Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session F75: New Fluxonium ArchitecturesFocus
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Sponsoring Units: DQI Chair: Peter Groszkowski, Oak Ridge National Laboratory Room: Room 401/402 |
Tuesday, March 7, 2023 8:00AM - 8:36AM |
F75.00001: Toward a scalable and high-fidelity quantum computing system with fluxonium qubits Invited Speaker: Chunqing Deng Fluxonium has been attracting increasing interest from the superconducting qubit community as an alternative qubit platform for high-fidelity quantum computing. However, advancements in several aspects, including device fabrications for a high qubit yield, maintaining high coherence while allowing for fast operations, and enabling a scalable processor architecture, are still needed before fluxonium can be adopted as a mainstream of choice. At Alibaba Quantum Laboratory, we study these problems using a full-stack approach: from searching robust new materials for the superinductor, to investigating the decoherence mechanisms of the qubit, and lastly to exploring quantum instruction sets for a fluxonium quantum processor to improve system performance. We summarize our progress in the above aspects and discuss our perspectives toward realizing a scalable and high-fidelity quantum computing system with fluxonium qubits. |
Tuesday, March 7, 2023 8:36AM - 8:48AM |
F75.00002: The Cold Echo Qubit: a Novel Small Logical Qubit Architecture Inspired by AC-Driven Quantum Annealing Eliot Kapit, Vadim Oganesyan We describe a novel superconducting logical qubit architecture, called the Cold Echo Qubit (CEQ), which is capable of preserving quantum information for much longer timescales than any of its component parts. The CEQ operates fully autonomously, requiring no measurement or feedback, and is compatible with strong interaction elements, allowing for fast, high fidelity logical gates between multiple CEQ's. Its quantum state is protected by a combination of strong interactions and microwave driving, which implements a form of many-body dynamical decoupling to suppress phase noise. Estimates based on careful theoretical analysis and numerical simulations predict improvements in lifetimes and gate fidelities by an order of magnitude or more compared to the current state of the art, assuming no improvements in base coherence. This talk focuses on the simplest possible implementation of the CEQ, using a pair of fluxonium qubits shunted through a shared mutual inductance. This version is the easiest to test experimentally and should display coherence well past breakeven (as compared to the limiting coherence times of its components). A more complex three-node circuit is also discussed; it is expected to roughly double the coherence of its two-fluxonium counterpart. |
Tuesday, March 7, 2023 8:48AM - 9:00AM |
F75.00003: An Architecture for High-Fidelity, Robust Two-Qubit Fluxonium Gates with a Transmon Coupler Leon Ding, Max Hays, Youngkyu Sung, Bharath Kannan, Junyoung An, Agustin Di Paolo, Roni Winik, Kate Azar, Thomas M Hazard, David K Kim, Bethany M Niedzielski, Alexander Melville, Mollie E Schwartz, Jonilyn L Yoder, Devin L Underwood, Terry P Orlando, Simon Gustavsson, Jeffrey A Grover, Kyle Serniak, William D Oliver Qubit lifetimes and weak anharmonicities in superconducting transmon-based quantum computers are leading causes of gate infidelity. The fluxonium qubit is a promising alternative to transmons, with coherence times reaching milliseconds and anharmonicities of several gigahertz. In this work, we present an architecture consisting of fluxonium qubits coupled via a tunable-transmon coupler (FTF, for fluxonium-transmon-fluxonium). FTF provides two very important benefits: (1) it allows for stronger couplings for non-computational state gates, and (2) suppresses the static ZZ down the kHz levels without requiring strict parameter matching. We take advantage of these strong couplings by performing a microwave-activated CZ gate, achieving high-fidelities with clear paths forward toward 99.9%. Furthermore, the frequency at which the gate is driven can be tuned over a large range, corresponding to the coupler spectrum. |
Tuesday, March 7, 2023 9:00AM - 9:12AM |
