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arXiv:2208.05571 · Physical Review Research

Tunable coupler for mediating interactions between a two-level system and a waveguide from a decoupled state to the ultrastrong coupling regime

N. Janzen, Xi Dai, S. Ren, Jiahao Shi, Adrian Lupaşcu

Published Sep 5, 2023• 14 citations
arXivHTMLPDF

Abstract

Two-level systems (TLSs) coupled to waveguides are a fundamental paradigm for light-matter interactions and quantum networks. We introduce and experimentally demonstrate a method to tune the interaction between a TLS, implemented as a flux qubit, and a transmission line waveguide from a decoupled state to a coupling strength that is a significant fraction of the TLS transition frequency, near the ultrastrong coupling regime. The coupling, controlled via magnetic flux, is described by a normalized coupling strength $\ensuremath{\alpha}$ that is measured to range between $6.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ and $2.19\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$, with larger attainable maximum values predicted by a circuit model of the device. This system enables future investigations in the dynamics of the spin-boson model, microwave photonics, and relativistic quantum information.

DOI
10.1103/physrevresearch.5.033155

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Two-level systems (TLSs) coupled to waveguides are a fundamental paradigm for light-matter interactions and quantum networks. We introduce and experimentally demonstrate a method to tune the interaction between a TLS, implemented as a flux qubit, and a transmission line waveguide from a decoupled state to a coupling strength that is a significant fraction of the TLS transition frequency, near the ultrastrong coupling regime. The coupling, controlled via magnetic flux, is described by a normalized coupling strength \ensuremathα\ensuremath{\alpha}\ensuremathα that is measured to range between 6.2\ifmmode×\else\texttimes\fi10\ensuremath−56.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}6.2\ifmmode×\else\texttimes\fi10\ensuremath−5 and 2.19\ifmmode×\else\texttimes\fi10\ensuremath−22.19\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}2.19\ifmmode×\else\texttimes\fi10\ensuremath−2, with larger attainable maximum values predicted by a circuit model of the device. This system enables future investigations in the dynamics of the spin-boson model, microwave photonics, and relativistic quantum information.
This thesis consists of two parts: the main part is a theoretical investigation of the ultrastrong coupling regime for atom-light coupling in superconducting circuits, and the second part is concerned with the development of a new high coherence flux qubit design. \n \nThe spin-boson model describes the interaction between a quantum two-level system and a continuum of bosonic modes. When the interaction strength becomes comparable to the system frequency, the system enters the ultrastrong coupling (USC) regime, where the rotating wave approximation breaks and the system dynamics need to be described nonperturbatively. Recently, the ultrastrong coupling has been achieved on a device consisting of a superconducting flux qubit coupled to an electromagnetic continuum, with the coupling strength being verified in experiments by the standard transmission method. The first project in this thesis aims to measure the dynamics of the spin-boson model in a direct and controllable way when the coupling strength is in the USC regime. We propose three experiments to measure the coherence of the two-level system including the relaxation and dephasing, and the the renormalized tunneling frequencies in the ultrastrong coupling regime. The controllable measurements are realized with a fast-switchable ultrastrong coupling system consisting of a two-loop flux qubit galvanically coupled to an open transmission line, flux bias and driving lines, and readout circuits. The design and model of the full device are presented. We demonstrate that the three proposed experiments can be well implemented based on the simulations of qubit properties and coupling strengths. \n \nThe second part of the thesis presents work on the design of a new type of capacitively shunted flux qubit. This work is motivated by improving qubit coherence time and anharmonicity, which is essential for speeding up qubit gates and enhancing the capability of quantum computing. It was demonstrated that adding shunting pads to flux qubits can drastically improve the coherence time and the reliability of qubit fabrication, but the CSFQ was shown with moderate anharmonicities. In this project, we present the a new design of CSFQ and its circuit model, which contains three Josephson junctions and three shunting pads. In experiments, the qubit spectroscopy matches well with the circuit model, which takes into account all the capacitances between the qubit and other circuits. The qubit is found to have both large anharmonicity A = ω₁₂ - ω₀₁ = 2π ⨯ 3.69 GHz, and high coherence with T₁ = 40 ± 5 µs. We also present experiments on the multilevel quantum control and multilevel relaxation measurement. We perform qutrit state tomography to reconstruct the full density matrix with the tomography fidelity reaching 99.2%. We are able to extract the time-dependent state populations for the relaxation process of a qutrit, and extract the exact transition rates between the three levels.
