Motivation
Positive maps give tests for entanglement in bipartite quantum states. A density matrix can pass one positive-map test and fail another, so the choice of map changes which states can be detected. Maps that behave predictably under unitary changes of basis are particularly structured, but that symmetry can appear restrictive. Bardet, Collins, and Sapra ask whether requiring unitary equivariance loses the ability to detect states that a given positive map detects. Their Theorem 5.1 says that it does not, provided one permits a sequence of larger output matrix algebras. Bardet, Collins, and Sapra, §5
The question sits between two familiar perspectives. The positive-map criterion tests a state by applying a map to one subsystem; unitary equivariance controls how that map reacts when the subsystem is rotated. The paper's result shows that a specified detector can be recovered, in the limit of a nested family, within this symmetric class. It addresses a fixed input map and a fixed ancillary dimension. The paper separately states a more uniform corollary about Schmidt number, but that corollary is outside this mission. Bardet, Collins, and Sapra, Theorem 5.1 and Corollary 5.4
Setting
Write Mn(C) for complex n×n matrices and Un for their unitary group. A complex linear map ϕ:Mn(C)→MN(C) is t-positive if idt⊗ϕ sends every positive semidefinite matrix on Ct⊗Cn to a positive semidefinite matrix. It is completely positive if it is r-positive for every natural number r. A map Φ:Mn(C)→MD(C) is unitarily equivariant if, for each U∈Un, there is one unitary V∈UD satisfying Φ(UXU∗)=VΦ(X)V∗ for every X∈Mn(C). The choice of V may depend on U and Φ, while the equality must hold for all X. Bardet, Collins, and Sapra, Definition 2.1
A density matrix on Cm⊗Cn is positive semidefinite and has trace one. The detection set of ϕ at ancillary dimension m is
Sm(ϕ)={ρ:ρ≥0, Trρ=1, (idm⊗ϕ)(ρ)≥0}.
Here ϕ acts on the second tensor factor. A state outside Sm(ϕ) is detected by ϕ: applying idm⊗ϕ produces an operator that is not positive semidefinite. No trace preservation or unitality is required of ϕ. Bardet, Collins, and Sapra, equation (17)
Formalization targets
Main theorem
Fix m,n,N,t with 1≤t≤min{m,n} and a t-positive map ϕ:Mn(C)→MN(C) that is not completely positive. The target is one family Φa:Mn(C)→MNa(C), chosen before any state ρ, for which each member is unitarily equivariant and t-positive, Na→∞, and
Sm(Φa+1)⊆Sm(Φa),a≥1⋂Sm(Φa)⊆Sm(ϕ).
Thus the family eventually detects every state detected by ϕ, and detection persists at later indices. The ancillary dimension m is fixed before the family is chosen. Bardet, Collins, and Sapra, Theorem 5.1
Intermediate targets
The milestone statements cover the infinite dimensional lift of a linear map to H=L2(Un,CN), its inclusion Sm(Φ)⊆Sm(ϕ), the finite dimensional invariant subspaces supplied by the Peter–Weyl decomposition, and the compression of the lift to such a subspace, which is a unitarily equivariant t-positive matrix map Ψ with Sm(Φ)⊆Sm(Ψ). These are the source's Lemma 5.3, Theorem 5.2 for its explicit witness, the consequence of equations (24)–(25) used in §5.2, and the compression step of the proof of Theorem 5.1 on pp. 14–15. The milestone for Theorem 5.2 names the particular fibrewise lift constructed in the paper as its witness and asserts its equivariance, its t-positivity and the inclusion of detection sets. Bardet, Collins, and Sapra, §5.1–5.2
Significance
The theorem shows that symmetry alone does not prevent a positive map from detecting the states detected by a specified input map. It turns a single detector into a nested family with growing finite output dimensions, while keeping the same positivity order. The conclusion is pointwise in states but uses one sequence uniformly across all density matrices at the fixed ancillary dimension: each state outside Sm(ϕ) is excluded from every sufficiently late Sm(Φa). The result does not assert that any one finite output dimension suffices for every state. Bardet, Collins, and Sapra, Theorem 5.1
The paper proves this mathematical result. This mission records its statements in Lean, leaving proofs of the drafted goal and milestones to formalization work. The finite matrix definitions and the infinite dimensional L2 interface can be reused for other questions about positive maps, regular representations, and finite dimensional compression. The published tensor superoperator and complete positivity definitions are shared with other quantum information developments rather than redefined here.
Difficulty
The immediate detector ϕ need not be unitarily equivariant. Replacing it by a single symmetric finite matrix map therefore gives no direct reason to preserve all of its detected states. The source passes through a fibrewise map indexed by every input unitary, so the intermediate output space is infinite dimensional. The remaining difficulty is to return to finite matrix outputs while retaining both equivariance and t-positivity, and to choose the outputs so their detection sets are nested. A finite dimensional truncation without invariance under the regular action would not establish the claimed symmetry. Bardet, Collins, and Sapra, §5.1–5.2
Formalization scope
Finite matrices are Matrix (Fin n) (Fin n) ℂ; positive semidefiniteness is Matrix.PosSemidef. The ancillary factor is first in Fin m × Fin n, and the existing QAlgorithms.idTensorSuperop applies the map to the second factor. Complete positivity is the published QAlgorithms.Entanglement.IsCompletelyPositive. The paper's indices a≥1 become Lean natural numbers starting at zero. The density matrix test includes both positivity and trace one. The output dimensions form an actual function tending to infinity, and the maps form a dependent family whose codomain uses that function. The goal keeps the printed “not completely positive” hypothesis and all five stated conclusions. In particular, choosing detectors separately for each state would not meet it.
For the intermediate result, H is a complex L2 space over a Borel measure on Un. The lift is encoded by its sesquilinear form, whose kernel at U is ϕ(U∗XU); positivity on Ct⊗H is tested by all block quadratic forms. A finite measure and two L2 vectors make these integrals integrable because the matrix kernel is bounded. The regular action uses left invariance; the detection inclusion uses positivity of nonempty open sets; the Peter–Weyl milestone uses Haar probability measure. These are properties of the paper's Haar measure. The source's standing convention that linear maps preserve adjoints is already implied by t-positivity for the goal and is not a separate assumption there. The general linear map in the lift milestones is stated exactly where the source permits it.
The development needs the shared matrix-map definitions, the L2 and measure interfaces, and finite dimensional unitary representation theory. Work on the bounded-operator interpretation of the form, invariant truncations, and compression can be reused beyond this mission. The paper's inconsistent phrase “Schmidt number smaller than t” is not encoded, because the target uses only detection sets. Bardet, Collins, and Sapra, pp. 12–15
Selected references
- I. Bardet, B. Collins, and G. Sapra, Characterization of equivariant maps and application to entanglement detection, arXiv preprint, 2019, arXiv:1811.08193v3.