Computer scientists say they’ve cracked the science behind error-correction in quantum computers thanks to new “4D codes.”
Developed by Microsoft, the new codes were revealed in a blog post published June 19 and purport to address the problem of fault tolerance — arguably quantum computing’s biggest bottleneck.
All computers can produce errors. In classical computing, error correction is achieved by making multiple copies of every bit of information that’s sent. If one or more bits are lost or corrupted, the remaining bits still contain the original information.
Qubits, however, can’t be copied. They also cannot be measured without experiencing what’s called “collapse.” This makes it much more challenging to detect and mitigate errors (which occur at a significantly higher rate than in classical bits) as they happen.
A typical quantum error-correction setup involves the addition of extra “physical” qubits to a system. These qubits are entangled with the “logical” qubits that typically carry quantum information. Instead of measuring the logical qubits, thus causing this collapse, scientists can check for errors by measuring the entangled physical qubits. This allows the computation process to continue.
Scientists typically employ 4D codes in the quantum error-correction process by recreating the topology of quantum processing surfaces on a four-dimensional lattice. This creates a self-correcting form of quantum memory.
Related: ‘The science is solved’: IBM to build monster 10,000-qubit quantum computer by 2029
The trouble is that most current error-correction techniques are either difficult to scale, resource-intensive, or both. The more physical qubits required to provide fault tolerance for a quantum system, and the more error-correction passes needed, the more energy is required for computation.
“Microsoft’s novel four-dimensional geometric codes require very few physical qubits per logical qubit, can check for errors in a single shot, and exhibit a 1,000-fold reduction in error rates,” said technical fellow of advanced quantum development at Microsoft Quantum, Krysta Svore, in the blog post.
A twist to quantum error-correction
The findings, uploaded June 18 to the arXiv preprint database, center on putting a literal twist on the torus-shaped 4D geometric code used for error-correction in certain quantum computing systems.
The scientists developed geometric code that could be overlaid in a system to detect errors using a four-dimensional topography. This 4D code connects the sample space (where the correction codes run) to the operational space (where the qubits contain information) via entanglement.
It works in four dimensions using a mathematical expression that, essentially, allows entanglement points to make connections over the surface of a “torus,” which can be imagined as a donut shape.
While 4D codes have been used to create self-correcting quantum memory in the past, their use here is considered novel because the researchers calculated a “twist” in the geometry that allows the same amount of code to cover the same amount of system space using fewer physical qubit entanglements.
By “twisting” the geometry, the 4D code overlay creates a larger representational space that reflects a greater portion of the quantum state of the actual qubits in use. Doing so allows researchers to detect errors in the code without disturbing the actual quantum processes occurring within the system.
The researchers ran their new “twisted” code on existing quantum computers and experimentally confirmed their theories in a separate preprint paper, published to the arXiv preprint server on June 13. Neither paper has yet been peer reviewed.
“Universal fault-tolerant quantum computers may be realized using 4D geometric codes, which are designed to enable efficiently realizing an increasing number of logical qubits with a modest number of physical qubits, while enabling low-depth logical cycles and universal fault tolerance,” the scientists said in the study.
Furthermore, the researchers purportedly demonstrated a groundbreaking technique for “replacing” the atoms used as qubits when they’re lost. In certain quantum computing systems, qubits are created by snagging neutral atoms with laser tweezers and trapping them in place. During computations, these atoms can be lost or dropped.
The researchers say they could replace lost atoms mid-cycle using an atomic beam to force new atoms into the array without disrupting the calculations — a first, the scientists said in the study.
Based on the findings, the new 4D code family could represent the second breakthrough in quantum error-correction in as many weeks. On June 10, IBM made a similar statement when it announced that it had developed quantum error-correction techniques that will lead to the development of a demonstrably useful quantum computer by 2029.
Where IBM’s new method utilizes a top-down development approach that takes advantage of its bespoke hardware, Microsoft’s is built from the bottom up to address fault tolerance using an approach that may have other applications beyond the hardware and use-cases it was tested on.
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