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 T75: Understanding and Mitigating Decoherence in Superconducting QubitsFocus
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Sponsoring Units: DQI Chair: Joao Basso, University of California, Berkeley Room: Room 401/402 |
Thursday, March 9, 2023 11:30AM - 12:06PM |
T75.00001: Suppression of crosstalk in superconducting qubits using dynamical decoupling Invited Speaker: Vinay Tripathi Currently available superconducting quantum processors with interconnected transmon qubits are noisy and prone to various errors. The errors can be attributed to sources such as open quantum system effects and spurious inter-qubit couplings (crosstalk). The ZZ-coupling between qubits in fixed frequency transmon architectures is always present and contributes to both coherent and incoherent crosstalk errors. Its suppression is therefore a key step towards enhancing the fidelity of quantum computation using transmons. Here we propose the use of dynamical decoupling to suppress the crosstalk and demonstrate the success of this scheme through experiments performed on several IBM quantum cloud processors. We demonstrate improvements in quantum memory as well as the performance of single-qubit and two-qubit gate operations. We perform open quantum system simulations of the multi-qubit processors and find good agreement with the experimental results. We analyze the performance of the protocol based on a simple analytical model and elucidate the importance of the qubit drive frequency in interpreting the results. We demonstrate that the XY4 dynamical decoupling sequence loses its universality if the drive frequency is not much larger than the system-bath coupling strength. Our work demonstrates that dynamical decoupling is an effective and practical way to suppress crosstalk and open system effects, thus paving the way towards higher-fidelity logic gates in transmon-based quantum computers. |
Thursday, March 9, 2023 12:06PM - 12:18PM |
T75.00002: Effective qubit dephasing induced by spectator-qubit relaxation Petar Jurcevic, Luke C Govia In many leading architectures for quantum computing, it remains to be understood if we can equate single-qubit coherence times measured in isolation, to that of coherence times measured in multi-qubit devices. On a multi-qubit superconducting circuit platform we show an increase in the dephasing rate of a control qubit due to the spontaneous relaxation of spectator qubits coupled to the control qubit. We attribute this increased dephasing to random in time Z-phase kicks on the control qubit due to the interplay between spectator relaxation and the control-spectator ZZ-interaction. We measure the magnitude of this extra dephasing using Ramsey decay experiments, show how it can be corrected via dynamical decoupling pulse sequences, and demonstrate that randomized benchmarking is insensitive to the effect. Our experimental results are supported by a robust theoretical model that captures an arbitrary number of spectator qubits, and gives a simple, intuitive picture for the mechanism behind the enhanced dephasing. |
Thursday, March 9, 2023 12:18PM - 12:30PM |
T75.00003: Modeling decoherence processes in high-dimensional superconducting circuits using master equations and quantum trajectory approach Kangdi Yu, Murat C Sarihan, Madeline Taylor, Jin Ho Kang, Ananyo Banerjee, Cody S Fan, Jonathan L DuBois, Yaniv J Rosen, Chee Wei Wong Recent advances in Josephson-junction-based superconducting quantum computing have focused on scaling the number of logical qubits via error mitigation and corrections. Another parallel approach is to increase the size of the Hilbert space in a single device, i.e., with high-dimensional qubits, also known as qudits. It has been shown that one can drive higher-order transitions in a transmon or design new multimode superconducting circuits, called multimons, to expand the computational basis. To systematically analyze the decoherence of an artificial atom with more than two levels, we adopt an effective Lindblad master equation and stochastic master equation (SME) to model various dephasing mechanisms in a qutrit, a three-level qudit, when dispersively coupled to a readout resonator. The effectiveness of dynamical decoupling pulses on a qutrit is shown by solving the Lindblad master equation. In addition, the quantum trajectories induced by the quantum nondemolition measurement are simulated using the qutrit SME at various readout frequencies. Sample paths of the heterodyne measurements are also computed along with the quantum trajectories to find an optimal measurement time and readout frequency for multiple-state classification and to compare with real experiments conducted on a 3D transmon qutrit. We also propose that the SME can be heuristically generalized for a multimon based on the cross-Kerr matrix measured. |
