Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session S41: Novel Superconducting Qubits IFocus Recordings Available

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Sponsoring Units: DQI Chair: Peter Groszkowski, University of Chicago Room: McCormick Place W196C 
Thursday, March 17, 2022 8:00AM  8:12AM 
S41.00001: Improving quantum control of the soft 0pi qubit through device engineering Junyoung An, Agustin Di Paolo, Roni Winik, Leon Ding, Kyle Serniak, Thomas M Hazard, Terry P Orlando, Simon Gustavsson, William D Oliver Ideally, 0Pi qubit is fully protected against both depolarization and dephasing noise. However, realizing it is a difficult task because of the strict devicefabrication constraints. Therefore, soft 0Pi qubit has been introduced as an alternative, which is achievable with current technology at the expense of weaker fluxnoise protection. However, implementing fast qubit control while avoiding leakage is one of the big challenges that soft 0Pi qubit faces. In this talk, we explore the parameter space of the soft 0Pi qubit to maximize coherence and gate speed through lowlying Raman transitions. Moreover, we discuss fabrication constraints and technological considerations that could help push the qubit's design beyond the soft 0Pi qubit. 
Thursday, March 17, 2022 8:12AM  8:24AM 
S41.00002: Bifluxon Qubit with Reduce Sensitivity to Charge Noise Francesco Vischi, Konstantin Kalashnikov, Wenyuan Zhang, Michael Gershenson, Matthew T Bell The Bifluxon qubit is a symmetryprotected qubit whose states are encoded in the parity of fluxons in a superconducting loop [1]. Protection of fluxonparity in the Bifluxon qubit is preserved through AharonovCasher destructive interference of singlefluxon tunneling events. Charge noise detrimentally affects parity protection by increasing the probability of singlefluxon tunneling. With the aim of protection improvement, we propose a modified design of the Bifluxon qubit which consists of two parallel splitCooperpair boxes shunted by a superinductor arranged in a superconducting loop. We show that the addition of a second Cooperpair box suppresses the effect of charge noise and results in improved protection against energy relaxation. We will present preliminary results on the implementation of this Bifluxon qubit with the gate control of induced charges on two superconducting islands; the data demonstrate exceptional charge stability and suppressed quasiparticle poisoning due to the gap engineering in circuit elements. The spectroscopy data will be compared with numerical diagonalization of circuit Hamiltonian. 
Thursday, March 17, 2022 8:24AM  8:36AM 
S41.00003: Experimental Verification of TimeDependent Flux Allocation in Fluxonium Jacob Bryon, Daniel K Weiss, Xinyuan You, Ziwen Huang, Jens Koch, Andrew A Houck External flux in the fluxonium Hamiltonian is generally allocated entirely to the inductor or to the Josephson junction, both of which are equally valid in the case of static external flux. However, recent theory work has shown that given a timedependent external flux, allocation of the flux to the junction leads to terms proportional to the time derivative of the flux appearing in the Hamiltonian [1]. When the flux is allocated to the inductor, the usual fluxonium Hamiltonian is recovered. Here, we experimentally verify the theory predictions by rapidly modulating the external flux and recording qubit state transfer. We find results consistent with grouping the flux with the inductor, and inconsistent with grouping the flux with the junction while ignoring the derivative term. 
Thursday, March 17, 2022 8:36AM  9:12AM 
S41.00004: Blochnium: A fluxinsensitive qubit with fluxtunable interactions Invited Speaker: Ray A Mencia We introduce a fluxlike qubit, nicknamed 'blochnium', whose 01 transition frequency is nearly insensitive to the external flux bias and is dual to a transmon. Blochnium is created by shunting a small Josephson junction with a hyperinductance, that is  a maximal perunitlength inductance that has minimal stray capacitance and is probably the highest impedance electromagnetic structure available today (L_{tot} > 1 μH, Z > 200 kΩ). To achieve such inductances, we release the entire circuit from the high dielectric substrate and suspend it almost entirely in vacuum. The qubit’s spectrum reveals a 01 flux dispersion on the order of a hundred MHz which becomes exponentially suppressed when decreasing the ratio EL/EC providing protection against 1/f flux noise. However, the higher transitions remain fluxsensitive allowing for multiqubit operations. We report on recent coherence properties of blochnium with varying etching techniques. 
