Researchers have set a new record for quantum entanglement — bringing reliable quantum computers a step closer to reality. The scientists successfully entangled 24 "logical qubits" — low-error quantum bits of information created by combining multiple physical qubits. This is the highest number ever achieved to date.
They also demonstrated that logical qubits can maintain error correction as the number of qubits increases, a crucial step toward larger, more fault-tolerant quantum systems. The researchers detailed their work in a study published Nov. 18 on the preprint database arXiv.
Despite the incredible promise of quantum computing, several key barriers stand in the way of replacing classical computing. One of these obstacles is controlling qubits — the basic units of quantum information — which is extremely difficult.
Unlike the binary 1s and 0s of traditional computer bits, qubits operate on an entirely different set of mechanics — quantum mechanics, to be precise. While qubits can exist as 1s and 0s, they can also exist as both at the same time, a phenomenon known as superposition. This makes measuring qubits a major challenge.
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Two other quantum phenomena, coherence and entanglement, throw additional spanners into the works. Coherence is a measure of how long qubits retain the desired state needed to process quantum calculations. Coherence times are usually measured in fractions of a second and can be disrupted by the tiniest of environmental factors.
When qubits lose coherence, they often also lose entanglement — a mechanism whereby the state of one qubit is tied directly to that of another. This loss of coherence and entanglement adversely affects the ability of quantum computers to perform calculations accurately and reliably.
Enter the logical qubit
In recent years, researchers have increasingly focused on logical qubits as a means of overcoming the fragility of physical qubits.
While physical qubits are typically made of charged particles like ions or superconducting circuits, logical qubits are created by encoding quantum information across multiple physical qubits. This architecture provides an error-correction system, so that if one qubit becomes unstable or loses information, the other qubits can detect and correct it.
The scientists successfully entangled their record-breaking 24 logical qubits using Atom Computing's "neutral-atom quantum processor," which processes and stores quantum information by manipulating individual atoms with lasers, and Microsoft's "qubit-virtualization system," a software platform that helps manage and stabilize qubits by detecting and correcting errors in real time.
While 24 may not seem like a huge number, the ability to entangle this many logical qubits represents a key milestone toward creating scalable, fault-tolerant quantum systems, the researchers said.
"Fault tolerant quantum computing is essential for being able to solve large computational problems that enable scientific and economic value beyond classical computing, and it requires the integration of multiple advanced technologies and quantum error correction algorithms to provide sufficient reliable computing resources in a sustainable way," Atom representatives said in a statement. "With these results [we] have now demonstrated all of the key ingredients necessary for supporting quantum error correction."
The researchers also showed how logical qubits can perform complex tasks and keep errors in check as quantum computers scale. Using the same Atom system, they created and ran computations on 28 logical qubits, proving that it's possible to maintain error correction as quantum systems get more powerful and complex.
"By coupling our state-of-the-art neutral-atom qubits with Microsoft's qubit-virtualization system, we are now able to offer reliable logical qubits on a commercial quantum machine," Ben Bloom, founder and CEO of Atom Computing, said in a statement. "This system will enable rapid progress in multiple fields including chemistry and materials science."
