Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session D75: Superconducting Qubit Optimal Control IFocus

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Sponsoring Units: DQI Chair: Fnu Setiawan, University of Chicago Room: Room 401/402 
Monday, March 6, 2023 3:00PM  3:36PM 
D75.00001: Fluxpulse optimization for entangling gates on a tunablecoupler architecture Invited Speaker: Nicolas Didier Improving the performance of entangling gates at scale is important to achieve quantum advantage and perform quantum error correction. The implementation of tunable couplers made of fluxtunable transmons has led to substantial progress in this direction. In this talk, we present optimal flux control techniques for superconducting qubits, focusing on the engineering of dynamical sweet spots. 
Monday, March 6, 2023 3:36PM  3:48PM 
D75.00002: Demonstrating twoqubit gates at the quantum speed limit using superconducting qubits. Bora Basyildiz, Joel Howard, Alexander Lidiak, Casey W Jameson, Kyle Clark, Tongyu Zhao, Mustafa Bal, Junling Long, David Pappas, Meenakshi Singh, Zhexuan Gong The speed of elementary quantum gates, particularly twoqubit gates, ultimately sets the limit on the speed at which quantum circuits can operate. In this work, we experimentally demonstrate commonly used twoqubit gates at nearly the fastest possible speed allowed by the physical interaction strength between two superconducting transmon qubits. We achieve this quantum speed limit by implementing experimental gates designed using a machine learninginspired optimal control method. The machine learningbased algorithm can achieve the speed limit of various twoqubit gates in an Nqubit system through the optimization of singlequbit pulses, and this algorithm significantly outperforms standard optimal control algorithms such as GRAPE. Importantly, our method only requires the singlequbit drive strength to be moderately larger than the interaction strength to achieve an arbitrary twoqubit gate close to its analytical speed limit with high fidelity. Thus, the method is applicable to a variety of platforms including those with comparable singlequbit and twoqubit gate speeds, or those with alwayson interactions. 
Monday, March 6, 2023 3:48PM  4:00PM 
D75.00003: Demonstration of highfidelity parametricresonance gates at scale Eyob A Sete, Angela Q Chen, Riccardo Manenti, Shobhan Kulshreshtha, Stefano Poletto We demonstrate highfidelity parametricresonance entangling gates in multiqubit floating tunable coupler architectures. We considered two configurations of the tunable coupler pads where the zerocoupling coupling can be achieved when the coupler frequency is either above (asymmetric configuration) or below (symmetric configuration) the qubits frequencies. We realized iSWAP and CZ twoqubit gates on both configurations and obtained highfidelity values with interleaved randomized benchmarking. 
Monday, March 6, 2023 4:00PM  4:12PM 
D75.00004: Novel Superconducting Qubit Discovery via an Enumeration Based Approach Eli J Weissler, Zhenxing Liu, Joshua L Combes, Mohit Bhat Long term, noise protected qubits have the potential to be more error resistant than the qubits used today. We will report progress on a circuit enumerationbased procedure to discover new noise protected superconducting qubits. We will discuss tradeoffs in designing the procedure and efforts to streamline the approach by reducing the number of duplicate circuits considered. 
Monday, March 6, 2023 4:12PM  4:24PM 
D75.00005: Error suppression in the crossresonance gate via recursive DRAG. Boxi Li, Tommaso Calarco, Felix Motzoi The crossresonance gate is one of the widely used twoqubit gates for superconducting qubits. The stateoftheart experiments have demonstrated highfidelity gate operations and it is also the default twoqubit gate on the IBM NISQ devices. The crossresonance gate is known to be subjected to several coherent errors such as nonadiabatic transitions on the control qubit and ZZ phase error. They are often suppressed through long pulse ramping time and echoed gate design, which inevitably increases the gate time. 
