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Walther-Meißner-Institut (WMI), Bayerische Akademie der Wissenschaften
Chair for Technical Physics (E23), Technische Universität München

Seminar on
Superconducting Quantum Circuits
SS 2021


Lectures & Exercises
Practical Training
Lecture Notes
Talks & Tutorials

Tuesday, 14:30 - 16:00 h
Place: please check below

virtual (Zoom), link available via TUM Online

Date Speaker Title
F. Deppe, A. Marx, R. Gross
Bayerische Akademie der Wissenschaften (BAdW) and Technische Universität München (TUM)
Preliminary discussion and assignment of topics
Patrick Missale
Josephson junctions for superconducting qubits
Franz von Silva-Tarouca
Experimental quantum teleportation of propagating microwaves (Advisor: Michael Renger)
Shawn Storm
Demonstration of two-qubit algorithms with a superconducting quantum processor (Advisor: Federico Roy)
Christian Mang
New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds (Advisor: Leon Koch)
Christian Mang
New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds (Advisor: Leon Koch)
Kevin Shen
Compiling quantum algorithms for architectures with multi-qubit gates (Advisor: Malay Singh)
Lukas Vetter
Simple pulses for elimination of leakage in weakly nonlinear qubits (Advisor: Max Werninghaus)
Oliver Kuijpers
Deterministic multi-qubit entanglement in a quantum network (Advisor: Fabian Kronowetter)
Michal Cherczynski
Characterizing and optimizing qubit coherence based on SQUID geometry (Advisor: Ivan Tsitsilin)
Florian Maier
Two-level systems in superconducting quantum devices due to trapped quasiparticles (Advisor: Kedar Honasoge)
Gerhard Huber
Multiqubit coupler with superconducting circuits
Niklas Bruckmoser
Optimization od superconducting Nb resonators
Leonhard Hölscher
Optimization of Nb/Al interfaces
Niklas Glaser
Optimization of qubit gates

Seminar description:

Superconducting circuits have evolved from a toy to study fundamental light-matter interaction into a prime candidate for scalable quantum computing. In addition to university groups, industry has started to enter the field (Goolge, IBM, Microsoft, D-Wave Systems, Rigetti quantum computing etc.). As of 2018, chips with several tens of coherent superconducting qubits have been reported, either as open or commercial platforms. The next big challenges are the demonstration of a quantum advantage and useful quantum error correction.

Within the seminar, students give talks on the latest developments in quantum computing with superconducting circuits and related areas such as spin systems or nanomechanics. The seminar is relevant for the special courses on "Superconductivity and Low Temperature Physics" and "Applied Superconductivity". The seminar is suitable for bachelor and master students in the 6. semester and higher. Seminar talks can be given either in English or in German.

List of open topics for seminar talks in SS 2021:

