# High impedance superconducting technology for hybrid devices and analog quantum simulation

## abstract:

Photonic cavity arrays form the basis of one of the most promising paradigms for quantum simulation to study complex many-body physics [1]. We developed a nontrivial structured photonic environment that could enable a multimode strong and ultra-strong coupling with a qubit. This platform consists of a unidimensional metamaterial implemented by an array of coupled superconducting microwave cavities made from thin Niobium Nitride (NbN) films. Such a disordered superconductor allows to reach a very high kinetic inductance, which presents a two-fold advantage: a) It allows to reach ultra-strong coupling with an artificial atom as the amplitude of the zero-point voltage fluctuation is proportional to the square root of the resonators’ impedance [2]; b) It allows to reduce the resonator/metamaterial footprint strongly. Furthermore, working with a metamaterial allows engineering a nontrivial photonic dispersion relation, where it is possible to obtain states displaying topological properties (SSH-states [3]). We have fabricated and characterized unidimensional metamaterials of up to 88 ultra-compact resonators. We are currently expanding this technology to 2D metamaterials, where we expect to engineer further topological edge states.

In addition, high impedance and compact resonators are an ideal platform for coupling microwave photons to qubits defined in semiconducting quantum dots (QDs) in the form of a hybrid device. We have realized a proof-of-concept experiment where virtual microwave photon excitations in a high impedance SQUID array resonator mediate the coupling between a transmon and a double QD (DQD). The high impedance resonator acts as a quantum bus enabling long-range coupling between dissimilar qubits [4]. Similarly, we achieved coherent coupling between two DQD charge qubits separated by approximately ~50 um [5]. We have further investigated how to in-situ tune the strength of the electric dipole interaction between the DQD qubit and the resonator [6]. By employing a Josephson junction array resonator with an impedance of ∼4 kΩ and a resonance frequency of ωr/2π∼5.6 GHz, we observed a coupling strength of g/2π∼630 MHz, demonstrating the possibility of achieving the ultrastrong coupling regime (USC) for electrons hosted in a semiconductor DQD.

References

[1] D. L. Underwood, W. E. Shanks, J. Koch, and A. A. Houck, Phys. Rev. A 86, 023837 (2012).

[2] N. Samkharadze, A. Bruno, P. Scarlino, G. Zheng, D. P. DiVincenzo, L. DiCarlo, and L. M. K. Vandersypen, Phys. Rev. Appl. 5, 044004 (2016).

[3] E. Kim, X. Zhang, V. S. Ferreira, J. Banker, J. K. Iverson, A. Sipahigil, M. Bello, A. González-Tudela, M. Mirhosseini, and O. Painter, Phys. Rev. X 11, 011015 (2021).

[4] P. Scarlino, D. J. van Woerkom, et al., Nat. Comm. 10, 3011 (2019).

[5] D. J. van Woerkom, P. Scarlino, et al., Phys. Rev. X 8, 041018 (2018).

[6] P. Scarlino, et al., arXiv:2104.03045.