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Enhancing Quantum Error Correction Effectiveness

In a study published in the journal Nature Physics, a team of scientists led by researchers from the University of Chicago’s Pritzker School of Molecular Engineering (PME) created the blueprint for a quantum computer that can fix errors more efficiently.

Although quantum computers are an extremely potent computational tool, their delicate qubits challenge engineers: how can they design useful, functional quantum systems using bits that are easily disrupted and erased of data by minute changes in their environment?

Engineers have long grappled with how to make quantum computers less error-prone, frequently creating methods to identify and rectify problems rather than preventing them in the first place. However, many of these error-correction systems entail replicating information over hundreds or thousands of physical qubits simultaneously, making it difficult to scale up efficiently.

The system makes use of reconfigurable atom array hardware, which enables qubits to communicate with more neighbors and, consequently, allows the qLDPC data to be encoded in fewer qubits, as well as a new framework based on quantum low-density party-check (qLDPC) codes, which can detect errors by examining the relationship between bits.

With this proposed blueprint, we have reduced the overhead required for quantum error correction, which opens new avenues for scaling up quantum computers.

Liang Jiang, Study Senior Author and Professor, Pritzker School of Molecular Engineering, University of Chicago

Intrinsic Noise

While standard computers rely on digital bits—in an on or off position—to encode data, qubits can exist in states of superposition, giving them the ability to tackle new computational problems. However, qubits’ unique properties also make them incredibly sensitive to their environment; they change states based on the surrounding temperature and electromagnetism.

Quantum systems are intrinsically noisy. There’s really no way to build a quantum machine that won’t have error. You need to have a way of doing active error correction if you want to scale up your quantum system and make it useful for practical tasks.

Qian Xu, Graduate Student, Pritzker School of Molecular Engineering, University of Chicago

For the previous few decades, scientists have primarily relied on one type of error correction, known as surface codes, for quantum systems. In these systems, users encode the same logical information into several physical bits grouped in a wide two-dimensional grid. Errors can be detected by comparing qubits to their immediate neighbors. A mismatch indicates that one qubit misfired.

Xu added, “The problem with this is that you need a huge resource overhead. In some of these systems, you need one thousand physical qubits for every logical qubit, so in the long run, we don’t think we can scale this up to very large computers.

Lowering Redundancy

Jiang, Xu, and colleagues from Harvard University, Caltech, the University of Arizona, and QuEra Computing designed a novel method to fix errors using qLDPC codes. This type of error correction had long been contemplated but not included in a realistic plan.

With qLDPC codes, data in qubits is compared to both direct neighbors and more distant qubits. It enables a smaller grid of qubits to do the same number of comparisons for error correction. However, long-distance communication between qubits has always been a challenge when implementing qLDPC.

The researchers devised a solution in the form of new hardware: reconfigurable atoms that can be relocated using lasers to enable qubits to communicate with new partners.

With today’s reconfigurable atom array systems, we can control and manipulate more than a thousand physical qubits with high fidelity and connect qubits separated by a large distance. By matching the structure of quantum codes and these hardware capabilities, we can implement these more advanced qLDPC codes with only a few control lines, putting the realization of them within reach with today's experimental systems.

Harry Zhou, Ph.D. Student, Harvard University

When researchers paired qLDPC codes with reconfigurable neutral-atom arrays, they achieved a lower error rate than surface codes using only a few hundred physical qubits. When scaled up, quantum algorithms requiring thousands of logical qubits might be completed with fewer than 100,000 physical qubits, vastly outperforming the gold-standard surface codes.

There’s still redundancy in terms of encoding the data in multiple physical qubits, but the idea is that we have reduced that redundancy by a lot,” Xu added.

Though scientists are developing atom-array platforms quickly, the framework is still theoretical and represents a step toward the real-world use of error-corrected quantum computation. The PME team is now striving to improve its design even more and ensure that reconfigurable atom arrays and logical qubits relying on qLDPC codes can be employed in computation.

Xu concluded, “We think in the long run, this will allow us to build very large quantum computers with lower error rates.

Journal Reference:

Xu, Q., et. al. (2024) Constant-overhead fault-tolerant quantum computation with reconfigurable atom arrays. Nature Physics. doi:10.1038/s41567-024-02479-z

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