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 Z75: Superconducting Qubit Gate Performance, Control, Benchmarking and ScalingFocus
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Sponsoring Units: DQI Chair: Xiao Mi, Google Room: Room 401/402 |
Friday, March 10, 2023 11:30AM - 12:06PM |
Z75.00001: Quantum optimal control of superconducting circuits Invited Speaker: Simone Montangero Quantum technologies require high-fidelity processes and transformations. Quantum optimal control allows finding optimal strategies to drive quantum systems with precisions compatible with the fault tolerant threshold even in presence of moderate levels of leakage and noise, provided that faithful simulation methods are available. We review efficient algorithms to simulate and optimally control few- and many-body quantum dynamics and present an information theoretical analysis of quantum optimal control processes, highlighting the potential applications and the limits of optimal control methods arising from geometrical, energetic and information arguments. We finally review some theoretical and experimental applications of optimal control, ranging from the optimal control of circuit QED systems to optimal quantum annealing, an annealing process that goes beyond the adiabatic strategy. |
Friday, March 10, 2023 12:06PM - 12:18PM |
Z75.00002: Calibration methods for numerically optimized gates: Part 1 Vivek Maurya, Holger Haas, Daniel Puzzuoli, Zoé McIntyre, Lev Bishop Optimal control theory is often used to numerically design single qubit gates in simulations. Connecting this open-loop optimization approach to gradient-free calibration in experiments is difficult due to the very high dimensionality of the former. Moreover, numerically designed pulses usually show poor performances in an experimental setting due to their sensitive nature toward model parameters. In this two-part talk, we try to explore a new method of systematically connecting optimal control theory with gradient-free calibration in experiments. The first part focuses on building robust single-qubit gates for transmon qubits using perturbation theory techniques. We demonstrate that single qubit gates built using this approach are robust to instabilities in control amplitude, dephasing noise and leakage errors and compare the stability performance of our pulse with default DRAG and composite pulses. |
Friday, March 10, 2023 12:18PM - 12:30PM |
Z75.00003: Calibration methods for numerically optimized gates: Part II Zoé McIntyre, Holger Haas, Daniel Puzzuoli, Vivek Maurya, Lev Bishop Applying the techniques of optimal control theory to the design of high-fidelity quantum gates typically requires that the Hamiltonian used to model the system be highly accurate. However, unavoidable inaccuracies in the Hamiltonians used to model superconducting qubits (and their environments) make the direct application of optimal control theory to superconducting qubits particularly challenging. As a result, achieving high-fidelity quantum operations requires that the resulting gate be further "tuned up" or tailored to a specific quantum device through closed-loop experimental calibration. The high-dimensional pulse representations typically used for numerical gate design then become an obstacle to fast calibration; in Ref. [1], for instance, calibrating all 55 pulse parameters required approximately 25 hours. With the goal of systematically addressing this problem, we explore a new method for calibrating numerically optimized pulses and demonstrate the application of this procedure to the calibration of single qubit gates. |
Friday, March 10, 2023 12:30PM - 12:42PM |
Z75.00004: Coupler-mediated unconditional reset of fixed-frequency superconducting qubits Gerhard B Huber, Federico Roy, Max Werninghaus, Niklas Bruckmoser, Niklas Glaser, Leon Koch, Gleb Krylov, Joao Romeiro, Malay Singh, Ivan Tsitsilin, Stefan Filipp As coherence times of superconducting qubits increase, a method for fast and reliable reset is needed to achieve high repetition rates. Moreover, for mid-circuit readout and error correcting schemes high fidelity active qubit reset is required. Here, we present an unconditional reset protocol for fixed frequency qubits in a flux tunable multi-qubit coupler architecture. Our reset protocol is based on a single parametric flux pulse, transferring the qubit excitation coherently to a cold and rapidly decaying transmission-line resonator, which is directly coupled to a 50 Ω environment. After the reset pulse with a typical duration of 400 ns the remaining excited state population is reduced to 0.5%, decreasing the effective qubit temperature from ≈50 mK to ≈35 mK. This multi-qubit coupler based reset method may be extended to realize the simultaneous reset of multiple qubits using a single parametric flux pulse. |
Friday, March 10, 2023 12:42PM - 12:54PM |
Z75.00005: Quantum-mechanical effects of the driving field on a superconducting qubit Aashish Sah, Suman Kundu, Timm F Mörstedt, Florian Blanchet, András Gunyhó, Giacomo Catto, Priyank Singh, Jian Ma, Aarne Keränen, Santos T Wallace, Arman Alizadeh, Mikko Möttönen Heisenberg-like uncertainty in photon-number and phase of the driving field that implements single-qubit gates may lead to significant infidelity of the resulting gate operation. This infidelity scales inversely to the average photon-number in the field [1,2]. We study the effect of quantum fluctuations in the driving field on a superconducting transmon qubit. To observe this quantum-mechanical effect experimentally, we design a single-qubit device with strongly coupled drive line and on-chip filters to suppress the enhanced Purcell decay due to the drive line. We present randomized-benchmarking results characterizing the infidelity due to the quantum fluctuations in the driving field.