Monday, March 6, 2023 4:24PM  4:36PM 
D75.00006: Numerically modeling the Hamiltonian of a microwavedriven superconducting circuit Yao Lu, Kevin C Smith, Daniel K Weiss, Xinyuan You, Yaxing Zhang, Suhas S Ganjam, Aniket Maiti, John W Garmon, Ian M Shem, Jens Koch, Steven M Girvin, Robert J Schoelkopf Modeling the timedependent Hamiltonian of a driven Josephson circuit is imperative to superconducting quantum computation. So far, static circuit Hamiltonians have been modeled by numerical schemes such as BBQ or EPR. In contrast, numerical modeling of the driven Hamiltonian in the presence of external voltage or flux modulation, without relying on a lumpedelement circuit model, has been largely unexplored. Here, we present a numerical method that leverages finiteelement simulation to obtain the lowenergy timedependent Hamiltonian of a multimode and multijunction Josephson device with complex geometry in the presence of external drives. Our scheme serves as a promising toolbox for characterizing the driven properties of realistic circuit devices in complicated electromagnetic environments  a task not typically amenable to standard lumpedelement circuit analysis. Consequently, our technique should have wide application to the optimization of various circuit designs. 
Monday, March 6, 2023 4:36PM  4:48PM 
D75.00007: Quantum state transfer between two superconducting resonators by parametric voltage modulation on gated Josephson junctions Yinqi Chen, Javad Shabani, Hugh O Churchill, Vladimir E Manucharyan, Maxim G Vavilov Quantum state transfer between elements of superconducting circuits is crucial for quantum information applications, such as initialization, control, and readout of qubit systems. In this talk, we analyze the exchange of excitations between microwave cavities based on parametric resonance. We consider two coupled superconducting coplanar waveguide resonators connected to gated superconductorsemiconductor Josephson junctions on one of their terminals. Periodical modulation of the gate voltage of one of the junctions causes the photon excitations to transfer from one resonator to another. We evaluate the role of nonlinearity and dissipation introduced by the supersemi junctions on the accuracy of state transfer. In particular, as the resonators acquire the signatures of the gatemon energy spectrum at strong anharmonicity, such parametric resonance can be used to generate fast, highfidelity two qubit gates. 
Monday, March 6, 2023 4:48PM  5:00PM 
D75.00008: A strongly coupled twoqubit system with weak quantum crosstalk Konstantin Nesterov, Denis Chevallier, Chiara Pelletti, Larry Chen, Bingcheng Qing, Ravi K Naik, David I Santiago, Irfan Siddiqi, Alexei Marchenkov In microwaveactivated twoqubit gate schemes, a strong static exchange interaction between superconducting qubits leads to a strong hybridization between their eigenstates and thus enables fast operations. However, a larger exchangeinteraction strength generally increases spurious quantum (ZZ) crosstalk, diminishing addressability for singlequbit gates. In this talk, we discuss the design of a system of transmons with multiple coupling paths between qubits, which provides a high entanglinggate rate with reduced static quantum crosstalk. We optimize the layout configuration of such a system by means of a finiteelement analysis combined with the energyparticipationratio technique [1]. This approach allows us to find the quantum Hamiltonian for a given physical layout and therefore accurately estimate the ZZ coupling magnitude as well as the rates of twoqubit gates. 
Monday, March 6, 2023 5:00PM  5:12PM 
D75.00009: Analysis of underlying mechanisms and alleviation of static ZZ coupling Simon Pettersson Fors, Jorge FernándezPendás, Anton Frisk Kockum Gate fidelities and gate times are continuously being improved in the ongoing attempts to create a superconducting quantum computer. However, the ZZ coupling remains as a fundamental obstacle to further improve the performance of current superconducting qubit systems. Here, we present a theoretical analysis of the underlying mechanisms which cause the static ZZ coupling, with the aim of both alleviating and utilizing the effect in single and twoqubit gates. This analysis uses perturbation theory to study a simplified model and validates the results via a more detailed model and numerical investigations. We expect that these insights into the static ZZ coupling will contribute to improving gate fidelities in twoqubit entangling gates. 
