Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S26: Superconducting Circuits: Fluxonium and Superinductance Devices 
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Sponsoring Units: DQI Chair: Hanhee Paik, IBM Thomas J. Watson Research Center Room: BCEC 160B 
Thursday, March 7, 2019 11:15AM  11:27AM 
S26.00001: Cavityless circuit quantum electrodynamics of a fluxonium artificial atom – Design Haonan Xiong, YenHsiang Lin, Nathanael Cottet, Long Nguyen, Ray Mencia, Aaron Somoroff, Vladimir Manucharyan In circuit quantum electrodynamics (cQED), a highquality factor cavity plays a central role: it protects a qubit from spontaneous emission and acts as a buffer system enabling a quantum nondemolition (QND) dispersive readout of the qubit state. Here we experiment with a fluxonium artificial atom capacitively connected to a 1D transmission line. We simultaneously achieved a strong coupling of a highfrequency “cycling” transition (0→3 or 1→2) to the traveling waves and a complete suppression of the spontaneous emission of the lowfrequency qubit transition (0→1). This allowed implementing the fluorescence “shelving” readout of a highlycoherent and fully controllable qubit. Unlike in conventional cQED, here the cycling dynamics during the readout is confined to a small Hilbert space and can be understood within a simple optical pumping model. Our system realizes a hardwareefficient interface between qubits and photons and hence can be useful in constructing quantum networks. It can also help understanding the processes leading to the loss of QNDness in cQED at relatively low photon numbers. 
Thursday, March 7, 2019 11:27AM  11:39AM 
S26.00002: Cavityless circuit quantum electrodynamics of a fluxonium artificial atom  Experiment YenHsiang Lin, Haonan Xiong, Nathanael Cottet, Long Nguyen, Ray Mencia, Aaron Somoroff, Vladimir Manucharyan In circuit quantum electrodynamics (cQED), a highquality factor cavity plays a central role: it protects a qubit from spontaneous emission and acts as a buffer system enabling a quantum nondemolition (QND) dispersive readout of the qubit state. Here we experiment with a fluxonium artificial atom capacitively connected to a 1D transmission line. We simultaneously achieved a strong coupling of a highfrequency “cycling” transition (0→3 or 1→2) to the traveling waves and a complete suppression of the spontaneous emission of the lowfrequency qubit transition (0→1). This allowed implementing the fluorescence “shelving” readout of a highlycoherent and fully controllable qubit. Unlike in conventional cQED, here the cycling dynamics during the readout is confined to a small Hilbert space and can be understood within a simple optical pumping model. Our system realizes a hardwareefficient interface between qubits and photons and hence can be useful in constructing quantum networks. It can also help understanding the processes leading to the loss of QNDness in cQED at relatively low photon numbers. 
Thursday, March 7, 2019 11:39AM  11:51AM 
S26.00003: SingleShot Readout of Fluxonium Qubits. Konstantin Nesterov, Ivan Pechenezhskiy, Long Nguyen, YenHsiang Lin, Aaron Somoroff, Ray Mencia, Vladimir Manucharyan, Maxim Vavilov We discuss the possibility of the singleshot readout of fluxonium superconducting qubits [1] based on the preparation of the "bright" and "dark" cavity states. With a proper choice of parameters, the dispersive shift of the cavity (the distance between its dressed and bare resonance frequencies) for the qubit 0 state can be made much smaller than this shift for the qubit 1 state. Thus, the cavity nonlinearity can be made much weaker for the qubit 0 than for the qubit 1 states. This has a potential to improve two types of singleshot qubit readout schemes that are used in the transmon readout, when the cavity nonlinearity is comparable for the two transmon states. First, for the readout based on microwave photon counters [2], one can achieve a higher cavity occupation in its bright state. Second, for the highpower readout [3], the onset of the bright transmission for one of the qubit states can occur at lower power. 
Thursday, March 7, 2019 11:51AM  12:03PM 
S26.00004: Nanowire Superinductance Fluxonium Qubit Thomas Hazard, Andras Gyenis, Agustin Di Paolo, Abraham Asfaw, Alexandre Blais, Stephen Aplin Lyon, Andrew Houck We characterize a fluxonium qubit consisting of a Josephson junction inductively shunted with a NbTiN nanowire superinductance. We explain the measured energy spectrum by means of a multimode theory accounting for the distributed nature of the superinductance and the effect of the circuit nonlinearity to all orders in the Josephson potential. Using multiphoton Raman spectroscopy, we address multiple fluxonium transitions, observe multilevel AutlerTownes splitting and measure an excited state lifetime of T_{1} = 20 μs. By measuring T_{1} at different magnetic flux values, we find a crossover in the lifetime limiting mechanism from capacitive to inductive losses. 
