Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L33: Fluxonium and Flux Tunable QubitsFocus

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Sponsoring Units: DQI Chair: Michel Devoret, Yale Univ Room: LACC 408B 
Wednesday, March 7, 2018 11:15AM  11:51AM 
L33.00001: TBA Invited Speaker: Manucharyan Vladimir TBA 
Wednesday, March 7, 2018 11:51AM  12:03PM 
L33.00002: Capacitively shunted flux qubits embeded in a 3D cavity Shiro Saito, Imran Mahboob, Hiraku Toida, Yuichiro Matsuzaki, Kosuke Kakuyanagi, William Munro, Yasunobu Nakamura, Hiroshi Yamaguchi Signigicant worldwide efforts have been undertaken to improve the coherence time of superconducting qubits. Recently a 3D cavity architecture has been developed to drastically elongate the coherence time of transmon qubits to 92 μs [1]. On the other hand, flux qubits were also improved using high quality shunt capacitors [2]. A coherence time of 85 μs has been oberved. In this case the Cshunt flux qubit was coupled to a 2D coplanar waveguide resonator to perform a dispersive qubit readout. A 3D cavity is expected to provide a cleaner electromagnetic environment for qubits compared to the typical coplanar resonator. Therefore we put a Cshunt flux qubit into a 3D cavity to improve its coherence. We have fabricated a Cshunt flux qubit on a sapphire substrate. The third junction of the qubit is 0.45 times smaller than the other two. From spectroscopy measurements, we obtain energy of a typical junction E_{J}/h = 192 GHz, charging energy E_{C}/h = 2 GHz, and a shunt capacitor C_{S} = 50 fF. These are close to the designed values. In this talk I will discuss about coherence properties of the Cshunt flux qubit. 
Wednesday, March 7, 2018 12:03PM  12:15PM 
L33.00003: Superconducting circuit with chargeparity protection: Theory Xu Xiao, Clarke Smith, Angela Kou, Ioannis Tsioutsios, Uri Vool, Jayameenakshi Venkatraman, Kyle Serniak, Shyam Shankar, Michel Devoret Superconducting qubits that encode quantum information over multiple local degrees of freedom should exhibit improved coherence times (protected qubits). However, the limitation of current fabrication techniques hinders their experimental realization. We propose an experimentally accessible prototype of a protected qubit. It resembles a transmon qubit, with its Josephson junction replaced by an element that exclusively permits tunneling of pairs of Cooper pairs. This renders the lowest two energy eigenstates nearly degenerate, owing to the conservation of Cooper pair number parity. Here, the operator that maps one ground state to the other can only be expressed as a linear combination of local degrees of freedom, providing a primitive form of protection. Numerical simulations of the energy spectrum, wavefunctions, and coherence times validate this analysis. 
Wednesday, March 7, 2018 12:15PM  12:27PM 
L33.00004: Superconducting circuit with chargeparity protection: Experiment Clarke Smith, Angela Kou, Xu Xiao, Ioannis Tsioutsios, Uri Vool, Jayameenakshi Venkatraman, Kyle Serniak, Shyam Shankar, Michel Devoret Highfidelity measurement and control of protected superconducting qubits constitutes a serious challenge for Hamiltonian quantum error correction. Such protected qubits benefit from their immunity to environmental fluctuations, but suffer from difficulties in implementing readout and manipulation. We propose two strategies for overcoming these difficulties: cascaded dispersive readout and indirect transitions. In our experiment, the realization of these strategies relies on the introduction of an ancillary mode containing a Superconducting Nonlinear Asymmetric Inductive eLement (SNAIL). The principles underlying these readout and control schemes as well as experimental evidence for their efficacy will be presented. 