Thursday, March 9, 2023 12:30PM - 12:42PM |
T75.00004: Decoherence bounds on fidelity of operations going outside of the computational subspace Tahereh Abad, Göran Johansson, Anton Frisk Kockum The fidelity of operations performed on qubits is often limited by incoherent errors, which typically can be modeled by fundamental Markovian noise processes such as amplitude damping and dephasing. In [Phys. Rev. Lett. 129, 150504 (2021)], a simple formula for the fidelity reduction of a general multiqubit operation in terms of the dissipative rates and the corresponding Lindblad jump operators is presented. This reduction is independent of the specific operation when the ideal evolution is confined to the computational subspace. Here we address quantum operations that utilize the states from outside of the computational subspace and find a closed expression for the fidelity reduction under the dissipation acting independently on the individual qubits. Using this expression we calculate the correction to the fidelity for a few well-known operations under various dissipation processes. These results are useful for understanding the error budgets of quantum gates while scaling up quantum computers. |
Thursday, March 9, 2023 12:42PM - 12:54PM |
T75.00005: Correlated fluctuation in relaxation times of superconducting qubits with T1 > 400 μs Shingo Kono, Jiahe Pan, Mahdi Chegnizadeh, Xuan Yang, Yang Xu, Xuxin Wang, Marco Scigliuzzo, Tobias J Kippenberg Scalable quantum computing with superconducting qubits requires stringent constraints on their coherence times, not only on long relaxation times but crucially on their stability. However, a longer relaxation time is more sensitive to small fluctuations in its environment. Nevertheless, a detailed analysis of the fluctuations may provide us with additional information about the loss mechanisms, enabling us to further improve the relaxation times. Here, we develop superconducting transmon qubits with a maximum relaxation time exceeding > 400 μs and average > 200 μs, enabled by a single Al/Ox/Al Josephson junction shunted by Nb capacitance electrodes on a Si substrate, and measure the stability of the relaxation times of multiple qubits over a long period of time (> 1 week). We observe that the fluctuations in the relaxation times are correlated, implying that the relaxation times can be limited by two-level systems, whose frequencies are scrambled by a common process, such as background ionizing radiations. |
Thursday, March 9, 2023 12:54PM - 1:06PM |
T75.00006: Enhancing the coherence of superconducting qubits with electric fields Jürgen Lisenfeld, Alexander Bilmes, Alexey V Ustinov Superconducting qubits are severely hampered by decoherence, of which a major part originates from tunneling defects at qubit circuit interfaces. |
Thursday, March 9, 2023 1:06PM - 1:18PM |
T75.00007: Quasiparticle effects in transmons with gap-asymmetric junctions Giampiero Marchegiani, Luigi Amico, Gianluigi Catelani Single-particle excitations, known as Bogoliubov quasiparticles, threaten the operation of superconducting qubits. We model the qubit-quasiparticle coupling in terms of quasiparticle densities, accounting for the gap asymmetry in Josephson junctions, which naturally arise from the deposition of aluminum layers with different thicknesses. |
Thursday, March 9, 2023 1:18PM - 1:30PM |