F75.00004: Small-scale quantum processor with Heavy-Fluxonium Qubit Gaurav Bothara, Kishor V Salunkhe, Meghan P Patankar, Rajamani Vijayaraghavan Among the various platforms for quantum computation and information processing, superconducting qubits have been a promising candidate for fault-tolerant computation. In the past, multi-qubit processors have only used transmon qubit designs. However, transmon has a fundamental limitation, it sacrifices anharmonicity, a precious quantum resource. Transmon's weak anharmonicity leads to slower two-qubit gates making it prone to decoherence errors. It also limits the scalability of quantum processors, a direct consequence of restricted parameter space of operation, thus motivating us to look for alternatives. Recently, fluxonium qubit has emerged as a serious contender for building a superconducting quantum processor. Fluxonium qubits have the potential to excel over transmons due to their inherent advantages of high coherence times and higher anharmonicity. One of the crucial steps in building a fault-tolerant quantum processor is implementing high-fidelity single- and multi-qubit gates. In addition, it is also necessary to have a high-fidelity, quantum non-demolition (QND) readout. Here, we will discuss our implementation of a two-qubit fluxonium gate and experiments to characterize and optimize high-fidelity readout. We will also describe a multi-qubit architecture to build a small-scale quantum processor using fluxonium qubits. |
Tuesday, March 7, 2023 9:12AM - 9:24AM |
F75.00005: Measurement of Planar Fluxonia Kate Azar, Kyle Serniak, Thomas M Hazard, Leon Ding, Agustin Di Paolo, Max Hays, Junyoung An, Ilan Rosen, David K Kim, Bethany M Niedzielski, Alexander Melville, Jeffrey A Grover, Mollie E Schwartz, Jonilyn L Yoder, William D Oliver Superconducting qubits are a promising approach to realizing quantum computers. One challenge to this realization is the finite coherence times of these qubits. Fluxonium, with its long coherence times, is a promising qubit modality to use to build up such a quantum processor. Here we measure various fluxonia of different circuit parameters and geometries, characterizing the coherence and single qubit gate fidelities of planar aluminum on silicon designs. |
Tuesday, March 7, 2023 9:24AM - 9:36AM |
F75.00006: Experimental Investigation of a Protected Qubit Subspace Within a Fluxonium Molecule Shashwat Kumar, Xanthe Croot, Sara F Sussman, Xinyuan You, Anjali Premkumar, Tianpu Zhao, Jens Koch, Andrew A Houck Protection against depolarization and pure dephasing processes is desirable for long coherence times in qubits. The disjoint support of the logical wavefunctions and sweet spots in the qubit energy landscape protect against spontaneous qubit relaxation and pure dephasing, respectively. We employ a fluxonium molecule [1] circuit to engineer protection against noise within a novel subspace. This subspace has disjoint support of the logical wavefunctions and a sweet spot in the energy spectrum. Here we present our recent progress, including working towards accessing full flux quanta along both control axes. |
Tuesday, March 7, 2023 9:36AM - 9:48AM |
F75.00007: Towards high-fidelity two-qubit gates on strongly coupled fluxonium qubits Wei-Ju Lin, Haonan Xiong, Vladimir E Manucharyan Microwave activated two-qubit gate schemes for fluxonium qubits have been demonstrated using the transitions outside computational space [1,2], where the gate fidelity is limited by the decoherence of these higher levels. Two-qubit gate using only computational states of fluxoniums was also achieved recently with high fidelity [3]. Here we demonstrate the implementation of controlled-Z gate utilizing strong coupling between two fluxoniums, where the large induced static ZZ term can still be cancelled. This scheme for high fidelity gate operation can benefit the development of fluxonium based quantum processors and universal quantum computation. |
Tuesday, March 7, 2023 9:48AM - 10:00AM |
F75.00008: Mitigation of ZZ Coupling on Fluxonium-based Multiqubit Systems Rafael Alapisco, Maxim G Vavilov Due to their scalability, high gate fidelities, and fast gate times, superconductor circuits are considered good candidates for the development of a quantum processor. This quantum computing approach is often one of the top choices in Industry. In the field of superconducting quantum computing there has been significant progress in reducing single-qubit gate errors however, two-qubit gates still suffer from error rates that are considerably higher. One of the main sources for these error rates is the unwanted ZZ interaction that arises when two qubits are coupled to generate entanglement. This unwanted ZZ quantum crosstalk is responsible for error rates in qubit systems. In recent years it has been shown that the ZZ coupling can be eliminated [1,2], achieving high-fidelity operations. In this talk, we will analyze the reduction of ZZ coupling in multi-fluxonium systems with capacitive interactions between qubits. We will theoretically show that with an optimal choice of fluxonium macroscopic parameters the ZZ quantum crosstalk can be reduced. We further demonstrate that microwave off-resonant drive of the multi-fluxonium system reduces ZZ crosstalk as it was demonstrated for a two fluxonium circuit [2]. |
