Publications Lukas DEEG

2026

Article

• R. Garcia-Belles, A. Anferov, L. Deeg, L. Colicchio, A. Brooks, T. Schattenburg, M. Drimmer, G. Kirchmair, Y. Chu Loss Mechanisms in High-coherence Multimode Mechanical Resonators Coupled to Superconducting Circuits, PRX Quantum 7 3313 (2026-06-17), http://dx.doi.org/10.1103/vszq-z1sh doi:10.1103/vszq-z1sh
  Circuit quantum acoustodynamics (cQAD) devices have a wide range of applications in quantum science, all of which depend crucially on the quantum coherence of the mechanical subsystem. In this context, high-overtone bulk acoustic-wave resonators (HBARs) are particularly promising, since they have shown very high quality factors with negligible dephasing. However, the introduction of piezoelectric films, which are necessary for coupling to a superconducting circuit, can lead to additional loss channels, such as surface scattering and two-level systems. Here, we study the acoustic dissipation of HBAR resonators in cQAD systems and find that the defect density of the piezoelectric material and its interface with the bulk are limiting factors for the coherence. We measure acoustic modes with phonon lifetimes up to 400  μ⁢s and lifetime-limited coherence times approaching 1 ms in the quantum regime. When coupled to a superconducting qubit, this leads to a hybrid system with a large quantum coherence cooperativity of 2 =1.1 ×105. These results represent a new milestone for the performance of cQAD devices and offer concrete paths forward for further improvements.

2025

Articles

• N. Diaz-Naufal, L. Deeg, D. Zoepfl, C. Schneider, M. L. Juan, G. Kirchmair, A. Metelmann Kerr enhanced optomechanical cooling in the unresolved sideband regime, Phys. Rev. A 111 53505 (2025-05-25), http://dx.doi.org/10.1103/PhysRevA.111.053505 doi:10.1103/PhysRevA.111.053505
  Dynamical backaction cooling has been demonstrated to be a successful method for achieving the motional quantum ground state of a mechanical oscillator in the resolved-sideband regime, where the mechanical frequency is significantly larger than the cavity decay rate. Nevertheless, as mechanical systems increase in size, their frequencies naturally decrease, thus bringing them into the unresolved-sideband regime, where the effectiveness of the sideband cooling approach decreases. Here we demonstrate, however, that this cooling technique in the unresolved-sideband regime can be significantly enhanced by utilizing a nonlinear cavity as shown in the experimental work of Zoepfl et al. [Phys. Rev. Lett. 130, 033601 (2023)]. The above arises due to the increased asymmetry between the cooling and heating processes, thereby improving the cooling efficiency. In addition, we show that injecting a squeezed vacuum into the nonlinear cavity paves the way to ground-state cooling of the mechanical mode. Notably, the required squeezing parameters are far less stringent than in the linear case, simplifying experimental implementation.
• L. Deeg, D. Zoepfl, N. Diaz-Naufal, M. L. Juan, A. Metelmann, G. Kirchmair Optomechanical Backaction in the Bistable Regime, Phys. Rev. Applied 23 (2025-01-31), http://dx.doi.org/10.1103/PhysRevApplied.23.014082 doi:10.1103/PhysRevApplied.23.014082
  With a variety of realizations, optomechanics utilizes its light-matter interaction to test fundamental physics. By coupling the phonons of a mechanical resonator to the photons in a high-quality cavity, control of increasingly macroscopic objects has become feasible. In such systems, state manipulation of the mechanical mode is achieved by driving the cavity. To be able to achieve high drive powers the system is typically designed such that it remains in a linear response regime when driven. A nonlinear response, and especially bistability, in a driven cavity is often considered detrimental to cooling and state preparation in optomechanical systems and is avoided in experiments. Here we show that with an intrinsic nonlinear cavity backaction cooling of a mechanical resonator is feasible operating deep within the nonlinear regime of the cavity. With our theory taking the nonlinearity into account, precise predictions on backaction cooling can be achieved even with a cavity beyond the bifurcation point, where the cavity photon number spectrum starts to deviate from a typical Lorentzian shape.

Preprint

• A. Paradakar, P. Nicaise, K. Dakroury, F. Resare, C. Dejaco, L. Deeg, G. Kirchmair, W. Wieczorek Efficient flip-chip and on-chip-based modulation of flux-tunable superconducting resonators, (2025-05-28), arXiv:2512.23119 arXiv:2512.23119