[1] Gea-Banacloche, J. & Miller, M. Quantum logic with quantized control fields beyond the 1/n limit: mathematically possible, physically unlikely. Phys. Rev. A 78, 032331 (2008)
[2] Ikonen, J., Salmilehto, J. & Möttönen, M. Energy-efficient quantum computing. njp Quant. Inf. 3, 17 (2017)
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Friday, March 10, 2023 12:54PM - 1:06PM |
Z75.00006: Using non-standard and non-uniform 2-qubit basis gates Sophia F Lin, Sara F Sussman, Casey Duckering, Pranav S Mundada, Jonathan M Baker, Rohan Kumar, Andrew A Houck, Frederic T Chong Near-term quantum computers are primarily limited by errors in 2-qubit (2Q) gates. A physical machine typically provides a set of basis gates that include primitive 2Q and 1Q gates that can be implemented in a given technology. 2Q entangling gates, coupled with some 1Q gates, allow for universal quantum computation. In superconducting technologies, the current state of the art is to implement the same 2Q gate between every pair of qubits (typically an XX- or XY-type gate). This strict hardware uniformity requirement for 2Q gates in a large quantum computer has made scaling up a time and resource-intensive endeavor in the lab.We argue for a radical idea – allow the 2Q basis gate(s) to differ between every pair of qubits, selecting the best entangling gates that can be calibrated between given pairs of qubits. We develop a theoretical framework for identifying good 2Q basis gates on "nonstandard" Cartan trajectories that deviate from "standard" trajectories like XX and XY. We then discuss the calibration and compilation with nonstandard 2Q gates, and introduce practical methods. We hope our work will enable the use of a much broader variety of novel 2Q gates for quantum computing. |
Friday, March 10, 2023 1:06PM - 1:18PM |
Z75.00007: Progress on a tunable coupler architecture for parametric gates between far-detuned fixed-frequency transmon qubits: Part 1 Sara F Sussman, Charles Guinn, Pranav S Mundada, Andrei Vrajitoarea, Catherine Leroux, Alexander P Place, Camille Le Calonnec, Agustin Di Paolo, Alexandru Petrescu, Alexandre Blais, Andrew A Houck Two-qubit gate performance is a major challenge in high fidelity operation of superconducting quantum processors. Designing an architecture to optimize two-qubit gates is a delicate balance between achieving fast gate speeds while minimizing coupling to the environment and unwanted interactions. Parametric gates are a promising method for entanglement, allowing large on-off ratios between detuned qubits. In this work we discuss progress on an architecture that uses a generalized flux qubit to couple two far-detuned fixed-frequency transmon qubits. AC flux modulation of the coupler allows for fast parametric gates while a DC flux bias allows the device to be tuned to a regime with zero static-ZZ crosstalk between the data qubits. Part 1: Circuit design and fabrication |
Friday, March 10, 2023 1:18PM - 1:30PM |
Z75.00008: Progress on a tunable coupler architecture for parametric gates between far-detuned fixed-frequency transmon qubits: Part 2 Charles Guinn, Sara F Sussman, Pranav S Mundada, Andrei Vrajitoarea, Catherine Leroux, Alexander P Place, Camille Le Calonnec, Agustin Di Paolo, Alexandru Petrescu, Alexandre Blais, Andrew A Houck Two-qubit gate performance is a major challenge in high fidelity operation of superconducting quantum processors. Designing an architecture to optimize two-qubit gates is a delicate balance between achieving fast gate speeds while minimizing coupling to the environment and unwanted interactions. Parametric gates are a promising method for entanglement, allowing large on-off ratios between detuned qubits. In this work we discuss progress on an architecture that uses a generalized flux qubit to couple two far-detuned fixed-frequency transmon qubits. AC flux modulation of the coupler allows for fast parametric gates while a DC flux bias allows the device to be tuned to a regime with zero static-ZZ crosstalk between the data qubits. Part 2: Performance benchmarking. |
Friday, March 10, 2023 1:30PM - 1:42PM |
Z75.00009: Long-distance transmon coupler as a building block for quantum processors Antti Vepsäläinen, Fabian Marxer, Shan W Jolin, Jani Tuorila, Alessandro Landra, Caspar Ockeloen-Korppi, Wei Liu, Kuan Y Tan, Juha Hassel, Mikko Möttönen, Johannes Heinsoo Tunable coupling of superconducting qubits has been widely studied due to its importance forisolated gate operations in scalable quantum processor architectures. Here, we demonstrate a tunablequbit-qubit coupler based on a floating transmon device which allows us to place qubits at least 2 mm apart from each other while maintaining over 50 MHz coupling between the coupler and the qubits. In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings. This leaves space for each qubit to have an individual readout resonator and a Purcell filter needed for fast high-fidelity readout. In addition, simulations show that the large qubit-qubit distance reduces unwanted non-nearest neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk. |
