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Q39 / Beyond qubits / question

Which compact-group irrep measurements support universal computation?

Replace qubits and SU(2)\mathrm{SU}(2) by a fixed representation of a compact group. Classify the computation generated by measuring irreducible labels on small subsets.

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Working mathematical formulation

Fix a compact group GG, a finite-dimensional unitary representation VV, and a constant arity kk. On NN systems initially in the maximally mixed state on V⊗NV^{\otimes N}, permit adaptive projective measurements onto the isotypic components of V⊗rV^{\otimes r} on selected subsets, r≤kr\le k, with uniform polynomial-time classical control. Preserve coherence within each reported component, including multiplicity. Require an effective description of the representation: each permitted constant-arity instrument must admit a uniform circuit approximation of cost poly⁡(log⁡(1/ε))\operatorname{poly}(\log(1/\varepsilon)) at error ε\varepsilon, or its implementation is a separately declared oracle resource. Which triples (G,V,k)(G,V,k) give universal bounded-error quantum computation under an explicit encoding, which admit efficient classical simulation, and which yield intermediate models? Count all input preparation, readout, ancillary systems, and approximation costs.

Scope & assumptions

The group and representation are fixed model data here. A growing representation or a black-box group requires separate input and precision conventions. Projecting onto an isotypic component differs from resolving individual multiplicity copies.

Why this is interesting

This asks whether small-subsystem irrep measurement defines a general computational primitive. Qudits in the fundamental representation and physical higher spins are particular cases of this broader question.

A place to start

Work out the two- and three-system decompositions for a chosen representation, determine the algebra generated by its isotypic projectors, and compare the reachable instruments with an encoded universal gate set.

Related work

These works provide context and established results. They need not be the origin of this exact formulation.

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