Entangled qubits may suffer strong joint noise challenging quantum computers

The Fully Depolarizing Noise Conjecture for Entangled Physical States: A Twenty-Year Perspective

Computational Complexity

Summary

Quantum computers rely on delicate qubits that can be linked together in a process called entanglement. The author revisits a long-standing idea that in real quantum devices, the noise affecting these linked qubits might be much stronger and more intertwined than usually assumed. This extra noise, called a fully depolarizing component, could make it much harder to build large-scale, reliable quantum computers. While the idea is still unproven, new experiments could soon test it. The paper also discusses related topics about how noise impacts the performance of near-term quantum devices and how to judge claims of quantum advantage.

What this means in practice

  • For quantum hardware engineers: Design experiments to measure joint fully depolarizing noise in entangled qubits to assess error models beyond standard assumptions.
  • For quantum algorithm developers: Evaluate algorithm robustness against correlated noise models that include fully depolarizing components affecting entangled qubits.

A position paper. It proposes an approach and reports no results.

Authors

Gil Kalai

Abstract

In this paper I revisit my 2006 conjecture on correlated errors in entangled physical qubits, originally proposed as a potential obstruction to quantum fault tolerance. The conjecture asserts that, in any physical implementation of a quantum computer, the effective noise channel acting on entangled physical qubits contains a joint fully depolarizing component, with a rate comparable to that of two-qubit gate errors. This hypothesized structural constraint goes beyond standard noise models and, if valid, would pose a significant challenge to scalable quantum fault tolerance. The conjecture remains open, but recent advances in experimental quantum computing bring it within reach of empirical testing on current devices. I also discuss two related directions in my critical study of quantum computation: the role of noise sensitivity and computational complexity in noisy intermediate-scale quantum systems, and the statistical analysis of experimental claims of quantum advantage. Finally, since this paper is written for a volume honoring Yuri Gurevich, I include some reflections on the ways in which my scientific and personal trajectory became intertwined with Yuri's.
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