Sumit Kumar

Fig. 1. A quantized vortex in superfluid helium, drawn as half a million particles riding its surface. Flow near the core circles faster than flow farther out, winding surface waves into spiral arms that feed a dark funnel. Close to the core the flow outruns the waves themselves, the same principle behind analogue black-hole horizons made with third sound. Move the pointer across the surface to stir it, or click to drop a ripple. Rendered live with WebGPU; not to scale.

Using microwaves to listen to superfluids, and superfluids to probe quantum fields.

I build cavity optomechanical systems, superconducting microwave resonators coupled to nanomechanical strings, drumheads and superfluid ⁴He, to probe mechanical motion at the quantum limit, simulate cosmological analogues, and search for gravitational waves above the LIGO band.

Sumit Kumar, Assistant Professor, IIT (ISM) Dhanbad

About

I am an experimental physicist working at the intersection of nanomechanics, microwave quantum measurement and low-temperature physics. My doctoral work at Institut Néel (CNRS, Grenoble) focused on cavity optomechanics with silicon-nitride strings and aluminium drumheads, including the first demonstration of single-mode passive cooling of a macroscopic object into its motional ground state.

As a postdoctoral researcher at Royal Holloway, University of London, I developed superfluid ⁴He optomechanics, using third sound for analogue black-hole simulations and fourth-sound resonators to study acoustic non-linearities in phononic crystals, and contributed to a proposal for high-frequency gravitational-wave detection in a microwave re-entrant geometry.

I joined IIT (ISM) Dhanbad as Assistant Professor in July 2025, where I am building a low-temperature quantum optomechanics laboratory and continuing collaborations on superfluid optomechanics and gravitational-wave detection.

  1. 2025–Assistant Professor, IIT (ISM) Dhanbad
  2. 2021–2025Postdoctoral researcher, Royal Holloway, University of London
  3. 2017–2021PhD, Institut Néel, CNRS / Université Grenoble Alpes
  4. 2015–2017M.Tech, Centre for Nanoscience and Engineering, IISc Bengaluru
  5. 2010–2014B.Tech, Engineering Physics, IIT Delhi

Research

Superconducting microwave optomechanics

A mechanical oscillator, such as a silicon-nitride string or an aluminium drumhead, is coupled to a superconducting microwave resonator: its motion shifts the resonance, and microwave photons push back on it. In my doctoral work at Institut Néel we used this to passively cool a macroscopic object into its quantum ground state of motion, and to study optomechanical coupling beyond the linear regime.

The same measurement chain turns these devices into sensors of weak forces and displacement, and into building blocks for quantum transduction.

DirectionsGround-state cooling and quantum-limited readout, non-linear optomechanics, sensing and quantum transduction.

Fig. 2. A drum resonator in its fundamental mode below a fixed electrode. As the drum rises the gap narrows and the microwave field in it (gold) intensifies, shifting the circuit's resonance; that shift is how the motion is read out.

Superconducting circuits: amplifiers and qubits

Every experiment in the lab rests on the same enabling hardware: high-Q superconducting microwave resonators, low-loss filters and routing, and parametric amplifiers operating near the standard quantum limit. We design, fabricate and characterise these components in coplanar-waveguide and three-dimensional re-entrant architectures, and study their dominant losses: two-level systems at dielectric interfaces, vortex motion in superconducting films, and radiation.

We also work on making complex circuits behave inside dilution refrigerators, through thermalisation, packaging and shielding, and on integrating them, together with qubits, with mechanical resonators and superfluid helium.

DirectionsLoss-engineered resonators, compact low-noise parametric amplifiers, cryogenic packaging, hybrid integration with qubits.

Fig. 3. A qubit on the Bloch sphere. A resonant microwave drive rotates the state from |0⟩ at the top to |1⟩ at the bottom and back, a Rabi oscillation, while it precesses about the vertical axis. The gold trail shows the last few seconds.

Superfluid optomechanics for analogue cosmology

Superfluid ⁴He hosts mechanical modes with extraordinarily low loss: third sound on thin films, fourth sound in porous media and sonic crystals, and acoustic resonances in helium-filled cavities. Coupled to superconducting microwave cavities, they become quantum-limited mechanical oscillators at millikelvin temperatures.

