French startup claims to have worked out how to massively reduce error correction overhead for quantum computers

A technology breakthrough could bring the world closer to usable quantum computers this year, a French technology startup claims.

French quantum hardware developer Alice & Bob collaborated with the country’s Institut national de recherche en sciences et technologies du numérique (INRIA) to develop a new quantum error correction architecture.

It has published a theoretical paper which is a preprint, and not yet peer reviewed.

The promise of quantum computers is that once they are functional, researchers expect them to outperform traditional binary logic computers, being able to solve large-scale problems and simulations that currently would take too long on existing hardware.

Technology giants such as IBM and Google are trying to develop working quantum computers that are practical and usable, but progress in the field has been slow.

That's because quantum computers require error correction to be useful, which has turned out to be a difficult problem for researchers to solve.

“Over 90% of quantum computing value depends on strong error correction, which is currently many years away from meaningful computations,” said Jean-François Bobier, partner and director at the Boston Consulting Group, in Massachusetts, United States.

Alice & Bob and INRA are looking to use a new architecture that comprises low-density parity check codes, which is an existing protocol to correct for signal errors in noisy environments, but which has been difficult to implement and is computationally expensive as well. 

LDPCs as the codes are abbreviated as, are mandatory for the current WiFi-6 wireless networking specification.

The LDPCs are then used with “cat qubits”. These are logical qubits (quantum bits) that are built with physical, hardware qubits. Cat qubits have inherent resistance to bit-flip errors, which allowed the researchers to massively reduce the hardware overhead required for error correction in quantum computers.

The cat cubits are named after Austro-Irish physicist Erwin Schrödinger, who used a fictional feline to explain the strange notion of superposition; in the example, the cat is both dead and alive.

“This new architecture using LDPC codes and cat qubits could run Shor’s algorithm with less than 100,000 physical qubits, a 200-fold improvement over competing approaches’ 20 million qubit requirement.” Théau Peronnin, the chief executive of Alice & Bob claimed.

Shor’s factoring algorithm is used to find the prime numbers of an integer, and it is considered as a demonstration of how quantum computing can speed up tasks compared to classical approaches.

Alice & Bob said a standard approach quantum computer would require massive infrastructure: to run Shor’s algorithm, 100s of Megawatts would be required to power them. 

If the LDPC and cat cubit combination becomes reality, and only 100,000 or so qubits are required, the 200 times reduction would mean far lower power demand, and, the company said, quantum computers could be available to the industry by the end of 2024.
 

Speaking to interest.co.nz, Scott Parkins, associate professor of physics at the University of Auckland, said that although this is a theory paper, the authors should be pretty aware of the experimental situation which should keep them somewhat down to earth.

Parkins said Alice & Bob is indeed a significant player in the quantum computing industry.

“Some of their people have come out of the Yale University group, which pioneered circuit quantum electrodynamics (QED), and some of the key platforms and ideas that Alice & Bob are pursuing,” Parkins said.

However, Parkins is sceptical about the advances claimed by the French quantum company.

“Scaling these platforms up to hundreds of logical qubits, all subject to the same degree of control necessary for high-fidelity quantum logic operations and quantum error correction, is a formidable task,” he said.

“My understanding of the IBM-type systems that have more than a thousand physical qubits is that they have nowhere near the degree of control required for general quantum operation,” Parkins added.

That said, there have been impressive experimental demonstrations of cat cubits that encode qubits in Schrödinger cat states of quantised electromagnetic field modes and of monitoring and manipulating them for error correction, Parkins said.

Scaling the systems, which have been at the level of single or few-mode, up to hundreds or thousands or more modes, which is required to build a useful, fault-tolerant quantum computer, is an issue that remains, he said.

Nevertheless, if someone can design an architecture that reduces such overheads in a significant way, then that’s a very important result too, Parkins said.

As a related aside, the French company has a name with pedigree for IT geeks. Alice and Bob are two fictional characters that appeared for the first time in 1978, in a paper on cryptography written by the Ron Rivest, Adi Shamir and Leonard Adleman, who went on to form a well-known security firm, and who devised one of the earliest encryption systems for data transmission.

Alice and Bob, along with multiple other fictional characters such as Mallory, have since been used to illustrate examples in physics, mathematical models and quantum cryptography.