T75.00008: Mitigating quasiparticle tunneling of the transmon qudit with charge parity measurements Xingrui Song, Patrick M Harrington, Luis A Martinez, Kristin M Beck, Jonathan L DuBois, Kater Murch As a promising platform for quantum computing, superconducting transmon circuits are widely used as qubits, with the lowest two energy levels as the computational basis. By design, the 0-1 transition is insensitive to the offset charge fluctuation in the standard transmon regime. As a natural extension, accessing the upper energy levels can be an attractive direction to efficiently encode quantum information in the transmon circuit. However, the upper energy levels are more vulnerable to offset charge fluctuation, which limits their application as a coherent quantum resource. We study one of the dominant mechanisms of offset charge fluctuations, the tunneling event of a quasiparticle across the Josephson junction, which switches the charge parity, shifts the frequencies of the energy levels, and alters the coupling strength. We take advantage of this effect by utilizing the third excited energy level for the direct dispersive readout of the charge parity state. Additionally, we mitigate the effects of quasiparticle tunneling by using the charge parity measurement result to pre-select the quantum evolution. This method gives us effectively deterministic quantum evolution unaffected by the charge parity switching. We demonstrate this method with a high-quality tantalum transmon in the standard charge insensitive regime (EJ / EC = 50) with a common cavity-QED readout setup, available through remotely accessing the LLNL quantum design and integration testbed (QuDIT facility). This method enables us to access the lowest four energy levels of the transmon qudit encoding two qubits of quantum information. Our work improves the performance of a transmon circuit as a qudit system. |
Thursday, March 9, 2023 1:30PM - 1:42PM Author not Attending |
T75.00009: Noise-bias preserving gate with qubits encoded in an superconducting Ising chain Gabriel Ethier-Majcher, Clauderic Ouellet-Plamondon, Marcelo Wu, Alireza Najafi-Yazdi Building qubits with intrinsic protection against noise is a promising approach to realize a useful quantum computer. It is possible to encode protected quantum information in Majorana zero modes found in the Kitaev model, but their implementation remains experimentally challenging. Alternatively, a qubit can be encoded in the degenerate ground states of the transverse field Ising model, the spin counterpart of the Kitaev model. Similar to cat qubits [1], the Ising chain qubit is noise-biased; it is immune to bit flips but sensitive to phase flips. This property can be used to implement more efficient error correcting codes [2]. |
Thursday, March 9, 2023 1:42PM - 1:54PM |
T75.00010: Noise Resilient Edge Modes on a Chain of Superconducting Qubits Xiao Mi, Dmitry Abanin, Vadim Smelyanskiy, Pedram Roushan, Michael Sonner, Murphy Yuezhen Niu, Kenneth Lee, Brooks Foxen Inherent symmetry of a quantum system may protect its otherwise fragile states. Leveraging such protection requires testing its robustness against uncontrolled environmental interactions. Using 47 superconducting qubits, we implement the one-dimensional kicked Ising model which exhibits non-local Majorana edge modes (MEMs) with $mathbb{Z}_2$ parity symmetry. Remarkably, we find that any multi-qubit Pauli operator overlapping with the MEMs exhibits a uniform late-time decay rate comparable to single-qubit relaxation rates, irrespective of its size or composition. This characteristic allows us to accurately reconstruct the exponentially localized spatial profiles of the MEMs. Furthermore, the MEMs are found to be resilient against certain symmetry-breaking noise owing to a prethermalization mechanism. Our work elucidates the complex interplay between noise and symmetry-protected edge modes in a solid-state environment. |
Thursday, March 9, 2023 1:54PM - 2:06PM |
T75.00011: Dynamics of Non-equilibrium Quasiparticles in Transmon Qubits Yizhou Huang, Zachary Steffen, Haozhi Wang, Yi-Hsiang Huang, Frederick C Wellstood, Benjamin S Palmer We have studied the diffusion, trapping, and recombination dynamics of non-equilibrium quasiparticles for different transmon qubits. To generate an excess density of low energy quasiparticles in the transmon device, the read-out resonator was pumped with a large amplitude rf signal, which induced quasiparticle-quasiparticle tunneling through the transmon's Al/AlOx/Al junction and thus generating an excess density near the junction. By modeling the recovery in time of T1 and T2 after initially injecting the quasiparticles and simulating the data using a simple numerical model, we can determine the rates for quasiparticle diffusion, trapping, and recombination. We will discuss observed differences in recovery characteristics and induced rates between transmons with and without a direct galvanic connection to ground, and between shunting capacitors made of Al and Ta. |
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