Long-wavelength sound on a flowing superfluid obeys the same equations as a field in curved spacetime, so third sound lets us build tabletop analogues of black-hole horizons and expanding cosmologies, and look for Hawking-like radiation and cosmological particle production. The same physics underlies our proposal for detecting high-frequency gravitational waves, in the kilohertz band above LIGO, with superfluid ⁴He in microwave re-entrant cavities.

DirectionsHawking-like radiation in third-sound horizons, engineered expansion histories, fourth-sound non-linearities, high-frequency gravitational waves.

Fig. 4. An expanding analogue universe (illustration). Every ring moves outward at a speed proportional to its distance, a Hubble law, and the ripples carried along are stretched to longer wavelengths and grow, much as quantum fluctuations were in the early universe.

Quantized vortices and two-dimensional turbulence

In a superfluid, rotation cannot vary smoothly. It concentrates into vortices that each carry one quantum of circulation. In thin ⁴He films, the same films that carry third sound, these vortices move, cluster and annihilate almost like an ideal two-dimensional gas, a clean testbed for long-standing predictions about turbulence in two dimensions.

Because vortices and their flow shift the third-sound modes, the microwave readout we use for superfluid optomechanics offers a way to follow their dynamics.

DirectionsVortex dynamics in thin films, Onsager vortex clusters, quantum turbulence.

Fig. 5. Interference fringes from a two-dimensional superfluid. Each fork in the fringes is a quantized vortex, where the phase winds by exactly 2π; gold and blue mark opposite circulation. When opposite vortices meet they annihilate and release a pulse of sound. Click to create a pair, or drag to aim it.

Room-temperature optomechanics

Not every application can wait for a dilution refrigerator. We developed a new three-dimensional microwave cavity architecture for optomechanics at room temperature, and used it to demonstrate optomechanically induced transparency at ambient conditions. Room-temperature setups also let us prototype and benchmark cavity designs quickly before taking them into the cryostat.

They are the starting point for practical devices: with our spin-out enthrophy.com we are turning laboratory-grade measurement chains into compact, robust sensors for inertial measurement, weak forces and material characterisation.

Directions3D-cavity optomechanics at ambient temperature, deployable precision sensors.

Fig. 6. A high-Q mechanical resonator (illustration). A patterned frame, a phononic crystal, forbids vibrations in a band of frequencies, so a mode trapped at the centre cannot leak away and keeps ringing, which matters most when there is no refrigerator to help.

Machine learning for physics

We use machine learning and automation to design and run experiments: surrogate models and Bayesian optimisation to search resonator and circuit design spaces, automated tuning of measurement chains, and learned models that pull physics out of noisy data.

DirectionsDesign optimisation, Python-based experiment automation, data analysis.

Fig. 7. An optimiser searching a loss landscape. Each run starts somewhere new and follows the slope, with momentum, into a minimum. The same idea guides searches over resonator and circuit designs.

Publications

In preparation

  1. A proposal for gravitational wave detection using superfluid ⁴He in a microwave re-entrant geometry Sean Hibbit, Sumit Kumar, et al.
    Manuscript in preparation
  2. Superfluid optomechanics with third sound on ⁴He for analogue black-hole simulations Sumit Kumar, et al.
    Manuscript in preparation
  3. Non-linearities in fourth sound of superfluid ⁴He sonic crystal resonator Sumit Kumar, et al.
    Manuscript in preparation

