Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T41: Novel Superconducting Qubits IIFocus Recordings Available

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Sponsoring Units: DQI Chair: Setiawan Wenming, University of Chicago Room: McCormick Place W196C 
Thursday, March 17, 2022 11:30AM  11:42AM 
T41.00001: Improved Readout with Active Reset in a Heavy Fluxonium Circuit Chunyang Ding, Helin Zhang, Daniel K Weiss, Jens Koch, David Schuster

Thursday, March 17, 2022 11:42AM  11:54AM 
T41.00002: Heaviness dependent characterization of coherence properties of Fluxnoium Qubit Parth K Jatakia, Anjali Premkumar, Xanthe Croot, Sara F Sussman, Andrew A Houck

Thursday, March 17, 2022 11:54AM  12:06PM 
T41.00003: Progress Towards a Protected Qubit Subspace within a Fluxonium Molecule Xanthe Croot, Xinyuan You, Anjali Premkumar, Jens Koch, Andrew A Houck 
Thursday, March 17, 2022 12:06PM  12:42PM 
T41.00004: Fast control of lowfrequency fluxonium qubits Invited Speaker: Helin Zhang The fluxonium qubit is a promising superconducting qubit for high fidelity quantum computing systems, due to its long coherence times. Fluxonium qubits operate at lower frequencies than typical superconducting qubits, yet have larger anharmonicities. We adapt the methods of readout, initialization, control, and two qubit logic to this unique regime. In this talk, we present a design of two inductively coupled fluxonium qubits with a tunable coupler that can realize 060 MHz coupling rate in a 2D structure. We demonstrate a set of control protocols for low frequency fluxonium qubits, including state initialization based on readout feedback, fast single and two qubit gates with flux pulses, and high fidelity single shot readout. With these control options, we can take full advantage of the large anharmonicity and long coherence times of fluxonium qubits, and demonstrate its potential for realizing large scale quantum processors. 
Thursday, March 17, 2022 12:42PM  12:54PM 
T41.00005: Highfidelity entangling gates for fluxonium qubits via flux modulation of a tunable coupler Daniel K Weiss, Helin Zhang, Chunyang Ding, David Schuster, Jens Koch The enhancement of coherence times in fluxonium qubits renders them an interesting alternative to transmons. To achieve highfidelity twoqubit gates, here we propose connecting two fluxonia with a tunable coupler. Each fluxonium is galvanically linked to two coupler degrees of freedom, one that is harmonic and the other that is fluxonium like. We show that for this circuit, static ZZ coupling is suppressed and the desired XX coupling can be tuned through zero. We present flux pulses for a √iSWAP gate predicted to achieve fidelities as high as 0.9999 under realistic conditions. 
Thursday, March 17, 2022 12:54PM  1:06PM 
T41.00006: Fast Flux Entangling Gate for Fluxonium Circuits Yinqi Chen, Konstantin Nesterov, Vladimir Manucharyan, Maxim G Vavilov The superconducting fluxonium circuit with its strong anharmonicity and long coherence time is a promising candidate for qubit implementation in a quantum processor [1]. Experimentally realized twoqubit gates with fluxoniums are based on microwave irradiation leading to excitations of higher noncomputational levels [2, 3]. Alternatively, the full advantage of the long lifetime can be utilized by always staying in the computational subspace with possible twoqubit gate schemes based on the crossresonance effect [4] or on driving a twophoton transition [5]. Here we analyze a highly accurate twoqubit gate with fluxoniums using fast flux pulses, which can be realized by temporarily detuning magnetic flux through the fluxonium loop away from the half flux quantum sweet spot [6]. We evaluate the effect of the flux noise and qubit relaxation on the gate fidelity and demonstrate that the gate error remains below 10^{4}. 
Thursday, March 17, 2022 1:06PM  1:18PM 
T41.00007: Towards a 2D tantalum Kerrcat qubit Xuan Hoang Le, Sara F Sussman, Xanthe Croot, Andrew A Houck Increasing the coherence time of superconducting qubits will enable errorcorrected multiqubit processors that can perform meaningful computations. To improve coherence, advances in qubit encoding and qubit fabrication could independently contribute. The recent Kerrcat qubit, where the logical qubit is encoded in cat states of a capacitively shunted flux qubit with a 3D readout cavity [1], significantly reduces phaseflip error compared to the standard encoding scheme using Fock states. Separately, using tantalum results in record coherence times in conventional 2D transmon qubits [2,3]. Here we present preliminary work towards a 2D tantalum Kerrcat qubit, combining these two developments. 
Thursday, March 17, 2022 1:18PM  1:30PM 
T41.00008: The Quantromon – a qubitcavity system with orthogonal qubit and readout modes Kishor V Salunkhe, Nicolas Gheeraert, Meghan P Patankar, Rajamani Vijayaraghavan The multimodal circuit nicknamed Quantromon has two modes: a transmon qubit and a linear LC oscillator coupled to each other via crossKerr coupling. An integrated qubitcavity system is realized using these modes with the linear LC oscillator playing the role of the readout cavity. We previously demonstrated a high measurement fidelity of 97.63% without using the Josephson parametric amplifier due to the possibility of using higher photon numbers in the Quantromon. Another unique feature is the inherent orthogonality of the qubit and readout mode which allows strong qubitreadout coupling without being limited by Purcell decay. This also enables the qubit and readout frequencies to be independent of each other, unlike the standard dipolar coupling scenario. We study this effect by designing a Quantromon device with multiple SQUID loops to tune the qubit frequency over a large range with only small changes to the readout frequency. We also discuss the possibility of using such designs for enabling improved readout multiplexing capabilities in a multiqubit architecture. 