Thursday, March 17, 2022 9:12AM  9:24AM 
S41.00005: Chargeparity qubits based on concatenation of πperiodic Josephson elements: Part 2 Kenneth R Dodge, Yebin Liu, Bradley G Cole, Abigail J Shearrow, Matthew Snyder, Emma Brann, Andrey Klots, Lara Faoro, Lev B Ioffe, Robert McDermott, B.L.T. Plourde For chargeparity qubits formed from concatenated πperiodic plaquette elements, the dispersion of the qubit energy bands with flux depends on the path through the multidimensional flux bias space. With only one plaquette biased near πperiodicity, the dispersion is linear near frustration, while biasing two concatenated plaquettes near πperiodicity results in a quadratic dispersion. Here, we describe our experimental characterization of devices with three concatenated πperiodic elements through spectroscopy and timedomain measurements at various flux and charge bias regimes. 
Thursday, March 17, 2022 9:24AM  9:36AM 
S41.00006: Chargeparity qubits based on concatenation of \piperiodic Josephson elements: Part 1 Yebin Liu, Kenneth R Dodge, Bradley G Cole, Jaseung Ku, Abigail J Shearrow, Matthew Snyder, Emma Brann, Andrey Klots, Lara Faoro, Lev B Ioffe, Robert McDermott, Britton L Plourde Superconducting qubits with topological protection against local noise hold the promise of significantly enhanced coherence times and higher gate fidelities than are possible with conventional qubits. We are pursuing a design that uses compact, high inductances and conventional Josephson junctions with individual flux control for tuning each plaquette to reach a regime with a \piperiodic Josephson energy. Concatenating multiple \piperiodic elements further enhances the degree of protection that can be achieved. Here we describe the design, fabrication, and flux characterization of such a qubit. 
Thursday, March 17, 2022 9:36AM  9:48AM 
S41.00007: Implementation of cross resonance gate for a capacitively coupled two fluxonium device in 3D cavity Ebru Dogan, Dario Rosenstock, Loïck Le Guevel, Quentin Ficheux, Haonan Xiong, Aaron Somoroff, Ray A Mencia, Konstantin Nesterov, Maxim G Vavilov, Vladimir Manucharyan, Chen Wang Fluxonium qubits stand out with their high anharmonicity and longer coherence times [1,2]. For this reason they are promising candidates for cross resonance gate applications where the weak transmon anharmonicity is one of the main limitations [3]. Our work focuses on the implementation of the cross resonance gate on a capacitively coupled two fluxonium system and suggests an 'all microwave' scheme in a simple 3D resonator geometry. We will present our improved toolbox for the realization of the CR gate in our system and report recent benchmarking results. In particular we will discuss observations of the effect of strong drives on fluxonium qubits in a 3D cavity. 
Thursday, March 17, 2022 9:48AM  10:00AM 
S41.00008: Inductively shunted transmon: a qubit with phase flip protected plasmon and bit flip protected fluxon states Farid Hassani Bijarbooneh, Matilda Peruzzo, Lucky Kapoor, Martin Zemlicka, Andrea Trioni, Johannes M Fink The inductively shunted transmon (IST) is a superconducting qubit with exponentially suppressed fluxon transitions and a plasmon spectrum approximating that of the transmon. It shares many characteristics with the transmon but offers charge offset insensitivity for all levels and precise flux tunability with quadratic flux noise suppression. In this work we propose and realize IST qubits deep in the transmon limit where the large geometric inductance acts as a mere perturbation. With a flux dispersion of only 5 MHz we reach the 'sweet spot everywhere' regime of a SQUID device with a measured constant coherence time over the full flux range. At the flux degeneracy point the device reveals tunneling physics between the two quasidegenerate ground states with typical observed lifetimes on the order of hours. In the future, a smaller inductor could be used to avoid leakage to unconfined transmon states in high power readout or drivendissipative bosonic qubit realizations. Moreover, the combination of well controllable plasmon transitions together with stable fluxon states in a single device might offer a way forward towards improved qubit encoding schemes. 
