Controlling Electron-Phonon Interaction in Nanocircuits Strong Coupling Regime

• Applicants: Privatdozent Dr. Stefan Ludwig; Professor Dr. Felix von Oppen; Dr. Paulo V. Santos
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: from 2018 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 405619279

• Final Report Year: 2025

Final Report Abstract

Phonons are the most fundamental vibration modes of solids. In the field of quantum information processing, they still have a bad reputation, because an uncontrolled (thermal) phonon bath is a source of decoherence. So-far, applications are restricted either to classical surface acoustic waves (corresponding to the regime of highly excited phonon modes) or nanomechanical resonators with well isolated non-thermal mechanical modes. But the fact is that non-thermal acoustic phonons can be coherent quantum states themselves, which could be employed for quantum technology applications. In the project we are reporting on, we explored surface acoustic phonon cavities to pave the way for the coherent coupling of distant quantum bits (qubits). Compared to phonon cavities, photon cavities (including the microwave range) have already been studied much more for coupling distant qubits. Like photons, low energy acoustic phonons have a linear dispersion relation, ω = 2πv/λ, but the speed of sound, v, is orders of magnitude smaller than that of light. Hence, for a given energy, ω, an acoustic phonon has a much shorter wavelength, λ, than a photon. This enables a different regime of interaction with a qubit: A phonon and a nanosize solid state qubit can be in resonance, while at the same time the phonon wavelength matches the size of the nanostructure. The result can be a maximal coupling between a qubit and a phonon cavity state, while the overlap integral becomes dependent on the phase difference between the qubit versus phonon wave functions. These properties extend the flexibility for controlling qubits compared to the possibilities of photons. The control of acoustic phonons turns out to be more complicated than that of photons: reflecting the crystal symmetries, the sound velocity and electron-phonon coupling are anisotropic and described by multi-level tensors. Further, their intrinsic coupling to free charge carriers cause decoherence of phonons. As a consequence, before a coherent qubit-phonon hybrid-state can be useful for quantum technology applications, the electron-phonon interaction as well as the phonon spectrum itself must be well thought out and engineered. For instance, away from the qubit, the carrier-phonon interaction should be minimal, while being maximal at the qubit. To this end, we have explored a variety of methods including transmission spectroscopy, atomic force microscopy and scanning X-ray microscopy to characterize and optimize phonon cavities. First transport spectroscopy measurements of a double quantum dot embedded inside a phonon cavity failed so-far, because of fabrication problems. The results obtained so far in this project proof the general suitability of acoustic phonon cavities for the coupling of solid state qubits.

Publications

• Acoustic Field for the Control of Electronic Excitations in Semiconductor Nanostructures. 2018 IEEE International Ultrasonics Symposium (IUS), 1-4. IEEE.
  Santos, Paulo V.; Msall, Madeleine & Ludwig, Stefan
  
• Focusing Surface-Acoustic-Wave Microcavities on GaAs. Physical Review Applied, 13(1).
  Msall, Madeleine E. & Santos, Paulo V.
  
• Determining Amplitudes of Standing Surface Acoustic Waves via Atomic Force Microscopy. Physical Review Applied, 17(4).
  Hellemann, Jan; Müller, Filipp; Msall, Madeleine; Santos, Paulo V. & Ludwig, Stefan
  
• Scanning X-Ray Diffraction Microscopy of a 6-GHz Surface Acoustic Wave. Physical Review Applied, 19(2).
  Hanke, M.; Ashurbekov, N.; Zatterin, E.; Msall, M.E.; Hellemann, J.; Santos, P.V.; Schulli, T.U. & Ludwig, S.