The work that has been performed from the start of the project to the end has lead to several results an publications:
WAVEGUDIE QUANTUM ELECTRO DYNAMICS
*Characterization of low loss microstrip resonators as a building block for circuit QED in a 3D waveguide- published in AIP Advances 7, 085118 (2017)
In this paper we present the microwave characterization of microstrip microwave resonators, made from aluminum and niobium, inside a 3D microwave waveguide. In the low temperature, low power limit internal quality factors of up to one million were reached. The setup presented here is appealing for testing materials and structures, as it is free of wire bonds and offers a well controlled microwave environment. In combination with transmon qubits, these resonators serve as a building block for a novel circuit QED architecture inside a rectangular waveguide (Fig1).
*Coherent control of a multi-qubit dark state in waveguide quantum electrodynamics - published in Nat. Phys. 18, 538 (2022).
Superconducting qubits in a waveguide have long-range interactions mediated by photons that cause the emergence of collective states. Destructive interference between the qubits decouples the collective dark states from the waveguide environment. Their inability to emit photons into the waveguide render dark states a valuable resource for preparing long-lived quantum many-body states and realizing quantum information protocols in open quantum systems. Here we demonstrate the coherent control of a collective dark state that is realized by controlling the interactions between four superconducting transmon qubits (Fig2) and local drives. The dark state’s protection against decoherence results in decay times that exceed those of the waveguide-limited single qubits by more than two orders of magnitude (Fig3). Moreover, we perform a phase-sensitive spectroscopy of the two-excitation manifold and reveal bosonic many-body statistics in the transmon array.
* Visualizing the emission of a single photon with frequency and time resolved spectroscopy - published in Quantum 5, 474 (2021).
At the dawn of Quantum Physics, Wigner and Weisskopf obtained a full analytical description (a photon portrait) of the emission of a single photon by a two-level system, using the basis of frequency modes (Weisskopf and Wigner, "Zeitschrift für Physik", 63, 1930). A direct experimental reconstruction of this portrait demands an accurate measurement of a time resolved fluorescence spectrum, with high sensitivity to the off-resonant frequencies and ultrafast dynamics describing the photon creation. In this work we demonstrate such an experimental technique in a superconducting waveguide Quantum Electrodynamics (wQED) platform, using a single transmon qubit and two coupled transmon qubits as quantum emitters. The photon portraits agree quantitatively with the predictions of the input-output theory and qualitatively with Wigner-Weisskopf theory (Fig4).
* Collective bosonic effects in an array of transmon devices - Phys. Rev. A 105, 063701 (2022)
Multiple emitters coherently interacting with an electromagnetic mode give rise to collective effects such as correlated decay and coherent exchange interaction, depending on the separation of the emitters. As an experimentally relevant setup of bosonic waveguide QED, we focus on a theoretical analysis of arrays of transmon devices embedded inside a rectangular waveguide. Specifically, we study the setup of two transmon pairs realized experimentally in Zanner et al. [Nat. Phys. 18, 538 (2022)] and show that it is necessary to consider transmons as bosonic multilevel emitters to accurately recover correct collective effects for the higher excitation manifolds.
* Control of Localized Single- and Many-Body Dark States in Waveguide QED - Phys. Rev. Lett. 129, 253601 (2022)
Here we identify a new class of quasilocalized dark states with up to half of the qubits excited, which only appear for lattice constants of an integer multiple of the wavelength. These states allow for a high-fidelity preparation and minimally invasive readout in state-of-the-art setups. In particular, we suggest an experimental implementation using a coplanar waveguide coupled to superconducting transmon qubits on a chip. With minimal free space and intrinsic losses, virtually perfect dark states can be achieved for a low number of qubits featuring fast preparation and precise manipulation.
COHERENT CONTROL OF INTERACTING TRANSMON QUBITS
*Quantum simulation of spin chain physics
Fig1. shows a conceptual schematic to implement a system for analog quantum simulation of a spin lattice, where the microstrip resonators developed in (AIP Advances 7, 085118 ) serve as a means to readout the state of qubits embedded in the waveguide. These resonators will provide means for locally probing the excitations and correlations in a spin chain or ladder geometry. Meanwhile we have realized this setup experimentally as can be seen in Fig. 5. and characterized it's properties.
*Fast flux control of 3D transmon qubits using a magnetic hose - published in Appl. Phys. Lett. 118, 012601 (2021)
An important feature in investigating different spin models is the possibility to change the frequency of SQUID-based transmon qubits rapidly by applying magnetic flux. We demonstrated a novel approach for fast flux control of a transmon qubit in a 3D cavity architecture. We use a magnetic hose to guide fast flux pulse from the outside to the inside of a microwave cavity (Fig6). The transition frequency can be tuned, using a simple square pulse, with a typical rise time of about 1 µs, which is much faster than the typical qubit decoherence. By measuring the transfer function of the system and applying a deconvolution kernel, the effective qubit response time can be further reduced, reaching about 50 ns (Fig7).
