.webp)
A physical qubit is an actual piece of quantum hardware — in QuEra's systems, a single neutral atom held in optical tweezers. A logical qubit is an error-corrected qubit encoded across many physical qubits, so that errors in individual atoms can be detected and fixed without destroying the computation. Physical qubits are fragile; logical qubits are what make quantum computers reliable enough to run useful applications.
Why can't we compute directly on physical qubits?
Every physical qubit is subject to noise. Even the best systems today reach two-qubit gate error rates around one in a thousand — QuEra's latest machines have demonstrated 99.77% two-qubit gate fidelity. That sounds high, but a commercially relevant algorithm may require millions or billions of operations. At physical error rates, the computation degrades into noise long before it finishes.
Quantum error correction (QEC) solves this by spreading one qubit's worth of information across a block of physical qubits. Repeated "syndrome" measurements reveal when and where errors occur, and a classical decoder determines how to correct them. The result is a logical qubit whose error rate can be made far lower than that of any individual atom — and, crucially, one that improves exponentially as the code grows, once the hardware operates below the QEC threshold.
How many physical qubits does one logical qubit require?
It depends on the error-correcting code. The widely studied surface code can require hundreds to a thousand physical qubits per logical qubit. QuEra's architecture uses high-rate qLDPC codes, enabled by the ability to move atoms and connect any qubit to any other, which reduces that overhead dramatically — peer-reviewed work in Nature Physics showed a more than 10x reduction in physical qubits compared to the surface code at the 3,000-qubit scale.
In 2023, QuEra and its Harvard and MIT collaborators demonstrated the world's first logical quantum processor, running 48 logical qubits on fewer than 300 physical qubits. QuEra's upcoming Libra system (link: roadmap page) will encode up to 256 logical qubits from just over 10,000 physical qubits at a logical error rate of 10⁻⁶ — one error per million logical operations.
Which number should you pay attention to?
Logical qubits. Physical qubit counts make headlines, but applications run on logical qubits, and performance is set by the logical error rate and how fast logical operations execute. A machine with many noisy physical qubits and no error correction cannot run deep circuits; a machine with hundreds of high-quality logical qubits can. That is why QuEra's roadmap is expressed in logical terms: 256 logical qubits at 10⁻⁶ error rates with Libra in 2028, and 1,000+ logical qubits at 10⁻⁹ with the gigaquop-class generation that follows.