Thursday, March 7, 2019 12:03PM  12:15PM 
S26.00005: Fast qubit reset of a low frequency fluxonium circuit Nathanael Cottet, Jeremy Stevens, Benjamin Huard The improvement of the lifetime and coherence time of superconducting circuits is a critical step towards the development of quantum processing of information. Recent results demonstrated coherence times and lifetimes above 100 microseconds on the first transition of fluxonium circuits biased at half flux quantum.These timescales can be improved tenfold by decreasing the transition frequencies below 500 MHz, without compromising on the gate time. The drawback is that the qubit is thermally excited at thermal equilibrium in a dilution refrigerator. Fast and efficient initialization processes are therefore crucial to follow that road. 
Thursday, March 7, 2019 12:15PM  12:27PM 
S26.00006: Towards a fluxoniumbased quantum processor I: noninteracting qubits Aaron Somoroff, Long Nguyen, YenHsiang Lin, Ray Mencia, Ivan Pechenezhskiy, Konstantin Nesterov, Maxim Vavilov, Vladimir Manucharyan We describe our progress in experimentally realizing a microwaveactivated twoqubit gate with capacitively coupled fluxonium qubits. When biased at the flux sweetspot, the individual qubits have frequencies around 500 MHz and reproducibly reach long coherence times in excess of 100 us (the best device had T2 > 300 us) [1]. A cPhase gate can be achieved by sending a short 2πpulse at the frequency near the 1→2 transition of the target qubit [2]. Our work includes characterization of coherence and parameter fluctuations in multiqubit chips, modeling and experimentally validating the twoqubit interactions, optimizing the joint readout, and benchmarking of the gate operations. 
Thursday, March 7, 2019 12:27PM  12:39PM 
S26.00007: Towards a fluxoniumbased quantum processor II: interacting qubits Long Nguyen, Aaron Somoroff, YenHsiang Lin, Ray Mencia, Ivan Pechenezhskiy, Konstantin Nesterov, Maxim Vavilov, Vladimir Manucharyan

Thursday, March 7, 2019 12:39PM  12:51PM 
S26.00008: Coherence of a granular aluminum fluxonium qubit Martin Spiecker, Lukas Gruenhaupt, Daria Gusenkova, Nataliya Maleeva, Sebastian T. Skacel, Ivan Takmakov, Francesco Valenti, Patrick Winkel, Hannes Rotzinger, Alexey Ustinov, IoanMihai Pop A promising alternative for the implementation of superinductors, compared to the predominantly used mesoscopic Josephson junction arrays, is granular aluminum (grAl), with a microstructure consisting of pure aluminum grains embedded in an AlOx matrix, effectively forming a selfassembled Josephson junction network [1]. This material offers a large kinetic inductance, while its nonlinearity is orders of magnitude smaller than that of Josephson junction arrays [2]. We present a fluxonium qubit employing a granular aluminum superinductor with coherence times T_{1} up to 23 μs and T_{2}^{R} up to 30 μs at the flux bias sweet spot. The measured T_{2}^{Echo} approaches the limit 2 × T_{1 }[3]. These coherence times recommend granular aluminum for increasingly complex protected superconducting quantum circuits, while they also evidence the need to further investigate and mitigate loss mechanisms in high impedance qubits. 
Thursday, March 7, 2019 12:51PM  1:03PM 
S26.00009: Design and fabrication of a granular aluminum fluxonium qubit Lukas Gruenhaupt, Martin Spiecker, Daria Gusenkova, Nataliya Maleeva, Sebastian T. Skacel, Ivan Takmakov, Francesco Valenti, Patrick Winkel, Hannes Rotzinger, Alexey Ustinov, IoanMihai Pop Superconducting materials with low microwave losses and high kinetic inductance are a valuable resource in quantum circuit design, enabling the design of socalled superinductors, which can provide electromagnetic environments with characteristic impedance larger than the resistance quantum R_{Q }= 6.5 kΩ. To implement superinductors, a promising alternative to the predominantly used mesoscopic Josephson junction arrays is granular aluminum (grAl) [1]. Its microstructure consists of pure aluminum grains embedded in an AlOx matrix, effectively forming a compact selfassembled Josephson junction network [2]. We present a superconducting fluxonium qubit employing a superinductor with impedance Z > R_{Q}, fabricated from a grAl thin film, insitu integrated with a conventional Al/AlOx/Al Josephson junction. The measured qubit spectrum is in good agreement with the fluxonium Hamiltonian [3]. 
Thursday, March 7, 2019 1:03PM  1:15PM 
S26.00010: CircuitQED Studies of Josephson Junction Arrays in the Quantum Regime Hiroki Ikegami, Cosmic Raj, Yasunobu Nakamura Josephson junction arrays (JJAs) in superconducting circuits offer model systems for studying various manybody phenomena both in the classical and quantum regimes. One of the remarkable phenomena observed in JJAs is the quantum phase transition between superconducting and insulating phases associated with the competition between the Josephson energy and the charging energy. In our previous study, we used a circuitQED technique to study dynamics of the superconductormetal transition of a classical JJA at a single photon level and found that the internal loss of the cavity shows a peak at the transition temperature. Here we use the experimental technique to explore JJAs in the quantum regime. We find that the temperature at which a peak appears in the cavity loss decreases when the charging energy becomes more dominant than the Josephson energy, i.e., the quantum effect becomes more significant. In the talk, we will discuss the result in connection with the quantum phase transition between the superconducting and insulating phases. 
