricated device platforms for manipulating light and sound waves at the nanoscale and engineering controlled interactions between them, and other solid state systems. My current research thrusts are along three main directions, all underpinned by advances made in my group on pushing the operation frequency and efficiency of light-sound interactions in guided wave devices: building resonant acousto-optic quantum transducers to translate quantum states between the microwave and optical frequency domains for connecting distributed superconducting, spin and trapped ion qubit platforms [Balram et al., Nat. Phot. 2016; Valle et al., Opt. Lett. 2019; Wu et al. Phys. Rev. Appl. 2020]; shrinking mobile RF front-ends by ~100x by applying ideas from integrated photonics to guided wave acoustics [Valle et al., Appl. Phys. Lett. 2019]; and applying modern developments in RF engineering to an old problem (spin detection) and providing a route towards improved sensitivity by ~8 orders of magnitude.

My work pushes the state of the art in nanofabrication methods and, the performance of devices and systems enabled by this establishing Bristol as a centre for excellence in nanofabrication. To enable this, my group also develops novel metrology tools that allows us to probe and quantify wave phenomena at the nanoscale. My interest in metrology is a natural outgrowth of spending three very enjoyable years as a postdoctoral fellow, working with Kartik Srinivasan at NIST Gaithersburg. Before that, I was a PhD student with David Miller at Stanford.

You can find more about my work at my personal webpage, or check out my papers.

PhD projects:

If you are interested in building nanoscale devices to engineer efficient light matter interactions, please drop me an email. A list of PhD projects I am actively looking to recruit on:

  1. Photonic-phononic integrated circuits for (quantum) microwave to optical signal transduction:
One of the key challenges facing quantum information processing platforms is the lack of an ideal qubit platform that satisfies all the requirements. Every platform (whether superconducting qubit, trapped ion or solid state photonic) has its shortcomings and one natural way to address this issue is to go for a best of all worlds approach and pick the best elements to implement each function and then figure out a way to wire everything up together. Such a hybrid system approach [Kurizki, PNAS 2015] has some obvious performance advantages, but to realize them requires the presence of efficient quantum transducers that can interface efficiently between these disparate systems. In this project, we will explore the development of efficient (quantum) microwave to optical signal transducers, using nanoscale acousto-optics, with a view towards linking superconducting qubits with telecom photons. Our approach is outlined in [Balram et al., Nat. Phot. 2016] and [Wu et al., Phys. Rev. Applied 2020]. See also [Srinivasan et al., J. Phys. D.] for background outlining some of the main challenges hindering high transduction efficiency.
This work is funded by my ERC starting grant award.
  1.  Cavity QMD: Piezoelectric micro-resonators as efficient near field transducers for readout and manipulation of nanoscale spin systems:
Piezoelectric surface acoustic wave (SAW) micro-resonators can generate strong, localized surface magnetic fields at GHz frequencies, resonantly enhanced by the acoustic quality factor. This provides a novel and efficient experimental route for nanoscale spin manipulation and (electrical) readout. In particular, the spin detection sensitivity is enhanced by almost eight orders of magnitude over existing inductive detection methods. This project aims to push the limit of the detection sensitivity by engineering the near fields of these resonators with the ultimate goal of achieving single spin electrical readout at cryogenic temperatures. For related ideas using superconducting cavities, see Bienfait et al., Nature 2016.
This work is funded by a prestigious New Horizons award from EPSRC.

Structured keywords and research groupings

  • QETLabs
  • photonics and quantum
  • Quantum Engineering Centre for Doctoral Training


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