  1. New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds (Alex P. M. Place et. al., arxiv:2003.00024 (2020)):
    Why are tantalum qubits suddenly better than aluminum or niobium qubits and what should the qubit community learn from this.
  2. Kerr-Free Three-Wave Mixing in Superconducting Quantum Circuits (V. V. Sivak et al., Phys. Rev Applied 11, 054060 (2019)):
    By connecting several Josephson junctions, it is possible to engineer an effective potential that would not otherwise have been realizable using a single junction alone.
  3. Experimental quantum teleportation of propagating microwaves (K. G. Fedorov et al., arxiv:2103.04155 (2021)):
    In this paper, the authors demonstrate the first experimental realization of continuous variable quantum teleportation of propagating quantum microwave states. By achieving fidelities beyond the no-cloning threshold, perfect security of the protocol is proven. Furthermore, a detailed theory to describe the Bell measurement as well as a numerical model which successfully describes the experimental results, are provided.
  4. High-accuracy Ising machine using Kerr-nonlinear parametric oscillators with local four-body interactions (T. Kanao and H. Goto, npj quantum information 7, 18 (2021)):
    A two-dimensional array of Kerr-nonlinear parametric oscillators (KPOs) with local four-body interactions is a promising candidate for realizing an Ising machine with all-to-all spin couplings, based on adiabatic quantum computation in the Lechner–Hauke–Zoller (LHZ) scheme. However, it has been unclear whether such an Ising machine works in general cases with asymmetric networks. They find that the asymmetry in the four-body interactions causes inhomogeneity in photon numbers and hence degrades the performance. They then propose a method for reducing the inhomogeneity, where the discrepancies of the photon numbers are corrected by tuning the detunings of KPOs depending on their positions, without monitoring their states during adiabatic time evolution.
  5. Cavity electromechanics with parametric mechanical driving (D. Bothner et al., Nat. Comm., 11 1589 (2020)):
    The authors investigate an electro-mechanical hybrid system consisting of a superconducting microwave resonator with a mechanically modulated capacitance based on a nanomechanical string. Using a parametric drive of the mechanical oscillator, squeezing of the mechanical thermal noise as well as amplification of the microwave transmission are demonstrated.
  6. Compiling quantum algorithms for architectures with multi-qubit gates ()E. A Martinez, New J. Phys. 18 063029 (2016):
    In recent years, small-scale quantum information processors have been realized in multiple physical architectures. These systems provide a universal set of gates that allow one to implement any given unitary operation. The decomposition of a particular algorithm into a sequence of these available gates is not unique. Thus, the fidelity of the implementation of an algorithm can be increased by choosing an optimized decomposition into available gates. Here, we present a method to find such a decomposition, where a small-scale ion trap quantum information processor is used as an example. We demonstrate a numerical optimization protocol that minimizes the number of required multi-qubit entangling gates by design. Furthermore, we adapt the method for state preparation, and quantum algorithms including in-sequence measurements.
  7. Characterizing and Optimizing Qubit Coherence Based on SQUID Geometry (J. Braumüller et al., Phys. Rev. Applied 13, 054079 (2020)):
    The dominant source of decoherence in contemporary frequency-tunable superconducting qubits is 1/f flux noise. To understand its origin and find ways to minimize its impact, they systematically studied flux noise amplitudes in more than 50 flux qubits with varied SQUID geometry parameters and compared their results to a microscopic model of magnetic spin defects located at the interfaces surrounding the SQUID loops. Their results and detailed model provide a guide for minimizing the flux noise susceptibility in future flux-tunable circuits
  8. Two-level systems in superconducting quantum devices due to trapped quasiparticles (S. E. de Graaf, Science Advances 6, eabc5055 (2020)):
    A major challenge in superconducting quantum circuits is the interaction with interfacial two level system (TLS) defects which lead to qubit parameter fluctuations and relaxations. Another challenge is from the nonequilibrium quasiparticles that result in qubit relaxation and dephasing. In this paper, the authors explore decoherence arising from a new type of TLS originating from trapped quasiparticles which can induce qubit relaxation. Using spectral, temporal, thermal, and magnetic mapping of TLS induced fluctuations in resonators, they identify a coherent subset of TLS population that persists to low temperatures (~300 mK) with a nonuniform density of states. An understanding of these properties is discussed from TLS formed by QPs trapped in shallow subgap states formed by the spatial fluctuations of the superconducting order parameter. This implies that even rare QP burts will afftect coherence over exponentially long time scales.
  9. Continuous-variable entanglement distillation by cascaded photon replacement (Y. Mardani, Phys. Rev. A 102, 012407 (2020)):
    In this paper, the authors investigate an entanglement distillation process in continuous-variable systems using photon replacement protocols on a two-mode squeezed vacuum state. Entanglement distillation is an important resource for quantum communication, providing enhanced security and extended communication range. Here, the authors show that by cascading photon replacement operation, entanglement is increased with the number of repetitions. Additionally, under suitable experimental parameters, this cascaded photon replacement protocol outperforms the well-known photon addition and photon subtraction entanglement distillation protocols. Finally, the results remains valid for the case of imperfect detectors under the assumption of high efficiency. Detailed calculations of entanglement of formation calculation are provided.
  10. Deterministic multi-qubit entanglement in a quantum network (Y. Zhong, Nature 590, 571(2021)):
    In this paper, the authors investigate distributed multi-qubit entanglement for quantum communication and computational networks. They report a quantum network comprising two superconducting quantum nodes connected by a one-metre-long superconducting coaxial cable, where each node includes three interconnected qubits. By directly connecting the cable to one qubit in each node, quantum states are transferred between the nodes with a process fidelity of > 0.9. Further, a globally distributed two-node, six-qubit GHZ state with a state fidelity of > 0.7 is generated. The fidelities are above the threshold of 0.5 for genuine multipartite entanglement, indicating that their architecture can be used to coherently link together multiple superconducting quantum processors.
  11. Demonstration of two-qubit algorithms with a superconducting quantum processor (L. DiCarlo, Nature 460, 240(2009)):
    In this paper the authors demonstrate a two-qubit superconducting processor and the implementation of the Grover search and Deutsch–Jozsa quantum algorithms. They use a two-qubit interaction, tunable in strength by two orders of magnitude on nanosecond timescales, which is mediated by a cavity bus in a circuit quantum electrodynamics architecture. This interaction allows the generation of highly entangled states with concurrence up to 94 per cent. Although this processor constitutes an important step in quantum computing with integrated circuits, continuing efforts to increase qubit coherence times, gate performance and register size will be required to fulfil the promise of a scalable technology.
  12. Simple pulses for elimination of leakage in weakly nonlinear qubits (F. Motzoi, Phys. Rev. Lett. 103, 110501 (2009)):
    In realizations of quantum computing, a two-level system (qubit) is often singled out from the many levels of an anharmonic oscillator. In these cases, simple qubit control fails on short time scales because of coupling to leakage levels. We provide an easy to implement analytic formula that inhibits this leakage from any single-control analog or pixelated pulse. It is based on adding a second control that is proportional to the time derivative of the first. For realistic parameters of superconducting qubits, this strategy reduces the error by an order of magnitude relative to the state of the art, all based on smooth and feasible pulse shapes. These results show that even weak anharmonicity is sufficient and in general not a limiting factor for implementing quantum gates.

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