Achintya Paradkar, Forskningsavdelningen Kvantteknologi

Main supervisor: Prof Witlef Wieczorek, QT lab
Assist Supervisor: Assoc Prof Thilo Bauch, QDP
Examiner: Prof Per Delsing, QT lab
Opponent: Prof Dag Winkler, QDP

Sammanfattning: 

Magnetic levitation of a superconducting microparticle provides a tunable and passive trapping with ultra-low dissipation of its center-of-mass (COM) motion [1]. This experimental platform paves the way for realizing macroscopic quantum states, relevant for fundamental physics and sensing applications. Previously, we demonstrated an on-chip magnetic trap [2] with a flux-based readout of the particle's motion by coupling it to a DC-SQUID [3]. To enhance control over the particle's motion, we explore flux-based coupling to flux-tunable superconducting resonators (FTRs), enabling the use of cavity magnetomechanics techniques [4] for ground-state cooling of the particle’s COM motion.
To realize an efficient coupling between the levitated particle and the FTR, we have developed novel methods for modulating FTRs using both flip-chip and on-chip configurations. We first present a simplified fabrication and bonding process for superconducting flip-chip devices using indium microspheres as superconducting interconnects [5]. This flip-chip assembly utilizes Au-passivated superconducting Nb or NbN, achieving chip separations of 20–50 μm with high alignment accuracy, and supports supercurrents of tens of milliamperes at millikelvin temperatures.
We then present our results on FTRs realized as quarter-wave coplanar waveguide resonators terminated by a DC-SQUID, featuring large SQUID loops (100 μm) and large Josephson junctions (1 μm). We demonstrate flux modulation of the FTRs via an input coil placed above the SQUID loop via flip-chip bonding. Additionally, we demonstrate an on-chip flux modulation approach using an air-bridge-based input coil concentric to the SQUID. Future work aims to couple the FTR to the COM motion of the levitated superconducting microparticle, thereby aiming toward its quantum control.

[1] Romero-Isart, O., Clemente, L., Navau, C., Sanchez, A., & Cirac, J. I., Quantum Magnetomechanics with Levitating Superconducting Microspheres, Physical Review Letters, 109, 147205 (2012).
[2] Latorre, M. G., Paradkar, A., Hambraeus, D., Higgins, G., & Wieczorek, W., A Chip-Based Superconducting magnetic trap for levitating superconducting microparticles, IEEE Transactions on Applied Superconductivity, 32, 1 (2022).
[3] Latorre, M. G., Higgins, G., Paradkar, A., Bauch, T., & Wieczorek, W., Superconducting Microsphere Magnetically Levitated in an Anharmonic Potential with Integrated Magnetic Readout, Physical Review Applied, 19, 054047 (2023).
[4] Zoepfl, D., Juan, M. L., Diaz-Naufal, N., Schneider, C. M. F., Deeg, L. F., Sharafiev, A., Metelmann, A., & Kirchmair, G., Kerr enhanced backaction cooling in magnetomechanics, Physical Review Letters, 130, 033601 (2023).
[5] Paradkar, A., Nicaise, P., Dakroury, K., Resare, F., & Wieczorek, W., Superconducting
flip-chip devices using indium microspheres on Au-passivated Nb or NbN as under-
bump metallization layer, arxiv.2408.14655 (2024).