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arXiv:2410.13619

Co-designing Transmon devices for control with simple pulses

Nicolas Wittler, Shai Machnes, Frank K. Wilhelm

Published Oct 17, 2024• 0 citations
arXivHTMLPDF

Abstract

In the current NISQ era, there is demand for functional quantum devices to solve relevant computational problems, which motivates a utilitarian perspective on device design: The goal is to create a device that is able to run a given algorithm with state-of-the-art performance. In this work, we use optimal control tools to derive the gate set required by a toy algorithm and, in tandem, explore the model space of superconducting quantum computer design, from dispersively coupled to stronger interacting qubits, to maximize gate fidelity. We employ perfect entangler theory to provide flexibility in the search for a two-qubit gate on a given platform and to compare designs with different entangling mechanisms, e.g., $\texttt{CPHASE}$ and $\sqrt{\texttt{iSWAP}}$. To ensure the applicability of our investigation, we limit ourselves to "simple" (i.e., sparse parametrization) pulses and quantify, where results differ from using the full complexity of piecewise constant controls.

DOI
10.48550/arxiv.2410.13619

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In the current NISQ era, there is demand for functional quantum devices to solve relevant computational problems, which motivates a utilitarian perspective on device design: The goal is to create a device that is able to run a given algorithm with state-of-the-art performance. In this work, we use optimal control tools to derive the gate set required by a toy algorithm and, in tandem, explore the model space of superconducting quantum computer design, from dispersively coupled to stronger interacting qubits, to maximize gate fidelity. We employ perfect entangler theory to provide flexibility in the search for a two-qubit gate on a given platform and to compare designs with different entangling mechanisms, e.g., CPHASE\texttt{CPHASE}CPHASE and iSWAP\sqrt{\texttt{iSWAP}}iSWAP​. To ensure the applicability of our investigation, we limit ourselves to "simple" (i.e., sparse parametrization) pulses and quantify, where results differ from using the full complexity of piecewise constant controls.
The effective use of current Noisy Intermediate-Scale Quantum (NISQ) devices is often limited by the noise which is caused by interaction with the environment and affects the fidelity of quantum gates. In transmon qubit systems, the quantum gate fidelity can be improved by applying control pulses that can minimize the effects of the environmental noise. In this work, we employ physics-guided quantum optimal control strategies to design optimal pulses driving quantum gates on superconducting qubit systems. We test our results by conducting experiments on the IBM quantum hardware using their OpenPulse API. We compare the performance of our pulse-optimized quantum gates against the default quantum gates and show that the optimized pulses improve the fidelity of the quantum gates, in particular the single-qubit gates. We discuss the challenges we encountered in our work and point to possible future improvements.
The strong requirement for high-performing quantum computing led to intensive research on novel quantum platforms in the last decades. The circuital nature of Josephson-based quantum superconducting systems powerfully supports massive circuital freedom, which allowed for the implementation of a wide range of qubit designs, and an easy interface with the quantum processing unit. However, this unavoidably introduces a coupling with the environment, and thus to extra decoherence sources. Moreover, at the time of writing, control and readout protocols mainly use analogue microwave electronics, which limit the otherwise reasonable scalability in superconducting quantum circuits. Within the future perspective to improve scalability by integrating novel control energy-efficient superconducting electronics at the quantum stage in a multi-chip module, we report on an all-microwave characterization of a planar two-transmon qubits device, which involves state-of-the-art control pulses optimization. We demonstrate that the single-qubit average gate fidelity is mainly limited by the gate pulse duration and the quality of the optimization, and thus does not preclude the integration in novel hybrid quantum-classical superconducting devices.
Abstract With recent improvements in coherence times, superconducting transmon qubits have become a promising platform for quantum computing. They can be flexibly engineered over a wide range of parameters, but also require us to identify an efficient operating regime. Using state-of-the-art quantum optimal control techniques, we exhaustively explore the landscape for creation and removal of entanglement over a wide range of design parameters. We identify an optimal operating region outside of the usually considered strongly dispersive regime, where multiple sources of entanglement interfere simultaneously, which we name the quasi-dispersive straddling qutrits regime. At a chosen point in this region, a universal gate set is realized by applying microwave fields for gate durations of 50 ns, with errors approaching the limit of intrinsic transmon coherence. Our systematic quantum optimal control approach is easily adapted to explore the parameter landscape of other quantum technology platforms.