Tuesday, March 7, 2023 10:00AM - 10:12AM |
F75.00009: Improving T1 in dielectric-loss-limited fluxonium qubits Parth K Jatakia, Jacob Bryon, Anjali Premkumar, Lev Krayzman, Shashwat Kumar, Nathalie P de Leon, Andrew A Houck The long coherence times and high anharmonicity of fluxonium qubits make it an attractive candidate for a superconducting quantum computer. However, past generations of high-coherence fluxonium qubits [1,2] have been limited by the dielectric loss associated with lossy capacitors. Recent studies on tantalum-based transmons have shown improvement in dielectric-loss-limited coherence times, realizing new records in device coherence [3]. Utilizing the same approach, we present a study of the coherence properties of fluxonium while being informed by the material surface participation ratio of the electric field in tantalum. As a result of our study, we showcase improvements in the coherence times of current 2D fluxonium qubits. |
Tuesday, March 7, 2023 10:12AM - 10:24AM |
F75.00010: Dephasing in Fluxonium Qubits from Coherent Quantum Phase Slips Kyle Serniak, Thomas M Hazard, Agustin Di Paolo, Kate Azar, Leon Ding, Max Hays, Junyoung An, Ilan T Rosen, David K Kim, Alexander Melville, Bethany M Niedzielski, Jeffrey A Grover, Mollie E Schwartz, Jonilyn L Yoder, William D Oliver Recent experiments on fluxonium qubits in planar circuit architectures have demonstrated near-record coherence times and gate fidelities. In order to improve these metrics further, we seek to develop a holistic model of decoherence in fluxonium qubits. Here we focus on one dephasing mechanism inherent to Josephson junction arrays which arises from the Aharonov-Casher effect and can limit fluxonium coherence times in circuits with large superinductors. In this talk, we report coherence times in fluxonium qubits specifically engineered to be sensitive to this dephasing mechanism and compare with theoretical models. |
Tuesday, March 7, 2023 10:24AM - 10:36AM |
F75.00011: Optimized Readout of a Fluxonium Qubit Taryn V Stefanski, Siddharth Singh, Figen Yilmaz, Martijn F. S. Zwanenburg, Christian Kraglund Andersen Much attention has focused on the transmon architecture for large-scale quantum devices, however, the fluxonium qubit has emerged as a possible successor. With an additional shunting inductor in parallel, the fluxonium offers larger anharmonicity and stronger protection against dialectric loss, leading to higher coherence times. The extra inductive element of the fluxonium qubit leads to a rich dispersive shift landscape when tuning the external flux. Here we propose to exploit the features in the dispersive shift to improve qubit readout using fast flux pulsing. Specifically, we report on theoretical simulations showing improved readout times and error rates by performing the readout at a flux bias point with large dispersive shift. We suggest optimal energy parameters for the fluxonium architecture that will allow for the implementation of our proposed readout scheme. |
Tuesday, March 7, 2023 10:36AM - 10:48AM |
F75.00012: Tunable coupler for high-fidelity two-qubit gates in fluxonium Noah J Stevenson, Zahra Pedramrazi, Noah Goss, Abhishek Chakraborty, Bibek Bhandari, Lucas Burns, Long B Nguyen, Ravi K Naik, Andrew N Jordan, Justin G Dressel, David I Santiago, Irfan Siddiqi The superconducting fluxonium qubit has emerged as a promising alternative to the widely-studied transmon qubit due to increased coherence times at the half-flux quantum sweet-spot, large anharmonicity, and robust charge-noise insensitivity. Scaling to multi-qubit fluxonium systems requires implementation of fast, high-fidelity, and highly expressive quantum gates, with small residual coupling when the gate is off. In this work we present the design of and experimental progress towards realizing a 2D tunable coupler composed of a tunable fluxonium element and a direct coupling path achieving these requirements. We study the family of gates realizable with charge and flux control, and investigate their limits with regard to gate time, leakage, and drive-induced decoherence. |
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