What is the difference between physical qubits and logical qubits in scaling?

All answers to this question.

This is the "billion-dollar question" in the industry. A physical qubit is the actual hardware (like a superconducting loop), while a logical qubit is a collection of physical qubits working together using error correction codes (like the Surface Code). Currently, depending on the error rate of the hardware, it can take anywhere from 100 to 1,000 physical qubits to create just one reliable logical qubit. This is why a 1,121-qubit chip is impressive but still only represents a handful of "clean" qubits for actual logic. We are likely 5–10 years away from the millions needed for Shor's algorithm.

   Answered 2025-05-10 by Kimberly White

Does this mean we should be looking at "Logical Qubit Roadmaps" instead of just raw qubit counts when evaluating quantum vendors?

   Answered 2025-06-02 by Kevin Parker

• Absolutely, Kevin. A vendor with 50 high-fidelity physical qubits might actually be closer to a stable logical qubit than one with 500 noisy ones. The focus has definitely shifted from "quantity" to "quality" in the last year, which is why you see companies like Microsoft and Quantinuum emphasizing their logical qubit breakthroughs over raw numbers lately.

     Commented 2025-06-08 by Patricia Moore

Think of it like "RAM" vs "ECC RAM"; logical qubits are the ones that actually stay stable long enough to finish a complex calculation without crashing.

   Answered 2025-07-15 by Daniel Evans

• That's a perfect analogy, Daniel. Without logical qubits, we're basically trying to solve a puzzle while someone keeps shaking the table—it just doesn't work for long.

     Commented 2025-07-20 by Kimberly White