Crosstalk diagnosis for the next generation of quantum processors

Robin Blume-Kohout, Mohan Sarovar, Erik Nielsen +3 more

Jaap J. Wesdorp, Eric Hyyppä, Joona Andersson +61 more

Pranav Mundada, Gengyan Zhang, Thomas Hazard +1 more

No talking! The authors solve the critical scalability issue of multiqubit crosstalk in quantum processors, by harnessing destructive interference. Their architecture involves linking two superconducting qubits with both a bus cavity and a tunable coupler; the tunability enables simultaneous suppression of crosstalk and realization of high-fidelity two-qubit gates. This result paves the way for the next generation of crosstalk-free multiqubit systems.

Kenneth Rudinger, Mohan Sarovar, Dylan Langharst +4 more

Xuexin Xu, Manabputra, Chloé Vignes +2 more

This study's modeling of superconducting quantum processors reveals that, contrary to popular belief, three-body ZZZ stray couplings can surpass parasitic ZZ interactions, posing a serious threat to gate fidelity and crosstalk. Even more striking, strongly decoupled qubits---long thought to be ideal---can be more problematic than loosely coupled ones, even when idle. These findings overturn traditional design principles and force us to rethink next-generation architectures for quantum processors.

Mohan Sarovar, Timothy J Proctor, Kenneth Rudinger +3 more

Crosstalk occurs in most quantum computing systems with more than one qubit. It can cause a variety of correlated and nonlocal crosstalk errors that can be especially harmful to fault-tolerant quantum error correction, which generally relies on errors being local and relatively predictable. Mitigating crosstalk errors requires understanding, modeling, and detecting them. In this paper, we introduce a comprehensive framework for crosstalk errors and a protocol for detecting and localizing them. We give a rigorous definition of crosstalk errors that captures a wide range of disparate physical phenomena that have been called ``crosstalk'', and a concrete model for crosstalk-free quantum processors. Errors that violate this model are crosstalk errors. Next, we give an equivalent but purely operational (model-independent) definition of crosstalk errors. Using this definition, we construct a protocol for detecting a large class of crosstalk errors in a multi-qubit processor by finding conditional dependencies between observed experimental probabilities. It is highly efficient, in the sense that the number of unique experiments required scales at most cubically, and very often quadratically, with the number of qubits. We demonstrate the protocol using simulations of 2-qubit and 6-qubit processors.