Peer-reviewed

  1. Optomechanically induced transparency in a 3D microwave cavity architecture at ambient temperature Sumit Kumar, et al.
    AIP Advances 14.3 (2024)
  2. Nano-beam clamping revisited I. Golokolenov, Sumit Kumar, et al.
    Journal of Applied Physics 133.12, 124302 (2023)
  3. A novel architecture for room-temperature microwave optomechanical experiments Sumit Kumar, et al.
    Journal of Applied Physics 133.9, 094501 (2023)
  4. Microwave optomechanically induced transparency and absorption between 250 and 450 mK Sumit Kumar, et al.
    Journal of Low Temperature Physics (2022)
  5. Microwave optomechanical measurement of non-metallized SiN strings at mK temperatures Sumit Kumar, et al.
    arXiv:2110.00228
    arXiv
  6. A macroscopic object passively cooled into its quantum ground state of motion beyond single-mode cooling D. Cattiaux, I. Golokolenov, S. Kumar, M. Sillanpää, L. Mercier de Lépinay, R. R. Gazizulin, X. Zhou, A. D. Armour, O. Bourgeois, A. Fefferman, E. Collin
    Nature Communications 12, 6182 (2021)
  7. Microwave single-tone optomechanics in the classical regime I. Golokolenov, D. Cattiaux, Sumit Kumar, M. Sillanpää, L. Mercier de Lépinay, A. Fefferman, E. Collin
    New Journal of Physics 23, 053008 (2021)
  8. Beyond linear coupling in microwave optomechanics D. Cattiaux, X. Zhou, S. Kumar, I. Golokolenov, R. R. Gazizulin, A. Luck, L. Mercier de Lépinay, M. Sillanpää, A. Armour, A. Fefferman, E. Collin
    Physical Review Research 2, 033480 (2020)
  9. Geometrical nonlinearity of circular plates and membranes D. Cattiaux, S. Kumar, X. Zhou, A. Fefferman, E. Collin
    Journal of Applied Physics 128, 104501 (2020)
  10. A 10 mK hermetic cell for eliminating parasitic heating in cryogen-free dilution refrigerators D. Schmoranzer, Sumit Kumar, S. Triqueneaux, X. Liu, T. Metcalf, G. Jernigan, E. Collin, A. Fefferman
    Cryogenics 110, 103162 (2020)
  11. Observations on thermal coupling of silicon oscillators in cryogen-free dilution refrigerators D. Schmoranzer, Sumit Kumar, A. Luck, E. Collin, A. Fefferman, X. Liu, T. Metcalf, G. Jernigan
    Journal of Low Temperature Physics 196, 268 (2019)
  12. Improved field emission from indium decorated multi-walled carbon nanotubes M. Sreekanth, S. Ghosh, P. Biswas, S. Kumar, P. Srivastava
    Applied Surface Science 383, 84–89 (2016)
  13. Inverse Leidenfrost effect: levitating drops on liquid nitrogen M. Adda-Bedia, S. Kumar, F. Lechenault, S. Moulinet, M. Schillaci, D. Vella
    Langmuir 32(17), 4179–4188 (2016)

Group

The Quantum Optomechanics group at IIT (ISM) Dhanbad is forming. We bring together experimentalists, theorists and engineers, with expertise in low-temperature physics, microwave measurement, nanofabrication and superfluid helium, and we are recruiting at every level.

Open positions

  • Postdoctoral researcher. Experimental background in microwave measurement, dilution refrigerators or superfluid ⁴He. Funded position contingent on grant.
  • PhD, superfluid optomechanics. Cavity optomechanics with superfluid ⁴He at millikelvin temperatures.
  • PhD, high-frequency gravitational waves. Detector development using superfluid ⁴He in microwave re-entrant cavities.
  • PhD, theory and simulation. Optomechanical dynamics, non-linear resonators, and analogue cosmology with third-sound systems.
  • Master's and M.Tech projects. Superconducting circuit design, microwave resonator fabrication, cryogenic instrumentation, and Python-based experiment automation.

Candidates from physics, electrical engineering and applied physics with interests in quantum measurement, low-temperature physics, microwave engineering or theory are encouraged to get in touch.

Collaborators

  • Dr. Xavier RojasRoyal Holloway, University of London. Superfluid optomechanics, fourth sound in phononic crystals, gravitational-wave detection.
  • Dr. Andrew FeffermanInstitut Néel, CNRS, Grenoble. Low-temperature microwave optomechanics, cryogenic instrumentation.
  • Dr. Eddy CollinInstitut Néel, CNRS, Grenoble. Ground-state cooling of macroscopic objects, non-linear optomechanics.
  • enthrophy.comSpin-out and industry partner, turning lab-grade precision sensing into deployable instruments.

Contact

Open to collaborations, student enquiries and seminar invitations.

Location

IIT (ISM) Dhanbad
Dhanbad, Jharkhand, India