Thursday, March 17, 2022 10:00AM  10:12AM 
S41.00009: Design of a granular aluminum fluxonium qubit in a coplanar waveguide architecture Patrick Paluch, Martin Spiecker, Nicolas Gosling, Alexandru Ionita, Simon Günzler, Daria Gusenkova, Dennis Rieger, Ivan Takmakov, Francesco Valenti, Patrick Winkel, Wolfgang Wernsdorfer, IoanMihai Pop Fluxonium qubits are often embedded in rectangular waveguides which dilute the electric field and favor high coherence [1,2]. However, this configuration complicates insitu flux gates and multiqubit experiments. Here, we present a fluxonium qubit placed in a coplanar waveguide architecture with an integrated fastflux coil, surrounded by a normal metal ground plane. The superinductor is made out of granular aluminum (grAl) [3] and the use of a comparably large silver ground plane potentially decreases the number of quasiparticles in the system via phonon trapping [4]. 
Thursday, March 17, 2022 10:12AM  10:24AM 
S41.00010: A microwaveactivated controlledphase gate between a transmon and a fluxonium Alessandro Ciani, Boris Varbanov, Nicolas Jolly, Christian Kraglund Andersen, Barbara M Terhal In the quest for high qubit coherence, fluxonium qubits have emerged as promising candidates for 
Thursday, March 17, 2022 10:24AM  10:36AM 
S41.00011: Highfidelity QND readout of HeavyFluxonium Qubit Gaurav Bothara, Kishor V Salunkhe, Meghan P Patankar, Rajamani Vijayaraghavan Heavy fluxonium qubits with their rich anharmonic spectrum, high coherence,and highfidelity twoqubit gates are emerging as a potential alternative to transmon qubits for building superconducting quantum processors. In addition,it is also necessary to have a highfidelity, quantum nondemolition (QND) readout to realize a quantum processor. In principle, the fidelity of the circuitQED based readout for the fluxonium qubit can be increased by using larger microwave power. But in some earlier work, it was observed that the qubit lifetime decreases significantly with increasing readout power [1] which resulted in lower fidelity.This effect also takes place in the transmon qubit readout but was observed to be stronger in fluxonium qubits. Recently, high fidelity readout was demonstrated in fluxonium qubit with a superinductance made using granular Aluminium [2]. Here, we describe a series of experiments to characterize and optimize measurement fidelity in heavyfluxoium qubits which use the conventional Josephsonjunction array for the superinductance. We will discuss the relaxation induced by cavity photons, signaltonoise ratio, and the QND aspect of the readout in the presence of a high readout photon number. 
Thursday, March 17, 2022 10:36AM  10:48AM 
S41.00012: Understanding and mitigating decoherence in fluxonium qubits Gengyan Zhang, Hantao Sun, Feng Wu, Ran Gao, Xizheng Ma The fluxonium qubit, with its long coherence time and large anharmonicity, is a promising candidate for the physical realization of quantum computation. Stateoftheart devices have achieved a relaxation time (T_{1}) and a coherence time (T_{2}) in the millisecond regime, and various decoherence mechanisms have been studied. We present experimental results from an tunableE_{J} device to study dissipation in fluxonium qubits, using highbandwidth flux controls to tune the device and measure the relaxation time in a large parameter space. We then analyze the data to characterize the noise spectrum and identify dissipation sources such as tunneling twolevel systems and quasiparticles. Statistics and temporal fluctuations of coherence times in fixedE_{J} devices will also be discussed. This work deepens our understanding of decoherence and enables its mitigation, thus paves the way for the implementation of highfidelity fluxonium quantum processors. 
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