Thursday, March 7, 2019 1:15PM  1:27PM 
S26.00011: Implementation of πperiodic Josephson Elements for Topologically Protected ChargeParity Qubits Yebin Liu, Kenneth Dodge, Michael Anthony Senatore, Shaojiang Zhu, FNU Naveen, Abigail J Shearrow, Francisco Schlenker, Andrey Klots, Lara Faoro, Lev B Ioffe, Robert F McDermott, B.L.T. Plourde Superconducting qubits with topological protection against local noise hold the promise of significantly enhanced coherence times and higher gate fidelities than is possible with conventional qubits. We are developing one such protected design — the hybrid chargeparity qubit that combines arrays of high kinetic inductance nanowires and conventional Josephson junctions and involves the individual flux control in each plaquette. For appropriate values of the Josephson energy and charging energy of the junctions and the inductive energy of the nanowires, the arrays can be tuned between a periodicity of 2π and π by varying the external magnetic flux through the array. We describe the fabrication of these arrays and experiments to characterize the periodicity with phase by incorporating the array into an rf SQUID. 
Thursday, March 7, 2019 1:27PM  1:39PM 
S26.00012: Granular aluminum: A source of nonlinearity for superconducting quantum circuits Patrick Winkel, Dennis Rieger, Lukas Gruenhaupt, Kiril Borisov, Nataliya Maleeva, Martin Spiecker, Alexey V. Ustinov, Wolfgang Wernsdorfer, IoanMihai Pop Superconducting granular aluminum (grAl) has already proven its applicability as linear inductor in kinetic inductance detectors and Fluxonium qubit designs [1]. Evaporated in an oxygen atmosphere, aluminum selfassembles into crystalline grains separated by amorphous aluminum oxide, resulting in highly inductive and lowloss superconducting grAl films [2]. We model the cQED properties of grAl microwave resonators using an effective array of Josephson junctions, and obtain selfKerr coefficients that are inversely proportional to the grAl volume and the critical current density [3]. By shunting a small grAl volume with a thin film aluminum capacitor, we enhance the selfKerr nonlinearity of the resulting LC mode, K_{11}, up to values much larger than the spectral linewidth of the fundamental mode, κ, with K_{11} ≈ 100×κ. By driving the resonator with increasingly larger power, we observe up to 30 multiphoton transitions between the levels of the fundamental mode, from which we extract a value of K_{11} in the MHz range. 
Thursday, March 7, 2019 1:39PM  1:51PM 
S26.00013: Phase Transitions and Edge States in Fluxonium Qubit Systems A. Baris Ozguler, Vladimir Manucharyan, Mark Dykman, Maxim Vavilov A chain of fluxonium qubits provides the means for simulating quantum manybody phenomena in spin1/2 magnets. The available controls allow us to map a qubit chain on an Ising chain in a transverse magnetic field with variable parameters. The role of the transverse field is played by the tunnelinduced splitting between the lowest energy states at the halfflux sweet spot [1]. The interaction comes from the inductive qubit coupling between fluxoniums' superinductors and can exceed the level splitting [2]. The magnetic flux detuning from the sweet spot plays the role of the longitudinal field for an Ising spin. In this talk, we discuss the phase diagram of the fluxonium chain. We demonstrate the quantum phase transition with the varying level splitting and the emergence of the edge states. We compare these states to the Majorana states in a fermion chain. We study how the edge state localization length depends on the disorder in the random longitudinal and transverse fields. 
Thursday, March 7, 2019 1:51PM  2:03PM 
S26.00014: Intrinsically Error Protected Superconducting Architecture Based on Superinductance Andras Gyenis, Thomas Hazard, Agustin Di Paolo, Andrei Vrajitoarea, Alexandre Blais, Jens Koch, Andrew Houck Significant effort has been recently devoted to develop qubits with hardwarelevel protection, where the disjoint nature of the qubit wavefunctions offers protection against various relaxation mechanisms. Among the superconducting architectures, the socalled 0π qubit [PRA 87, 052306 (2013)] is a promising candidate for realizing such a system. Here, we introduce the soft0π qubit: a twist on the original 0π qubit proposal that relaxes some of the constraints on the qubit design parameters. In this talk, we present spectroscopic and timedomain measurements on this device. Our approach exploits an exponentially small overlap between the qubit logical wave functions and flux sweet spots to render the soft0π qubit noiseprotected. 
Thursday, March 7, 2019 2:03PM  2:15PM 
S26.00015: Characterizing Granular Aluminum in Superconducting Circuits Alexander Place, Thomas Hazard, Andras Gyenis, Andrew Houck High kinetic inductance materials are promising candidates for implementing large inductances which are crucial for several proposed qubit designs. These materials also allow for a distributed nonlinear element, opening the door for a new family of qubits. Here, we present a new method to deposit granular aluminum in addition to spectroscopic and time domain measurements of granular aluminumbased superconducting qubits. 
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