Thursday, March 17, 2022 1:30PM  1:42PM 
T41.00009: A YuShibaRusinov qubit Archana Mishra, Pascal Simon, Timo Hyart, Mircea Trif Magnetic impurities in swave superconductors lead to spinpolarized YuShibaRusinov (YSR) ingap states. Chains of magnetic impurities offer one of the most viable routes for the realization of Majorana bound states which hold a promise for topological quantum computing. However, this ambitious goal looks distant since no quantum coherent degrees of freedom have yet been identified in these systems. To fill this gap we propose an effective twolevel system, a YSR qubit, stemming from two nearby impurities. Using a timedependent wavefunction approach, we derive an effective Hamiltonian describing the YSR qubit evolution as a function of distance between the impurity spins, their relative orientations, and their dynamics. We show that the YSR qubit can be controlled and read out using stateoftheart experimental techniques for manipulation of the spins. Finally, we address the effect of spin noise on the coherence properties of the YSR qubit, and show a robust behavior for a wide range of experimentally relevant parameters. Looking forward, the YSR qubit could facilitate the implementation of a universal set of quantum gates in hybrid systems where they are coupled to topological Majorana qubits. 
Thursday, March 17, 2022 1:42PM  1:54PM 
T41.00010: Unimon: an islandfree superconducting qubit (Part 1/2) Eric Hyyppä, Suman Kundu, Juha Hassel, Alessandro Landra, Wei Liu, Jani Tuorila, Caspar F OckeloenKorppi, Akseli Mäkinen, Mario Palma, Tianyi Li, Brian Tarasinski, Roope Kokkoniemi, Fabian Marxer, András Gunyhó, Juho Hotari, Joni Ikonen, Jeffrey Chan, JeanLuc Orgiazzi, Johannes Heinsoo, Kuan Y Tan, Mikko Möttönen In the design of superconducting qubits, one strives for a circuit that is insensitive to common sources of decoherence while retaining a high anharmonicity and a high controllability. We propose a novel superconducting qubit that is insensitive to lowfrequency charge noise thanks to the islandfree structure of the circuit. The proposed qubit, nicknamed as the unimon, is constructed of a grounded coplanar waveguide resonator incorporating a single embedded Josephson junction in its center conductor. In addition to being insensitive to charge noise, the unimon qubit has a fluxinsensitive sweet spot and a pair of gradiometric loops that provide protection against magnetic flux noise. At the sweet spot, the unimon qubit has an anharmonicity that is positive and a few times higher than in the conventionally used transmon qubits. We present a theoretical treatment of the circuit and first experimental results demonstrating that it is possible to implement fast and highfidelity singlequbit gates using the unimon qubit. 
Thursday, March 17, 2022 1:54PM  2:06PM 
T41.00011: Unimon: an islandfree superconducting qubit (Part 2/2) Suman Kundu, Eric Hyyppä, Juha Hassel, Alessandro Landra, Wei Liu, Jani Tuorila, Caspar Ockeloen Korppi, Akseli Mäkinen, Mario Palma, Tianyi Li, Brian Tarasinski, Roope Kokkoniemi, Fabian Marxer, András Gunyhó, Juho Hotari, Joni Ikonen, Jeffrey Chan, JeanLuc Orgiazzi, Johannes Heinsoo, Kuan Y Tan, Mikko Möttönen In the design of superconducting qubits, one strives for a circuit that is insensitive to common sources of decoherence while retaining a high anharmonicity and a high controllability. We propose a novel superconducting qubit that is insensitive to lowfrequency charge noise thanks to the islandfree structure of the circuit. The proposed qubit, nicknamed as the unimon, is constructed of a grounded coplanar waveguide resonator incorporating a single embedded Josephson junction in its center conductor. In addition to being insensitive to charge noise, the unimon qubit has a fluxinsensitive sweet spot and a pair of gradiometric loops that provide protection against magnetic flux noise. At the sweet spot, the unimon qubit has an anharmonicity that is positive and a few times higher than in the conventionally used transmon qubits. We present a theoretical treatment of the circuit and first experimental results demonstrating that it is possible to implement fast and highfidelity singlequbit gates using the unimon qubit. 
Thursday, March 17, 2022 2:06PM  2:18PM 
T41.00012: Nonlocal dynamics in a superconducting circuit Clarke Smith, Alvise Borgognoni, Camille Berdou, Marius Villiers, Aron Vanselow, Natalia Pankratova, José Palomo, Aurélie Pierret, Takis Kontos, Mazyar Mirrahimi, Benoit Douçot, Philippe CampagneIbarcq, Zaki Leghtas Quantum harmonic oscillators are prized in the field of superconducting qubits for their long lifetimes, which make them attractive for encoding quantum information in multiphoton states. These bosonic encoded qubits rely on residual small nonlinearities for state preparation, manipulation, and readout. For example, an oscillator coupled to a transmon qubit inherits a small twophoton interaction Hamiltonian, enabling the creation of nonclassical states of light [1], but are there others? We present the design and experimental status of an alternative species of weaklyanharmonic oscillator where the nonlinearity comprises large displacements in phase space. Physically, this oscillator is made by combining extremely small Josephson energies and extremely large quantum phase fluctuations. Such oscillators could prove essential for error syndrome measurements in concatenated cat codes. 
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