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IEEE International Conference on Quantum Computing and Engineering (QCE)

Global Control in a Superconducting Quantum Computer

Marco Riccardi, Roberto Menta, Francesco Cioni, Riccardo Aiudi, Marco Polini, Vittorio Giovannetti

Published Sep 15, 2024• 1 citation
Publisher

Abstract

We propose a platform to implement a scalable universal quantum computer based on a 2D array of superconducting qubits. Notably, calculations in our scheme are enabled by a set of only three global driving signals, irrespective of the number of qubits, ensuring full scalability of the computation. In stark contrast with the existing literature, global control in our scheme is achieved by leveraging the longitudinal ZZ coupling between neighboring qubits. We show how this enables information flow, one-and two-qubit gates through a series of three global pulses that act together on different subsets of the physical qubits in our system. These findings provide an alternative route to the realization of scalable and fault-tolerant superconducting quantum computers with reduced wiring, bringing them one step closer to solving practical problems in our daily lives.

DOI
10.1109/qce60285.2024.10323

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We propose a platform to implement a scalable universal quantum computer based on a 2D array of superconducting qubits. Notably, calculations in our scheme are enabled by a set of only three global driving signals, irrespective of the number of qubits, ensuring full scalability of the computation. In stark contrast with the existing literature, global control in our scheme is achieved by leveraging the longitudinal ZZ coupling between neighboring qubits. We show how this enables information flow, one-and two-qubit gates through a series of three global pulses that act together on different subsets of the physical qubits in our system. These findings provide an alternative route to the realization of scalable and fault-tolerant superconducting quantum computers with reduced wiring, bringing them one step closer to solving practical problems in our daily lives.
Reaching fault tolerance means scaling qubit counts by orders of magnitude, a jump that conventional superconducting architectures cannot sustain without solving the so-called `wiring problem'. Global control sidesteps this bottleneck, but implementing quantum error correction (QEC) on previously proposed global architectures incurs extremely steep overhead costs, due to the need for separate correction procedures for the computational and auxiliary qubits that comprise the global device. We resolve this by introducing the first globally-controlled architecture with zero qubit overhead. Every physical qubit is a computational qubit, and thus, every qubit is protected under a single error correcting scheme. We identify a class of cyclic stabilizer codes realizable through global iSWAP and single-qubit gates, yielding QEC thresholds nearly seven orders of magnitude larger than previous estimates for globally-controlled arrays. We further show these thresholds improve systematically as the global architecture is augmented with a limited amount of local measurement sites, demonstrating a trade-off between wiring simplicity and fault-tolerant performance.
Current superconducting quantum computing architectures are designed in such a way that each qubit is locally controlled with a dedicated drive and, sometimes, flux line. Scaling up these architectures to larger chips sizes and qubit counts is problematic, as the increased number of wires in the system would produce additional crosstalk and thermal load, as well as increasing costs. Recently, there have been a number of proposals to overcome this wiring bottleneck. These proposals rely on engineering a largeZZ\mathbf{Z Z}ZZinteraction between neighboring qubits in order to enter the so-called blockade regime, which enables the execution of logical operations with only three global drive lines regardless of the number of qubits on the chip. In these architectures, the entire dynamics is governed by the ZZ interaction, which needs to be the dominant energy scale to preserve the blockade regime. We present here preliminary results showing the possibility of retaining a largeZZ\mathbf{Z Z}ZZcoupling between two strongly detuned qubits, with an increase of several orders of magnitude with respect to what has been reported so far in the literature. In particular, we measure theZZ\mathbf{Z Z}ZZcoupling between two capacitively-coupled transmon qubits and extract a ZZ strength of 22 MHz for a detuning of 2.4 GHz, placing our system well into the blockade regime. These findings provide an experimental validation of the working principle of several global quantum computing architectures and represent a first step towards their practical realization.
Globally controlled superconducting qubit systems are regaining popularity due to their potential to solve one of the problems that most threatens the scalability of these platforms: the wiring problem. In fact, current quantum computers based on this technology rely on a local approach to qubit control, which, as the number of qubits increases, results in a cable density that interferes with the processor’s proper functioning. Adopting a global control approach makes it possible to reduce the number of cables and thus improve scalability. Since globally controlled architectures capable of universal quantum computation have already been proposed, the natural next step in the study of these platforms is the application of optimal control techniques. Indeed, given the global nature of the controls, optimization takes place across the entire system simultaneously, offering possibilities that would otherwise be unattainable with local control. The main objective of this work is to investigate these possible applications, focusing primarily on three aspects: digital quantum computation, quantum simulation, and the scalability of optimized pulses.
The processing unit of a solid-state quantum computer consists of an array of coupled qubits, each driven with on-chip microwave lines that route control signals to the qubits in order to perform logical operations. This approach to quantum computing comes with two major problems. On the one hand, it greatly hampers scalability towards fault-tolerant quantum computers, which are estimated to need a number of qubits—and therefore driving lines—on the order of 10 6 . On the other hand, these lines are a source of electromagnetic noise, exacerbating frequency crowding and crosstalk, while also contributing to power dissipation inside the dilution fridge. We here tackle these two challenges by presenting a quantum processing unit (QPU) for a universal quantum computer which is (rather than ) driven. Our QPU relies on a closed loop of superconducting qubits with always-on ZZ interactions which we dub the “conveyor belt.” Notably, this architecture requires only O ( N ) physical qubits to run a computation on N computational qubits, in contrast to previous O ( N 2 ) proposals for global quantum computation. Universality is achieved via the implementation of single-qubit gates and a Toffoli gate. The ability to perform multiqubit operations in a single step could vastly improve the fidelity and execution time of many algorithms.
We present a comprehensive framework for constructing various architectures of globally driven quantum computers, with a focus on superconducting qubits. Our approach leverages static inhomogeneities in the Rabi frequencies of qubits controlled by a common classical pulse -- a technique we refer to as the "crossed-qubit" method. We detail the essential components and design principles required to realize such systems, highlighting how global control can be harnessed to perform local operations, enabling universal quantum computation. This framework offers a scalable pathway toward quantum processors by striking a balance between wiring complexity and computational efficiency, with potential applications in addressing current challenges to scalability.