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SUMMARY:IQOQI Dissertationspreis: Coherent control of a multi-qubit dark state in waveguide quantum electrodynamics
DESCRIPTION:The coherence properties of an atom or superconducting qubit strongly depend on the electromagnetic environment. Typical circuit QED experiments protect the qubit mode from decay into dissipative modes by placing it into a cavity. Effectively, a reduction of the available mode density reduces the free-space spontaneous emission rate of the qubit. In waveguide QED the qubit is strongly coupled to a continuous mode spectrum, thus it decays rapidly. Collective effects between multiple qubits can be utilized to create subradiant states that decouple from the dissipative waveguide environment. In our experiment we strongly couple two pairs of transmon qubits to the fundamental propagating mode of a rectangular waveguide. We show that the decay of the four qubit dark state is strongly suppressed, exceeding the waveguide-limited lifetimes of the individual qubits by two orders of magnitude [1]. We characterize the dark state by measuring the coherence time in a Ramsey experiment and perform a pulsed spectroscopy into the two-excitation manifold, which can only be accurately modeled by taking into the account the bosonic nature of the transmons [2]. [1] Zanner et al., Nat. Phys. 18, 538–543 (2022) (Experiment)[2] Orell et al., Phys. Rev. A 105, 063701 (Theory) 
LOCATION:Erwin Schrödinger Saal
DTSTART:20230411T113000
DTEND:20230411T130000
TZID: Europe/Vienna
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