Friday, March 10, 2023 1:42PM - 1:54PM |
Z75.00010: Implementing and characterizing high-fidelity two-qubit gates with long-distance transmon coupler Fabian Marxer, Antti Vepsäläinen, Shan W Jolin, Jani Tuorila, Alessandro Landra, Caspar Ockeloen- Korppi, Wei Liu, Kuan Y Tan, Juha Hassel, Mikko Möttönen, Johannes Heinsoo High-fidelity two-qubit gates are an essential requirement for any quantum computing application. However, as two-qubit-gate fidelities approach the level of single-qubit-gate fidelities, standard characterization tools, such as interleaved randomized benchmarking, become unreliable. In a device featuring the long-distance transmon coupler, we have achieved a controlled-Z (CZ) gate with (99.81 ± 0.02)% fidelity [1]. To improve the estimate of this fidelity, we utilize iterative interleaved randomized benchmarking [2], which amplifies the CZ gate error and additionally reveals coherent errors. We furthermore compare the fidelity estimate to a detailed CZ gate error-budget. We see that the fidelity is limited by the qubit coherence rather than the coupler coherence, which we attribute to having a floating transmon as a coupler. |
Friday, March 10, 2023 1:54PM - 2:06PM |
Z75.00011: Characterisation of a 3D-integrated 16-qubit superconducting circuit Vivek Chidambaram, Peter Spring, Giulio Campanaro, Shuxiang Cao, Simone D Fasciati, James F Wills, Mustafa S Bakr, Boris Shteynas, Peter J Leek Scaling up superconducting circuits can decrease device performance due to fabrication complexity reducing yield and making frequency targeting more challenging [1], and the introduction of spurious modes in larger device enclosures [2]. High coherence and low crosstalk were recently shown in an uncoupled 4-qubit prototype of a tileable 3D-integrated circuit architecture [3] which should maintain performance at larger scale due to the simple tileable design and the use of off-chip inductive shunting to remove spurious enclosure modes. |
Friday, March 10, 2023 2:06PM - 2:18PM |
Z75.00012: Modular flip-chip architecture for generalized flux qubits Simon Geisert, Soeren Ihssen, Patrick Winkel, Martin Spiecker, Patrick Paluch, Dennis Rieger, Simon Guenzler, Nicolas Zapata, Nicolas Gosling, Wolfgang Wernsdorfer, Ioan M Pop Superconducting circuits are a promising and widely-used platform to implement quantum information processing hardware. However, scaling up to more sophisticated devices requires major engineering efforts due to the complexity of the mandatory coupling, readout and control circuitry. To investigate innovative coupling and scaling strategies, we developed a modular flip-chip architecture, in which the various circuit elements reside on dedicated chips that are capacitively coupled. A unit cell of our architecture consists of a qubit chip that is flipped above a control chip. The qubit chip contains a single generalized flux qubit (GFQ) and a harmonic readout mode, through which dispersive readout is possible. The control chip is used to excite, read out and flux bias the qubit. We tested our architecture by characterizing the GFQs in all conventional flux qubit regimes by modifying the qubit loop inductance as well as the shunt capacitance and the Josephson energy of the alpha junction. This resulted in qubit frequencies between 150 MHz and 7.5 GHz, and dispersive shifts of 60 kHz to 6 MHz. Coupling between unit cells may be achieved through coupler chips, so that our unit cell can be used as a basic building block of a scalable qubit array. |
Friday, March 10, 2023 2:18PM - 2:30PM |
Z75.00013: Two-qubit gate demonstration on superconducting overlap qubits compatible with cryo-CMOS multiplexer control Anton Potocnik, Rohith Acharya, Tsvetan Ivanov, A. M. Vadiraj, Jacques Van Damme, Shana Massar, Daniel Perez Lozano, Massimo Mongillo, Danny Wan, Kristiaan De Greve Scaling-up superconducting quantum processors is hindered by insufficient yield and uniformity of superconducting qubit parameters. Device variability is dominated by the Josephson junction, in particular by surface and line-edge roughness of junction electrodes as well as barrier thickness variation. These fabrication limitations can be overcome by foundry-standard fabrication processes, which are responsible for the success of the microelectronics industry. A fully foundry-compatible qubit fabrication process based on the overlap technique has already been used to demonstrate high-coherence fixed-frequency transmon qubits1. In this talk we will present a two-qubit device with flux-tunable coupler fabricated with the overlap technique. Using a CPHASE gate we benchmark the performance of the overlap multiqubit device and discuss scaling perspectives including the compatibility with cryo-CMOS multiplexers. |
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