What is a logical qubit?
A logical qubit is one unit of quantum information stored across a group of error-prone physical qubits. That way, faults in individual qubits can be detected and corrected before they spoil a calculation.
Also known as: error-corrected qubit, encoded qubit
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How it works
The best quantum computers get about one operation in a thousand wrong, according to a NIST explainer updated in May 2026. Logical qubits are the workaround. IBM defines a logical qubit as the building block of an error-corrected machine. Several physical qubits together hold one qubit's worth of information and check one another for errors, it says.
In the surface code, a square grid of physical qubits represents one logical qubit. Google researchers tested grids of 3 by 3, 5 by 5 and 7 by 7. They said each step up cut the encoded error rate by a factor of 2.14. Bigger grids help only on good enough hardware. The December 2024 Nature paper on that work says exponential improvement holds only if the physical error rate is below a critical threshold. Its largest code used 101 qubits and had a 0.143 percent error rate per correction cycle. It outlasted its best physical qubit by a factor of 2.4.
Why counts can mislead
Logical qubits differ in quality, so counts alone say little. In December 2023, Harvard University researchers and collaborators reported in Nature circuits with up to 48 logical qubits on a neutral-atom processor running up to 280 physical qubits. Quantinuum said in November 2025 that its 98-qubit Helios machine produced 94 logical qubits in one entangled state, 50 error-detected ones and 48 fully error-corrected ones. It said using fewer gives higher fidelity.
Where things stand in 2026
IEEE Spectrum reported in June 2026 that the record claims belonged to QuEra, with 96 logical qubits, and Quantinuum, with 94. Jay Sau of the University of Maryland told the magazine the label now seems to mean a group of qubits only somewhat better than one. Edward Parker of the Rand Corporation called hundreds of logical qubits by 2028 plausible. But Parker said they would still be noisy.
The U.S. Department of Energy said on June 23, 2026 it would hold a competition to demonstrate fault-tolerant systems with logical qubits in the low hundreds in 2028. It opened the contest on September 17, 2026, with up to $215 million in planned funding for machines with at least 100 logical qubits. It set an application deadline of October 19, 2026. IBM said in June 2025 it plans to deliver Starling, with 200 logical qubits able to run 100 million operations, by 2029. Those are goals, not delivered machines.
Sources
- Quantum Computing Explained, National Institute of Standards and Technology (NIST)
- IBM Sets the Course to Build World's First Large-Scale, Fault-Tolerant Quantum Computer at New IBM Quantum Data Center, IBM
- Making quantum error correction work, Google Research
- Quantum error correction below the surface code threshold, Nature
- Logical quantum processor based on reconfigurable atom arrays, Nature
- Introducing Helios: The Most Accurate Quantum Computer in the World, Quantinuum
- The Trump Administration Doubles Down on Quantum, IEEE Spectrum
- Energy Department Announces Initiative to Create and Deploy the World’s First Scientifically Relevant, Fault-Tolerant Quantum Computers, U.S. Department of Energy
- DOE Launches Competition to Accelerate Development of World’s First Fault-Tolerant Quantum Computer, U.S. Department of Energy
