IBM Says Decoder Could Push Quantum Computing Toward Fault Tolerance

NEW YORK, Aug. 5, 2025 — The field of quantum computing till recent past has struggled to find a decoder powerful enough to fix the errors of a large-scale, fault-tolerant quantum computer in real time.

However, a little over a year ago, IBM researchers across multiple disciplines, from electrical engineering to many-body physics, put their heads together and began tackling this challenge, said a company statement.

Last month, they published a preprint detailing their new algorithm, Relay-BP, which achieves logical error rates that are orders of magnitude better than all prior state-of-the-art decoders for qLDPC codes, it said.

Furthermore, it is both flexible and compact, making it ideal for decoding quantum memory experiments in an FPGA, a computation device which can be reconfigured after manufacturing.

In their paper, the researchers show Relay-BP to be the most performant real-time decoder for qLDPC codes to date in terms of flexibility, compactness, speed, and accuracy. Its arrival marks yet another step in the direction of fault-tolerant quantum computing.

The error correction decoder is designed to detect errors during quantum computation. It is an algorithm implemented on classical hardware attached to a quantum computer, and is part of a larger family of components working together to store and process information, find errors, and correct them in quantum computers, the statement said.

How Does the Decoder Work?  

The decoder consumes all of the physical measurements throughout a logical circuit to identify the most probable course of errors. The decoder, along with all the other components, is necessary to achieve fault-tolerant quantum computing. Finding a good decoderQuantum error correction (QEC) refers to the methods by which we protect quantum data when storing, transmitting, and manipulating it, says the statement.

Qubits are particularly sensitive to their environment, so we must devise a way to protect them if we want them to maintain their quantum state. If we are able to produce a quantum computer with sufficiently low logical error rates (i.e. with minimal disturbance to our qubits), we can achieve fault-tolerant quantum computing. QEC is an essential part of this mission.

In practice, QEC uses a code that brings together a bunch of unreliable physical qubits to create reliable logical qubits. Due to the laws of quantum physics, we cannot extract error information from logical qubits directly, as doing so would destroy their quantum information, so we measure properties of the unreliable qubits to form a kind of "signature" of the underlying error.

These measurements are known as syndromes; pieces of information that give us clues as to what’s causing the error. Then, it’s up to the decoder to decipher the syndrome data and determine how to correct the problem.

Things to Know About the Decoder

It becomes imperative to various important aspects of decoders:

Different Decoders: A fault-tolerant quantum computer requires a decoder that is flexible, accurate, fast and compact. That is, it must be able to decode a full suite of quantum circuits accurately, it must decipher errors fast enough so that it does not create an ever-increase backlog of undecoded syndromes, and it should only use a small number of computational resources.

Decoders exist in classical computation as well, and classical algorithms such as belief propagation (BP) can shed light on how quantum decoders work. How does it work? BP is what’s known as a message-passing algorithm, as it transmits information between a collection of compute units.

Functioning of Units: Each unit acts like an individual person, and when grouped, they behave as several people talking to one another. With error correction decoding, it’s as though the people are trying to figure out who among them is responsible for a crime—that is, the source of the computational error, which could be from some qubit’s failure or from the act of measuring the qubit itself.

Each person holds their own belief about the probability of some person being responsible, and by talking to each other and disseminating their knowledge, they can come to a conclusion and locate the perpetrator, said the statement.

However, in many cases standard BP struggles to converge to a solution on qLDPC codes, sometimes oscillating between two solutions, and at other times converging to incorrect or uncertain solutions.

Until the arrival of Relay-BP, researchers who wanted the most accurate decoder available usually chose BP+OSD, belief propagation working in tandem with another algorithm known as ordered statistics decoding (OSD). However, OSD relies on expensive mathematical calculations, so even though it can be accurate, it is difficult to implement in real-time in an efficient and cost-effective way.

Relay-BP Decoder: For these reasons and more, Relay-BP outperforms all other qLDPC decoders across the four key metrics we use to judge a decoder’s performance: flexibility, compactness, speed, and accuracy. Flexibility assesses how well the decoder works across different kinds of qLDPC codes. Compactness determines resources used in an implementation on a CPU or FPGA. Speed evaluates the deftness with which it avoids generating a backlog of error syndromes. Accuracy measures its logical error rate.

Relay-BP builds heavily on BP, which on its own is quite fast but not as accurate as Relay-BP. BP+OSD is much more accurate than BP, but falls short on compactness and speed. In their paper, the IBM researchers show that Relay-BP delivers a roughly 10x improvement in accuracy when compared to BP+OSD, while maintaining and in some cases improving upon the speed of BP. Relay-BP showed similar performance improvements over a range of leading alternative methods the research team examined. This means that, to our knowledge, Relay-BP is the only real-time qLDPC decoder that hits all four nails on the head. It is not only flexible and compact, but also faster and more accurate than all known alternative methods.

In their paper, the IBM researchers show that Relay-BP delivers a roughly 10x improvement in accuracy when compared to BP+OSD, while maintaining and in some cases improving upon the speed of BP.

Inputs from Saqib Malik

Editing by David Ryder