2026 Quantum Error Correction Milestones: What You Need to Know

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1. Which organisation reported in 2024 that scaling a surface code from distance-3 to distance-5 to distance-7 roughly halved the logical error rate at each step, placing the code 'below threshold'?

   • A. Google Quantum AI
   • B. IBM Quantum
   • C. IonQ
   • D. Rigetti Computing

2. In the surface code, what happens to the logical error rate as the code distance is increased, provided the physical error rate is below threshold?

   • A. It is suppressed exponentially with the code distance
   • B. It falls linearly with the code distance
   • C. It stays constant regardless of code distance
   • D. It rises, because more physical qubits mean more total errors

3. What property of AWS's cat qubits allows them to be protected by a simpler, hardware-efficient error-correcting code?

   • A. Their noise is biased: bit-flip errors are exponentially suppressed, so mainly phase flips must be corrected
   • B. They are immune to all errors, so no code is needed at all
   • C. They operate at room temperature, eliminating thermal noise
   • D. They store a logical qubit in a single two-level atom

4. Which decoding algorithm is the long-standing baseline for surface-code syndrome decoding, pairing up detection events on a graph?

   • A. Minimum-weight perfect matching
   • B. Union-Find decoder
   • C. Neural network decoder
   • D. Belief propagation decoder

5. How many physical qubits does a single distance-7 rotated surface code logical qubit require?

   • A. 49
   • B. 97
   • C. 145
   • D. 200

6. Fault-tolerant operations on a logical qubit encoded in the [[7,1,3]] colour (Steane) code were first demonstrated on which hardware platform?

   • A. Trapped-ion processors, by groups such as Innsbruck and Quantinuum
   • B. Superconducting transmon processors at Yale
   • C. Photonic chips at Xanadu
   • D. Silicon spin qubits at Delft

7. What is the main appeal of bosonic codes such as cat and GKP codes compared with 2D qubit-array codes?

   • A. They build the redundancy into a single harmonic-oscillator mode, cutting the number of physical components needed
   • B. They remove the need for any active syndrome measurement or feedback
   • C. They tolerate physical error rates above 50%
   • D. They can be operated at room temperature

8. Which condition defines the 'break-even' point for quantum error correction?

   • A. Physical gate fidelity exceeds 99.9%
   • B. The encoded logical qubit outperforms the best physical qubit in the same system
   • C. Syndrome measurement cycle time falls below 1 μs
   • D. The processor holds more than 10 logical qubits

9. Which error source in superconducting processors, documented by Google, violates the surface code's assumption that errors are independent?

   • A. Slow drift in qubit frequency calibration
   • B. Correlated errors from cosmic-ray and quasiparticle burst events
   • C. Amplitude damping during idle periods
   • D. Readout classification errors on individual qubits

10. Why cannot the surface code on its own provide a universal set of logical gates?

    • A. Its fault-tolerant native gates are limited to Clifford operations, so a non-Clifford gate such as T must be supplied by magic-state distillation
    • B. It cannot perform any logical two-qubit gates
    • C. Its syndrome extraction is too slow for any logical gate
    • D. It can only ever encode one logical qubit per chip

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