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.

The seminar takes place in a hybrid mode. Both in-presence participation (3G rules, QRONITON) and online participation are possible.

List of open topics for seminar talks in WS 2021/22:

1. Laser-annealing Josephson Junctions for yielding scaled-up (J. B. Hertsberg et al., npj Quantum Information 7, 129 (2021)):
   To enable the fabrication of lage quantum processors frequency crowding is one of the main probles that need to be solved. In this paper the laser annealing approach is chosen and shows promising results.
2. Surpassing the Resistance Quantum with a Geometric Superinductor (M. Peruzzo et al., Phys. Rev. Applied 14, 044055 (2020)):
   A large inductor is reported which has been regarded impossible in the literature.
3. Superconducting qubit to optical photon transduction (Mohammad Mirhosseini et al., Nature 588, 599 (2020)):
   The efficient transduction from microwave to optical frequencies while keeping the quantum properties of light is still an outstanding problem in quantum information processing. In this article, the authors experimentally demonstrate such a transduction with an efficiency of 10^-5.
4. Enhancing quantum annealing performance by a degenerate twolevel system (Shohei Watabe et al., Scientific Reports 10, 146 (2020)):
   Quantum annealing is an innovative idea and method for avoiding the increase of the calculation cost of the combinatorial optimization problem. However, the conventional quantum annealing machine may not have a high success probability for fnding the solution because the energy gap closes exponentially as a function of the system size. Here they show that a degenerate two-level system provides the higher success probability than the conventional spin-1/2 model in a weak longitudinal magnetic feld region.
5. Direct observation of deterministic macroscopic entanglement (S. Kotler et al., Science 372, 622 (2021)):
   The laws of quantum mechanics presumably apply to objects of all sizes. However, observing quantum mechanical effects becomes increasingly hard as masses increase, requiring the experimentalists to keep measurement and control errors vanishingly small. In this work, the authors demonstrate generation and observation of quantum entanglement between two mechanical resonators with comparatively large masses of several tens of picograms.
6. Digital-analog quantum computation (Adrian Parra-Rodriguez et al., Phys. Rev. A 101, 022305 (2020)):
   Digitial single qubit operations and Analog blocks of multi-qubit entangling operations to simulate Hamiltonians of interest.
7. Moving beyond the Transmon: Noise-Protected Superconducting Quantum Circuits (András Gyenis et al., PRX Quantum 2, 030101 (2021)):
   The noise-protected devices constitute a new class of qubits in which the computational states are largely decoupled from local noise channels. The main challenges in engineering such systems are simultaneously guarding against both bit- and phase-flip errors, and also ensuring high-fidelity qubit control. The complete protection can only be fulfilled by implementing multimode or hybrid circuits. This paper reviews the theoretical principles of these new qubits, describes recent experiments.
8. Microwave Package Design for Superconducting Quantum Processors (Sihao Huang et al., PRX Quantum 2, 020306 (2021)):
   Describe microwave package design guidelines that can support qubit lifetimes up 350 µs. The guidelines are validated through simulations and measurements. The approach encompasses material choices, signal line engineering, and spurious mode suppression.
9. Experimental demonstration of entanglement-enabled universal quantum cloning in a circuit (Zhen-Biao Yang et al., npj Quantum Information 7, 44 (2021) ):
   Due to the impossibility of producing perfect clones of quantum states, much interest has been given to finding optimal quantum cloning machines. In this paper, the authors present an experimental implementation of a superconducting Xmon qubits based circuit, which serves as a universal quantum-cloning machine. In their experiment, they show to be able to approach the optimal cloning fidelity of 5/6. Furthermore, the measured entanglement between original qubits and their respective copies demonstrates the cloning process to be input state independent while highlighting its universal quantum behavior.
10. Implementation of a canonical phase measurement with quantum feedback (Leigh S. Martin et al., Nature Physics 16, 1046 (2020)):
    In this paper, the authors present a communication system base on a superconducting transmon qubit and a Josephson parametric amplifier (JPA). This system allows them to estimate the phase of a single-microwave-photon signal by continuously adapting the measurement basis. To this end, the JPA is used to adjust, in a feedback loop, the measurement axis after each photon detection. This method leads to an improved phase sensibility, reducing the measurement error by 15% compared to a standard heterodyne detection. Additional, they report a quantum detection efficiency of 0.4 and a JPA gain of 6 dB, both of which could be optimized for better results.
11. Improving qubit coherence using closed-loop feedback (Antti Vepsäläinen et al., arxiv:2105:01107 (2021)):
    Supercondunting qubit gate fidelities are nowadays in many cases limited by the coherence times of the qubits. In this paper the authors experimentally employ closed-loop feedback to stabilize the frequency fluctuations of a superconducting transmon qubit, thereby increasing its coherence time by 26% and reducing the single-qubit error rate from (8.5±2.1)×10−4 to (5.9±0.7)×10−4. Their approach is simple and elegant and leads to very interesting results where the best fidelities are obtained away form the flux sweetspot.
12. Quantum Information Scrambling on a Superconducting Qutrit Processor (M. S. Blok et al., Phys. Rev. X 11, 021010 (2021)):
    To date, verified experimental implementations of scrambling have dealt only with systems comprised of two-level qubits. Higher-dimensional quantum systems, however, may exhibit different scrambling modalities and are predicted to saturate conjectured speed limits on the rate of quantum information scrambling. The authors take the first steps toward accessing such phenomena, by realizing a quantum processor based on superconducting qutrits (three-level quantum systems). Their teleportation algorithm, which connects to recent proposals for studying traversable wormholes in the laboratory, demonstrates how quantum information processing technology based on higher dimensional systems can exploit a larger and more connected state space to achieve the resource efficient encoding of complex quantum circuits.
13. Spin-Resonance Linewidths of Bismuth Donors in Silicon Coupled to Planar Microresonators (James O’Sullivan et al., Phys. Rev. Applied 14, 064050 (2020)):
    Ensembles of bismuth-donor spins in silicon are promising storage elements for microwave quantum memories (QM) due to their long coherence times exceeding seconds. For achieving critical coupling between the spin ensemble and a suitable high-quality factor resonator, a thorough understanding of the line shapes for the relevant spin-resonance transitions is required. Via pulsed ESR, spin transitions across a range of frequencies and fields are studied. Based on the findings a route to achieve sufficiently strong coupling, as required for a quantum memory is discussed.
14. A reversed Kerr traveling wave parametric amplifier (Arpit Ranadive et al., arxiv:2101.05815 (2021)):
    Controlling non-linear coefficients with the external magnetic flux allows to achieve 4WM with avoiding the presence of gaps in transmission, reducing gain ripples, and allowing in situ tunability of the amplification band over an unprecedented wide range