UCL and NQCC researchers measure one logical qubit inside a QLDPC block on Quantinuum H2
A preprint reports QLDPC surgery on Quantinuum's H2-2 trapped-ion processor, reading one of two encoded qubits in a code block while leaving the other intact.
Codes that pack several logical qubits into one block promise to cut the hardware cost of fault-tolerant quantum computing, but only if each encoded qubit can be operated on individually. A new preprint reports a hardware test of exactly that. Surabhi Luthra of University College London and Alex Owens of the UK's National Quantum Computing Centre (NQCC) used Quantinuum's H2-2 trapped-ion processor to measure one logical qubit inside a two-qubit code block without disturbing its neighbour, a technique known as QLDPC surgery.
What was announced
The paper, Individually Addressed Logical Pauli Measurements by QLDPC Surgery on a Trapped Ion Quantum Computer, was submitted to arXiv on 28 October 2026. The authors say that, to their knowledge, measuring individual logical qubits within a multi-qubit block by QLDPC surgery had not previously been done on hardware. Earlier surgery experiments, they note, used lattice surgery on code blocks that each held a single logical qubit.
The technical details
The team encoded two logical qubits in ten physical data qubits using a [[10,2,3]] generalised bicycle code, a member of the quantum low-density parity-check (QLDPC) family. To read out a chosen logical operator, the code block is temporarily linked to an ancillary "gadget" made of extra qubits. Checks measured on the gadget reveal the value of the targeted logical operator, and the block stays in its code space afterwards. That makes the measurement non-destructive, unlike a transversal readout, which collapses the encoded state.
According to the paper, the results were:
- A single round of surgery with seven gadget qubits measured the first logical qubit's X operator with a 0.72% error rate, and agreed with a later transversal measurement with 1.03% error.
- Decoding across three rounds of syndrome extraction pushed the gadget measurement error below 0.26%.
- The unaddressed "spectator" logical qubit was preserved with an error of 0.21% during the three-round measurement, and the syndrome after surgery confirmed that the block had returned to the code space.
- Reusing the same gadget qubits to measure X on the first logical qubit and then Z on the second gave a 1.12% error on the second measurement.
The authors note that their gadget construction applies to arbitrary QLDPC codes and underpins the processing blocks of a proposed architecture called Pinnacle.
Why it matters
The paper notes that fault tolerance built on the surface code needs hundreds to thousands of physical qubits for each logical qubit. QLDPC codes can lower that overhead by storing several logical qubits per block, but they need connections beyond nearest neighbours and a way to address each encoded qubit. Non-destructive, individually addressed Pauli measurements, combined with a supply of magic states, are enough for universal computation in the model known as Pauli-based computing. The authors describe their results as a hardware validation of the surgery primitive that QLDPC architectures depend on.
What to watch
The work is a preprint and has not been peer reviewed, and it uses a small distance-3 code. The open questions are whether the same gadget approach holds up on larger, higher-distance codes, how it combines with magic-state supply for full logical computation, and whether hardware with fixed rather than reconfigurable connectivity can support it at reasonable qubit cost.
Sources
Facts on this page come from the sources below, retrieved by DesignQuantum.