Monday, March 6, 2023 5:12PM  5:24PM 
D75.00010: Planar multimode superconducting circuit design for highdimensional computation Murat C Sarihan, Kangdi Yu, Madeline K Taylor, Ananyo Banerjee, Jin Ho Kang, Cody S Fan, KaiChi chang, Chee Wei Wong Transmon qubits have recently been advanced as the building blocks of noisy intermediatescale quantum processors. In these quantum processors and building blocks, multiqubit gate fidelity and interqubit connectivity becomes crucial, wherein transmons create a bottleneck due to weak transverse coupling among nearestneighbor qubits. Multimode superconducting qubits, or multimons, are demonstrated as an alternative, where fullyconnected qubits with strong longitudinal coupling form hybridized modes to achieve highfidelity multiqubit gates over a larger Hilbert space. With this approach, a threequbit system is demonstrated with a complete gate set using 3D superconducting cavities, the latter of which can be less susceptible for largescale integration [1,2,3]. For this purpose, we propose planar coupling strategies for multimons inspired by stateoftheart planar topologies of Josephson ring modulatorbased devices [4, 5]. In this work, we successfully coupled two pairs of coplanar waveguide resonators to a trimon with equal strength in a differential configuration to effectively control and readout a threequbit system through the two hybridized longitudinal modes. We also proposed a coupling scheme between multiple trimons for scalable quantum processors. 
Monday, March 6, 2023 5:24PM  5:36PM 
D75.00011: Efficient Machine Learning Systems for HighFidelity Qubit Readout Satvik Maurya, Chaithanya N Mude, William D Oliver, Benjamin Lienhard, Swamit Tannu Multiqubit readout is among the most errorprone operations in superconducting quantum computing systems. These errors occur for various reasons, including but not limited to: crosstalk between the readout tones in a frequencymultiplexed readout scheme, spontaneous state transitions during the measurement, excitations caused by the readout pulse, and thermal noise added to the readout signal as it travels from the refrigerator to the roomtemperature electronics. Prior works on reducing readout errors include machine learningassisted readout, where a neural network is used for more robust discrimination by compensating for crosstalk errors. However, the neural network size can limit systems' scalability, especially if fast hardware discrimination is required. This work presents a scalable approach for mitigating singleshot readout errors by using a matched filter in conjunction with a significantly smaller and scalable neural network for qubitstate discrimination. In addition, we optimize the training of the matched filter and neural network by using a preclassifier that filters incorrectly labeled training data. Fast and accurate discrimination of qubit states is essential for deploying quantum error correction codes. To that end, we investigate computationally efficient and scalable machine learning algorithms for enabling highfidelity multiquit readout that are readily implementable on FPGAs. 
Monday, March 6, 2023 5:36PM  5:48PM 
D75.00012: Highfidelity qutrit entangling gates with superconducting circuits based on parametric coupling Mahadevan Subramanian, Adrian Lupascu Recently, significant progress has been made in the demonstration of single qutrit and coupled qutrit gates with superconducting circuits. Coupled qutrit gates have significantly lower fidelity than single qutrit gates, owing to long implementation times. We present a protocol to implement CZ gates using two fixed frequency transmons capacitively coupled to a tunable transmon. We make use of fundamental gates that are iSWAPlike exchanges between the two qutrit states 01> to 10> and the states 12> to 21>. Parametric coupling is used to conduct these exchanges by driving an AC flux in the tunable coupler of an appropriate frequency which is different for the two kinds of exchanges. We show that these gates have high expressibility when combined with single qutrit rotations. We show that a highfidelity CZ gate can be obtained by combining these entangling gates with single qutrit rotations. 
Monday, March 6, 2023 5:48PM  6:00PM 
D75.00013: MultiQubit coupler for superconducting circuits with controllable inductive interactions Klaus Liegener, Stefan Filipp Multiqubit couplers are envisioned to provide fast gates between nexttonearest neighbor qubits as well as between multiple qubits in one step. Common proposals, such as a capacitively coupled squid loop acting as a tunable element which however suffer from residual qubitqubit interactions. Instead, this talk focuses on an architecture where all qubits connect inductively via squid loops to an intermediate node. In the limit of negligible capacitive coupling between the qubits and the coupler node, said intermediate node becomes frozen and enforces the appearance of interaction terms between all qubits connected to the coupler. We find a suitable configuration in parameter space, where by controlling the coupler with two external flux bias lines, pairwise interactions can be activated while other qubits remain decoupled. We present a derivation of the relevant circuit QED Hamiltonian and simulate parameter regimes that are experimentally accessible. We further explore its potential for creating multiqubit gates. 
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