Control of Localized Dark States in Collective Waveguide QED

Julian Daser,

Teresa Hönigl-Decrinis, Gerhard Kirchmair

Institute for Experimental Physics, University of Innsbruck, 6020 Innsbruck, Austria



Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, 6020 Innsbruck, Austria

julian.daser@uibk.ac.at,

Microwave photons propagating in waveguides couple efficiently to superconducting qubits and mediate long-range interactions between distant qubits, causing the emergence of collective states due to interference effects [1]. Of particular interest are dark or subradiant states, which are protected from decoherence by decoupling from the waveguide environment and thus exhibit long lifetimes. This makes them promising candidates for photon storage, excitation transfer and photon-photon gates [2]. However, the protection from decoherence comes at the cost that the control of such states is challenging.

Only recently was a collective dark state - formed by two transmon pairs, each pair supporting its own local dark state - probed experimentally by selective excitation through individual drive ports in a rectangular waveguide [3]. To extend the system to larger arrays of transmon qubits [2,4], we are pursuing a planar implementation. Our current system consists of eight transmon qubits coupled to a coplanar waveguide, with qubits spaced 8 mm apart, corresponding to half the emission wavelength. We observe collective bright and dark states at frequencies around 7.7815 GHz. In this architecture, it is theoretically possible to realize subradiant states with up to four excitations [2]. To date, we have performed time-domain measurements on a two-qubit dark state with a single excitation. Ongoing work focuses on stabilizing the system to achieve longer coherence times and to realize an eight-qubit dark state.



References

[1] Sheremet, A. S., et al. Rev. Mod. Phys. 95, 015002 (2023)

[2] Holzinger, R., et al., Phys. Rev. Let. 129, 253601 (2022)

[3] Zanner, M., et al., Nature Physics 18, 538-543 (2022)

[4] Orell, T., et al. Phys. Rev. A 105.6, 063701 (2022)