Wednesday, March 7, 2018 12:27PM  12:39PM 
L33.00005: Integrating Flux Tunability in Superconducting 3D cQED Devices Parker Henry, Luke Burkhart, Wolfgang Pfaff, Christopher Axline, Nicholas Frattini, Kevin Chou, Jacob Blumoff, Luigi Frunzio, Michel Devoret, Robert Schoelkopf Circuit quantum electrodynamics (cQED) experiments have demonstrated excellent quantum control and coherence in both planar and 3D architectures. Long coherence times in cQED architectures have been achieved by enclosing onchip Josephson junctionbased circuit elements in protective superconducting cavities. Superconducting cavities provide an electromagnetic shield for coherent elements, as well as functioning as highly coherent memories themselves. Flux tunability is routinely used as an experimental tool in planar devices and in normal metal 3D cavities. However, the same superconducting shield that protects coherent circuit elements has made it difficult to implement flux tunability, due to the Meissner effect. In this work, we implement DC flux tunability in a superconducting coaxial transmission line architecture, while still retaining a high Q factor. We present an outlook for integrating this architecture with future 3D cQED experiments. 
Wednesday, March 7, 2018 12:39PM  12:51PM 
L33.00006: Highcoherence fluxonium qubit Long Nguyen, YenHsiang Lin, Nicholas Grabon, Raymond Mencia, Aaron Somoroff, Vladimir Manucharyan

Wednesday, March 7, 2018 12:51PM  1:03PM 
L33.00007: FluxInsensitive Fluxonium Qubit with FluxTunable Coupling Ivan Pechenezhskiy, Raymond Mencia, Long Nguyen, YenHsiang Lin, Vladimir Manucharyan With its intrinsic charge noise protection and demonstrated long coherence times, a fluxonium qubit is a promising building block for quantum information processing in superconducting systems. In a fluxonium, the qubit Josephson junction is shunted by a large linear inductance which, in practice, is realized using a chain of Josephson junctions. In this study, we explore the limit of the fluxonium qubit when the value of shunt inductance is pushed to higher values, from roughly one hundred nanohenry (for a typical fluxonium device) to more than a microhenry. Such an order of magnitude increase in shunting inductance makes the 01 qubit transition nearly fluxinsensitive, thereby substantially reducing the qubit dephasing due to flux noise. However, the transitions to and between higher, noncomputational qubit states still remain fluxsensitive yielding a direct way to control coupling between the qubits with flux as a tunning knob while also maintaining the qubit protection from flux noise. In this talk, we report our progress towards experimental realization of a fluxinsensitive fluxonium qubit. 
Wednesday, March 7, 2018 1:03PM  1:15PM 
L33.00008: Capacitively Shunted Flux Qubits and Asymmetric Transmons for MultiQubit Operations Jaseung Ku, Yebin Liu, Britton Plourde, Jared Hertzberg, Markus Brink, Jerry Chow Capacitively shunted flux qubits (CSFQs) and asymmetric transmons (ATs) are both fluxtunable qubits with their own merits. CSFQs with high coherence hold promise for improved single and twoqubit gate operations due to their relatively large and positive anharmonicity. Meanwhile, ATs with optimized asymmetry provide flexibility on frequency tuning without excessive penalties on qubit coherence. Pairing a CSFQ and AT on the same chip may provide a novel regime for twoqubit operations. We present measurements of the coherence of CSFQs and ATs, and also discuss the possibility of twoqubit crossresonance (CR) gates between them. 
Wednesday, March 7, 2018 1:15PM  1:27PM 
L33.00009: Coherent Revival of Ramsey Oscillations in the Fluxonium Qubit Coupled to a bath of Harmonic Oscillators Farshad Foroughi, Mattia Mantovani, Remy Dassonneville, Luca Planat, Javier Puertas, Sebastien Leger, Etienne Dumur, yuriy krupko, Wolfgang Belzig, cecile naud, Olivier Buisson, Nicolas Roch, Frank Hekking, Gianluca Rastelli, Wiebke Guichard We have realized different Fluxonium qubits in a 2D and 3D structure and find state of the art coherence and relaxation times. We observed an increase of the relaxation time both in 3D and 2D at the optimum point of the qubit, when dissipative quasiparticle tunneling is suppressed. 