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We propose different designs of switchable coupling between a superconducting flux qubit and a microwave transmission line. They are based on two or more loops of Josephson junctions which are directly connected to a closed (cavity) or open transmission line. In both cases the circuit induces a coupling that can be modulated in strength, reaching the so-called ultrastrong coupling regime in which the coupling is comparable to the qubit and photon frequencies. Furthermore, we suggest a wide set of applications for the introduced architectures.
Light-Matter Interaction – from Strong to Ultrastrong Coupling in Superconducting Quantum Circuits Rudolf Gross TU Munchen / Walther-Meisner-Institut fur Tieftemperaturforschung Superconducting quantum circuits imple-mented by nanostructured superconduc-tors allow for the investigation of funda-mental quantum phenomena on a macro-scopic scale and the implementation of solid state quantum information systems. A big advantage of superconducting quantum two-level systems (qubits) over natural atoms is their design flexibility and wide tunability by means of external control parameters such as magnetic fields. We address the coupling of superconducting flux qubits [1,2] to on-chip microwave resonators, giv-ing rise to the prospering field of superconducting circuit quantum electrodynamics (c-QED), which allows us to study the fundamental interaction between artificial solid state atoms and single microwave photons as the basis for communicating quantum information. Recently, we succeeded to realize for the first time c-QED systems operating in the ultra-strong coupling re-gime, where the atom-cavity coupling rate reaches a considerable fraction of the atom transi-tion frequency [3]. In this regime new objects are formed consisting of matter and light. We present spectroscopy data on these new objects providing insight into novel phenomena that could not be studied in atom cavity-QED so far. We also address the interplay of multi-photon processes and symmetries in a qubit-resonator system by spectroscopically analyzing a super-conducting qubit-resonator system under oneand two-photon driving [4]. [1] T. Niemczyk et al., Supercond. Sci. Techn. 22, 034009 (2009). [2] F. Deppe et al., PRB 76, 214503 (2007); K. Kakuyanagi, et al., PRL 98, 047004 (2007). [3] T. Niemczyk et al., Nature Physics 6, 772-776 (2010). [4] F. Deppe et al., Nature Physics 4, 686 (2008); T. Niemczyk et al., arXiv:1107.0810v1 (2011). Gastgeber: Prof. Martin Dressel, Universitat Stuttgart, Telefon: 0711 685-64946
We realize a device allowing for tunable and switchable coupling between two frequency-degenerate superconducting resonators mediated by an artificial atom. For the latter, we utilize a persistent current flux qubit. We characterize the tunable and switchable coupling in the frequency and time domains and find that the coupling between the relevant modes can be varied in a controlled way. Specifically, the coupling can be tuned by adjusting the flux through the qubit loop or by controlling the qubit population via a microwave drive. Our measurements allow us to find parameter regimes for optimal coupler performance and quantify the tunability range.
In the circuit quantum electrodynamics architecture, both the resonance frequency and the coupling of superconducting qubits to microwave field modes can be controlled via external electric and magnetic fields to explore qubit-photon dynamics in a wide parameter range. Here, we experimentally demonstrate and analyze a scheme for tuning the coupling between a transmon qubit and a microwave resonator using a single coherent drive tone. We treat the transmon as a three-level system, with the qubit subspace defined by the ground and the second excited states. If the drive frequency matches the difference between the resonator and the qubit frequencies, a Jaynes-Cummings-type interaction is induced, which is tunable in both amplitude and phase. We show that coupling strengths of about 10 MHz can be achieved in our setup, limited only by the anharmonicity of the transmon qubit. This scheme has been successfully used to generate microwave photons with a controlled temporal shape [M. Pechal et al., Phys. Rev. X 4, 041010 (2014)] and can be directly implemented with superconducting quantum devices featuring larger anharmonicity for higher coupling strengths.