Quantum experiment generates long-range entanglement in 54-qubit system

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The operation and performance of quantum computers relies on the ability to realize and control entanglement between multiple qubits. Yet entanglement between many qubits is inherently susceptible to noise and imperfections in quantum gates.

In recent years, quantum physicists and engineers worldwide have thus been trying to develop more robust protocols to realize and control entanglement. To be most effective for real-world applications, these approaches should reliably support long-range entanglement, or in other words ensure that qubits remain entangled even when they are separated by large distances.

Researchers at IBM Quantum, University of Cologne and Harvard University set out to demonstrate one of these protocols in an experimental setting.

Their paper, published in Nature Physics, demonstrates the generation of long-range entanglement and a mixed state phase transition in a system comprised of 54 qubits on the ibm_sherbrooke quantum computer, which is powered by a 127-qubit IBM Eagle quantum processor.

"In 2022, teams at the University of Cologne's Institute for Theoretical Physics and Harvard's Department of Physics posted an arXiv pre-print in which they proposed a theoretical protocol to produce a widely studied long-range entangled state," Edward H. Chen, co-first author of the paper and researcher at IBM Quantum, told Phys.org.

"This paper caught the attention of our team at IBM. The protocol it introduced was not only relevant for the broad condensed matter community, but it also had relevance to the long-term interest of performing quantum error correction."

The recent study by Chen and his colleagues builds on the previous theoretical work of researchers at University of Cologne and Harvard, who introduced a new protocol to realize a long-range entangled state, known as the Ising order in a system with multiple qubits.

As IBM Quantum has an active collaboration with Harvard, they reached out to the university and initiated a collaboration with some of the researchers who introduced the protocol.

"The protocol entails the use of very few layers of quantum gates to create a minimal amount of entanglement between two sets of qubits and then measure out one set of them so as to consolidate the other set, leveraging the backaction of quantum measurements," explained Gui-Yi Zhu, co-first author of the paper.

"Another indispensable ingredient is the communication between the measured classical bits and the unmeasured qubits, akin to the idea of quantum teleportation, which was essentially the game that Ed and I were playing."

In their experiments, Chen, Zhu and their colleagues used classical decoding to extract and stabilize the quantum order in their 54-qubit system that was masked by noise. They found that this order persisted despite the errors in the system, up until the system entered a so-called Nishimori transition.

This critical phase transition was rare and classical systems require fine-tuning to reach this state. In contrast, the researchers found that this state emerged naturally in their quantum system, due to a rule that governs quantum measurement, known as Born's rule. Moreover, this phenomenon, which is rare in classical systems, was recently found to be ubiquitous in the open quantum systems, such as quantum error correction and topological phase of matter.

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"Ours is the first quantum experiment that harnesses Born's rule to create the celebrated Nishimori critical state without the need of fine-tuning as in the classical world," said Zhu.

"In retrospect, it realizes a generalized version of the spontaneous symmetry breaking in the open quantum system, dubbed strong-to-weak spontaneous symmetry breaking, which is a hot topic in theoretical physics recently motivated by the advent of quantum technologies."

The findings of this recent study could have important implications for the development of quantum computing systems. In the future, the protocol and methods used by Chen, Zhu and their colleagues could contribute to the scaling of quantum processors, while also informing research assessing the performance of large quantum computers in noisy environments.

"As authorities at the intersection of quantum information and condensed matter physics, Guo-Yi Zhu and Simon Trebst extended the insights from this work to consider the impact of sequential rounds of the protocol in different bases and the possibility of simulating spin liquids," added Chen.

"Ruben Verresen, along with Nathanan Tantivasadakarn and Ashvin Vishwanath, had the foresight years ago to study the impact of measurements and feed-forward in general for strongly interacting systems, and are also pioneers in this area. We are currently considering ways to mitigate the leading errors so that we could, in fact, perform multiple rounds of the protocol on our latest devices."

Chen and his colleagues at IBM Quantum are now working on improving the protocol they employed, while also developing new hardware that will be released over the next few years. By 2029, they expect to realize a highly performing quantum system that is capable of error correction.

More information: Edward H. Chen et al, Nishimori transition across the error threshold for constant-depth quantum circuits, Nature Physics (2024). DOI: 10.1038/s41567-024-02696-6

Journal information: Nature Physics , arXiv

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