Wednesday, March 7, 2018 1:27PM  1:39PM 
L33.00010: Raman Transitions in a Capacitively shunted Fluxonium Circuit Nathan Earnest, Srivatsan Chakram, Yao Lu, Nicholas Irons, Ravi Naik, Nelson Leung, Leonidas Ocola, David Czaplewski, Brian Baker, Walter Lawrence, Jens Koch, David Schuster Superconducting qubits are a promising technology for quantum information processing, but still require improvements in coherence times for fault tolerant quantum computation. One promising path to enhance lifetimes is to engineer a circuit with suppressed transition matrix elements between the qubit states, increasing robustness to environmental noise. We realize this suppression by adding a capacitive shunt to a fluxonium circuit, obtaining lifetimes up to 8\,ms. The reduced transition matrix elements, however, make direct coherent operations a challenge. We overcome this challenge using a threephoton Raman scheme in a $\Lambda$ system realized in the circuit, demonstrating a 500$\times$ decrease in gate times. We will also discuss ongoing research in development of qubits that are protected from both decay and dephasing. 
Wednesday, March 7, 2018 1:39PM  1:51PM 
L33.00011: FluxTunable Dissipation for Superconducting Quantum Circuits Matti Partanen, Kuan Tan, Shumpei Masuda, Joonas Govenius, Russell Lake, Leif Grönberg, Juha Hassel, Slavomir Simbierowicz, Visa Vesterinen, Jani Tuorila, Tapio AlaNissila, Mikko Möttönen Circuit quantum electrodynamics is one of the most promising approaches for building a largescale quantum computer. Despite the intensive research in the field, further development steps are still required to obtain this goal. Especially, fast initialization of qubits is beneficial for various error correction codes. Here, we experimentally demonstrate a fluxtunable environment that can potentially be used for qubit initialization [J. Tuorila, et al., npj Quantum Information 3, 27 (2017)]. Our system consists of two coupled resonators: one with a high quality factor, and the other with a low quality factor and a tunable resonance frequency achieved with a superconducting quantum interference device. We can tune the loaded quality factor of the system from above 10^{5} down to a few thousand. 
Wednesday, March 7, 2018 1:51PM  2:03PM 
L33.00012: MicrowaveActivated ControlZ Gate for FixedFrequency Fluxonium Qubits. Konstantin Nesterov, Ivan Pechenezhskiy, Chen Wang, Vladimir Manucharyan, Maxim Vavilov A unique feature of a fluxonium superconducting circuit at half flux quantum is the hierarchy of its low energy transitions: the transition frequency between the states 1 and 2 can be an order of magnitude higher than between states 0 and 1. This feature allows combination of long information storage with fast information processing in a single circuit. We propose a microwave activated controlz (CZ) gate for two fixedfrequency (i.e., parked at flux sweet spot) fluxonium qubits coupled via a common capacitance or inductance. The hybridization of noncomputational states 21 and 12 by capacitive coupling or hybridization of computational states 10 and 01 by inductive coupling lifts the degeneracy between 1020 and 1121 transitions. As a result, resonant driving of 1121 transition can lead to an extra πphase accumulation for the state 11, which is equivalent to the CZ gate. Furthermore, for the case of 1001 inductive coupling, the gate can be implemented by driving 1002 transition, which reduces undesired effects on the remaining computational states since the transition is well separated in frequency. Our simulations show that these CZ gates have fidelity well above 99% with gate time below 100 ns. We discuss the application of the proposed gates to both 2D and 3D circuit QED. 
Wednesday, March 7, 2018 2:03PM  2:15PM 
L33.00013: Tunable XXCoupling Between High Coherence Flux Qubits Gabriel Samach, Steven Weber, David Hover, Danna Rosenberg, Jonilyn Yoder, David Kim, William Oliver, Andrew Kerman The first generation of quantum annealers based on Josephson junction technology successfully represent arrays of spins in the quantum transversefield Ising model. However, to date, no annealing architecture has emulated the more sophisticated nonstoquastic Hamiltonians of interest for next generation quantum annealing. Here, we present our recent results for tunable XXcoupling between high coherence superconducting flux qubits, as well as robust device simulations of these coupled quantum systems. We consider the capabilities and limitations of annealing architectures based on these twoqubit building blocks, and we address specifically our efforts to engineer strong XXcoupling in the absence of local qubit fields, an inherent limitation of all existing flux qubitbased annealing systems. 
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