The discovery of quantum mechanics in the 20th century forms the basis of many of the technologies that define our lives today. The ability to engineer and manipulate individual quantum systems -- even create artificial atoms -- promises a similar revolutionary leap in technology. A quantum technology of particular interest is quantum computing, which has the potential to solve problems that are intractable for classical computers, opening up new domains of computation. Meanwhile, an attractive approach to creating engineered quantum systems is circuit quantum electrodynamics (cQED). Where research initially focused on understanding the physics of cQED devices, focus has shifted to building systems capable of performing useful computations. However, this remains extremely challenging, in part due to the inherently fragile nature of the individual quantum bits, but also due to difficulties in controlling and scaling up these systems. This thesis focuses on the control aspects of building an extensible full-stack quantum computer based on superconducting transmon qubits. We define the demonstration of quantum fault-tolerance as our target application to give focus to our efforts. The QuSurf architecture for a full-stack quantum computer presented in this thesis is designed with this application in mind. We provide a detailed study of the error sources present in this system and give an overview of the relevant characterization techniques. In the second part of this thesis, we address several key challenges in the control of a quantum computer. To realize high-fidelity coherence limited gates, we present a novel tuneup protocol that achieves a tenfold speedup over the state-of-the-art. This is realized by eliminating the need for qubit initialization. We demonstrate this protocol by calibrating single-qubit gates to a coherence limited Clifford fidelity of 99.9% in one minute. Performing repeated parity checks, as is required for quantum error correction, requires reusing qubits quickly after they have been measured. By introducing a numerically optimized depletion pulse we are able to speeds up the depletion of measurement photons in a readout resonator without having to rely on specific symmetry conditions. Using this technique speed up photon depletion by more than six inverse resonator linewidths, reducing the error rate in an emulated ancilla parity check by a factor 75. Flux-pulsing based two-qubit gates are the fastest two-qubit gates. However, they are also very technically demanding. The key challenge in performing these gates is addressing the distortions that control signals experience as they traverse various electrical components. We have developed Cryoscope (short for cryogenic oscilloscope) to characterize and correct these distortions. Cryoscope is an in-situ technique that uses the qubit to sample control pulses of arbitrary shape. Even when correcting distortions to within sim0.1\\sim 0.1\\%sim0.1 two-qubit gates are history-dependent due to the long timescale upon which some of these distortions act. We have invented Net-Zero, a new type of flux-pulsing based two-qubit gate, to address this problem. It makes use of a symmetry condition of the transmon to have net-zero integral, making the gate resilient to long-timescale distortions. The gate suppresses leakage out of the computational subspace to 0.1% by making use of leakage interference and has a built-in echo effect that enhances the coherence of the gate, achieving a two-qubit gate fidelity of 99.1%. Custom software is required to perform the physics experiments needed to build and operate a quantum computer. PycQED is an open-source software framework we have developed for this purpose. We discuss the design choices and concepts of PycQED before turning our focus to characterization and calibration. Here we introduced dependency graphs as a useful abstraction and system emulation as an essential development tool for automating the characterization and calibration process. We conclude the thesis by reflecting on the limitations of our architecture and providing an outlook on the grand challenges of building a useful kilo-qubit sized quantum computer. We define these challenges as The Application Problem, The Fabrication Problem, and The Calibration Problem.
With recent improvements in coherence times, superconducting transmon qubits have become a promising platform for quantum computing. They can be flexibly engineered over a wide range of parameters, but also require us to identify an efficient operating regime. Using state-of-the-art quantum optimal control techniques, we exhaustively explore the landscape for creation and removal of entanglement over a wide range of design parameters. We identify an optimal operating region outside of the usually considered strongly dispersive regime, where multiple sources of entanglement interfere simultaneously, which we name the quasi-dispersive straddling qutrits regime. At a chosen point in this region, a universal gate set is realized by applying microwave fields for gate durations of 50 ns, with errors approaching the limit of intrinsic transmon coherence. Our systematic quantum optimal control approach is easily adapted to explore the parameter landscape of other quantum technology platforms.
Abstract The utility of a quantum computer is highly dependent on the ability to reliably perform accurate quantum logic operations. For finding optimal control solutions, it is of particular interest to explore model-free approaches, since their quality is not constrained by the limited accuracy of theoretical models for the quantum processor—in contrast to many established gate implementation strategies. In this work, we utilize a continuous control reinforcement learning algorithm to design entangling two-qubit gates for superconducting qubits; specifically, our agent constructs cross-resonance and CNOT gates without any prior information about the physical system. Using a simulated environment of fixed-frequency fixed-coupling transmon qubits, we demonstrate the capability to generate novel pulse sequences that outperform the standard cross-resonance gates in both fidelity and gate duration, while maintaining a comparable susceptibility to stochastic unitary noise. We further showcase an augmentation in training and input information that allows our agent to adapt its pulse design abilities to drifting hardware characteristics, importantly, with little to no additional optimization. Our results exhibit clearly the advantages of unbiased adaptive-feedback learning-based optimization methods for transmon gate design.