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TheoremStatement: Literature-sourcedProof: AI-generatedPipeline-generatedjudge pass (gpt-6-luna)
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Exact and approximate success of complete-graph continuous-time search

Statement

For every integer N≥2, every marked label w∈[N], and every encoded complete-graph search instance of Complete-graph continuous-time search whose supplied preparation produces es=N−1/2∑xeenc(x), exact evolution under H=−A/N−∣eenc(w)⟩⟨eenc(w)∣ at t=πN/2 places the search register in the marked basis ray with probability exactly 1. If that evolution is replaced by a unitary V with ∥V−UH(t)∥op≤ρ for ρ≥0, the measured probability of the marked label is at least 1−2ρ.

Facts & Assumptions

Given: The finite encoded register, N≥2, marked label w, complete-graph generator and uniform state from the algorithm, real time t=πN/2, and, for the approximation clause, a unitary simulator V with the stated operator-norm error bound.

[F1]

Complete-graph continuous-time search: on the active label subspace, Aeenc(x)=∑y≠xeenc(y), the marked projector is Pw=∣eenc(w)⟩⟨eenc(w)∣, H=−A/N−Pw, and es=N−1/2∑xeenc(x).

[F2]

Finite Hermitian matrix dynamics: for a finite self-adjoint matrix, UH(t) is the spectral sum with coefficient e−itλ on the λ-eigenspace.

[F4]

The computational basis is orthonormal: for every d≥1, the ordered computational basis of Qd is orthonormal; in particular, each qubit factor basis (e0,e1) is orthonormal.

[F5]

Dirac Notation Conversion: ∣x⟩⟨y∣ acts by z↦⟨z,y⟩x.

[F8]

Quarter-turn values and shifts by pi/2 and pi: cos⁡(π/2)=0 and sin⁡(π/2)=1.

[F9]

The measurement law for a finite continuous-time walk: for a unit register state, the probability of basis outcome y is the squared modulus of its y-coordinate.

[F10]

Operator-norm error bounds a basis-outcome probability change: for unitary outputs with operator-norm separation at most ρ, any one basis-outcome probability changes by at most 2ρ.

[F11]

The adjoint T∗:W→V is characterised by ⟨Tv,w⟩W=⟨v,T∗w⟩V: the adjoint is characterized by ⟨Tx,y⟩=⟨x,T∗y⟩.

[F12]

Ordered tensor registers, their inner product, and wire permutations: an ordered tensor register uses the lexicographic product basis and product inner product; with the orthonormal qubit bases from [F4], the full computational basis is orthonormal.

Proof

technique · direct
1.1givenF1F4F5F11F12algebra

Write ∣w⟩:=eenc(w) and ∣r⟩:=(N−1)−1/2∑x≠weenc(x). By [F4], [F12] these are orthonormal; the sum defining ∣r⟩ has N−1≥1 terms. In their ordered basis, [F1] gives A∣w⟩=N−1∣r⟩ and A∣r⟩=N−1∣w⟩+(N−2)∣r⟩. By [F5], Pw∣w⟩=∣w⟩ and Pw∣r⟩=0, so its restricted matrix is diag⁡(1,0). Thus their span S is invariant under H, and the restricted matrices are A=(0N−1N−1N−2),H=(−1−N−1/N−N−1/N−(N−2)/N). In the full orthonormal computational basis [F12], A has real symmetric entries between distinct encoded labels and zero entries elsewhere; [F5] makes Pw diagonal with a single 1. Hence the matrix of H=−A/N−Pw is real symmetric, so ⟨Hx,y⟩=⟨x,Hy⟩ for all x,y and [F11] gives H=H∗. For z∈S⊥ and v∈S, self-adjointness and invariance of S give ⟨Hz,v⟩=⟨z,Hv⟩=0, so S⊥ is invariant too. Thus H splits into self-adjoint blocks on S and S⊥. The input state is ∣s⟩=N−1/2∣w⟩+(N−1)/N∣r⟩, which is unit because ∣w⟩,∣r⟩ are orthonormal and the squared coefficients sum to 1.

2.1step 1.1algebra

On this two-dimensional invariant span put K:=H+(N−1)I/N. Then K=1N(−1−N−1−N−11),K2=I/N,NK∣s⟩=−∣w⟩. The first two equalities follow by direct multiplication; for the last, the first coordinate of K∣s⟩ is −1/N and the second is 0.

3.1step 1.1step 2.1F2F6F7algebra

Let P±=(I±NK)/2. Step 2.1 and self-adjointness of K give P±2=P±=P±∗, P+P−=0, P++P−=I, and KP±=±P±/N. By step 1.1, each eigenspace of H is the orthogonal sum of the corresponding eigenspaces of its two blocks: projecting Hz=λz onto either invariant summand preserves that equation. Therefore the spectral sum [F2] restricts on S to the sum determined by P±. On S, H=K−(N−1)I/N, so [F2] and [F6] give, for a=t/N, UH(t)∣S=eit(N−1)/N(e−iaP++eiaP−). By [F7], this is eit(N−1)/N(cos⁡a I−iNsin⁡a K).

4.1step 2.1step 3.1F7F8F9algebra

At t=πN/2, [F8] gives a=π/2, so step 3.1 and NK∣s⟩=−∣w⟩ give UH(t)∣s⟩=ieit(N−1)/N∣w⟩. The coefficient has modulus one by [F7], hence [F9] assigns probability exactly 1 to the marked label.

5.1givenF3F10step 4.1algebra∎

In the approximate branch, both V and UH(t) are unitary and the prepared input ∣s⟩ is unit. Applying [F10] to the marked basis outcome shows its probability differs from the exact probability in step 4.1 by at most 2ρ. Since that exact probability is 1, the simulated success probability is at least 1−2ρ.

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